Digestive System
A digestive system is a group of organs that break down food (physically and chemically) so that it can be used by the body cells.
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| Fig. 1: The digestive system |
The organs of the digestive system can be broadly divided into two:
Gastrointestinal (GI) tract
The GI tract or alimentary canal is a 5-7 meters long continuous tube that begins at the mouth and ends at the anus. The GI tract passes through the thoracic and abdominopelvic cavities.
Several organs constitute the GI tract:
- Mouth
- Most part of the pharynx
- Esophagus
- Stomach
- Small intestine
- Large intestine
The walls of the GI tract produce muscular contraction to:
- Physically break down the food by churning
- Propel the food from the esophagus to the anus
- Mix the food with the secretions in the GI tract
Accessory digestive organs
The accessory digestive organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
Teeth: help in physical breakdown of food
Tongue: helps in chewing and swallowing food
Other accessory digestive organs: produce or store secretions that help in the chemical breakdown of food
Mainly, the digestive system performs six elementary processes:
- Ingestion or eating: This process of taking foods and liquids into the mouth.
- Secretion: Cells of the walls of the GI tract and accessory digestive organs secrete water, mucous, acid, buffers, and enzymes into the lumen of the tract.
- Mixing and propulsion: The GI tract mixes the food and secretion and propel the mixture towards the anus. This capability of the GI is called motility. This is achieved by the alternating contractions and relaxations of smooth muscle present in the walls of the GI tract.
- Digestion: Digestion is the physical and chemical breakdown of the ingested food into small molecules. In physical digestion, teeth cut and grind the food and smooth muscles of the GI tract churn the food so that the food becomes liquid after mixing with the secretion of the GI tract. The secretions of the GI tract contain digestive enzymes that cause hydrolysis (chemical digestion) of the large molecules present in the food. e.g., carbohydrates, lipids, proteins, and nucleic acid. The hydrolyzed molecules are easily absorbed from the GI tract. Substances like vitamins, ions, cholesterol, and water are absorbed without digestion.
- Absorption: Transportation of digested food, ions, and water into the blood via the epithelial cells lining the GI tract.
- Defecation: Unabsorbed substances, sloughed cells of the GI tract, and bacteria exit the GI tract through the anus. The excreted material is called feces.
Peritoneum
- Simple squamous epithelium (mesothelium)
- Areolar connective tissue
- Parietal peritoneum (lines the wall of the abdominopelvic cavity)
- Visceral peritoneum (covers some of the organs in the cavity)
Ascites: accumulation of several liters of fluid in the peritoneal cavity.
The peritoneum contains large folds that interlace between the viscera and keep the organs together and with the walls of the abdominal cavity.
There are five major peritoneal folds:
- Greater omentum (drapes over the transverse colon and coils of the small intestine like a “fatty apron”)
- Falciform ligament (attaches the liver to the anterior abdominal wall and diaphragm)
- Lesser omentum (suspends the stomach and duodenum from the liver)
- Mesentery (binds the jejunum and ileum of the small intestine to the posterior abdominal wall)
- Mesocolon (bind transverse colon and sigmoid colon to the posterior abdominal wall).
Layers of the GI Tract
The tissues of the GI tract are arranged in four basic layers from the lower esophagus to the anal canal.
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| Fig. 2: The four layers of the alimentary canal |
The four layers of the tract (from deep to superficial) are
- Mucosa
- Submucosa
- Muscularis
- Serosa or adventitia
Mucosa
The mucosa is the innermost lining of the GI tract and is a mucous membrane. It is composed of three-layer of tissue:
- Epithelium tissue (the innermost layer)
- Connective tissue called the lamina propria (middle layer)
- Smooth muscle called the muscularis mucosae (outer layer)
In the mouth, pharynx, esophagus, and anal canal, the epithelium tissue is chiefly non-keratinized stratified squamous epithelium that has a protective role.
The stomach and intestines are lined by simple columnar epithelium that helps in secretion and absorption.
The epithelial cells renew every 5-7 days. Amongst epithelial cells, several exocrine and endocrine cells (enteroendocrine cells) are present which secret mucous and hormone, respectively.
Areolar connective tissue constitutes the lamina propria layer. This layer contains blood vessels and lymphatic vessels that play a significant role in absorption. This layer binds the epithelium to the muscularis mucosae. This layer also contains the mucosa-associated lymphatic tissue (MALT) cells that contain immune cells and protect against disease.
Muscularis mucosae is a thin layer of smooth muscle fibers. In the stomach and intestine, muscularis mucosae provide several folds to the mucosa to increase the surface area for digestion and absorption.
Submucosa
Areolar connective tissue forms the submucosa that adheres the mucosa to the muscularis.
The submucosa contains blood and lymphatic vessels that transport absorbed food molecules. An extensive network of neurons, the submucosal plexus, is also present in the submucosa. Glands and lymphatic tissues are also present in the submucosa.
Muscularis
The muscularis layer is made up of muscular tissue.
Skeletal muscles of the muscularis layer of the mouth, pharynx, and superior and middle parts of the esophagus helps with voluntary swallowing. The external anal sphincter is also made up of skeletal muscle that allows voluntary control of defecation.
All over the rest of the GI tract, the muscularis comprises of smooth muscle that is normally available in two layers:
- Inner layer of circular fibers
- Outer layer of longitudinal fibers
Involuntary contraction and relaxation of the smooth muscle break down food, mix it with secretions of the GI tract, and push it along the GI tract.
Another network of neurons, the myenteric plexus is present between the two layers of the muscularis.
Serosa
Serosa is a serous membrane consists of simple squamous epithelium (mesothelium) and areolar connective tissue.
It is the superficial layer on the portions of the GI tract that are suspended in the abdominopelvic cavity.
The serosa is also called the visceral peritoneum because it forms a part of the peritoneum.
The esophagus only has a single layer of areolar connective tissue called the adventitia.
Regulations of GI tract
Activities of the GI tract is regulated by two sets of nerves:
- Enteric nervous system (intrinsic set of nerves)
- Autonomic nervous system (extrinsic set of nerves)
Enteric Nervous System (ENS)
ENS is also called the “brain of the gut” and involves about 100 million neurons that extend from the esophagus to the anus.
The neurons of the ENS are arranged into two plexuses:
- Myenteric plexus
- Submucosal plexus
Auerbach's plexus or myenteric plexus is located between the circular and longitudinal smooth muscle layers of the muscularis. It regulates the frequency and strength of contraction of the muscularis.
The submucosal plexus or plexus of Meissner is situated within the submucosa. It regulates the secretions of the organs of the GI tract.
The plexuses of the ENS comprise sensory neurons, interneurons, and motor neurons.
The sensory neurons have two chief sensory receptors present in the mucosal epithelium:
- Chemoreceptors (respond to certain chemicals in the food)
- Mechanoreceptors (stretch receptors, that are activated when food stretches the wall of a GI organ)
The interneurons of the ENS interconnect the neurons of the myenteric and submucosal plexuses.
Autonomic Nervous System (ANS)
Although the neurons of the ENS can function autonomously, they are subject to regulation by the neurons of the ANS.
The vagus (X) nerves supply parasympathetic fibers to most parts of the GI tract, except for the last half of the large intestine, which is innervated with parasympathetic fibers from the sacral spinal cord.
In general, the parasympathetic nerves stimulation causes an increase in GI secretion and motility by increasing the activity of ENS neurons.
Sympathetic nerves from the thoracic and upper lumbar regions of the spinal cord supply the GI tract.
In general, the sympathetic nerves that supply the GI tract cause a decrease in GI secretion and motility by inhibiting the neurons of the ENS.
Emotions such as anger, fear, and anxiety may slow digestion because they stimulate the sympathetic nerves that supply the GI tract
The organs of the GI tract
Mouth
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| Fig. 3: The oral cavity |
The mouth also called the oral or buccal cavity is made up of the cheeks, palates (hard and soft), and tongue.
The cheeks are lateral walls of the oral cavity and internally covered by mucous membrane.
The mucous membrane is made up of non-keratinized stratified squamous epithelial tissue.
Buccinator muscles and connective tissue is present between the mucous membrane and external skin of the cheeks.
The anterior portions of the cheeks start from the lips i.e., opening of the mouth. Lips have the orbicularis oris muscle.
The labial frenulum, a fold of mucous membrane, attaches the lips with respective gum.
Fauces is opening between the oral cavity and oropharynx (throat).
The palate forms the roof of the mouth and separates the nasal cavity from the oral cavity. Breathing at the time of chewing is possible because of the palate. The palate is of two types
- Hard palate
- Soft palate
The hard palate (anterior portion) is made up of the maxillae and palatine bones that are covered by a mucous membrane.
The soft palate (posterior portion) is an arch-shaped muscular partition between the oropharynx and nasopharynx that is lined with mucous membrane.
Uvula, conical muscular process, hangs from the soft palate. During swallowing, the soft palate and uvula are drawn superiorly to prevent the entry of swallowed food and liquid into the nasal cavity.
Lateral to the base of the uvula are two muscular folds that run down the lateral sides of the soft palate:
palatoglossal arch (anterior and extend to side of the base of the tongue)
palatopharyngeal arch (posterior and reaches to the side of the pharynx)
The palatine tonsils are positioned between the arches, and the lingual tonsils are located at the tongue base.
The mouth opens into the oropharynx through the fauces at the posterior border of the soft palate.
Salivary Glands
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| Fig. 4: The salivary glands |
A salivary gland secrets saliva into the oral cavity.
Normally, just enough saliva is secreted to moisten the mucous membranes oral cavity and cleanse the mouth and teeth. However, after the entry of food into the mouth, secretion of saliva increases to lubricate, dissolve, and to begin the chemical digestion of the food.
Several small salivary glands are present on the mucous membrane of the mouth and tongue. These salivary glands open directly or indirectly (via short ducts) into the oral cavity.
These glands make small contribution to saliva and include
Labial salivary glands: Lips
Buccal salivary glands: Cheeks
Palatal salivary glands: Palate
Lingual salivary glands: Tongue
Most saliva is secreted by three major salivary glands that lie beyond the oral mucosa:
Parotid gland
Submandibular gland
sublingual glands
The parotid glands located inferior and anterior to the ears, between the skin and the masseter muscle. These glands secrete saliva into the oral cavity via a parotid duct that pierces the buccinator muscle to open into the vestibule opposite the second maxillary (upper) molar tooth.
The submandibular glands are situated in the floor of the mouth, medial and partly inferior to the body of the mandible. Their ducts, the submandibular ducts, run under the mucosa and enter the oral cavity on the sides of the lingual frenulum.
The sublingual glands are beneath the tongue and superior to the submandibular glands. Their ducts, the sublingual ducts, open into the floor of the mouth in the oral cavity.
Composition and Functions of Saliva
Chemically, saliva is 99.5% water and 0.5% solutes.
Among the solutes are ions (Na+, K+, Cl-, HCO3-, PO42-), dissolved gases, and organic substances (urea, uric acid, mucus, immunoglobulin A, lysozyme, and salivary amylase)
The water acts as a medium for dissolving foods so that gustatory receptors becomes stimulated and digestive reactions can begin.
Chloride ions activate salivary amylase, an enzyme that digest the starch.
Bicarbonate and phosphate ions act as buffer so that pH of the saliva is remains slightly acidic (pH 6.35–6.85).
Urea and uric acids are the waste material.
Mucus lubricates food so it can be swallowed easily.
Immunoglobulin A (IgA) prevents anchoring of microbes into the epithelium (small quantities only).
Lysozyme kills bacteria (small quantities only)
Salivation
Salivation, secretion of saliva, is controlled by the autonomic nervous system. The average daily secretion of saliva is 1000–1500 mL.
Ordinarily, parasympathetic stimulation triggers continuous secretion of a moderate amount of saliva that moisturizes the mucous membranes and lubricates the tongue and lips during speech.
The saliva is then swallowed and helps moistening the esophagus.
During stress, sympathetic stimulation results in dryness of the mouth.
During dehydration, the salivary glands stop secreting saliva to conserve water; the subsequent dryness in the mouth imparts to the sensation of thirst.
The thought, feel, smell, sight, sound and taste of food are potent stimulators of salivation. Chemicals in the food stimulate taste buds receptors on the tongue, and impulses are conveyed to the two salivary nuclei in the brain stem (superior and inferior salivatory nuclei).
Returning parasympathetic impulses in fibers of the facial (VII) and glossopharyngeal (IX) nerves stimulate the secretion of saliva.
Tongue
The tongue is composed of skeletal muscle covered with mucous membrane. It along with associated muscles, forms the floor of the oral cavity.
Median septum divides the tongue into symmetrical halves.
Tongue is attached inferiorly to the hyoid bone, styloid process of the temporal bone, and mandible.
Each half of the tongue comprises of
Extrinsic muscle
Intrinsic muscles
The extrinsic muscles originate outside the tongue insert into connective tissues of the tongue. Extrinsic muscle helps in side to side and in and out movement of tongue.
The hyoglossus, genioglossus, and styloglossus muscles are extrinsic muscles of tongue. These form the floor of the mouth and keep the tongue in its position.
The intrinsic muscles originate in tongue and remains associated with connective of tissue of tongue. Intrinsic muscles works during speaking and swallowing.
The longitudinalis superior, longitudinalis inferior, transversus linguae, and verticalis linguae muscles.
The lingual frenulum is a midline fold of mucous membrane under the tongue. It attaches the tongue to the floor and limits the posterior movement of tongue.
Ankyloglossia is the condition when the lingual frenulum is abnormally short or rigid that results in impairment of speech.
Papillae are nipple shaped projections that are present on the dorsum (upper surface) and lateral surfaces of the tongue.
Many papillae contain receptors for gustation called taste buds.
Lingual glands of the tongue secrete both mucus and a watery serous fluid having the enzyme lingual lipase.
Lipase enzymes hydrolyses the triglycerides.
Teeth
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| Fig. 5: Sagittal section of lower molar |
The teeth or dentes are placed in sockets of the alveolar processes of the mandible and maxillae.
Gingivae or gums cover the alveolar processes
Periodontal ligament (dense fibrous connective tissue) lines the sockets of alveolar processes.
A tooth has three major external regions:
• Crown
• Root (1-3)
• Neck
Internally, dentin (calcified connective tissue) forms the major portion of the tooth and provide the basic shape and rigidity to the tooth.
Enamel covers the dentin of the crown and it primarily contains calcium phosphate and calcium carbonate.
Enamel is the hardest substance in the body and it protects the tooth from the wear and tear of chewing.
Cementum covers the dentin and attaches the root to the periodontal ligament.
Pulp, connective tissue having blood vessels, nerves, and lymphatic vessels, filled the pulp cavity (space within the crown).
Root canals are the narrow extensions of pulp cavity and have opening at their bases, apical foramen, through which blood vessels, lymphatic vessels, and nerves enters the pulp cavity.
The different branches of dentistry are
Endodontics: associated with the diagnosis, prevention, and treatment of ailments that affect the root, pulp, periodontal ligament, and alveolar bone.
Orthodontics: associated with the prevention and correction of irregularly aligned teeth
Periodontics: associated with the treatment of ailments of the tissues immediately adjacent to the teeth, e.g., gingivitis (gum disease).
Humans have two dentitions, or sets of teeth:
Deciduous (primary teeth, baby teeth or milk teeth)
Permanent (secondary teeth)
The deciduous teeth (20 in number)
Incisors (for cutting into food)
Cuspids (canines) (for tearing and shredding food)
Molars (Maxillary molars have three roots; mandibular molars have two roots; for crushing and grinding the food).
The permanent dentition (32 teeth) erupt between age six and adulthood.
Also have incisors, cuspids and molars teeth.
First and second premolars replace the deciduous molars.
Jaw grows to accommodate secondary molars.
Pharynx
Pharynx (throat) is a funnel-shaped tube that connects larynx (anteriorly) to the esophagus (posteriorly).
The pharynx is made up of skeletal muscle and lined by mucous membrane
It is divided into three parts:
Nasopharynx
Oropharynx
Laryngopharynx
The nasopharynx play a role only in respiration, but both the oropharynx and laryngopharynx have functions in digestion and in respiration
Esophagus
The esophagus is a 25 cm long collapsible muscular tube.
It is positioned posterior to the trachea and anterior to the verterbral column and passes through mediastinum.
At esophageal hiatus, esophagus pierces the diaphragm and reaches to the superior portion of the stomach.
Layers of GI tract
Mucosa: nonkeratinized stratified squamous epithelium with lamina propria and muscularis mucosae
Submucosa: areolar connective tissue with blood vessels and mucous glands
Muscularis: First 1/3 skeletal muscle, last 1/3 smooth muscle and remaining 1/3 mix of skeletal and smooth muscle (middle portion)
At each end of the esophagus there are two sphincters
Upper esophageal sphincter (skeletal muscle)
Lower esophageal sphincter (smooth muscle)
Adventitia: (instead of serosa) connective tissue merged with the connective tissue of surrounding
Esophagus secretes mucus and transports food into the stomach.
Stomach
The stomach is a J-shaped enlargement of the GI tract directly inferior to the diaphragm in the epigastric, umbilical, and left hypochondriac regions of the abdomen.
The stomach connects the esophagus to the duodenum (the first part of the small intestine).
One of the functions of the stomach is to serve as a mixing chamber and holding reservoir.
In the stomach, digestion of starch continues, digestion of proteins and triglycerides begins, the semisolid bolus is converted to a liquid, and certain substances are absorbed.
Anatomy of the Stomach
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| Fig. 6: The anterior view of the stomach |
The stomach has four main regions:
Cardia
Fundus
Body
Pylorus (pyloric antrum and pyloric canal)
The pylorus communicates with the duodenum of the small intestine via a smooth muscle sphincter called the pyloric sphincter.
The concave medial border of the stomach is called the lesser curvature, and the convex lateral border is called the greater curvature.
Histology of the Stomach
The stomach wall is composed of the same basic layers as the rest of the GI tract, with certain modifications.
Mucosa: simple columnar epithelial cells called surface mucous cells. The mucosa contains a lamina propria (areolar connective tissue) and a muscularis mucosae (smooth muscle)
Epithelial cells extend down into the lamina propria, where they form columns of secretory cells called gastric glands.
Several gastric glands open into the bottom of narrow channels called gastric pits.
The gastric glands contain three types of exocrine gland cells that secrete their products into the stomach lumen:
Mucous neck cells: mucus
Chief cells: pepsinogen and gastric lipase
Parietal cells: intrinsic factor (needed for absorption of vitamin B12) and hydrochloric acid
The secretions of the mucous, parietal, and chief cells form gastric juice, which totals 2000–3000 mL per day.
In addition, gastric glands include a type of enteroendocrine cell
The G cell in pyloric antrum secretes the hormone gastrin into the bloodstream. This hormone stimulates several aspects of gastric activity.
The submucosa of the stomach is composed of areolar connective tissue.
The muscularis has three layers of smooth muscle:
Outer longitudinal layer
Middle circular layer
Inner oblique layer
The serosa is composed of simple squamous epithelium (mesothelium) and areolar connective tissue; the portion of the serosa covering the stomach is part of the visceral peritoneum.
The visceral peritoneum as lesser omentum connects the lesser curvature of the stomach with the liver.
The visceral peritoneum as greater omentum hangs from the greater curvature of the stomach and drapes over the intestines.
Mechanical and Chemical Digestion in the Stomach
After food enters the stomach, every 15-25 seconds gentle, rippling, peristaltic movements called mixing waves pass over the stomach.
These waves macerate food, mix it with secretions of the gastric glands, and reduce it to a soupy liquid called chyme.
The fundus has storage function so few mixing waves are observed in the fundus. As digestion proceeds, more strong mixing waves arise at the body of the stomach and strengthen as they reach the pylorus. The pyloric sphincter normally remains almost closed.
As food reaches the pylorus, each mixing wave at times empties about 3 mL of chyme into the duodenum across the pyloric sphincter. This phenomenon is termed as gastric emptying.
Most of the chyme is revert into the body of the stomach, where mixing continues.
The next wave pushes the chyme towards pyloric sphincter again and transports a little more into the duodenum.
The gastric content is thoroughly mixed because of these back and forth movements.
Fundus stores the food for about an hour without mixing with gastric juice and the salivary amylase remains active for that duration.
Once the food mixed with acidic gastric juice, salivary amylase becomes inactive and lingual lipase becomes active to digest the triglycerides.
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| Fig. 7: The parietal cell |
Parietal cells secrete hydrogen ions (H+) and chloride ions (Cl-) separately into the stomach lumen. These ions combined to form hydrochloric acid (HCl).
Proton pumps (H+/K+ ATPase) actively transport H+ into the stomach lumen while bringing potassium ions (K+) into the parietal cell.
The Cl- and K+ diffuse out into the stomach lumen through Cl- and K+ channels present in the apical membrane of parietal cell.
The enzyme carbonic anhydrase catalyzes the formation of carbonic acid (H2CO3) from water (H2O) and carbon dioxide (CO2).
The carbonic acid is source of H+ and bicarbonate ions (HCO3-).
The HCO3- exchanged for Cl¬- from blood via Cl-/HCO3- antiporters in the basolateral membrane (next to the lamina propria).
The HCO3- diffuses into nearby blood capillaries and elevate blood pH slightly and make urine more alkaline. This phenomenon is termed as “alkaline tide”.
HCl secretion by parietal cells can be stimulated by several sources:
Acetylcholine (ACh) released by parasympathetic neurons
Gastrin secreted by G cells
Histamine, a paracrine substance released by mast cells in the nearby lamina propria.
Histamine synergistically enhances the effects of acetylcholine and gastrin.
Receptors for all three substances are available in the plasma membrane of parietal cells.
The ACh acts through M1 (muscarinic) receptors.
The gastrin hormone acts through gastrin receptors.
The histamine acts through H2 receptors; action of histamine through H1 receptor is associated with allergic responses.
The strong acidic fluid of the stomach destroys many microbes in food and partially denatures (unfolds) proteins in food.
The chief cells secrete pepsinogen (zymogen = inactive enzyme) that is converted into active form pepsin at low pH.
Pepsin is a proteolytic enzyme that hydrolyses the peptide bonds between amino acids and breaks down the protein into smaller peptide fragments.
The alkaline mucus secreted by surface mucus cells and mucous neck cells protects the mucosa of stomach from the acid by forming a 1–3 mm thick layer mucus.
The enzyme gastric lipase separate the short-chain triglycerides present in fat molecules into fatty acids and monoglycerides. This enzyme has restricted role in adult stomach and performs optimally at pH 5-6.
Mucous cells of the stomach perform limited absorption of some water, ions, and short-chain fatty acids, as well as weakly acidic drugs (especially aspirin) and alcohol.
The stomach empties its content into duodenum within 2 to 4 hours of taking a meal.
Carbohydrate rich food spend the least time in the stomach and fat laden meal remains in the stomach for longer duration.
Pancreas
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| Fig. 8: The pancreas, liver and gallbladder |
The pancreas is a retroperitoneal gland about 12–15 cm long and 2.5 cm thick. It lies posterior to the greater curvature of the stomach.
The pancreas consists of a
• head
• body
• tail
The head is the expanded portion of the organ near the curve of the duodenum; superior to and to the left of the head are the central body and the tapering tail.
Pancreatic juices are secreted by exocrine cells into small ducts that ultimately unite to form two larger ducts
• Pancreatic duct or duct of Wirsung (VE-R-sung)
• Accessory duct
These in turn convey the secretions into the small intestine.
In most people, the pancreatic duct joins the common bile duct from the liver and gallbladder and enters the duodenum as a dilated common duct called the hepatopancreatic ampulla or ampulla of Vater (FAH-ter).
Pancreas is usually connected to the duodenum of the small intestine by two ducts.
• Hepatopancreatic ampulla
• Accessory duct
The ampulla opens on an elevation of the duodenal mucosa known as the major duodenal papilla, which lies about 10 cm inferior to the pyloric sphincter of the stomach.
The passage of pancreatic juice and bile through the hepatopancreatic ampulla into the duodenum of the small intestine is regulated by a mass of smooth muscle surrounding the ampulla known as the sphincter of the hepatopancreatic ampulla, or sphincter of Oddi (OD-ē).
The other major duct of the pancreas, the accessory duct (duct of Santorini), leads from the pancreas and empties into the duodenum about 2.5 cm superior to the hepatopancreatic ampulla.
Histology of the Pancreas
The pancreas is made up of small clusters of glandular epithelial cells. About 99% of the clusters, called acini (AS-i-nī), constitute the exocrine portion of the organ.
The cells within acini secrete a mixture of fluid and digestive enzymes called pancreatic juice.
The remaining 1% of the clusters, called pancreatic islets (islets of Langerhans), form the endocrine portion of the pancreas.
These cells secrete the hormones glucagon, insulin, somatostatin and pancreatic polypeptide.
Composition and Functions of Pancreatic Juice
Each day the pancreas produces 1200–1500 mL of pancreatic juice, a clear, colorless liquid consisting mostly of water, some salts, sodium bicarbonate, and several enzymes.
The sodium bicarbonate gives pancreatic juice a slightly alkaline pH (7.1–8.2) that buffers acidic gastric juice in chyme, stops the action of pepsin from the stomach, and creates the proper pH for the action of digestive enzymes in the small intestine.
The enzymes in pancreatic juice include a starch-digesting enzyme called pancreatic amylase; several enzymes that digest proteins into peptides called trypsin (TRIP-sin), chymotrypsin (kī′-mō-TRIP-sin), carboxypeptidase (kar-bok′-sē-PEP-ti-dās), and elastase (ē-LAS-tās); the principal triglyceride–digesting enzyme in adults, called pancreatic lipase; and nucleic acid–digesting enzymes called ribonuclease (rī′-bō-NOOklē- ās) and deoxyribonuclease (dē-oks-ē-rī′-bō-NOO-klē-ās) that digest ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) into nucleotides.
The protein-digesting enzymes of the pancreas are produced in an inactive form just as pepsin is produced in the stomach as pepsinogen. Because they are inactive, the enzymes do not digest cells of the pancreas itself. Trypsin is secreted in an inactive form called trypsinogen (trip-SIN-ō-jen).
Pancreatic acinar cells also secrete a protein called trypsin inhibitor that combines with any trypsin formed accidentally in the pancreas or in pancreatic juice and blocks its enzymatic activity. When trypsinogen reaches the lumen of the small intestine, it encounters an activating brush-border enzyme called enterokinase (en′-ter-ō-KI--nās), which splits off part of the trypsinogen molecule to form trypsin. In turn, trypsin acts on the inactive precursors (called chymotrypsinogen, procarboxypeptidase, and proelastase) to produce chymotrypsin, carboxypeptidase, and elastase, respectively.
Liver and gall bladder
The liver is the heaviest gland of the body, weighing about 1.4 kg in an average adult.
Of all of the organs of the body, it is second only to the skin in size. The liver is inferior to the diaphragm and occupies most of the right hypochondriac and part of the epigastric regions of the abdominopelvic cavity.
The gallbladder (gall- _ bile) is a pear-shaped sac that is located in a depression of the posterior surface of the liver. It is 7–10 cm long and typically hangs from the anterior inferior margin of the liver.
Anatomy of the Liver and Gallbladder
The liver is almost completely covered by visceral peritoneum and is completely covered by a dense irregular connective tissue layer that lies deep to the peritoneum.
The liver is divided into two principal lobes (separated by the falciform ligament, a fold of the mesentery)—
a large right lobe
a smaller left lobe
Although the right lobe is considered by many anatomists to include an inferior quadrate lobe and a posterior caudate lobe, based on internal morphology (primarily the distribution of blood vessels), the quadrate and caudate lobes more appropriately belong to the left lobe.
The falciform ligament extends from the under surface of the diaphragm between the two principal lobes of the liver to the superior surface of the liver, helping to suspend the liver in the abdominal cavity.
In the free border of the falciform ligament is the ligamentum teres (round ligament), a remnant of the umbilical vein of the fetus; this fibrous cord extends from the liver to the umbilicus.
The right and left coronary ligaments are narrow extensions of the parietal peritoneum that suspend the liver from the diaphragm.
The parts of the gallbladder include the broad fundus, which projects inferiorly beyond the inferior border of the liver; the body, the central portion; and the neck, the tapered portion. The body and neck project superiorly.
Histology of the Liver and Gallbladder Histologically, the liver is composed of several components
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| Fig. 9: Histology of the liver |
1. Hepatocytes (hepat- _ liver; -cytes _ cell). Hepatocytes are the major functional cells of the liver and perform a wide array of metabolic, secretory, and endocrine functions. These are specialized epithelial cells with 5 to 12 sides that make up about 80% of the volume of the liver.
Hepatocytes form complex three-dimensional arrangements called hepatic laminae. The hepatic laminae are plates of hepatocytes one cell thick bordered on either side by the endothelial-lined vascular spaces called hepatic sinusoids.
The hepatic laminae are highly branched, irregular structures. Grooves in the cell membranes between neighboring hepatocytes provide spaces for canaliculi into which the hepatocytes secrete bile.
Bile, a yellow, brownish, or olive-green liquid secreted by hepatocytes, serves as both an excretory product and a digestive secretion.
2. Bile canaliculi (kan-a-LIK-u- -li _ small canals). These are small ducts between hepatocytes that collect bile produced by the hepatocytes. From bile canaliculi, bile passes into bile ductules and then bile ducts. The bile ducts merge and eventually form the larger right and left hepatic ducts, which unite and exit the liver as the common hepatic duct The common hepatic duct joins the cystic duct (cystic _ bladder) from the gallbladder to form the common bile duct. From here, bile enters the small intestine to participate in digestion.
3. Hepatic sinusoids. These are highly permeable blood capillaries between rows of hepatocytes that receive oxygenated blood from branches of the hepatic artery and nutrient-rich deoxygenated blood from branches of the hepatic portal vein. Recall that the hepatic portal vein brings venous blood from the gastrointestinal organs and spleen into the liver. Hepatic sinusoids converge and deliver blood into a central vein. From central veins the blood flows into the hepatic veins, which drain into the inferior vena cava. In contrast to blood which flows toward a central vein, bile flows in the opposite direction.
Also present in the hepatic sinusoids are fixed phagocytes called stellate reticuloendothelial (Kupffer) cells, which destroy worn-out white and red blood cells, bacteria, and other foreign matter in the venous blood draining from the gastrointestinal tract. Together, a bile duct, branch of the hepatic artery, and branch of the hepatic vein are referred to as a portal triad (tri _ three). The hepatocytes, bile duct system, and hepatic sinusoids can be organized into anatomical and functional units in three different ways:
1. Hepatic lobule. For years, anatomists described the hepatic lobule as the functional unit of the liver. According to this model, each hepatic lobule is shaped like a hexagon (six-sided structure). Left at its center is the central vein, and radiating out from it are rows of hepatocytes and hepatic sinusoids. Located at three corners of the hexagon is a portal triad. This model is based on a description of the liver of adult pigs. In the human liver it is difficult to find such well-defined hepatic lobules surrounded by thick layers of connective tissue.
2. Portal lobule. This model emphasized the exocrine function of the liver, that is, bile secretion. Accordingly, the bile duct of a portal triad is taken as the center of the portal lobule. The portal lobule is triangular in shape and is defined by three imaginary straight lines that connect three central veins that are closest to the portal triad. This model has not gained widespread acceptance.
3. Hepatic acinus. In recent years, the preferred structural and functional unit of the liver is the hepatic acinus. Each hepatic acinus is an approximately oval mass that includes portions of two neighboring hepatic lobules. The short axis of the hepatic acinus is defined by branches of the portal triad— branches of the hepatic artery, vein, and bile ducts—that run along the border of the hepatic lobules. The long axis of the acinus is defined by two imaginary curved lines, which connect the two central veins closest to the short axis. Hepatocytes in the hepatic acinus are arranged in three zones around the short axis, with no sharp boundaries between. Cells in zone 1 are closest to the branches of the portal triad and the first to receive incoming oxygen, nutrients, and toxins from incoming blood. These cells are the first ones to take up glucose and store it as glycogen after a meal and break down glycogen to glucose during fasting. They are also the first to show morphological changes following bile duct obstruction or exposure to toxic substances. Zone 1 cells are the last ones to die if circulation is impaired and the first ones to regenerate. Cells in zone 3 are farthest from branches of the portal triad and are the last to show the effects of bile obstruction or exposure to toxins, the first ones to show the effects of impaired circulation, and the last ones to regenerate. Zone 3 cells also are the first to show evidence of fat accumulation. Cells in zone 2 have structural and functional characteristics intermediate between the cells in zones 1 and 3. The hepatic acinus is the smallest structural and functional unit of the liver. Its popularity and appeal are based on the fact that it provides a logical description and interpretation of (1) patterns of glycogen storage and release and (2) toxic effects, degeneration, and regeneration in the three zones of the hepatic acinus relative to the proximity of the zones to branches of the portal triad. The mucosa of the gallbladder consists of simple columnar epithelium arranged in rugae resembling those of the stomach. The wall of the gallbladder lacks a submucosa. The middle, muscular coat of the wall consists of smooth muscle fibers. Contraction of the smooth muscle fibers ejects the contents of the gallbladder into the cystic duct. The gallbladder’s outer coat is the visceral peritoneum. The functions of the gallbladder are to store and concentrate the bile produced by the liver (up to tenfold) until it is needed in the small intestine. In the concentration process, water and ions are absorbed by the gallbladder mucosa
Blood Supply of the Liver
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| Fig. 10: blood supply to the liver |
The liver receives blood from two sources (Figure 24.16). From the hepatic artery it obtains oxygenated blood, and from the hepatic portal vein it receives deoxygenated blood containing newly absorbed nutrients, drugs, and possibly microbes and toxins from the gastrointestinal tract (see Figure 21.28 on page 818). Branches of both the hepatic artery and the hepatic portal vein carry blood into liver sinusoids, where oxygen, most of the nutrients, and certain toxic substances are taken up by the hepatocytes. Products manufactured by the hepatocytes and nutrients needed by other cells are secreted back into the blood, which then drains into the central vein and eventually passes into a hepatic vein. Because blood from the gastrointestinal tract passes through the liver as part of the hepatic portal circulation, the liver is often a site for metastasis of cancer that originates in the GI tract.
Role and Composition of Bile Each day, hepatocytes secrete 800–1000 mL (about 1 qt) of bile, a yellow, brownish, or olive-green liquid. It has a pH of 7.6–8.6 and consists mostly of water, bile salts, cholesterol, a phospholipid called lecithin, bile pigments, and several ions. The principal bile pigment is bilirubin. The phagocytosis of aged red blood cells liberates iron, globin, and bilirubin (derived from heme). The iron and globin are recycled; the bilirubin is secreted into the bile and is eventually broken down in the intestine. One of its breakdown products—stercobilin—gives feces their normal brown color. Bile is partially an excretory product and partially a digestive secretion.
Bile salts, which are sodium salts and potassium salts of bile acids (mostly chenodeoxycholic acid and cholic acid), play a role in emulsification, the breakdown of large lipid globules into a suspension of small lipid globules. The small lipid globules present a very large surface area that allows pancreatic lipase to more rapidly accomplish digestion of triglycerides. Bile salts also aid in the absorption of lipids following their digestion. Although hepatocytes continually release bile, they increase production and secretion when the portal blood contains more bile acids; thus, as digestion and absorption continue in the small intestine, bile release increases. Between meals, after most absorption has occurred, bile flows into the gallbladder for storage because the sphincter of the hepatopancreatic ampulla (sphincter of Oddi;) closes off the entrance to the duodenum.
Functions of the Liver In addition to secreting bile, which is needed for absorption of dietary fats, the liver performs many other vital functions: • Carbohydrate metabolism. The liver is especially important in maintaining a normal blood glucose level. When blood glucose is low, the liver can break down glycogen to glucose and release the glucose into the bloodstream. The liver can also convert certain amino acids and lactic acid to glucose, and it can convert other sugars, such as fructose and galactose, into glucose. When blood glucose is high, as occurs just after eating a meal, the liver converts glucose to glycogen and triglycerides for storage.
• Lipid metabolism. Hepatocytes store some triglycerides; break down fatty acids to generate ATP; synthesize lipoproteins, which transport fatty acids, triglycerides, and cholesterol to and from body cells; synthesize cholesterol; and use cholesterol to make bile salts.
• Protein metabolism. Hepatocytes deaminate (remove the amino group, NH2, from) amino acids so that the amino acids can be used for ATP production or converted to carbohydrates or fats. The resulting toxic ammonia (NH3) is then converted into the much less toxic urea, which is excreted in urine. Hepatocytes also synthesize most plasma proteins, such as alpha and beta globulins, albumin, prothrombin, and fibrinogen.
• Processing of drugs and hormones. The liver can detoxify substances such as alcohol and excrete drugs such as penicillin, erythromycin, and sulfonamides into bile. It can also chemically alter or excrete thyroid hormones and steroid hormones such as estrogens and aldosterone.
• Excretion of bilirubin. As previously noted, bilirubin, derived from the heme of aged red blood cells, is absorbed by the liver from the blood and secreted into bile. Most of the bilirubin in bile is metabolized in the small intestine by bacteria and eliminated in feces.
• Synthesis of bile salts. Bile salts are used in the small intestine for the emulsification and absorption of lipids.
• Storage. In addition to glycogen, the liver is a prime storage site for certain vitamins (A, B12, D, E, and K) and minerals (iron and copper), which are released from the liver when needed elsewhere in the body. • Phagocytosis. The stellate reticuloendothelial (Kupffer) cells of the liver phagocytize aged red blood cells, white blood cells, and some bacteria. • Activation of vitamin D. The skin, liver, and kidneys participate in synthesizing the active form of vitamin D.
Small Intestine
The small intestine is a lengthy tube that is responsible for the majority of nutritional digestion and absorption.
The small intestine starts at the stomach's pyloric sphincter and finishes at the big intestine's entrance.
It has an average diameter of 2.5 cm and a length of roughly 3 m in a living human.
The small intestine's length provides a huge surface area for digestion and absorption, which is further boosted by circular folds, villi, and microvilli.
Anatomy of the Small Intestine
The small intestine is divided into three regions.
• Duodenum (25 cm)
• Jejunum (1 m)
• Ileum (2 m)
Histology of the Small Intestine
The mucosa is made up of a layer of epithelium, lamina propria, and muscularis mucosae. The epithelial layer consists of simple columnar epithelium that contains many types of cells.
Absorptive cells: Digest and absorb nutrients.
Goblet cells: Secrete mucus.
Paneth cells: Secrete lysozyme (bactericidal enzyme) that regulate the microbial population in the small intestine.
Three types of enteroendocrine cells that secrete hormones:
S cells: secretin
CCK cells: cholecystokinin
K cells: glucose-dependent insulinotropic peptide or GIP
The lamina propria of the mucosa contains areolar connective tissue and has an abundance of mucosa-associated lymphoid tissue (MALT).
Solitary lymphatic nodules are most abundant in the distal part of the small intestine.
Groups of lymphatic nodules referred to as aggregated lymphatic follicles (Peyer’s patches) are also present in the ileum.
The muscularis mucosae of the mucosa consists of smooth muscle.
The submucosa of the duodenum contains duodenal (Brunner’s) glands that secrete an alkaline mucus and neutralize gastric acid present in the chyme.
The muscularis of the small intestine consists of two layers of smooth muscle
• Outer, thinner layer contains longitudinal fibers;
• Inner, thicker layer contains circular fibers.
Except for a major part of the duodenum, the serosa (or visceral peritoneum) completely surrounds the small intestine.
The process of digestion and absorption in the intestine is facilitated by special structural features:
Circular folds
Villi
Microvilli
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| Fig. 11 a |
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| Fig. 11 b: histology of the small intestine |
Circular folds or plicae circulares are about 10 mm long folds of the mucosa and submucosa. Circular folds enhance absorption by increasing surface area and causing the chyme to spiral, instead of moving in a straight line.
Villi are 0.5 to 1 mm long fingerlike projections of the mucosa that vastly increases the surface area for absorption and digestion and gives the intestinal mucosa a velvety appearance. Each villus (singular form) is covered by epithelium and has a core of lamina propria with an arteriole, a venule, a blood capillary network, and a lacteal (lymphatic capillary).
Microvilli are 1 μm-long cylindrical, membrane-covered projections having a bundle of 20–30 actin filaments. In a light microscope, the microvilli appears as a fuzzy line therefore called the brush border. The microvilli greatly increase the surface area of the plasma membrane and also contain several digestive enzymes (brush-border enzymes).
Role of Intestinal Juice and Brush-Border Enzymes
Intestinal juice is a clear yellow fluid and about 1–2 liters is secreted each day. It has slightly alkaline pH (pH 7.6) and contains water and mucus.
Collectively, pancreatic and intestinal juices aids the absorption of substances from chyme in the small intestine.
The brush-border enzymes are synthesized by the absorptive cells of the small intestine that helps in the digestion of the chyme.
Four carbohydrate-digesting enzymes are dextrinase, maltase, sucrase, and lactase
Protein-digesting enzymes called peptidases (aminopeptidase and dipeptidase)
Nucleotide-digesting enzymes are nucleosidases and phosphatases.
Mechanical Digestion in the Small Intestine
The two types of movements of the small intestine that are governed by myenteric plexus:
Segmentations
Migrating Motility Complexes (a type of peristalsis)
Segmentations are limited mixing contractions that happen in small portions of intestine due to contraction of the circular muscles.
Migrating motility complex (MMC) is the peristalsis that starts from the lower portion of the stomach and pushes chyme forward along a short distance in small intestine before fading out. The MMC reaches the ileum in 90-120 minute and then another MMC starts. Altogether, chyme remains in the small intestine for 3–5 hours.
Chemical Digestion in the Small Intestine
Salivary amylase (in saliva) transforms starch (a polysaccharide) into maltose (a disaccharide), maltotriose (a trisaccharide), and dextrins (branched short-chain fragments of starch with 5–10 glucose units).
Pepsin (in stomach) hydrolyses proteins to peptides (small fragments of proteins).
Lingual and gastric lipases transform some triglycerides into fatty acids, diglycerides, and monoglycerides.
Thus, partially digested chyme enters the small intestine contains and further digestion is completed with the help of pancreatic juice, bile and intestinal juice.
Digestion of Carbohydrates: Pancreatic amylase from the pancreatic juice carry on the transformation of starch into maltose, maltotriose, and a-dextrins. It has no effect on cellulose, an indigestible plant fiber that is generally referred to as “roughage” as it moves through the digestive system. After that brush-border enzyme will carry forward the chemical digestion of the carbohydrate
Dextrinase acts on the dextrins
Sucrase breaks sucrose
Lactase digests lactose
Maltase splits maltose and maltotriose
Digestion of carbohydrates is completed on the generation of monosaccharides that can be absorbed from the small intestine.
Digestion of Proteins Enzymes present in pancreatic juice—trypsin, chymotrypsin, carboxypeptidase, and elastase will hydrolysed the peptide bonds of the protein. These enzymes cleave a peptide bond between a two specific amino acid.
Carboxypeptidase hydrolyses the peptide bond near to the carboxyl end of the protein. Protein digestion is finished by two peptidases present in the brush border: aminopeptidase and dipeptidase.
Aminopeptidase hydrolyses the peptide bond near to the amino end of the peptide.
Dipeptidase hydrolyses a dipeptides.
Digestion of Lipids: Lipases split the triglycerides and phospholipids. Pancreatic lipase hydrolyses triglycerides into fatty acids and monoglycerides. However, for the better activity of pancreatic lipase, emulsification of the lipids must be achieved in the small intestine with the help of bile salts present in the bile juice. The bile salts decrease the surface tension between the water and lipid molecule so as make them miscible.
Digestion of Nucleic Acids
Ribonuclease digests RNA
Deoxyribonuclease digests DNA.
The product of the action of these two nucleases are further digested by phosphatases and nucleosidases into phosphates, pentoses, and nitrogenous bases. These products are absorbed via active transport.
Absorption in the Small Intestine
The mechanical and chemical digestion change the food into forms that can be transported across the epithelial cells of the mucosa and into the lymphatic and blood vessels in submucosa (absorption).
Carbohydrates are converted into monosaccharides (glucose, fructose, and galactose.
Proteins are reduced to single amino acids, dipeptides, and tripeptides.
Triglycerides are split into fatty acids, monoglycerides, and glycerol.
Absorption of materials occurs via
Diffusion
Facilitated diffusion
Osmosis
Active transport
Absorption of Monosaccharides All carbohydrates are absorbed as monosaccharides except the indigestible cellulose and fibers. Monosaccharides are absorbed via facilitated diffusion or active transport.
Fructose via facilitated diffusion;
Glucose and galactose via secondary active transport coupled to the active transport of Na+.
The absorbed monosaccharides enters the capillaries of the villi via facilitated diffusion
Absorption of Amino Acids, Dipeptides, and Tripeptides
Most proteins are absorbed as amino acids via active transport processes.
Different transporters carry different types of amino acids.
Some amino acids enter get absorbed via Na+ dependent secondary active transport processes.
One symporter brings dipeptides and tripeptides along with H+ into the absorptive cells wherein the peptides are hydrolyzed to single amino acids. Amino acids move out into the capillaries of villus via diffusion.
Both monosaccharides and amino acids are transported in the blood to the liver by way of the hepatic portal system.
Absorption of Lipids
All dietary lipids are absorbed via simple diffusion.
The triglycerides are converted into monoglycerides and fatty acids.
Short-chain fatty acids (10–12 carbon atoms) are water soluble and get absorb via simple diffusion similar to monosaccharides.
Long-chain fatty acids (reaches to absorptive cells in form of micelles formed by bile salts and diffuse into the absorptive cells).
Inside the absorptive cells, the monoglycerides and long-chain fatty acids and monoglycerides reconstitute the triglycerides in spherical form that accumulate phospholipids, cholesterol, and proteins. These spherical masses having diameter of about 80 nm are called chylomicrons.
These chylomicrons enter in to lymphatic vessels called Lacteals and enter into the blood via the left subclavian vein.
Once inside the blood, these chylomicrons are modified by lipoprotein lipase present in the endothelial cells of the blood vessels of the liver
After completion of emulsification and absorption of lipids, more than 90% of bile salts are reabsorbed through active transport inside ileum and reaches to the liver via blood. The reabsorbed bile salts are again secreted in the bile by the hepatocytes and this cycle of secretion and reabsorption is known as the enterohepatic circulation.
Absorption of Electrolytes
Most of the electrolytes are absorbed by the small intestine through active transport or secondary active transport.
Absorption of Vitamins
Fat soluble vitamins such as vitamins A, D, E, and K are absorbed via simple diffusion from the micelles.
Water-soluble vitamins, including most B vitamins and vitamin C, also are absorbed via simple diffusion.
Vitamin B12 is absorbed through active transport in combined form with intrinsic factor secreted by parietal cells of the stomach.
Absorption of Water
All water absorption in the GI tract occurs via osmosis that depends on the absorption of electrolytes and nutrients.
LARGE INTESTINE
The large intestine is the terminal portion of the GI tract. The overall functions of the large intestine are
• Completion of absorption
• Production of certain vitamins
• Formation of feces
• Expulsion of feces from the body.
Anatomy of the Large Intestine
The large intestine, which is about 1.5 m long and 6.5 cm in diameter, extends from the ileum to the anus. It is attached to the posterior abdominal wall by its mesocolon, which is a double layer of peritoneum.
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| Fig. 12: The large intestine |
Structurally, the four major regions of the large intestine are the
• Cecum,
• Colon,
• Rectum,
• Anal Canal.
The opening from the ileum into the large intestine is guarded by a fold of mucous membrane called the ileocecal sphincter (valve), which allows materials from the small intestine to pass into the large intestine.
Hanging inferior to the ileocecal valve is the cecum, a small pouch about 6 cm long.
Attached to the cecum is a twisted, coiled tube, measuring about 8 cm (in length, called the appendix or vermiform appendix (vermiform _ worm-shaped; appendix _ appendage).
The mesentery of the appendix, called the mesoappendix, attaches the appendix to the inferior part of the mesentery of the ileum.
The open end of the cecum merges with a long tube called the colon, which is divided into
Ascending colon
Transverse colon
Descending colon
Sigmoid colon.
Both the ascending and descending colon are retroperitoneal; the transverse and sigmoid colon are not. True to its name, the ascending colon ascends on the right side of the abdomen, reaches the inferior surface of the liver, and turns abruptly to the left to form the right colic (hepatic) flexure.
The colon continues across the abdomen to the left side as the transverse colon. It curves beneath the inferior end of the spleen on the left side as the left colic (splenic) flexure and passes inferiorly to the level of the iliac crest as the descending colon.
The sigmoid colon (S-shaped) begins near the left iliac crest, projects medially to the midline, and terminates as the rectum at about the level of the third sacral vertebra.
The rectum, the last 20 cm of the GI tract, lies anterior to the sacrum and coccyx. The terminal 2–3 cm (1 in.) of the rectum is called the anal canal.
The mucous membrane of the anal canal is arranged in longitudinal folds called anal columns that contain a network of arteries and veins.
The opening of the anal canal to the exterior, called the anus, is guarded by an internal anal sphincter of smooth muscle (involuntary) and an external anal sphincter of skeletal muscle (voluntary). Normally these sphincters keep the anus closed except during the elimination of feces.
Histology of the Large Intestine
Mucosa consists of simple columnar epithelium, lamina propria (areolar connective tissue), and muscularis mucosae (smooth muscle).
The epithelium comprises of absorptive and goblet cells.
Both absorptive and goblet cells are located in long, straight, tubular intestinal glands (crypts of Lieberkühn) that extend the full thickness of the mucosa.
There are no circular folds or villi; however, microvilli are present on the absorptive cells.
The submucosa of the large intestine consists of areolar connective tissue.
The muscularis consists of an external thick layer of longitudinal smooth muscle and an internal layer of circular smooth muscle.
The longitudinal muscle layer forms three conspicuous bands called the teniae coli that run almost full length of the large intestine.
The serosa of the large intestine is part of the visceral peritoneum.
Mechanical Digestion in the Large Intestine
The passage of chyme from the ileum into the cecum is regulated by the action of the ileocecal sphincter. Normally, the valve remains partially closed so that the passage of chyme into the cecum usually occurs slowly. Immediately after a meal, a gastroileal reflex intensifies peristalsis in the ileum and forces any chyme into the cecum. The hormone gastrin also relaxes the sphincter. Whenever the cecum is distended, the degree of contraction of the ileocecal sphincter intensifies. ileocecal sphincter. Because chyme moves through the small intestine at a fairly constant rate, the time required for a meal to pass into the colon is determined by gastric emptying time. As food passes through the ileocecal sphincter, it fills the cecum and accumulates in the ascending colon. One movement characteristic of the large intestine is haustral churning. In this process, the haustra remain relaxed and become distended while they fill up. When the distension reaches a certain point, the walls contract and squeeze the contents into the next haustrum. Peristalsis also occurs, although at a slower rate (3–12 contractions per minute) than in more proximal portions of the tract. A final type of movement is mass peristalsis, a strong peristaltic wave that begins at about the middle of the transverse colon and quickly drives the contents of the colon into the rectum. Because food in the stomach initiates this gastrocolic reflex in the colon, mass peristalsis usually takes place three or four times a day, during or immediately after a meal.
Chemical Digestion in the Large Intestine
The final stage of digestion occurs in the colon through the activity of bacteria that inhabit the lumen. Mucus is secreted by the glands of the large intestine, but no enzymes are secreted. Chyme is prepared for elimination by the action of bacteria, which ferment any remaining carbohydrates and release hydrogen, carbon dioxide, and methane gases. These gases contribute to flatus (gas) in the colon, termed flatulence when it is excessive. Bacteria also convert any remaining proteins to amino acids and break down the amino acids into simpler substances: indole, skatole, hydrogen sulfide, and fatty acids. Some of the indole and skatole is eliminated in the feces and contributes to their odor; the rest is absorbed and transported to the liver, where these compounds are converted to less toxic compounds and excreted in the urine. Bacteria also decompose bilirubin to simpler pigments, including stercobilin, which gives feces their brown color. Bacterial products that are absorbed in the colon include several vitamins needed for normal metabolism, among them some B vitamins and vitamin K.
Absorption and Feces Formation in the Large Intestine By the time chyme has remained in the large intestine 3–10 hours, it has become solid or semisolid because of water absorption and is now called feces. Chemically, feces consist of water, inorganic salts, sloughed-off epithelial cells from the mucosa of the gastrointestinal tract, bacteria, products of bacterial decomposition, unabsorbed digested materials, and indigestible parts of food. Although 90% of all water absorption occurs in the small intestine, the large intestine absorbs enough to make it an important organ in maintaining the body’s water balance. Of the 0.5–1.0 liter of water that enters the large intestine, all but about 100–200 mL is normally absorbed via osmosis. The large intestine also absorbs ions, including sodium and chloride, and some vitamins.
The Defecation Reflex
Mass peristaltic movements push fecal material from the sigmoid colon into the rectum. The resulting distension of the rectal wall stimulates stretch receptors, which initiates a defecation reflex that empties the rectum. The defecation reflex occurs as follows: In response to distension of the rectal wall, the receptors send sensory nerve impulses to the sacral spinal cord. Motor impulses from the cord travel along parasympathetic nerves back to the descending colon, sigmoid colon, rectum, and anus. The resulting contraction of the longitudinal rectal muscles shortens the rectum, thereby increasing the pressure within it. This pressure, along with voluntary contractions of the diaphragm and abdominal muscles, plus parasympathetic stimulation, opens the internal anal sphincter. The external anal sphincter is voluntarily controlled. If it is voluntarily relaxed, defecation occurs and the feces are expelled through the anus; if it is voluntarily constricted, defecation can be postponed. Voluntary contractions of the diaphragm and abdominal muscles aid defecation by increasing the pressure within the abdomen, which pushes the walls of the sigmoid colon and rectum inward. If defecation does not occur, the feces back up into the sigmoid colon until the next wave of mass peristalsis stimulates the stretch receptors, again creating the urge to defecate. In infants, the defecation reflex causes automatic emptying of the rectum because voluntary control of the external anal sphincter has not yet developed. The amount of bowel movements that a person has over a given period of time depends on various factors such as diet, health, and stress. The normal range of bowel activity varies from two or three bowel movements per day to three or four bowel movements per week.
Diarrhea (dı¯-a-RE¯-a; dia- _ through; rrhea _ flow) is an increase in the frequency, volume, and fluid content of the feces caused by increased motility of and decreased absorption by the intestines. When chyme passes too quickly through the small intestine and feces pass too quickly through the large intestine, there is not enough time for absorption. Frequent diarrhea can result in dehydration and electrolyte imbalances. Excessive motility may be caused by lactose intolerance, stress, and microbes that irritate the gastrointestinal mucosa.
Constipation (kon-sti-PA¯ -shun; con- _ together; stip- _ to press) refers to infrequent or difficult defecation caused by decreased motility of the intestines. Because the feces remain in the colon for prolonged periods, excessive water absorption occurs, and the feces become dry and hard. Constipation may be caused by poor habits (delaying defecation), spasms of the colon, insufficient fiber in the diet, inadequate fluid intake, lack of exercise, emotional stress, and certain drugs. A common treatment is a mild laxative, such as milk of magnesia, which induces defecation. However, many physicians maintain that laxatives are habit-forming, and that adding fiber to the diet, increasing the amount of exercise, and increasing fluid intake are safer ways of controlling this common problem.
PHASES OF DIGESTION
Digestive activities occur in three overlapping phases: the cephalic phase, the gastric phase, and the intestinal phase.
Cephalic Phase During the cephalic phase of digestion, the smell, sight, thought, or initial taste of food activates neural centers in the cerebral cortex, hypothalamus, and brain stem. The brain stem then activates the facial (VII), glossopharyngeal (IX), and vagus (X) nerves. The facial and glossopharyngeal nerves stimulate the salivary glands to secrete saliva, while the vagus nerves stimulate the gastric glands to secrete gastric juice. The purpose of the cephalic phase of digestion is to prepare the mouth and stomach for food that is about to be eaten.
Gastric Phase Once food reaches the stomach, the gastric phase of digestion begins. Neural and hormonal mechanisms regulate the gastric phase of digestion to promote gastric secretion and gastric motility.
• Neural regulation. Food of any kind distends the stomach and stimulates stretch receptors in its walls. Chemoreceptors in the stomach monitor the pH of the stomach chyme. When the stomach walls are distended or pH increases because proteins have entered the stomach and buffered some of the stomach acid, the stretch receptors and chemoreceptors are activated, and a neural negative feedback loop is set in motion. From the stretch receptors and chemoreceptors, nerve impulses propagate to the submucosal plexus, where they activate parasympathetic and enteric neurons. The resulting nerve impulses cause waves of peristalsis and continue to stimulate the flow of gastric juice from gastric glands. The peristaltic waves mix the food with gastric juice; when the waves become strong enough, a small quantity of chyme undergoes gastric emptying into the duodenum. The pH of the stomach chyme decreases (becomes more acidic) and the distension of the stomach walls lessens because chyme has passed into the small intestine, suppressing secretion of gastric juice.
• Hormonal regulation. Gastric secretion during the gastric phase is also regulated by the hormone gastrin. Gastrin is released from the G cells of the gastric glands in response to several stimuli: distension of the stomach by chyme, partially digested proteins in chyme, the high pH of chyme due to the presence of food in the stomach, caffeine in gastric chyme, and acetycholine released from parasympathetic neurons. Once it is released, gastrin enters the bloodstream, makes a round-trip through the body, and finally reaches its target organs in the digestive system. Gastrin stimulates gastric glands to secrete large amounts of gastric juice. It also strengthens the contraction of the lower esophageal sphincter to prevent reflux of acid chyme into the esophagus, increases motility of the stomach, and relaxes the pyloric sphincter, which promotes gastric emptying. Gastrin secretion is inhibited when the pH of gastric juice drops below 2.0 and is stimulated when the pH rises. This negative feedback mechanism helps provide an optimal low pH for the functioning of pepsin, the killing of microbes, and the denaturing of proteins in the stomach.
Intestinal Phase
The intestinal phase of digestion begins once food enters the small intestine. In contrast to reflexes initiated during the cephalic and gastric phases, which stimulate stomach secretory activity and motility, those occurring during the intestinal phase have inhibitory effects that slow the exit of chyme from the stomach. This prevents the duodenum from being overloaded with more chyme than it can handle. In addition, responses occurring during the intestinal phase promote the continued digestion of foods that have reached the small intestine. These activities of the intestinal phase of digestion are regulated by neural and hormonal mechanisms.
• Neural regulation. Distension of the duodenum by the presence of chyme causes the enterogastric reflex. Stretch receptors in the duodenal wall send nerve impulses to the medulla oblongata, where they inhibit parasympathetic stimulation and stimulate the sympathetic nerves to the stomach. As a result, gastric motility is inhibited and there is an increase in the contraction of the pyloric sphincter, which decreases gastric emptying.
• Hormonal regulation. The intestinal phase of digestion is mediated by two major hormones secreted by the small intestine: cholecystokinin and secretin. Cholecystokinin (CCK) is secreted by the CCK cells of the small intestinal crypts of Lieberkühn in response to chyme containing amino acids from partially digested proteins and fatty acids from partially digested triglycerides. CCK stimulates secretion of pancreatic juice that is rich in digestive enzymes. It also causes contraction of the wall of the gallbladder, which squeezes stored bile out of the gallbladder into the cystic duct and through the common bile duct. In addition, CCK causes relaxation of the sphincter of the hepatopancreatic ampulla (sphincter of Oddi), which allows pancreatic juice and bile to flow into the duodenum. CCK also slows gastric emptying by promoting contraction of the pyloric sphincter, produces satiety (a feeling of fullness) by acting on the hypothalamus in the brain, promotes normal growth and maintenance of the pancreas, and enhances the effects of secretin. Acidic chyme entering the duodenum stimulates the release of secretin from the S cells of the small intestinal crypts of Lieberkühn. In turn, secretin stimulates the flow of pancreatic juice that is rich in bicarbonate (HCO3 _) ions to buffer the acidic chyme that enters the duodenum from the small intestine. Besides this major effect, secretin inhibits secretion of gastric juice, promotes normal growth and maintenance of the pancreas, and enhances the effects of CCK. Overall, secretin causes buffering of acid in chyme that reaches the duodenum and slows production of acid in the stomach.
Other Hormones of the Digestive System Besides gastrin, CCK, and secretin, at least 10 other so-called “gut hormones” are secreted by and have effects on the GI tract. They include motilin, substance P, and bombesin, which stimulate motility of the intestines; vasoactive intestinal polypeptide (VIP), which stimulates secretion of ions and water by the intestines and inhibits gastric acid secretion; gastrin-releasing peptide, which stimulates release of gastrin; and somatostatin, which inhibits gastrin release. Some of these hormones are thought to act as local hormones (paracrines), whereas others are secreted into the blood or even into the lumen of the GI tract. The physiological roles of these and other gut hormones are still under investigation.
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