Digestion

The digestive system has as its main function the absorption of nutrients. However, it also has an important role in excretion of waste products. Furthermore, the digestive tract is one of the three main routes of exposure to toxins from the environment. Digestion is the physical and chemical breakdown of food into components that can be absorbed and used. It is accomplished by the action of mechanical mixing, acid conditions (in the stomach), and digestive enzymes. Digestive enzymes may include:

• Protein-digesting proteases, which produce peptides and amino acids

• Fat-digesting lipases, which produce fatty acids

• Complex carbohydrate-digesting amylases, which produce simple sugars or small oligosaccharides

• Nucleic acid-digesting nucleases, which produce nucleotides

Some digestive enzymes are secreted in an inactive form called a zymogen or proenzyme, designated by the suffix ''-ogen'' or the prefix "pro-." The zymogen is then converted, or activated, in the lumen of the digestive tract. For example, the pancreas produces the zymogen trypsinogen, which is converted in the intestines to the protease trypsin.

The digestive system consists of the digestive tract and accessory organs. The digestive tract, or gut, is basically a muscular tube including the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory organs include the teeth, tongue, and several exocrine glandular organs, such as the salivary glands, liver, and pancreas. The digestive tract is lined with an epithelial tissue called the mucosa, or mucous membrane, underlain by loose connective tissue. The mucus protects the epithelium from digestive acids and enzymes.

Food is crushed by teeth in the oral cavity and mixed with saliva, which contains lubricating glycoproteins and amylase. Amylase begins the breakdown of polysaccharides into their component sugars. After swallowing, the ball of food, which is now called a bolus, is propelled down the esophagus to the stomach by a wavelike muscle contraction called peristalsis.

The stomach serves four main functions: (1) storage, (2) mechanical breakdown of food, (3) digestion by acids and enzymes, and (4) production of intrinsic factor, a glyco-protein essential for the absorption of vitamin B12 in the intestines. When full, the stomach can contain 1.0 to 1.5 L of material. The stomach produces about 2 L/day of hydrochloric acid, to maintain the pH at 1.5 to 2.0. The acidic conditions disinfect the food by killing microorganisms, denature enzymes and other proteins in the food, and break down plant cell walls and animal connective tissue in the food.

Hydrochloric acid is secreted by parietal cells by an interesting mechanism that avoids the production of the acid within the cells themselves (Figure 9.12). CO2 and water form carbonic acid, which dissociates into bicarbonate ions and hydrogen ions. The bicarbonate passes into the bloodstream by a passive membrane transport mechanism that is coupled with the transport of chloride ions out of the blood. (Thus, the alkalinity of the blood increases.) This maintains electroneutrality across the membrane. The chloride accumulation in the cell is secreted toward the stomach through calcium channels. At

*■ Diffusion ( ) Active transport

» Carrier-mediated Q Countertransport

Transport

Figure 9.12 Hydrochloric acid secretion by parietal cells of the stomach. (Based on Martini, 1998.)

the same time, active transport uses ATP to move the hydrogen ions from the carbonic acid toward the stomach.

The zymogen pepsinogen is secreted into the stomach and converted by the low pH into the protease pepsin. By secreting an inactive form, the cells of the stomach themselves are protected from being digested. The stomach also secretes hormones, including gastrin, which stimulates contraction of the stomach for mixing and the secretion of hydrochloric acid, intrinsic factor, and pepsinogen. Gastrin secretion is stimulated by protein in the food. It is also stimulated by coffee (both caffeinated and decaffeinated). Food stays in the stomach several hours. Then the mixture, now called chyme, is squirted into the beginning of the small intestine by repeated waves of stomach contractions. Proteins and complex carbohydrates in chyme are partially digested, and fats are undigested.

The small intestine averages 6 m in length and has three sections. In the first 25 cm of the small intestine, called the duodenum, chyme is mixed with digestive secretions from the liver and pancreas. The next 2.5 m is the jejunum, where most of the digestion and absorption occurs. The last 3.5 m is the ileum, which performs additional absorption. It is lined with lymph nodes that protect the small intestine against bacteria from the large intestine. The inside of the small intestine, especially the jejunum, has numerous folds. The surface, especially of the duodenum and jejunum, is covered with tiny fingerlike projections called villi. Furthermore, each epithelial cell on the villi has fingerlike projections of its plasma membrane called microvilli. Taken together, these projections greatly increase the intestinal surface area for absorption. This means, of course, not only adsorption of food, but potentially of toxins as well.

The pancreas produces about 1 L/day of digestive juices containing enzymes and zymogens. It also secretes bicarbonate alkalinity to neutralize the acid pH of the stomach by a mechanism similar to the stomach secretion of HCl in reverse. The enzymes produced by the pancreas or its secretions include the proteases trypsin, chymotrypsin, carboxypeptidase, and elastase. Each attacks peptide bonds between specific amino acids and leave the others, producing a mixture of dipeptides, tripeptides, and amino acids. It also produces amylase, lipase, and nucleases.

The liver, the largest visceral organ, is remarkable for the number of vital roles it plays in the body. Over 200 different functions have been identified. Besides its digestive function, it has major control of the following metabolic and blood functions:

Carbohydrate metabolism. Controls blood sugar by forming and breaking down glycogen reserves, or synthesizing glucose from fats, proteins, or other compounds.

Lipid metabolism. Maintains blood levels of triglycerides, fatty acids, and cholesterol.

Amino acid metabolism. Removes excess amino acids from circulation. Some may be destroyed by removal of the amine group, producing ammonia. This is then converted to urea, which is removed by the kidneys.

Vitamin storage. The fat-soluble vitamins A, D, E, K, and B12 are stored in reserve.

Mineral storage. Iron from the breakdown of red blood cells is stored in the liver.

Detoxification. Drugs and other toxins are transformed into easier-to-excrete forms.

Erythrocyte recycling. Red blood cells are broken down.

Production of plasma proteins. The liver produces albumins, various transport proteins, clotting proteins, and the complement proteins of the immune system.

Removal of hormones. The liver removes and recycles hormones such as epinephrine, norepinephrine, thyroid hormones, and steroid hormones.

Storage or excretion of toxins. Lipophilic toxins such as DDT are removed from circulation and stored in the liver. Others are excreted in the bile.

Production of bile. The last function returns us to the role of the liver in digestion. Bile is a digestive secretion of the liver. It is stored in the gallbladder, which releases it to the duodenum upon stimulation by cholecystokinin. Bile contains six major components: (1) bile salts (synthesized from cholesterol), (2) cholesterol, (3) lecithin (a phospholipid), (4) bicarbonate ions and other inorganic salts, (5) bile pigments such as bilirubin, and (6) trace amounts of metals. The first three act as an emulsifier for lipids in the food. The bicarbonate contributes to neutralizing stomach acid. The bile pigments and metals are excretions of the liver destined for elimination with the feces.

The blood supply of the liver is unusual. Besides the usual oxygenated blood, it receives all the deoxygenated blood from the intestines, and these are mixed in the liver. The liver then processes nutrients and may attack toxins. By receiving all the blood from the intestines, the liver serves as a first line of defense against toxins, before substances absorbed in the intestines are spread throughout the body. After they have done their work, most of the bile salts are absorbed in the ileum and recycled in the liver. The circulation of bile salts from liver to intestines and back to the liver via the blood is called the enterohepatic circulation. Enterohepatic circulation may also result in the recycling of lipophilic toxins.

It takes an average of 5 hours for food to move through the small intestine. Contractions gradually move the remaining unabsorbed food to the large intestine. The large intestine connects the small intestine to the anus. It is about 1.5 m long and 7.5 cm in diameter and consists of the cecum, the colon, and the rectum. The cecum is a pouch at the entrance attached to which is a small worm-shaped structure about 9 cm long called the appendix. The appendix is part of the lymphatic system and contains lymph nodes. The second, and major part of the large intestine is the colon, which absorbs water, vitamins, and minerals from the chyme. Bacteria are a natural component of the colon. They include Escherichia coli, Lactobacillus, and Streptococcus, collectively called the intestinal flora. These bacteria, especially E. coli, are used as indicators to detect fecal contamination of water. The bacteria in our gut benefit us by displacing harmful bacteria and by producing vitamins, including thiamine, riboflavin, Bi2, and K. Some materials in our food are poorly digested. For example, beans contain large amounts of indigestible polysaccharides. These are used by the intestinal flora, producing gas called flatus, consisting mostly of CO2 and nitrogen, plus smaller amounts of hydrogen, methane, and hydrogen sulfide.

Material takes 12 to 36 hours to move through the colon. The resulting product is about 150 g per day of feces, which is about two-thirds water. The 50 g of dry solids in feces is more than 60% bacteria, plus undigested polysaccharides (including dietary fiber), bile pigments, cholesterol, and salts such as potassium. The last 15 cm of the large intestine is the rectum, which stores feces until discharge through the anus (the exterior opening).

The colon absorbs water by first absorbing sodium, and the water then follows by osmosis. Some diseases, notably cholera, disable the main sodium transport mechanism, preventing water absorption. Cholera is transmitted by contaminated drinking water due to poor sanitary conditions. The result is diarrhea, the loss of large amounts of water and minerals with the feces. Diarrhea is the major cause of death in children in the developing world, killing about 4 million under the age of 5 each year. This makes the lack of proper sanitation the most important environmental problem in the world today.

Cholera by itself is not deadly. If the loss of water and minerals is compensated for, the prognosis is quite good. However, in the past this has meant intravenous injection, which was not widely available in poorer parts of the world. Over the last several decades a simpler treatment has been developed, called oral rehydration therapy (ORT). ORT is based on the observation that there are other mechanisms by which the intestines transport sodium that are not affected by cholera. Specifically, there is a mechanism involving the cotransport of glucose and sodium that can be stimulated by drinking a dilute solution containing both solutes, plus potassium chloride and triso-dium citrate. This kind of solution has reduced cholera mortality from 50 or 60% to less than 1%.

Popular soft drinks have concentrations of sugar and other solutes that make them hypertonic. Thus, by osmosis, they increase the flow of water into the gut, water which then has to be removed. To avoid this problem, "sports drinks'' are formulated to be isotonic.

The digestive system is controlled both by hormones (at least four, and perhaps up to 12) and neural control. As noted above, gastrin, is a hormone that stimulates stomach activity. The endocrine hormone secretin is secreted by the duodenum and stimulates the pancreas to start producing enzymes and bicarbonate alkalinity. Secretin was the first hormone to be discovered. The intestinal mucosa secrete the hormone cholecystokinin, which further stimulates the pancreatic secretions.

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