Region of Grinding and Early Digestion
Region of Grinding
and Early Digestion
In most vertebrates, and in some invertebrates, the stomach provides initial digestion as well as storage and mixing of food with digestive juices. Mechanical breakdown of food, especially plant food with its tough cellulose cell walls, often continues in herbivorous animals by grinding and crushing devices in the stomach. The muscular gizzard of terrestrial oligochaete worms and birds is assisted by stones and grit swallowed along with food or, in arthropods, by hardened linings (for example, chitinous teeth of the insect proventriculus [Figure 34-9], and calcareous teeth of the gastric mill of crustaceans).
Digestive diverticula—blind tubules or pouches arising from the main passage— often supplement the stomach of many invertebrates. They are usually lined with a multipurpose epithelium having cells specialized for secreting mucus or digestive enzymes, or absorption or storage. Examples include the ceca of polychaete annelids, digestive glands of bivalve molluscs, hepatopancreas of crustaceans, and pyloric ceca of sea stars.
Herbivorous vertebrates have evolved several strategies for exploiting cellulose-splitting microorganisms to derive maximal nutrition from plant food. Despite its abundance on earth, the woody cellulose that encloses plant cells can be broken down only by an enzyme, cellulase, that has limited distribution in the living world. No metazoan animals can produce intestinal cellulase for the direct digestion of cellulose. However many herbivorous metazoans harbor microorganisms (bacteria and protozoa) in their gut that do produce cellulase. These microorganisms ferment cellulose under the anaerobic conditions of the gut, producing fatty acids and sugars that the herbivore can use. While the ultimate fermentation machine is the multichambered stomach of the cudchewing ruminants described on, many other animals harbor microorganisms in other parts of the gut, such as the intestine proper or the cecum.
The stomach of carnivorous and omnivorous vertebrates is typically a U-shaped muscular tube provided with glands that produce proteolytic enzymes and strong acids, the latter an adaptation that probably arose for killing prey and halting bacterial activity. When food arrives at the stomach, the cardiac sphincter opens reflexively to allow the food to enter, then closes to prevent regurgitation back into the esophagus. In humans, gentle peristaltic waves pass over the filled stomach at the rate of approximately three each minute. Churning is most vigorous at the intestinal end where food is steadily released into the duodenum, first region of the small intestine. A pyloric sphincter regulates the flow of food into the intestine and prevents regurgitation in the stomach. Deep tubular glands in the stomach wall secrete gastric juice, in humans approximately 2 liters each day. Two types of cells line these glands: chief cells, which secrete pepsin, and parietal cells, which secrete hydrochloric acid. Pepsin is a protease (proteinsplitting enzyme) that acts only in an acid medium (pH 1.6 to 2.4). This highly specific enzyme splits large proteins by preferentially breaking down certain peptide bonds scattered along the peptide chain of the protein molecule. Although pepsin, because of its specificity, cannot completely degrade proteins, it effectively hydrolyzes them into smaller polypeptides. Other proteases that together can split all peptide bonds complete digestion of protein in the intestine. Pepsin is present in the stomachs of nearly all vertebrates.
That the stomach mucosa is not digested by its own powerful acid secretions results from another gastric secretion, mucin, a highly viscous organic compound that coats and protects the mucosa from both chemical and mechanical injury. We should note that despite the popular misconception that an “acid stomach” is unhealthy, a notion nourished in advertising, stomach acidity is normal and essential. Sometimes, however, the protective mucous coating fails. This failure is often associated with an infection from a bacterium (Helicobacter pylori) that secretes toxins causing inflammation of the stomach’s lining. This inflammation may lead to a stomach ulcer.
Rennin (not to be confused with renin, an enzyme produced by the kidney, ) is a milk-curdling enzyme found in the stomach of ruminant mammals. It probably occurs in many other mammals. By clotting and precipitating milk proteins, it slows the movement of milk through the stomach. Rennin extracted from stomachs of calves is used in making cheese. Human infants, lacking rennin, digest milk proteins with acidic pepsin, just as adults do.
The secretion of gastric juices is intermittent. Although a small volume of gastric juice is secreted continuously, even during prolonged periods of starvation, secretion normally increases when stimulated by the sight and smell of food, by presence of food in the stomach, and by emotional states such as anxiety and anger. A most unusual and classic investigation in the field of digestion was made by U.S. Army surgeon William Beaumont during the years 1825 to 1833. His subject was a young, hardliving French-Canadian voyageur named Alexis St. Martin, who in 1822 accidentally shot himself in the abdomen with a musket, the blast “blowing off integuments and muscles of the size of a man’s hand, fracturing and carrying away the anterior half of the sixth rib, fracturing the fifth, lacerating the lower portion of the left lobe of the lungs, the diaphragm, and perforating the stomach.” Miraculously the wound healed, but a permanent opening, or fistula, formed that permitted Beaumont to see directly into the stomach (Figure 34-11). St. Martin became a permanent, although temperamental, patient in Beaumont’s care, which included food and housing. Over a period of 8 years, Beaumont was able to observe and record how the lining of the stomach changed under different psychological and physiological conditions, how foods changed during digestion, the effect of emotional states on stomach motility, and many other facts about the digestive process of his famous patient.
In most vertebrates, and in some invertebrates, the stomach provides initial digestion as well as storage and mixing of food with digestive juices. Mechanical breakdown of food, especially plant food with its tough cellulose cell walls, often continues in herbivorous animals by grinding and crushing devices in the stomach. The muscular gizzard of terrestrial oligochaete worms and birds is assisted by stones and grit swallowed along with food or, in arthropods, by hardened linings (for example, chitinous teeth of the insect proventriculus [Figure 34-9], and calcareous teeth of the gastric mill of crustaceans).
Digestive diverticula—blind tubules or pouches arising from the main passage— often supplement the stomach of many invertebrates. They are usually lined with a multipurpose epithelium having cells specialized for secreting mucus or digestive enzymes, or absorption or storage. Examples include the ceca of polychaete annelids, digestive glands of bivalve molluscs, hepatopancreas of crustaceans, and pyloric ceca of sea stars.
Herbivorous vertebrates have evolved several strategies for exploiting cellulose-splitting microorganisms to derive maximal nutrition from plant food. Despite its abundance on earth, the woody cellulose that encloses plant cells can be broken down only by an enzyme, cellulase, that has limited distribution in the living world. No metazoan animals can produce intestinal cellulase for the direct digestion of cellulose. However many herbivorous metazoans harbor microorganisms (bacteria and protozoa) in their gut that do produce cellulase. These microorganisms ferment cellulose under the anaerobic conditions of the gut, producing fatty acids and sugars that the herbivore can use. While the ultimate fermentation machine is the multichambered stomach of the cudchewing ruminants described on, many other animals harbor microorganisms in other parts of the gut, such as the intestine proper or the cecum.
The stomach of carnivorous and omnivorous vertebrates is typically a U-shaped muscular tube provided with glands that produce proteolytic enzymes and strong acids, the latter an adaptation that probably arose for killing prey and halting bacterial activity. When food arrives at the stomach, the cardiac sphincter opens reflexively to allow the food to enter, then closes to prevent regurgitation back into the esophagus. In humans, gentle peristaltic waves pass over the filled stomach at the rate of approximately three each minute. Churning is most vigorous at the intestinal end where food is steadily released into the duodenum, first region of the small intestine. A pyloric sphincter regulates the flow of food into the intestine and prevents regurgitation in the stomach. Deep tubular glands in the stomach wall secrete gastric juice, in humans approximately 2 liters each day. Two types of cells line these glands: chief cells, which secrete pepsin, and parietal cells, which secrete hydrochloric acid. Pepsin is a protease (proteinsplitting enzyme) that acts only in an acid medium (pH 1.6 to 2.4). This highly specific enzyme splits large proteins by preferentially breaking down certain peptide bonds scattered along the peptide chain of the protein molecule. Although pepsin, because of its specificity, cannot completely degrade proteins, it effectively hydrolyzes them into smaller polypeptides. Other proteases that together can split all peptide bonds complete digestion of protein in the intestine. Pepsin is present in the stomachs of nearly all vertebrates.
That the stomach mucosa is not digested by its own powerful acid secretions results from another gastric secretion, mucin, a highly viscous organic compound that coats and protects the mucosa from both chemical and mechanical injury. We should note that despite the popular misconception that an “acid stomach” is unhealthy, a notion nourished in advertising, stomach acidity is normal and essential. Sometimes, however, the protective mucous coating fails. This failure is often associated with an infection from a bacterium (Helicobacter pylori) that secretes toxins causing inflammation of the stomach’s lining. This inflammation may lead to a stomach ulcer.
Rennin (not to be confused with renin, an enzyme produced by the kidney, ) is a milk-curdling enzyme found in the stomach of ruminant mammals. It probably occurs in many other mammals. By clotting and precipitating milk proteins, it slows the movement of milk through the stomach. Rennin extracted from stomachs of calves is used in making cheese. Human infants, lacking rennin, digest milk proteins with acidic pepsin, just as adults do.
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| Figure 34-11 Dr. William Beaumont at Fort Mackinac, Michigan Territory, collecting gastric juice from Alexis St. Martin. |
The secretion of gastric juices is intermittent. Although a small volume of gastric juice is secreted continuously, even during prolonged periods of starvation, secretion normally increases when stimulated by the sight and smell of food, by presence of food in the stomach, and by emotional states such as anxiety and anger. A most unusual and classic investigation in the field of digestion was made by U.S. Army surgeon William Beaumont during the years 1825 to 1833. His subject was a young, hardliving French-Canadian voyageur named Alexis St. Martin, who in 1822 accidentally shot himself in the abdomen with a musket, the blast “blowing off integuments and muscles of the size of a man’s hand, fracturing and carrying away the anterior half of the sixth rib, fracturing the fifth, lacerating the lower portion of the left lobe of the lungs, the diaphragm, and perforating the stomach.” Miraculously the wound healed, but a permanent opening, or fistula, formed that permitted Beaumont to see directly into the stomach (Figure 34-11). St. Martin became a permanent, although temperamental, patient in Beaumont’s care, which included food and housing. Over a period of 8 years, Beaumont was able to observe and record how the lining of the stomach changed under different psychological and physiological conditions, how foods changed during digestion, the effect of emotional states on stomach motility, and many other facts about the digestive process of his famous patient.
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