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18. 1 Introduction




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421


Table 18.4 The relative concentrations of some constituents of hepatic bile and gallbladder bile



Solute

Solute concentration ratio (gallbladder bile/hepatic bile)

Na*

1.7

Ca2

5.0

HCO3

0.2

ci-

0.2

Bile acids

8.9

Bile pigments Cholesterol

4.0 8.3

Lecithin

8.0

Bile osmolality

290-300 mosmoles kg-1

Secreted volume of gallbladder bile

500-1000 ml day-1

absorption of water from the gallbladder to the interstitial fluid. Figure 18.24 illustrates this mechanism of solute and water absorption by the gallbladder.

Contraction of the gallbladder forces bile into the duodenum

Within a few minutes of starting a meal, particularly one that is rich in fats, the muscle of the gallbladder contracts, providing a pressure which forces bile towards the duodenum. This initial response is mediated via the vagal nerves but the major stimulus for contraction is CCK. This hormone is secreted in response to the presence of fatty and acidic chyme in the intestine. CCK also stimulates pancreatic secretion and relaxes the sphincter of Oddi so that bile and pancreatic juice can enter the duodenum.



Fig. 18.24 The absorption of salt and water in the gallbladder, illustrating the standing gradient hypothesis. Sodium, chloride, and bicarbonate are transported across the basolateral membrane inro rhe interstitial space and water follows passively.

Parasympathetic vagal activity makes a relatively minor con­tribution to the stimulation of gallbladder contraction. Conversely, emptying of the gallbladder is suppressed by sym­pathetic activity. The gallbladder normally empties completely about an hour after a fat-rich meal. This maintains the level of bile acids in the duodenum above the critical micellar concentration.

Summary

  1. The liver secretes 500-1000 ml of bile each day. Bile is vital for the
    processing of fats by the small intestine. It is stored and concentrated
    in the gallbladder which contracts to deliver bile to the duodenum
    following a meal.

  2. Bile acids are important constituents of bile. They become con­
    jugated to ammo acids to form bile salts which are amphipathic
    (having both hydrophobic and hydrophilic regions). At high con­
    centration, bile salts aggregate together to form micelles. The entero-
    hepatic circulation returns to the liver about 94 per cent of the bile
    salts entering the small intestine.

  3. The formation of bile is stimulated by bile salts, secretin, glucagon
    and gastrin. The release of bile stored in the gallbladder is stimu­
    lated by CCK which is secreted in response to the presence of chyme
    in the duodenum.

  4. Bile pigments (the excretory products of heme) and other waste
    products are excreted in the bile. Bilirubin is the principal pigment.
    In the hepatocytes it is conjugated with glucuronic acid to form the
    water-soluble bilirubin diglucuronide which enters the bile. Failure
    to excrete bile pigments leads to their accumulation in the blood,
    and the development of jaundice.

^ 18.12 Absorption of digestion products in the small intestine

Absorption is the process by which the products of digestion are transported into the epithelial cells that line the GI tract and from there into the blood or lymph draining the tract. Each day about 8—10 liters of water and up to 1 kg of nutrients pass across the gut wall. Absorption through the gastrointestinal mucosa occurs by the processes of active transport and diffusion. Because the epithelial cells of the intestinal mucosa are joined at their apical (luminal) surfaces by tight junctions, nutrients cannot move between the cells. Instead they must pass through the cells and into the interstitial fluid abutting their basal membranes if they are to enter the capillary blood. This process is called transepithelial transport. The physical principles of active and passive transport are described together with epithelial transport in section 4.3. The mechanisms involved in the intestinal absorption of iron are described in detail in section 13.5. The intestinal absorption of calcium and its regulation by para­thyroid hormone and the metabolites of vitamin D are described in section 12.6.

422

18 The gut and nutrition






The absorption of monosaccharides, the digestion products of carbohydrate

Glucose, galactose, and fructose are absorbed largely in the duo­denum and upper jejunum, entering the blood of the hepatic portal vein. None remain in the chyme reaching the terminal ileum. Glucose and galactose are taken up into the epithelial cells against their concentration gradients by a sodium-dependent cotransport mechanism similar to that found in the proximal tubular cells of the renal nephron (see Chapter 17) and described in detail in Section 4.3. The sodium gradient that drives this transport is maintained by the Na+,K+-ATPase. The mono­saccharides leave the intestinal epithelial cell at the basolateral membrane by facilitated diffusion. Fructose is absorbed from the intestinal lumen by sodium-independent facilitated diffusion. It cannot be transported against a concentration gradient.

The absorption of peptides and amino acids, the digestion products of proteins


Fig. 18.25 The mechanisms by which amino acids and small peptides are absorbed in the small intestine.

Each day approximately 200 g of amino acids and small peptides are absorbed from the small intestine of an adult eating a normal mixed diet. At least 50 g must be absorbed each day to maintain a positive nitrogen balance and meet the needs of an adult body for tissue growth and repair. Large peptides and whole proteins are not normally absorbed, although small amounts may enter the bloodstream. Amino acids are absorbed at the brush border of the intestinal epithelial cells by a sodium-dependent cotrans­port mechanism similar to that utilized for the absorption of monosaccharides (Fig. 18.25). Three separate transporters exist for neutral, basic, and acidic amino acids. A fourth transporter carries proline and hydroxyproline. Once the amino acids have entered the enterocyte they pass across the basolateral surface into the blood capillaries of the villus and hence into the portal vein. Most of the amino acids are absorbed in the first part of the small intestine. A few may enter the colon where they are metabolized by the colonic bacteria.

Small peptides (mainly dipeptides) are transported into the enterocytes by another carrier which is not linked to sodium but is believed to be linked to the influx of hydrogen ions. This carrier is also responsible for the rapid uptake of certain drugs by the gut, such as the antihypertensive drug, captopril. Once the peptides enter the intracellular compartment they are broken down to their constituent amino acids. These leave the entero­cytes via the amino acid carrier system of the basolateral surface (Fig. 18.25). Up to half of the ingested protein is now believed to be absorbed in this way.

The absorption of monoglycerides

and free fatty acids, the digestion

products of fats

Because of their insolubility in water, fats pose a special problem for the GI tract in terms of both their digestion and absorption. Bile salts play an essential part in each of these processes. In the stomach, ingested fats form large fat globules. As these globules enter the duodenum they are coated with bile salts. The non-polar regions of the bile salts cling to the fat molecules while their hydrophilic polar regions allow them to repel each other and interact with water. As a result fatty droplets are pulled off the large fat globules and a stable emulsion is created. (An emul­sion is an aqueous suspension of fatty droplets each about 1 /лт in diameter.) This dispersal of fat molecules greatly increases the number of triglycerides exposed to the pancreatic lipases and facilitates their breakdown to monoglycerides and free fatty acids.

Each day about 80 g of fat are absorbed from the small intes­tine, largely in the jejunum. The monoglycerides and free fatty acids liberated by the activity of the pancreatic lipases become associated with bile salts and lecithin to form micelles, as described earlier. The nonpolar core of the micelle also contains cholesterol and fat-soluble vitamins. The hydrophilic outer region of the micelle enables it to enter the aqueous layer sur­rounding the microvilli that form the brush border of the enterocytes. Monoglycerides, free fatty acids, cholesterol, fat-soluble vitamins and lecithin then diffuse passively into the duo­denal cells while the bile salt portion of the micelle remains within the lumen of the gut until the terminal ileum. The majority of the bile salts entering the small intestine are recycled by the enterohepatic circulation.

A small amount of short-chain fatty acids is absorbed directly from the intestinal epithelial cells into the capillary blood, by passive diffusion. The majority of the products of fat digestion, however, undergo further chemical processing inside the entero­cytes. In the smooth endoplasmic reticulum, triglycerides are reformed by the re-esterificarion of monoglycerides, phospho-lipids are resynthesized, and much of the cholesterol also under­goes re-esterification. The lipids accumulate in the vesicles of the smooth ER to form chylomicrons which are released from the cells by exocytosis, into the lateral intercellular spaces. From here,

^ 18.12 Absorption in the small intestine

423


they enter the lacteals of the villi and leave the intestine in the lymph, from which they are released into the venous circulation via the thoracic duct. Lipids thus avoid the hepatic portal vein and bypass the liver in the short term. Figure 18.26 illus­trates the processes involved in the absorption of fat digestion products.

The feces contain about 5 per cent fat, most of which is derived from bacteria. Increased amounts of fat are found in the feces if bile production is diminished or if bile is prevented from entering the duodenum (biliary obstruction).

The absorption of fluid and electrolytes

Approximately 2 liters of fluid are ingested each day (although this may vary considerably depending upon thirst and social factors). The secretion of digestive juices adds a further 8-9 liters of fluid to the gastrointestinal lumen. Almost all of this fluid is absorbed from the small and large intestines, only 50-200 ml leaving the body with the feces. About 5—6 liters a

day are absorbed in the jejunum, 2 liters a day in the ileum, and between 400 ml and 1 liter a day in the colon. A summary of overall fluid balance in the GI tract is shown in Fig. 18.27.



Fig. 18.26 The key steps involved in the absorption of lipids by the small intestine.





Fig. 18.27 The overall balance between secretion and absorption of fluid in the GI tract.

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18 The gut and nutrition


Absorbed electrolytes originate both from ingested foods and gastrointestinal secretions. Most are actively absorbed along the length of the small intestine, although absorption of calcium and iron is restricted mainly to the duodenum. As described earlier, the absorption of sodium ions is coupled with the transport of both sugars and amino acids. There are numerous active sodium—potassium pumps in the basal membranes of the intesti­nal epithelial cells, which pump sodium out of the cells thereby creating a gradient that draws sodium passively into the cell from the intestinal lumen. The active transport of sodium is stimulated by aldosterone as it is in the distal tubule of the renal nephron. Some potassium is actively secreted into the gut, par­ticularly in mucus, but in general potassium is absorbed passively along a concentration gradient set up by the absorption of water.

For the most part, anions follow passively the electrical potential generated by the active transport of sodium. Chloride ions are also actively transported and in parts of the lower ileum bicarbonate ions are actively secreted into the intestinal lumen in exchange for chloride.

The absorption of water by the intestine occurs by osmosis into blood vessels in response to the gradients established by the absorption of nutrients and electrolytes. Water can also be transported by osmosis from the blood to the intestinal lumen when the chyme entering the duodenum becomes hypertonic as a result of the digestion of nutrients. In this way, isotonicity of the chyme is established rapidly and is then maintained along the length of the intestine—as nutrients and electrolytes are progressively absorbed, water follows almost instantaneously.

The absorption of vitamins

The fat-soluble vitamins are absorbed in the same way as the products of fat digestion, partitioning into micelles and passing into the lymph as described above.

Specific recognition molecules have been identified for most of the water-soluble vitamins and they may enter the intestinal epithelial cells by passive, facilitated, or active transport. Vitamin C, for example, is absorbed in the jejunum by sodium-dependent active transport by a mechanism similar to that described earlier for amino acids and monosaccharides.

Vitamin B12 (cyanocobalamin) is absorbed in the ileum by a specific mechanism involving intrinsic factor, a glycoprotein secreted by parietal cells in the gastric mucosa. In the lumen of the jejunum, intrinsic factor binds to vitamin B12. The result­ing complex is recognized by a receptor protein in the brush border membrane of ileal cells to which the complex binds before slowly entering the cell and eventually the blood. The mechanisms by which vitamin B12 crosses both the luminal membrane and the basal membrane of the ileal cell remain unclear. Vitamin B12 appears in the blood mostly bound to a plasma protein called transcobalamin II.

Summary

  1. Absorption is the process by which the products of digestion are
    transported into the epithelial cells of the GI tract and thence into
    the blood or lymph draining the gut. Almost all of the absorption of
    water, electrolytes, and nutrients occurs in the small intestine.

  2. Monosaccharides are absorbed in the duodenum and upper jejunum
    by sodium-dependent cotransport driven by the sodium—potassium
    pump. Amino acids utilize similar mechanisms although at least four
    separate transporters exist for the four types of amino acid.

  3. Amphipathic bile salts are essential for both the digestion and the
    absorption of fats and fat-soluble vitamins. They emulsify the fats in
    the small intestine, rendering them more accessible to the pancreatic
    lipases which break them down into fatty acids and monoglycerides.

  4. The products of fat digestion are incorporated into micelles along
    with bile salts, lecithin, cholesterol, and fat-soluble vitamins. In this
    way they are brought close to the enterocyte membrane and the fatty
    components of the micelle diffuse into the cells. Bile salts are recy­
    cled and the fats are reprocessed by the smooth ER to form chylo-
    microns. These are exocytosed across the basolateral cell membrane
    and enter the lacteals of the villi.

  5. The GI tract absorbs about 10 liters of fluid and electrolytes each day.
    The active transport of sodium and nutrients is followed by anion
    movement and the absorption of water by osmosis. Failure to absorb
    fluid results in potentially life-threatening diarrhea.

  6. Fat-soluble vitamins are absorbed along with the products of fat
    digestion. Most water-soluble vitamins are absorbed by facilitated
    transport. A specific uptake process involving gastric intrinsic factor
    is responsible for the absorption of vitamin B]2.

18.13 The large intestine

Around 1500 ml of chyme pass from the ileum into the cecum every day. Material then passes in sequence through the ascend­ing colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal (Fig. 18.28a). Semisolid waste material (feces) is eliminated from the body through the anus. In adults the large intestine is approximately 1.2 m long but its diameter is greater than that of the small intestine. It has a variety of functions. It stotes food residues prior to their elimination, it secretes mucus which lubricates the feces, and it absorbs any water and electrolytes remaining in the residue. In addition, bacteria that live in the colon synthesize vitamin К and some В vitamins and play a part in fermentation reactions.

Special histologic features and innervation of the large intestine

Structurally, the wall of the large intestine follows the basic plan of the GI tract. However, the longitudinal smooth muscle layer of the muscularis externa is thickened to form longitudinal bands called taeniae colt. Three of these are presenr in the cecum and the colon. In between the taeniae the longitudinal muscle layer is relatively thin. The tone of the smooth muscle in the taeniae causes the wall of the large intestine to pucker into

^ 18.13 The large intestine

425






Fig. 18.28 (a) The major structures of the large intestine, showing the principal blood supply, (b) Longitudinal section through the rectum and anal canal.

pocket-like sacs called haustra. In the rectum there are two broad bands of longitudinal muscle but haustra are absent.

The mucosal surface of the cecum, colon, and upper rectum is smooth and has no villi. However, large numbers of crypts are present. The mucous membrane consists of columnar absorptive cells, with many mucus-secreting goblet cells.

The anal canal, which is about 3 cm long and lies entirely outside the abdominal cavity, has internal and external sphinc­ters (Fig. 18.28b) which remain closed (acting rather like purse strings) except during defecation. The mucosa of the anal canal reflects the greater degree of abrasion that this area receives. It hangs in long folds called anal columns and contains stra-

tified squamous epithelium. Between the columns are the anal sinuses. Two superficial venous plexuses are associated with the anal canal. If these become inflamed, itchy varicosities called hemorrhoids form.

The large intestine receives both parasympathetic and sym­pathetic innervation. Vagal fibers supply the cecum and the colon as far as the distal third of the transverse region. Para­sympathetic fibers supplying the rest of the colon, rectum, and anal canal are from the pelvic nerves from the sacral spinal cord (nervi erigentes). The parasympathetic fibers end chiefly on neurons of the intramural plexuses. The sympathetic input is from the celiac and superior mesenteric ganglia (cecum and colon) and from sympathetic nerves from the lumbar spinal cord and the superior hypogastric plexus (rectum and anal canal). The external anal sphincter receives branches of somatic nerves arising from the sacral region of the spinal cord.

The cecum and appendix

The cecum is a blind-ended tube about 7 cm long leading from the ileo-cecal valve to the colon. Although it is important in cellulose digestion in herbivores, it has no significant digestive role in humans. Attached to the posteromedial surface of the cecum is the vermiform appendix, a small, blind pouch about the size of a finger, containing lymphoid tissue. Although it is parr of the mucosa-associated lymphatic tissue (MALT), it has no essential function in humans. Inflammation of the appendix is known as appendicitis and necessitates the surgical removal of the organ to prevent its rupture. If the appendix does rupture, the more serious condition of peritonitis develops as a con­sequence of the presence of fecal material containing bacteria in the abdominal cavity.

^ The colon

The colon acts as a reservoir, storing unabsorbed and unusable food residues. Although most of the residue is excreted within 72 hours of ingestion, up to 30 per cent of it may remain in the colon for a week or more.

Electrolyte and water absorption in the colon

Although large amounts of fluid are absorbed from chyme as it passes along the small intestine, the chyme entering the colon still contains appreciable quantities of water and electrolyres. Indeed, the colon absorbs 400-1000 ml of fluid each day. Failure to do so results in severe diarrhea.

Sodium ions are transported actively from the intestinal lumen to the blood. This absorption is sensitive to aldosterone. As in the lower ileum, the absorption of chloride ions is linked with the secretion of bicarbonate. This bicarbonate may help to neutralize the acidic end products of bacterial action (see below) and water accompanies the movement of salts.

^ The role of intestinal bacterial flora

Various bacteria colonize the large intestine, living symbiotic-ally with their human host. Some of these, such as Clostridium

426

18 The gut and nutrition


perfringens and Bacteroides fragilis, are anaerobic species, while others such as ^ Enterobacter aerogenes асе aerobic. The intestinal flora perform a number of functions within the large intestine. One of these is the fermentation of indigestible carbohydrates (notably cellulose) and lipids that enter the colon. As a result of these fermentation reactions short-chain fatty acids are produced, along with a number of gases (for example hydrogen, nitrogen, carbon dioxide, methane, hydrogen sulfide), which form about 500 ml of flatus each day (more if the diet is rich in indigestible carbohydrates such as cellulose). Short-chain fatty acids, includ­ing acetate, propionate, and butyrate are absorbed readily by the colon, stimulating water and sodium uptake at the same time. The colonocytes appear to utilize the short-chain fatty acids for energy.

Another action of the intestinal flora is the conversion of bilirubin to nonpigmented metabolites, the urobilinogens (Section 18.11).They are also able to degrade cholesterol, drugs, and certain food additives.

Finally, intestinal bacteria are able to synthesize certain vitamins, e.g. vitamin K, vitamin B12, thiamine, and riboflavin. Vitamin B12 can only be absorbed in the terminal ileum, so that which is synthesized in the colon is usually excreted and is of no value to the body.

^ Movements of the colon

The movements of the colon, like those of the small intestine may be defined as either mixing or propulsive movements. Since the role of the colon is to store food residues and to absorb water and electrolytes, the propulsive movements of the colon are relatively sluggish. Characteristically, material travels along the colon at 5—10 cm hour"1 and typically remains within the colon for 16—20 hours.

Mixing movements (haustmtions)

Contraction of the circular smooth muscle layer of the colon serves to constrict the lumen in much the same way as described above for the small intestine. This kind of segmental movement is called haustration in the colon because the seg­ments correspond to the smooth muscle thickenings called haustra. It is the predominant type of movement seen in the cecum and the proximal colon. The purpose of haustration seems to be to squeeze and roll the fecal material around so that every portion of it is exposed to the absorptive surfaces of the colonic mucosa, thus aiding the absorption of water and electrolytes.

Propulsive movements—peristalsis and mass movements

In addition to haustral contractions, short-range peristaltic waves are seen in the more distal parts of the colon (transverse and descending regions). These serve to propel the intestinal contents, now in the form of semisolid fecal material, towards the anus.

Several times a day, usually after meals, a more vigorous propulsive movement of the colon occurs in which a portion of the colon remains contracted for rather longer than during a

peristaltic wave. This is called a mass movement and results in the emptying of a large portion of the proximal colon. Mass movements are also seen in the transverse and descending colon. When they force a mass of fecal material into the rectum, the desire for defecation is experienced.

Mass movements are initiated, at least in part, by intrinsic reflex pathways resulting from distension of the stomach and duodenum. These are termed the gastro-colic and duodeno-colic reflexes. These intrinsic motor patterns are modified by auto-nomic nerves and by hormones. Vagal stimulation, for example, enhances colonic motility, while both gastrin and CCK increase the excitability of the colon and facilitate ileal emptying by causing the ileo-cecal sphincter to relax.

Analgesics such as morphine, codeine, and pethidine decrease the frequency of colonic mass movements. Other drugs, includ­ing aluminum-based antacids, have the same effect. People taking these drugs may therefore become constipated.

The role of dietary fiber in the large intestine

The time taken for food residues to be expelled from the body, after eating, varies considerably but appears to be related directly to the amount of dietary fiber ingested. Dietary fiber (or 'roughage') consists largely of cellulose. Humans are unable to digest this, so it remains in the intestine, adding bulk to the food residues. It tends to exert a hygroscopic effect, absorbing water so that stools with a high fiber content tend to be bulkier and softer, making them easier to expel. A shorter mouth-to-anus transit time is also believed to reduce the risk of develop­ing carcinoma of the large intestine and rectum. This may be due partly to a reduction in the time for which bacterial toxins and potentially harmful metabolites are in contact with the gut wall.

The rectum and defecation

The rectum is a muscular tube about 12-15 cm long. It is nor­mally empty but when a mass movement forces feces into the rectum, the urge to defecate is initiated. The rectum opens to the exterior via the anal canal which has both internal and exter­nal sphincters. The internal sphincter is not under voluntary control. It is supplied by both sympathetic and parasympathetic neurons. Contraction of the smooth muscle of the internal sphincter is initiated by sympathetic stimulation, and relaxation by parasympathetic stimulation. The external anal sphincter is composed of skeletal muscle. It is supplied by the pudendal nerve and is under learned voluntary control from the age of about 18 months. Both the anal sphincters are maintained in a tonic state of contraction.

About 100—150 g of feces are normally eliminated each day, consisting of 30—50 g of solids and 70-100 g of water. The solid portion consists largely of cellulose, epithelial cells shed from the lining of the GI tract, bacteria, some salts, and the

^ 18.13 The large intestine
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