The Digestive System
General Concepts Composition
The digestive system has two anatomical subdivisions: the accessory organs and the digestive tract.
- The accessory organs are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
- The digestive tract is a tube extending from mouth to anus. It includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
Functions
1. Digestion
2. Absorption
Digestion
the process to break ingested food into usable forms that can be absorbed into the blood. There are two stages of digestion: mechanical and chemical.
- Mechanical digestion is achieved by the cutting and grinding action of the teeth and the churning contractions of the stomach and small intestine.
- Chemical digestion consists solely of hydrolysis reactions that break the dietary macromolecules into their monomers. It is carried out by digestive enzymes produced by the salivary glands, stomach, pancreas, and small intestine.
Absorption
The process to take usable forms of nutrients from digestive tract into the blood
The Mouth
The mouth, or oral or buccal cavity, functions in the following:
- breaking food into pieces small enough to be swallowed (mastication)
- Sense of taste
- Swallowing
- Speech
Saliva and the Salivary Glands
• Saliva moistens the mouth, digests a small amount of starch and fat, cleanses the teeth, inhibits bacterial growth, dissolves molecules so they can stimulate taste buds, and moistens food and binds particles together to aid in swallowing. It is secreted by the salivary glands.
The Stomach
1. serves as a food storage organ. When empty, it has a volume of 50 mL. When very full, it may hold up to 4L.
2. breaks up food particles, liquefies the food, and begins the chemical digestion of proteins and a small amount of fat, producing a mixture of semidigested food called chyme.
The Stomach Wall
1. The gastric mucosa is pocked with depressions called gastric pits.
2. At the bottom of the pits lie glands.
The gastric glands have a greater variety of cell types and secretions:
- mucous neck cells: produce mucus
- chief cells: produce pepsinogen
- parietal cells: produce HCl and intrinsic factor
- endocrine (G) cells: produce hormones that regulate digestion
Gastric Secretion
The gastric glands produce 2-3 L of gastric juice daily, composed mainly of water, HCl, and pepsinogen.
Hydrochloric Acid
• Gastric juice has a high concentration of HCl and a pH as low as 0.8.
Stomach acid has several functions
- activates pepsinogen into the active enzyme pepsin.
- It breaks up connective tissues and plant cell walls.
- It converts ferric ions to ferrous ions.
- It destroys ingested bacteria and other pathogens.
Intrinsic Factor
1. a glycoprotein that is essential to the absorption of vitamin B12
2. The secretion of intrinsic factor is the only indispensable function of the stomach.
Pepsinogen
- Pepsinogen is the inactive precursor of the active enzyme pepsin. Inactive precursors of enzymes are called zymogens.
- Pepsinogen is activated by HCl or pepsin.
Gastrin
- a hormone that stimulates the secretion of HCl and pepsinogen, and stimulates motility of the large intestine.
- is secreted in the bloodstream or diffuses to nearby parietal and chief cells.
Gastric Motility
- During swallowing, signals from the swallowing center of the medulla oblongata stimulate the stomach to relax in preparation of the arrival of food. When food enters the stomach, it is stimulated to stretch further, a phenomenon known as the stress-relaxation response.
- Next, the stomach shows a rhythm of peristaltic contractions, initiated by pacemaker cells in the greater curvature.
- The antrum of the stomach holds about 30 mL of chyme. As a peristaltic wave passes down the antrum, it squirts about 3 mL of chyme into the duodenum.
Digestion and Absorption in Stomach
- Minor role in digestion or absorption of nutrients
- But, the stomach does absorb aspirin and some lipid-soluble drugs.
Protection of the Stomach
The living stomach is protected in three ways from the harsh chemical of its interior.
1. It has a highly alkaline mucous coat.
2. Epithelial cells are replaced every 3-6 days.
3. Tight junctions between epithelial cells prevent gastric juice from seeping between cells.
Regulation of Gastric Function
Gastric activity is divided into three stages called the cephalic, gastric, and intestinal phases.
The Cephalic Phase
• The cephalic phase is stimulated by the sight, smell, taste, or mere thought of food.
The Gastric Phase
• The gastric phase is stimulated by food in the stomach and accounts for two-thirds of gastric secretion.
The Intestinal Phase
- The intestinal phase is stimulated by chyme entering the duodenum.
- Chyme initially stimulates the secretion of intestinal gastrin which stimulates gastric secretion and mobility.
- Gastric secretion and mobility are then quickly inhibited via the enterogastric reflex which is initiated by HCl, fats, and peptides.
- Chyme in the duodenum also stimulates its enteroendocrine cells to release secretin, cholecystokinin, and gastric inhibitory peptide.
All the three hormones suppress gastric motility.
• Inhibition of gastric secretion and motility and contraction of the pyloric sphincter limit the addition of more chyme into the duodenum, giving the duodenum more time to work on the chyme.
The Liver, Gallbladder, and Pancreas
The small intestine receives not only chyme from the stomach but also secretions from the liver and pancreas.
The Liver
• The liver is the body's largest gland and performs a tremendous variety of functions, including the secretion of bile for digestive purposes.
• The liver parenchyma consists mostly of hepatocytes arranged in cylinders called hepatic lobules. Each lobule has a central vein passing through its core.
• The liver secretes bile into narrow channels, the bile canaliculi, between sheets of hepatocytes. Bile passes from there into the small bile ductules and then into the right and left hepatic ducts. These two ducts converge to form the common hepatic duct, which then joins the cystic duct coming from the gallbladder.
• The common bile duct descends through the lesser omentum and joins the duct of the pancreas, forming the hepatopancreatic ampulla. This ampulla contains the hepatopancreatic sphincter, which regulates the passage of bile and pancreatic secretion into the duodenum.
The Gallbladder and Bile
- When no chyme is in the small intestine, the hepatopancreatic sphincter is closed. Bile then fills up the common bile duct and spills over into the gallbladder, which absorbs water and stores the bile for later use.
- The liver produces 500-1,000 mL of bile per day. It is a yellow-green fluid containing minerals, bile pigments, bile salts, cholesterol, neutral fats, and phospholipids.
- Bile salts and phospholipids emulsifies fat globule into emulsification droplets which can be digested more easily by lipase. Bile salts also coat digested lipids and form micelles, facilitating in fat absorption.
- Bile salts and phospholipids are not excreted in the feces but are reabsorbed in the ileum and returned to the liver via enterohepatic circulation.
The Pancreas
The pancreas has both endocrine and exocrine functions. Exocrine means secretion into the lumen of a duct; endocrine means secretion into the blood.
Exocrine
• Most of the pancreas is exocrine tissue, which secretes 1,200-1,500 mL of pancreatic juice per day into the main pancreatic duct. It empties into the small intestine through the hepatopancreatic ampulla or by way of a smaller accessory pancreatic duct in some people.
Pancreatic juice is an alkaline mixture of:
- Water
- Sodium bicarbonate: buffers the hydrochloric acid from the stomach.
- Enzymes: pancreatic amylase (digesting starch), pancreatic lipase (digesting fat), ribonuclease (digesting RNA), and deoxyribonuclease (digesting DNA).
They are activated upon exposure to bile acid and ions in the intestinal lumen. zymogens: trypsinogen, chymotrypsinogen, and procarboxypeptidase (digesting proteins and peptides). They are activated by enterokinase located on the surface of intestine.
Regulation of Bile and Pancreatic Secretion
Bile and pancreatic juice are secreted in response to similar stimuli. During the cephalic and gastric phases of gastric secretion, the vagus nerves also stimulate pancreatic secretion.
- As chyme enters the duodenum laden with acid and fat, it stimulates the duodenal mucosa to secrete cholecystokinin (CCK).
- CCK stimulates relaxation of the hepatopancreatic sphincter, the contraction of the gallbladder, releasing more bile into the duodenum common bile duct, and secretion of pancreatic enzymes.
- Acidic chyme also stimulates the duodenum to release secretin, which stimulates the production of bicarbonate ions by both the hepatic and pancreatic ducts to neutralize stomach acid in the duodenum.
The Small Intestine
• Nearly all chemical digestion and nutrient absorption occur in the small intestine. The term "small" applies to its diameter, not its length. Circular folds, villi, and microvilli all serve to increase the surface area inside the small intestine.
Anatomy
The small intestine is divided into three regions.
- The Duodenum constitutes the first 25 cm. It receives the stomach contents, pancreatic juice, and bile. Stomach acid is neutralized here, pepsin is inactivated by the elevated pH, and pancreatic enzymes take over the job of chemical digestion.
- The Jejunum comprises the next 2.5 m.
- The Illeum forms the last 3.6 m and ends at the ileocecal junction.
Intestinal Secretion
The intestinal crypts secrete 1-2 L of intestinal juice per day, especially in
1. response to acid, hypertonic chyme, and intestinal distension.
2. The duodenum endocrine cells secret cholecystokinin (CCK) and secretin.
Intestinal Motility
Contractions of the small intestine serve three functions:
- to mix chyme with intestinal juice, bile, and pancreatic juice;
- to churn chyme and bring it in contact with the brush border for digestion and absorption;
- to move residue toward the large intestine.
- Ring-like constrictions appear at several places along the intestine and then relax while constrictions occur elsewhere
- The intensity (but not frequency) of the contractions is modified by nervous and hormonal influences.
- When most nutrients have been absorbed and little remains but residue, segmentation slows and peristalsis (synchronized contraction) begins.
At the ileocecal junction, the muscularis of the ileum is thickened to form a sphincter, the ileocecal valve.
The valve is usually closed.
- Food in the stomach triggers the release of gastrin as well as the gastroileal reflex, both of which enhance segmentation in the ileum and relax the valve.
- As the cecum fills with residue, the pressure pinches the valve shut, preventing the reflux of cecal contents into the ileum.
The Large Intestine
Absorption and Motility
- Each day, about 500 mL of food residue enters the large intestine. It undergoes no further chemical digestion, but its volume is reduced as it passes through the large intestine. The average adult voids about 150 mL of feces per day, consisting of 75% water and 25% solid matter, of which 30% is bacteria, and 30% undigested fiber.
- Strong contractions called mass movements occur one to three times a day, last about 15 minutes each, and occur especially an hour after breakfast.
Bacterial Flora and Intestinal Gas
- The large intestine is densely populated with several species of useful bacteria referred to collectively as the bacterial flora. They ferment cellulose and other undigested carbohydrates and synthesize B vitamins and vitamin K, which are absorbed by the colon.
- The average person expels about 500 mL of flatus per day. It is composed of nitrogen, carbon dioxide, hydrogen, methane, hydrogen sulfide, and indole and skatole.
Defecation
• In the intrinsic defecation reflex, stretch signals travel by the mesenteric nerve plexus to the muscularis of the descending and sigmoid colons and the rectum. This triggers a peristaltic wave that drives the feces downward, and it relaxes the internal anal sphincter. Defecation occurs only if the external anal sphincter is voluntarily relaxed at the same time.
Chemical Digestion and Absorption
Carbohydrates
• Most digestible dietary carbohydrate is starch.
Carbohydrate Digestion
- Starch is digested first to oligosaccharides three to eight glucose residues long, then into the disaccharide maltose, and finally to glucose, which is absorbed by the small intestine.
- The process of starch digestion begins in the mouth. Salivary amylase hydrolyzes starch into oligosaccharides, and functions best at the pH of the mouth cavity. It is denatured quickly upon contact with stomach acid.
- Starch digestion resumes in the small intestine when the chyme mixes with pancreatic amylase that entirely converts it to oligosaccharides and maltose within 10 minutes. Its digestion is completed by brush border enzymes.
Carbohydrate Absorption
- In the plasma membrane adjacent to the brush border enzymes, there are transport proteins that absorb monosaccharides as soon as they are produced. Most of the absorbed sugar is glucose, which is taken up by a sodium-dependent glucose transporter (SGLT).
- After a high-carbohydrate meal, two to three times as much glucose is absorbed by solvent drag as by SGLT.
Proteins
Enzymes that digest proteins are called proteases or peptidases. They are absent from the saliva but begin their job in the stomach.
- In the stomach, pepsin hydrolyzes any peptide bond between tyrosine and phenylalanine, thus digesting 10-15% of the dietary protein into shorter polypeptides.
- In the small intestine, the pancreatic enzymes trypsin and chymotrypsin take over protein digestion by hydrolyzing polypeptides into even shorter oligopeptides. Finally these are taken apart one amino acid at a time by carboxypeptidase, aminopeptidase, and dipeptidase. All three of these enzymes are found on the brush border.
Amino acid absorption is similar to that of monosaccharides. There are several sodium-dependent amino acid cotransporters for different classes of amino acids.
Lipids
1. Fats are digested by enzymes called lipases.
• Lingual lipase from the intrinsic salivary glands is activated by stomach acid, where it digests as much as 10% of the ingested fat.
• Most fat digestion occurs in the small intestine through the action of pancreatic lipase.
• In the small intestine, fats are first broken up into smaller emulsification droplets by lecithin and bile salts in the bile. When lipase digests fats, the products are two fatty acids (FFAs) and a monoglyceride. Bile salts coat these and other lipids and form droplets called micelles.
2. Micelles pass between the microvilli of the brush border, and upon reaching the surface of the epithelial cell, they release their lipids. The lipids diffuse freely across the phospholipid plasma membrane.
3. Within the cell, the FFAs and monoglycerides are resynthesized into triglycerides. They are coated with a thin film of protein, forming droplets called chylomicrons.
4. Although some free fatty acids enter the blood capillaries, chylomicrons are too large to do so and must be first transported in the lymphatic lacteal.
Nucleic Acids
• Nucleic acids are generally present in small quantities compared to the other polymers. The nucleases of pancreatic juice hydrolyze these to their component nucleotides. Nucleosidases and phosphatases of the brush border further break them down, and the products are transported across the intestinal epithelium by membrane carriers.
Vitamins
Vitamins are absorbed unchanged.
1. The fat-soluble vitamins are absorbed with other lipids.
2. Water soluble vitamins are absorbed by simple diffusion, with the exception of vitamin B12. This is an unusually large molecule that can only be absorbed if it binds to intrinsic factor from the stomach.
Minerals
Minerals (electrolytes) are absorbed along the entire length of small intestine.
• Iron and calcium are unusual in that they are absorbed in proportion to the body's need, whereas other minerals are absorbed at fairly constant rates regardless of need.
Water
• The digestive system is one of several systems involved in water balance.
• The digestive tract receives about 9 L of water per day - 0.7 L in food, 1.6 L in drink, 6.7 L in gastrointestinal secretions. About 8 L of this is absorbed by the small intestine and 0.8 L by the large intestine.
Water is absorbed by osmosis.
- Diarrhea occurs when the large intestine absorbs too little water from the feces.
- Constipation occurs when fecal movement is slow, too much water is reabsorbed, and the feces become hardened.
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