Lecture Notes for Digestion

Overview:

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At its simplest, the digestive system is a tube running from the mouth to the anus.

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The function of the digestive system is to prepare food, by the mechanical and chemical breakdown of the ingested food, for absorption into the blood and/or lymphatic system, where these newly altered molecules can be absorbed by the cells of the body. Think of the digestive tract as a "disassembly line."

©Kimballs' Biology Pages

Jobs of the organs of the digestive tract, and the accessory organs:

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Mouth—>Food is broken down mechanically by chewing; saliva moistens and lubricates the bolus (food packet); enzymes in saliva do minimal digestion of starch. There are three pairs of salivary glands, and their secretions are somewhat specialized. The glands also produce lysozymes which help inhibit some bacterial growth.

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Esophagus—>Transports food between mouth to the stomach.

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Stomach—>A "holding tank" or reservoir for food—approx. a 4 cup (1 L) capacity; food is turned into chyme (liquid food) by mixing with gastric juices. Protein digestion begins in the stomach with the release of the enzyme precursor pepsinogen into the acidic gastric juices. (HCl, hydrochloric acid) is needed to activate pepsinogen—>pepsin, the active form of the enzyme, which begins to cleave larger protein molecules into smaller ones). Food remains in the stomach fro about 2-3 hours (longer for high fat meals).

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Liver—>A major metabolic regulator of the body. In digestion, it primarily produces bile salts (which are stored in the gall bladder), and which are needed to emulsify fats (think of what a detergent does to greasy dishes) so they can be digested and absorbed. It also sets levels of circulating fatty acids in the bloodstream, makes transport proteins needed to move nutrients and other compounds in the blood, and plays an important role, along with the pancreas, in regulating blood glucose (blood sugar) levels.

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Pancreas—>Both an exocrine (produces enzymes and other substances secreted through ducts into the digestive tract), and an endocrine (hormone- producing organ, with the hormones being secreted into the bloodstream, and which affect targets like muscle and liver cells). Supplies important digestive enzymes to the small intestine (for the digestion of fats, carbs and proteins). As an endocrine organ, it secretes the hormones insulin and glucagon which regulate glucose metabolism by body cells.

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Kidney—>Mentioned here for its role in elimination of the nitrogen-containing waste products from protein digestion.

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Small Intestine—>Where the bulk of chemical digestion occurs, and where most absorption occurs. Has specialized structures not found elsewhere in the digestive tract for these processes to occur (circular folds, villi, microvilli, brush border enzymes). Food remains in the small intestine for approx. 3-10 hours

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Large Intestine—>Some remaining water from digestive secretions is reabsorbed here. Bacterial fermentation (digestion) of some food substances not otherwise digested occurs here. Feces are formed, compacted, moved along and eliminated. (Flatus, or gas, is produced primarily from the action of microbes fermenting soluble fiber, for example, from beans. Swallowing air is the cause of air in the stomach, and burping. A little air is swallowed with each mouthful that is swallowed).

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Sphincters—>Mentioned here in their role as "pinch valves" that close off areas of the digestive tract from other areas (for example, the stomach has sphincters at its upper and lower ends, etc).

 

Movement through the digestive tract:

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Food is moved through the digestive tract by involuntary muscular contractions of the smooth muscle tissue found in the walls of the organs of the tract. This movement is called peristalsis. Muscles that run longitudinally in the tract propel food forward. Contraction above the bolus, and relaxation below the bolus moves food forward. Muscle lying more at right angles, produces segmentation (a briefly isolated area of the small intestine where mixing of chyme with enzymes and fluids can occur).

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Peristalsis increases during the anticipation of food (cephalic phase response), upon actual receipt of food, and varies throughout the digestive tract dependent upon the activity occurring.

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In general, contractions are more forceful closer to the stomach, and may occur more frequently. Contractions tend to be slower as you reach the large intestine. Cramping is strong, painful peristalsis, often due to irritation of the GI tract.

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Vomiting, while making use of the smooth muscles of the digestive tract, is not a simple reversal of peristalsis, but is a complex interaction between the GI tract, and the vomition center of the brain.

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Diarrhea happens when movement through the GI tract, and particularly the colon, is too rapid for adequate absorption of water, and compaction of feces to occur, resulting in watery, frequent stools.

 

Control of digestive system functions:

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Control of digestive system function is managed by both the nervous system and the endocrine system.

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The digestive system has its own, local nervous system called the enteric nervous system. The body considers this important enough that this local nervous system has as many nerve cells (neurons) as the spinal cord! The enteric nervous system is made up of two sets or networks of neurons: 1) the myenteric plexus controls digestive tract motility (moving substances through the regions of the tract); 2) the submucous plexus senses the changes in the lumen of the digestive tract, and assists in regulating blood flow and control over functioning of the epithelial cells which line the GI tract (motor neurons, which can also control release of products from secretory cells — chief, parietal, mucous, etc.).

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The enteric nervous system communicates with and receives direction from the autonomic branch of the central nervous system (sympathetic signals—inhibitory signals which slow digestive activity, and parasympathetic signals—stimulate digestive activity).

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The second system which controls digestive activity is the enteric endocrine (hormone) system. Hormones are chemical messengers that are secreted into the blood, travel to target cells—cells with receptors (special molecules) on their surface or inside the cells that can "read" the hormone message—and change the metabolic activity of the cells (starting or speeding up some processes, or slowing down or stopping others).

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Cells in the enteric endocrine system don't pump out their "message" continuously—that wouldn't produce true control or regulation of activity. Instead, particular events trigger the release of the hormones.

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A table of major* hormones, their activities, and the triggers for their release:

Gastrin* Stimulates gastric acid secretion (from parietal cells, and gastric epithelial growth) Presence of proteins in stomach
Cholecystokinin (CCK)* Stimulates secretion of pancreatic enzymes, and contraction, emptying of gall bladder Presence of fats, proteins in the small intestine
Secretin* Stimulates secretions from pancreas, and from the bile ducts Acid chyme in lumen of small intestine
Motilin Probably stimulates "housekeeping" movements in digestive tract Trigger is unclear, but causes movement during periods between meals or when fasting.
Gastric Inhibitory Peptide* Inhibits gastric secretions and motility, and stimulates release of insulin from beta cells in the pancreas Presence of fats and glucose in the small intestine.
Gastric Lipase* Promotes digestion of milk fats in infants Presence of milk and milk fats in the stomach
Insulin* Plays a role in both glucose and lipid metabolism (lowers blood glucose levels by encouraging uptake by body cells). Released in response to elevated blood glucose levels
Glucagon* Stimulates release of glucose from the breakdown of glycogen Released in response to decreased level of glucose in bloodstream
Intrinsic Factor* Needed by the small intestine to facilitate abosrption of B-12. Secretion probably controlled by serum levels of B-12 (low B-12 stimulates release of Intrinsic Factor)
Somatostatin Secreted by a broad range of tissues, including the pancreas. Inhibits secretion of other GI hormones, and secretion of pancreatic enzymes. Released to inhibit the action of many other hormones, including human growth hormone.

 

Absorption of nutrients:

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Nearly all nutrients are absorbed into the blood (or lymphatic system, in the case of most fatty acids), through the small intestine. The small intestine also absorbs water and electrolytes (charged particles from the disassociation of salts, acids, bases—electrolytes can carry an electric current; this makes them important in cells that send and receive signals, such as muscle and nerve cells).

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The single most important process that allows absorption to occur in the small intestine is the establishment of a sodium-potassium gradient across the cell membrane of intestinal cells (the cells need to keep more sodium on the outside of the cell, and more potassium on the inside of the cell).

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To maintain this gradient, there are over 100,000 sodium pumps in the cell membrane of enterocytes (cells of the epithelial gut, particularly those of the small intestine). The reason the cell needs these pumps is that much of absorption will not occur without them. (More about this later—don't panic!).

Methods of movement of materials:

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osmosis—>the simple diffusion of water down its concentration gradient (movement through the cell membrane from the lumen of the GI tract into the cell's interior);

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passive diffusion—>movement of molecules through the cell membrane (directly through the phospholipid structure of the membrane); water, urea, CO2, and O2, and small lipid molecules. Requires no expenditure of energy (ATP) on the part of the cell.

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Movement of other materials requires protein structures in the cell membrane called channel proteins or carrier proteins. Channel proteins (and some carrier proteins) allow substances to pass through which couldn't make it through the lipid barrier of the cell membrane (passive diffusion or transport, doesn't require ATP).

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Carrier proteins can function in facilitated diffusion, which often means co-transporting a particle—like a sodium ion—which changes the shape of the carrier, and allows it to then bring across a glucose molecule (or some other important substance being transported out of the GI tract). Again, it doesn't require ATP. Sodium ions co-transported in this way, have to be pumped back out of the cell through the sodium pumps, or the fluid balance of cells would be disrupted.

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Other substances have to be moved out of the GI tract against their concentration gradient, and in these cases, the carrier proteins require the energy supplied by the hydrolysis (breaking) of ATP.

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To be absorbed, carbohydrates must be changed into simple sugars (glucose, fructose, galactose). To be absorbed, proteins must be broken down into their amino acids, or into di-peptides, and tri-peptides (two or three amino acids in the chain). For a few days after birth, babies can absorb intact proteins, which enables them to take in antibodies in breast milk.

Lipid absorption is more complex, more multistage:

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Bile salts break fats (triglycerides, primarily) in chyme into tiny droplets which can be attacked by pancreatic lipase. This produces two fatty acids and a monoglyceride (one fatty acid still attached to the glycerol backbone).

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The fatty acids, monoglycerides and bile salts form little bundles called micelles. These bump into the brush border, where the fatty acids and monoglycerides are absorbed into the intestinal cells.

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Once inside the cell, these bits and pieces are put back together as triglycerides!!! (This happens in the cell's endoplasmic reticulum). These rebuilt triglycerides are further packaged in the Golgi apparatus into bundles of fat carrying special proteins on the surface—little supertankers called chylomicrons.

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Chylomicrons are transported out of the cell (by exocytosis), and are transported into the lacteal, the lymphatic vessel inside each villus. Chylomicron-rich lymph (a liquid that circulates in the lymphatic vessels, and which is similar to blood serum) is fairly rapidly dumped into the bloodstream through the thoracic duct.

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These chylomicrons are transported to the liver where they are further modified into the serum lipoproteins we all need to be concerned about—LDLs, HDLs.

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Fat soluble vitamins follow the same process as triglycerides.

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Specialized structures of the small intestine that increase the surface area are the circular folds (plicae), villi, and microvilli (where the brush border enzymes are found). This gives the relatively short tube of the small intestine a surface area equivalent to a tennis court! The more surface area, the more opportunities for action by enzymes and absorption of digested molecules.

The role of the large intestine:

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Recovery of water and electrolytes (salts, etc.) from remaining food materials: By the time what is left of the chyme reaches the small intestine, about 90% of the water in it has already been reabsorbed, but there is still water, sodium, and chloride that will be absorbed in this last section of the GI tract. (Interestingly, the cells of the colon are better at absorbing water than those of the small intestine).

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Formation of feces (solid waste): The remains of food is dehydrated, mixed with bacteria (which live in the large intestine), and with mucus, and is formed into feces. Normal feces are about 75% water, and about 25% food waste and bacteria. Fecal color is due to compounds from bile not reabsorbed. Fecal odor is due mostly to compound produced by bacteria. Feces are compacted by a type of peristalsis not seen elsewhere in the GI tract. Called "giant migrating contractions" these intense contractions which last for quite some time apparently are intended to strip a section of the colon clean of its contents. These mass movements push feces into the rectum, and distension of this short end section of the colon stimulates the defecation reflex.

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Microbial fermentation: The large intestine contains a huge population of bacteria. Their role is to digest carbs left undigested by the small intestine (particularly soluble fiber). In this process, they produce the gases that can be socially embarassing for us, but they also synthesize some vitamin K, a substance needed for normal blood clotting and a few other substances.