CHAPTER 65 Digestion and Absorption in the Gastrointestinal Tract
The primary foods on which the body lives can be classified as carbohydrates, fats, and proteins. This chapter discusses (1) the digestion of carbohydrates, fats, and proteins and (2) the mechanisms by which the end products of digestion as well as water, electrolytes, and other substances are absorbed.
Saliva contains the enzyme ptyalin (an α-amylase), which hydrolyzes starch into maltose and other small polymers of glucose. Less than 5% of the starch content of a meal is hydrolyzed before swallowing. However, digestion can continue in the stomach for about 1 hour before the activity of salivary amylase is blocked by gastric acid. Nevertheless, α-amylase hydrolyzes as much as 30% to 40% of the starches to maltose.
The function of pancreatic α-amylase is almost identical to that of the α-amylase in saliva but is several times as powerful; therefore, soon after chyme empties into the duodenum and mixes with pancreatic juice, virtually all the starches are digested.
The microvilli brush border contains enzymes that split the disaccharides lactose, sucrose, and maltose as well as small glucose polymers into their constituent monosaccharides. Glucose usually represents more than 80% of the final products of carbohydrate digestion.
The ability of pepsin to digest collagen is especially important because the collagen fibers must be digested for enzymes to penetrate meats and digest cellular proteins.
Proteins leaving the stomach in the form of proteoses, peptones, and large polypeptides are digested into dipeptides, tripeptides, and some larger peptides by proteolytic pancreatic enzymes; only a small percentage of proteins are digested by pancreatic juices to form amino acids.
The last digestion of proteins in the intestinal lumen is achieved by enterocytes that line the villi.
Emulsification is the process by which fat globules are broken into smaller pieces by the detergent actions of bile salts and especially lecithin. The emulsification process increases the total surface area of the fats. The lipases are water-soluble enzymes and can attack fat globules only on their surfaces. Consequently, it can be readily understood how important this detergent action of bile salts and lecithin is for the digestion of fats.
The most important enzyme for digestion of triglycerides is pancreatic lipase. This is present in such enormous quantities in pancreatic juice that all triglycerides are digested into free fatty acids and 2-monoglycerides within a few minutes.
The hydrolysis of triglycerides is highly reversible; therefore, accumulation of monoglycerides and free fatty acids in the vicinity of digesting fats quickly blocks further digestion. Bile salts form micelles that remove monoglycerides and free fatty acids from the vicinity of the digesting fat globules. Micelles are composed of a central fat globule (containing monoglycerides and free fatty acids) with molecules of bile salt projecting outward to cover the surface of the micelle. The bile salt micelles also carry monoglycerides and free fatty acids to the brush borders of the intestinal epithelial cells.
The total area of the small intestinal mucosa is 250 square meters or more—about the surface area of a tennis court.
It is absorbed from the gut when the chyme is dilute and moves into the intestine when hyperosmotic solutions enter the duodenum. As dissolved substances are absorbed from the gut, the osmotic pressure of the chyme tends to decrease, but water diffuses so readily through the intestinal membrane that it almost instantaneously “follows” the absorbed substances into the blood. Thus the intestinal contents are always isotonic with the extracellular fluid.
Sodium is actively transported from inside the intestinal epithelial cells through the basal and side walls (basolateral membrane) of these cells into the paracellular spaces, which decreases the intracellular sodium concentration. This low concentration of sodium provides a steep electrochemical gradient for sodium movement from the chyme through the brush border into the epithelial cell cytoplasm. The osmotic gradient created by the high concentration of ions in the paracellular space causes water to move by osmosis through the tight junctions between the apical borders of the epithelial cells and, finally, into the circulating blood of the villi.
Dehydration leads to aldosterone secretion by the adrenal glands, which greatly enhances sodium absorption by the intestinal epithelial cells. The increased sodium absorption then causes secondary increased absorption of chloride ions, water, and some other substances. This effect of aldosterone is especially important in the colon.
The toxins of cholera and some other diarrheal bacteria can stimulate secretion of sodium chloride and water so greatly that as much as 5 to 10 L of water and salt can be lost each day as diarrhea. In most instances, the life of the cholera victim can be saved by simply administering large amounts of sodium chloride solution to make up for the losses.
The most abundant of the absorbed monosaccharides is glucose, usually accounting for more than 80% of the absorbed carbohydrate calories. Glucose is the final digestion product of our most abundant carbohydrate food, the starches.
Active transport of sodium through the basolateral membranes into the paracellular spaces depletes the sodium inside the cells. This decrease causes sodium to move through the brush border of the enterocyte to its interior by secondary active cotransport. The sodium combines with a transport protein that requires another substance, such as glucose, to bind simultaneously. When intestinal glucose combines with the transport protein, sodium and glucose are transported into the cell at the same time.
Galactose is transported by the exact same mechanism as glucose. In contrast, fructose is transported by facilitated diffusion all the way through the enterocyte but is not coupled with sodium transport. Much of the fructose is converted to glucose within the enterocyte and finally is transported to blood in the form of glucose.
The energy for most of this transport is supplied by sodium co-transport mechanisms in the same way that sodium co-transport of glucose and galactose occurs. A few amino acids do not require this sodium co-transport mechanism but, instead, are transported by special membrane transport proteins in the same way that fructose is transported—via facilitated diffusion.
Lipids are soluble in the enterocyte membrane. After entering the enterocyte, the fatty acids and monoglycerides are mainly recombined to form new triglycerides. A few of the monoglycerides are further digested into glycerol and fatty acids by an intracellular lipase. Triglycerides themselves cannot pass through the enterocyte membrane.
The reconstituted triglycerides aggregate within the Golgi apparatus into globules that contain cholesterol and phospholipids. The phospholipids arrange themselves with the fatty portions toward the center and the polar portions on the surface, providing an electrically charged surface that makes the globules miscible with water. The globules are released from the Golgi apparatus and are excreted by exocytosis into the basolateral spaces; from there, they pass into the lymph in the central lacteal of the villi. These globules are then called chylomicrons.
The mucosa of the large intestine has a high capability for active absorption of sodium, and the electrical potential created by absorption of sodium causes chloride absorption as well. The tight junctions between the epithelial cells are tighter than those of the small intestine, which decreases back-diffusion of ions through these junctions. This allows the large intestinal mucosa to absorb sodium ions against a higher concentration gradient than can occur in the small intestine. The absorption of sodium and chloride ions creates an osmotic gradient across the large intestinal mucosa, which in turn causes absorption of water.
When the total quantity entering the large intestine through the ileocecal valve or by way of large intestine secretion exceeds this maximum absorptive capacity, the excess appears in the feces as diarrhea.
The solid matter is composed of about 30% dead bacteria, 10% to 20% fat, 10% to 20% inorganic matter, 2% to 3% protein, and 30% undigested roughage of the food and dried constituents of digestive juices, such as bile pigment and sloughed epithelial cells. The brown color of feces is caused by stercobilin and urobilin, which are derivatives of bilirubin. The odor is caused principally by indole, skatole, mercaptan, and hydrogen sulfide.