Physiology and Digestion
The primary function of the gastrointestinal (or alimentary) tract is to provide the body with nutrients, water, and electrolytes from ingested foodstuffs. When food moves through the gastrointestinal tract (see box), it is broken down into small units that can be absorbed in a process called digestion. Absorption (i.e., the transport of nutrients from the intestine into the blood or lymph system) takes place in various parts of the gastrointestinal tract for different nutrients. Here, we give an overview of the anatomy and physiology of the gastrointestinal tract, the various digestion and absorption processes that take place within it, and the changes that occur during exercise.
Anatomy of the Gastrointestinal Tract
The gastrointestinal tract is a long tubular structure that reaches from the mouth to the anus and includes the esophagus, stomach, small intestine, large intestine, rectum, anus, and several accessory digestive glands such as the salivary glands, gallbladder, liver, and pancreas. In this six- to eight-meter tube, food is digested and nutrients are absorbed. The mouth, stomach, pancreas, and gallbladder have a predominantly digestive function, and most of the absorption occurs in the small and large intestine. After absorption, most nutrients are transported to the liver, and from there they enter the main circulation. Mouth chewing (or masticating) of food is the first step of digestion. It is often referred to as mechanical digestion. The anterior teeth or incisors provide strong cutting action, and the posterior teeth (molars) are used for grinding. The forces applied to cut and grind the food can be as much as 25 kg on the incisors and 90 kg on the molars.
Chewing food serves three major purposes:
(1) It mechanically reduces the size of the food particles, which increases the rate of gastric emptying.
(2) It increases the surface area of food, which in turn increases the contact area for digestive enzymes that are released from the salivary glands and stomach (enzymatic digestion). Increasing the total surface area of the food increases the rate of digestion.
(3) It mixes the food particles with saliva and digestive enzymes. The mouth has three pairs of salivary glands: the parotid glands, the sublingual glands, and the submandibular glands. Chewing is especially important for plant material (fruits and raw vegetables) because indigestible cellulose cell walls must be mechanically destroyed to release the nutrients.
When the food is small and soft enough to swallow, it moves past the pharynx at the back of the mouth into the esophagus. The esophagus moves the food particles to the stomach. This transport process is caused by rhythmic contractions and relaxations of the esophagus. The esophagus contains an inner layer of smooth muscle consisting of circular bands and an outer layer of smooth muscle that runs longitudinally. Contraction of these muscles causes peristalsis, a squeezing action involving progressive and recurring contractions, which mixes and moves the food to the stomach. This mechanism makes swallowing possible even when a person is hanging upside down or in space with zero gravity. At the end of the esophagus, a valve of smooth muscle called the esophageal sphincter relaxes to allow food into the stomach. After some food particles have passed the esophageal sphincter, it contracts, preventing reflux of food or fluids from the stomach into the esophagus. If this sphincter does not function properly, people may experience some acid leaking from the stomach, or heartburn. This gastrointestinal problem is common among runners and cyclists.
The stomach, which is about 20 to 25 cm long, is divided in three parts: the corpus, or body; the antrum; and the fundus. The corpus and the antrum have different physiological functions. Although the fundus is a different part of the stomach from an anatomical point of view, from a functional point of view it is considered part of the corpus. The end of the stomach is a circular valve called the pyloric sphincter (pylorus), which controls the emptying of food from the stomach into the small intestine. When this muscle relaxes, food leaves the stomach; when it contracts, food stays in the stomach. The functions of the stomach include:
- storage of large quantities of food until it can be accommodated in the intestine,
- mixing this food with gastric secretions to form a homogeneous, acidic, soup-like liquid, or paste called chyme, and
- regulation of the emptying of the chyme into the duodenum (upper part of the small intestine) at a rate suitable for proper digestion and absorption.
Normally, food that enters the stomach forms concentric circles in the corpus and fundus so that the latest food is closest to the esophagus and the oldest food is nearest to the wall of the corpus. The stomach volume is normally around 1.5 L, but this volume can change from almost nothing when the stomach is empty to about 6 L when the stomach is full. The muscular tone of the stomach wall decreases as soon as food enters the stomach, which allows the stomach wall to stretch outward to accommodate more food.
The wall of the corpus contains gastric glands that secrete digestive juices. These secretions come in contact with the food portions nearest the stomach wall.
The stomach can also contract and relax, which mixes the food into chyme. Chyme can be a fluid or a paste, depending on the relative amounts of food and secretions and the degree of digestion. The chyme is passed down to the small intestine by a strictly controlled process of gastric emptying. Little absorption takes place in the stomach (with the exception of some water and alcohol).
The small intestine is approximately 2 to 3 m in length and 3 to 5 cm in diameter, and it can be divided into the duodenum (about 30 cm), the jejunum (next 1 to 2 m), and the ileum (last 1 m). About 95% of all absorption takes place in the duodenum and jejunum. The intestinal mucosa of the duodenum and jejunum contains many folds called the folds of Kerkring.
These folds increase the surface area of the intestine about three times that of a similarly sized flat internal lining. These folds are covered by millions of small fingerlike structures called villi, which project about 1 mm from the surface of the mucosa. The villi increase the total surface area of the small intestine 10-fold. The intestinal cells that form the border of the villi are covered by a brush border consisting of about 600 microvilli, approximately 1 mm long. These microvilli increase the total surface area 20-fold further.
The highly specialized construction of the small intestine, including the folds of Kerkring, the villi, and the microvilli, increases the absorption about 600-fold compared with a simple tube with a flat internal surface. The total surface area of the small intestine may be as large as a regulation tennis court!
Villi are finger-shaped and highly . The wall of a villus consists of a layer of epithelial cells, each with its own brush border (brush border is the name for the microvilli-covered surface of the intestine). Water, water-soluble particles, and electrolytes require transport or diffusion across the luminal and contra luminal membranes of the epithelial cell into the blood vessels. These nutrients are then transported to the liver through the hepatic portal vein. Each villus also contains a lacteal, located in the central part of a villus. The lacteal transports particles that are not readily water soluble (e.g., long-chain fatty acids) via the lymphatic vessels. These vessels drain into large veins near the heart.
Motility and Transit Time
Food spends one to three days in the gastrointestinal tract before it is eliminated. The time spent in a section of the gastrointestinal tract is the transit time. For instance, the transit time in the small intestine is approximately 3 to 10 hours, depending on the composition of the food and the motility (movement of food) of the gastrointestinal tract. The small-intestine wall contains two layers of smooth muscle with longitudinal and circular muscle fibers that allow peristalsis and mixing contractions that push the chyme in the distal direction toward the large intestine (like squeezing toothpaste out of a tube). These contractions occur at a rate of 0.5 to 2.0 cm/s, with the fastest movement in the proximal intestine and the slowest movement in the distal intestine. The average speed of chyme along the small intestine is approximately 1 cm/s. Peristalsis increases after a meal and can increase greatly after intense irritation of the intestinal mucosa, such as during infectious diarrhea. Mixing, or segmentation, contractions differ from peristalsis. The circular muscles contract, giving the small intestine the look of linked sausages. These intermittent contractions (8/min to 12/min) cause the chyme to move both forward and backward. The chyme moves backward before it advances. The function of these circular contractions is to mix the chyme with bile from the gallbladder, pancreatic juices, and intestinal juices. The juices get extra time to digest the food, and the contact time and area are increased.
The gallbladder stores, concentrates, and releases bile. Bile, which is produced by liver cells, consists of water, electrolytes, bile salts, cholesterol, lecithin, and bilirubin. The gallbladder can store approximately 30 to 60 ml, but it secretes as much as 1,200 ml into the duodenum every day, storing up to 12 hours of bile secretion by concentrating the bile constituents. Bile facilitates the digestion and absorption of fat and is released through the hepatic duct, which joins the pancreatic duct just before entering the duodenum. Bile secretion increases after a meal, especially when the meal contains a large amount of fat.
The pancreas is a large gland situated parallel to and just beneath the stomach. It secretes sodium bicarbonate to buffer the hydrochloric acid of the stomach and digestive enzymes to break down carbohydrate, protein, and fat. Pancreatic juice is mainly secreted in response to chyme in the upper portions of the small intestine. The regulatory mechanisms for sodium bicarbonate secretion and digestive enzyme secretion are different, and the secretion rates are highly dependent on the type and amount of food ingested. The concentration of various enzymes in pancreatic juice also depends to some extent on the type of food ingested.
From the small intestine, the chyme moves into the large intestine through the ileocecal valve. This valve prevents backflow of indigestible fecal material into the small intestine. The valve can resist pressure equal to about 50 to 60 cm of water. The distal end of the small intestine, or ileum, has a thicker muscular coat that controls the emptying from the ileum. Contraction of the ileocecal sphincter is regulated by a variety of factors, including:
(1) distension of the cecum (a blind pouch, open only at one end, at the beginning of the large intestine),
(2) irritating substances in the cecum, and
(3) fluidity of the chyme.
An inflamed appendix (a nonfunctional part of the intestine that is short, thin, and outpouching from the cecum) restricts the emptying of the ileum. Increased fluidity of chyme, on the other hand, increases emptying from the ileum.
In the large intestine, the chyme is called feces. The large intestine consists of the colon, the rectum, and the anal canal. The colon is divided into the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The functions of the colon include absorption of water and electrolytes from the chyme and storage of feces until they can be expelled. Absorption takes place mainly in the first part of the colon, and storage mainly occurs in the distal parts. The peristaltic movements of the colon are slower than those of the small intestine. The colon also has circular and longitudinal smooth muscle layers and moves the feces toward the distal parts of the colon by rhythmic contractions. The fecal material is slowly rolled over and mixed so that contact with the surface of the large intestine increases and as much water as possible is absorbed. Normally only 80 to 150 ml of water are present in approximately 300 ml of feces.
Regulation of the Gastrointestinal Tract
The gastrointestinal tract is innervated by both the sympathetic and the parasympathetic components of the autonomic nervous system. Parasympathetic stimulation in general stimulates motility. The vagus nerve is the source of parasympathetic activity in the esophagus, stomach, pancreas, gallbladder, small intestine, and upper section of the large intestine. The lower portion of the large intestine receives parasympathetic innervation from spinal nerves in the sacral region (the lower end of the spine). Autonomic regulation, which is “extrinsic” to the gastrointestinal tract, is overruled by “intrinsic” modes of regulation. Sensory neurons in various parts of the gastrointestinal tract have their cell bodies in the gut wall but are not part of the autonomic nervous system. In addition, hormonal regulation plays an important role. Endocrine glands secrete hormones into circulation, whereas paracrine glands or cells secrete products that influence the secretion of another product discharged by a local gland or cell. For example, gastrin is a hormone secreted by the stomach that increases hydrochloric acid and pepsinogen secretion in the stomach. Another example is secretin, a hormone produced by the small intestine that increases water and bicarbonate secretion by the pancreas. Substances within the tissues of the gastrointestinal tract and hormones released by organs in the gastrointestinal tract affect secretion and motility.
Digestion starts the moment food is ingested and may take four to six hours to complete. Specific enzymes are responsible for the digestion of different macronutrients.
The digestion of carbohydrates starts in the mouth as saliva is added to food. Saliva is secreted from the parotid glands, the sublingual glands, and the submandibular glands. The daily secretion of saliva normally ranges between 800 and 1,500 ml. In the unstimulated state, saliva secretion rate is about 0.5 ml/min, but this rate can increase up to tenfold during the chewing of food. Saliva consists primarily of water (99.5%) derived from extracellular fluid. In addition, it contains a-amylase (also referred to as ptyalin), an enzyme responsible for the breakdown of starch into smaller units; mucoid proteins; bicarbonate; electrolytes; lysozymes, enzymes that break down proteins and attack bacteria; lingual lipase; and protein antibodies (the major secretory antibody being immunoglobulin, which helps to destroy oral bacteria). Thus, besides a digestive function, saliva has a protective function against invading bacteria. The mucoid proteins give saliva its viscous quality, which helps lubricate food and makes it easier to swallow. Chewing mixes saliva with the food and increases the contact area so amylase can start breaking down the glucose chain in starches. Prolonged chewing of a cracker will cause it to taste sweeter because some starch breaks down to disaccharide sugars such as maltose, which tastes much sweeter than starch. When food is swallowed and arrives in the acidic environment of the stomach, amylase activity decreases. Carbohydrate digestion still takes place but at a much slower rate. In the mouth and stomach, before the stomach content is completely mixed with gastric secretions, approximately 30 to 40% of the carbohydrate may be digested, predominantly to maltose, maltotrioses, and small oligosaccharides. When the carbohydrates are emptied from the stomach into the duodenum and the acid is neutralized by sodium bicarbonate from the pancreas, digestion again proceeds at a high rate. In the duodenum, additional a-amylase will be secreted in the pancreatic juice. This a-amylase, like salivary amylase, hydrolyzes the starches into small glucose polymers (dextrins) and maltose. Almost complete hydrolysis of all starches to maltose has taken place when the chyme enters the ileum. The disaccharides and small polysaccharides are further digested by specific enzymes located in the brush borders of intestinal epithelial cells. As soon as disaccharides come in contact with the brush border, they are digested by the enzymes lactase, sucrase, and maltase. Lactase breaks lactose down into glucose and galactose, sucrase breaks sucrose down into glucose and fructose, and maltase breaks maltose down into two glucose molecules. Problems with the digestive process can result when a deficiency of one or more of these enzymes exists. Lactose intolerance is caused by an absence or deficiency of the intestinal enzyme lactase. When lactose, the main carbohydrate component of milk, is not digested, diarrhea and fluid loss result. In addition, bacteria in the large intestine metabolizes the lactose to produce large quantities of gas, which causes bloating and pain.
Fiber, a form of dietary carbohydrate, contains cellulose, a structural component of plant cells which is resistant to human digestive enzymes. Cellulose can be excreted in the feces. Some of it, however, is fermented by the bacteria present in the large intestine. Similar to the way in which yeast ferments the sugars in grape juice to produce wine, the bacteria in the large intestine ferment cellulose to produce hydrogen and carbon dioxide gases, volatile FAs, and, in many instances, methane gas (which has an unpleasant odor). Changes in the diet or in the type of microorganisms can influence the amount of gas produced. Peristaltic movements push undigested carbohydrates, including fibrous substances, to the colon, where more digestion occurs. Indigestible carbohydrates (predominantly cellulose) move to the rectum for expulsion though the anus
Digestion of lipids begins in the mouth because saliva contains small amounts of lingual lipase, the enzyme that splits triacylglycerols (triglycerides) into FAs and glycerol. In the stomach, this acid-stable lipase continues to hydrolyze the triacylglycerols. Hydrolysis, however, is slow because triacylglycerols are not soluble in water and therefore do not mix well with the water fraction in which lipase is found. The lingual and gastric lipases act together but mainly on the short-chain (C4 to C6) and medium-chain (C8 to C10) triacylglycerols, whereas most of the fat (long-chain triacylglycerols, C12 to C24) is digested in the small intestine. Lingual lipase is responsible for 10 to 30% of the triacylglycerol digestion. When chyme enters the duodenum, bile is added and acts on the triacylglycerols, which by this time are organized into large lipid globules. Pancreatic lipase is secreted into the duodenum and further hydrolyzes the triacylglycerols. After initial hydrolysis starts and the triglycerides are converted into FAs, monoglycerides and diglycerides organize themselves into small emulsion droplets. The fat-soluble part of the FA faces inward, and the water-soluble part forms the core of these droplets. When bile salts (bile acids), stored in the gallbladder, are secreted into the duodenum, micelles are formed. Micelles are well-defined structures with a disk-like shape, on which phospholipids and FAs form a bilayer. The bile salts occupy the edge positions, rendering the edge of the disk hydrophilic (i.e., more attractive to water). The bile salts emulsify the lipids into small droplets, which increases the total surface area and thus facilitates the hydrolysis (breakdown) of triacylglycerols by pancreatic lipase.
Protein digestion breaks down the ingested proteins into simple amino acids, dipeptides, and tripeptides for absorption across the intestinal mucosa. This process (protein hydrolysis) takes place within the stomach and small intestine and depends on specific protein-digesting enzymes (proteases) and the acidity of the stomach. Specific cells produce and secrete hydrochloric acid (HCl), a strong acid, into the stomach. These parietal cells secrete an isotonic 160 mM HCl solution with a pH of about 0.8, illustrating its extreme acidity. The pH within the stomach and of the gastric contents is typically around 2.0.
HCl, and thereby the acidic ingested food, has various functions; among other things it:
- activates the protease enzyme pepsin,
- kills many pathogenic organisms,
- increases the absorption of iron and calcium, protein pepsin from stomach
- inactivates hormones of plant and animal origin, and
- denatures (breaks down) food proteins, making them more vulnerable to enzyme action.
Proteases are often stored in the form of an inactive precursor, but as soon as it is released into the stomach or small intestine it becomes active. This mechanism prevents the digesting of the cells in which the proteases are produced and stored.
Pepsin (an important group of proteases), secreted as its precursor pepsinogen from the cells of the stomach wall, is initially inactive. As soon as pepsinogen comes in contact with the HCl of the stomach, it is automatically converted into the active pepsin that breaks down protein. Pepsin degrades the collagenous connective tissue fibers of meat. After dismantling these fibers, other proteases can effectively digest the remaining animal protein.
Stomach enzymes and acids attack the long, complex protein strands and hydrolyze approximately 10 to 20% of the ingested proteins. The low pH causes denaturation of the protein, meaning that the three-dimensional structure of protein is uncoiled and breaks into smaller polypeptide and peptide units. When the chyme passes into the small intestine, pepsin becomes inactivated by the relatively high pH in the duodenum.
Other proteases (alkaline enzymes), including trypsin, are released and become active to digest the remaining proteins and polypeptides. The pancreatic juice is rich in precursors of endopeptidases, carboxypeptidases, enteropeptidases, trypsinogen, and trypsin. These proteases digest the polypeptides into tripeptides, dipeptides, and single amino acids. Amino acids, dipeptides, and tripeptides can be transported across the enterocyte.
Absorption of nutrients across the intestinal walls occurs either by active transport or by simple diffusion. Active transport requires energy and usually takes place against a concentration gradient or an electrical potential. Active transport often requires specialized carrier proteins. Diffusion is the movement of substances across a membrane along, rather than against, an electrochemical gradient. Simple diffusion does not require transport proteins or energy in the form of ATP, but many nutrients are transported by facilitated diffusion, which requires a protein transporter or channel.
Absorption of Carbohydrates
The major monosaccharides that result from digestion of polysaccharides and disaccharides are glucose, fructose, and galactose. These monosaccharides are absorbed by carrier-mediated transport processes. The transporters that mediate the uptake of monosaccharides in the epithelial cell are a sodium monosaccharide cotransporter (most commonly the sodium dependent glucose transporter [SGLT1]) and a sodium-independent facilitated-diffusion transporter with specificity for fructose (GLUT5). For each molecule of glucose, two sodium ions will be transported into the epithelial cell. The sodium is then actively transported back into the gut lumen through a Na +-K +-ATPase pump. Galactose also uses SGLT1. A separate monosaccharide transporter on the contraluminal side of the epithelial cell accepts all three monosaccharides (GLUT 2). The monosaccharides then enter the circulation in the hepatic portal vein, which will transport them to the liver.
Absorption of Fats
The monoacylglycerols and FAs incorporated into micelles are transported to the villi and move into the spaces between the microvilli. Here FAs diffuse across the membrane of the epithelium and enter the epithelial cell. The micelles then move away from the villi, incorporate new FAs, and transport them to the villi. Micelles formed within the intestinal lumen, therefore, perform an important ferrying function. In the presence of bile salts (and, thus, micelles), fat absorption is almost complete (97%), whereas in the absence of bile, only about 50% of the FAs are absorbed. The absorption of FAs through the epithelial membranes is by diffusion (because they are highly soluble in the lipid membranes). In the epithelial cell, FAs are altered to triacylglycerols in the endoplasmatic reticulum.
Once formed, triacylglycerols combine with cholesterol and phospholipids to form chylomicrons. In this chylomicron, the fatty sides of the phospholipids face toward the center, and the polar parts form the surface. Chylomicrons make possible the transport of fat in the aqueous environment of the lymph and blood plasma. These large molecules move toward the central lacteal of the villi and are slowly transported through the lymphatic system, reaching the circulation in the subclavian veins.
Short-chain and medium-chain FAs are more water soluble than long-chain FAs and therefore follow a slightly different route of absorption. They enter the epithelial cell, and, without being reesterified to triacylglycerols, they directly diffuse through the contraluminal membrane into the portal blood, where they are bound to the plasma protein albumin and passed to the liver via the hepatic portal vein. The bile salts are reabsorbed again in the intestinal mucosa of the distal ileum. They enter the portal blood and pass to the liver. In the liver, they are resecreted into the bile. In this way, 94% of bile salts are reutilized. The recirculation of bile salts is called the enterohepatic circulation.
Absorption of Amino Acids
Amino acids, dipeptides, and tripeptides are absorbed by active transport (coupled to the transport of sodium) in the small intestine and delivered to the liver via the hepatic portal vein. Dipeptides and tripeptides that have been transported across the epithelial membrane are broken down inside the cell into their amino acid constituents by specific dipeptidases and tripeptidases.
Most amino acids are transported across the epithelium against a concentration gradient, and, therefore, carrier-mediated transport is needed. At least seven brush border-specific transport proteins have been identified. The luminal membrane usually contains sodium-dependent transport systems, whereas the contraluminal membrane transport does not require sodium. The small intestine has a large and effective capacity to absorb amino acids and small peptides. Most amino acids can use more than one transporter for absorption. Less than 1% of the ingested protein is usually found in feces. After amino acids have passed the epithelium, they are transported to the liver where they can be converted to glucose, fat, or protein, or they can be released into the bloodstream as free amino acids.
Absorption of Water
Most water absorption (99%) take place in the small intestine, mainly in the duodenum (72%), entirely by simple diffusion. This absorption obeys the laws of osmosis. A membrane that is impermeable to solutes but permeable to water separates two compartments with the same amount of fluid but different numbers of solute particles.
Water diffuses across this membrane in both directions, but relatively more water flows toward the compartment with the lower water concentration (higher solute concentration). This net movement of water eventually results in a similar solute concentration on both sides of the membrane. But the amount of water in the compartment with the lower water concentration increases.
Absorption of Vitamins
Most vitamin absorption takes place in the jejunum and ileum and is usually a passive process (diffusion). Fat-soluble vitamins (A, D, E, and K) are absorbed along with FAs. They are also incorporated into chylomicrons and transported through the lymph system into the systemic circulation to liver and other tissues. Most of the absorption of fat-soluble vitamins takes place in the small intestine.
Water-soluble vitamins are also mostly absorbed in the small intestine by diffusion. The water-soluble vitamins are not retained to any great extent by different tissues, and when large amounts are ingested, they are mostly excreted in urine. Most vitamin C is absorbed in the distal portion of the small intestine. Excess intake of vitamin C (above approximately 1,200 mg/day) decreases the efficiency of renal reabsorption of vitamin C, and much of the excess intake appears in the urine. B vitamins are often ingested as part of coenzymes in food; digestion liberates the vitamins. For example, pantothenic acid is usually present in food as part of coenzyme A. Digestion releases the vitamin from its coenzyme, and absorption takes place. Thiamin and vitamin B6 are mainly absorbed in the jejunum. Biotin and riboflavin are mainly absorbed in the proximal part of the small intestine. Niacin is partly absorbed in the stomach but mostly in the small intestine. Vitamin B12 is mainly absorbed in the ileum. Its absorption is more complex, involving binding to a specific protein (called intrinsic factor, which is secreted by the parietal cells of the gastric mucosa). Absorption of folic acid depends on the presence of the intestinal enzyme conjugase, which facilitates the absorption of folic acid in the small intestine.
Minerals are not well absorbed in the human intestine. The intake in food is therefore usually far in excess of actual requirements. Mineral absorption often depends on its chemical form. The best-known example of this requirement is probably the difference in absorption between nonheme and heme iron. (Heme iron is found in meat; nonheme iron is obtained from plants.) About 15% of all ingested heme iron is absorbed in the small intestine, whereas only 2% to 10% of nonheme iron is absorbed. Absorption of other minerals is also relatively poor.
A maximum of only 35% of ingested calcium is absorbed, 20 to 30% of ingested magnesium is absorbed, 14 to 41% of ingested zinc is absorbed, and less than 2% of ingested chromium is absorbed. Besides poor absorption, excretion rates by urine are also high. About 65% of absorbed phosphorus and 50% of absorbed calcium is excreted by urine. When daily mineral intake is insufficient, increased intake may result in increased retention. For example, many women in Western countries have insufficient iron and calcium intake; increasing intake generally increases storage of these minerals.
Sodium is actively transported out of the epithelial cell into the portal circulation, a process requiring ATPase carrier enzymes and energy (in the form of ATP). The transport of sodium out of the epithelial cell creates a low sodium concentration in the cell, which increases diffusion of sodium from the gut lumen into the epithelial cell. About 30 g of sodium is secreted in intestinal secretions every day. Daily sodium ingestion is about 5 to 8 g. Thus, about 25 to 35 g of sodium must be reabsorbed each day, representing a large percentage of the body sodium stores.
This requirement explains why extreme diarrhea results in large sodium losses, which can be dangerous and even life threatening.
Function of Bacteria in the Colon
The adult human gut contains about 1 kg of various bacteria (colon bacilli). The gastrointestinal tract contains an immensely complex ecology of microorganisms. A typical person harbors more than 500 distinct species of bacteria. The composition and distribution of these microorganisms vary with age, state of health, and diet.
The number and type of bacteria in the gastrointestinal tract vary dramatically by region. In healthy people the stomach and proximal small intestine contain few microorganisms, largely a result of the bacteriocidal activity of gastric acid. One interesting testimony to the ability of gastric acid to suppress bacterial populations is seen in patients with achlorhydria, a genetic condition that prevents secretion of gastric acid. Patients with achlorhydria who are otherwise healthy may have as many as 10,000 to 100 million microorganisms per milliliter of stomach contents.
In sharp contrast to the stomach and small intestine, the colon literally teems with bacteria, predominantly strict anaerobes (bacteria that survive only in environments virtually devoid of oxygen). Between these two extremes is a transitional zone, usually in the ileum, where moderate numbers of both aerobic and anaerobic bacteria are found. The gastrointestinal tract is sterile at birth, but colonization typically begins within a few hours of birth, starting in the small intestine and progressing caudally over a period of several days. In most circumstances, a mature microbial flora is established by three to four weeks of age.
Bacterial populations in the large intestine digest carbohydrates, proteins, and lipids that escape digestion and absorption in the small intestine. The bacteria are responsible for the fermentation of small amounts of cellulose. More important, however, is the production of vitamin K, vitamin B12, thiamine, riboflavin, and other substances. Vitamin K is especially important because most people’s daily vitamin K intake in food is insufficient.
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