pertaining to digestion.
digestive system the organs that have as their particular function the ingestion, digestion, and absorption of food or nutritive elements. They include the
mouth,
teeth,
tongue,
pharynx,
esophagus,
stomach, and
intestines. The accessory organs of digestion, which contribute secretions important to digestion, include the
salivary glands,
pancreas,
liver, and
gallbladder. (See also color plates.)
Mouth. The mouth is the entrance to the alimentary canal; in it the teeth, tongue, and jaws begin the process of digestion by
mastication.
saliva is secreted into the mouth by three separate pairs of glands (the
salivary glands) located under the tongue, inside the lower jaw, and in the cheek. Saliva softens and lubricates the food, and dissolves some of it; it also contains an enzyme called
ptyalin that begins the conversion of starches into sugar. Saliva also moistens the inside of the mouth, the tongue, and the teeth, and rinses them after the food has departed on the next stage of its journey.
Four passageways meet at the back of the throat: the oral and nasal passages, the
larynx, and the
esophagus. In the act of
swallowing, the entrances to the nasal passages and the
larynx are each sealed off momentarily by the soft
palate and the
epiglottis, so that the food can pass into the
esophagus without straying into the
respiratory tract.
Stomach. Propelled by rhythmic muscular contractions called
peristalsis, the food moves rapidly through the
esophagus, past the
cardiac sphincter (a circular muscle at the base of the esophagus) and into the stomach. Here the peristaltic motions are stronger and more frequent, occurring at the rate of three per minute, churning, liquefying, and mixing the foods with the gastric juice. In the juice are the enzymes
pepsin and
lipase and, in infants,
rennin; a secretion called
mucin, which coats and protects the stomach lining; and
hydrochloric acid. Together the
pepsin and
hydrochloric acid begin the splitting of the proteins in the food. The
lipase in the stomach is a rather weak fat-splitting enzyme, able to act only on fats that are already emulsified, such as those in cream and the yolk of egg; the intestine has a stronger
lipase, and it is there that most fats are digested.
The average adult stomach holds about 1.5 liters. The stomach reaches its peak of digestive activity nearly 2 hours after a meal and may empty in 3 to 4½ hours; a heavy meal may take as long as 6 hours to pass into the small intestine.
Small Intestine. The food leaves the stomach in the form of
chyme, a thick, liquid mixture. It passes through the
pylorus, a sphincter muscle opening from the lower part of the stomach into the
duodenum. This sphincter is closed most of the time, opening each time a peristaltic wave passes over it. The stomach is much wider than the rest of the canal and also has a J-shaped curve at its bottom, so that the passage of food through the pylorus is automatically slowed until the food is of the right consistency to flow through the narrow opening into the intestine.
The small intestine is about 6 meters (20 feet) long. The lining of the small intestine has deep folds and fingerlike projections called
villi that give it a surface of about 9 square meters (100 square feet) through which absorption of food can take place.
The
duodenum, a C-shaped curve with a length of about 25 cm (10 in), is the first and widest part of the small intestine. Into it flows the pancreatic juice, with enzymes that break down starch, protein, and fats. The common bile duct also empties into the duodenum. The bile emulsifies fats for the action of the fat-splitting enzymes.
Just below the duodenum is the
jejunum, the longest portion of the small intestine, and beyond that is the
ileum, the last and narrowest section of the small intestine. Along this whole length, carbohydrates, proteins, and fats are broken down into sugars, amino acids, fatty acids, and glycerin. The lining of the small intestine absorbs these nutrient compounds as rapidly as they are produced. The bulky and unusable parts of the diet pass into the large intestine.
Large Intestine. At the junction of the small and large intestines is the
ileocecal valve, so called because it is at the end of the ileum and the beginning of the
cecum. A small blind tube called the
vermiform appendix is attached to the
cecum. The longer part of the large intestine is called the
colon and is divided into the ascending, transverse, and descending colon, and the sigmoid flexure, an S-shaped bend at the distal end of the colon. The sigmoid colon empties into the
rectum. Along the 1.7 meters (5.5 feet) or so of the large intestine, the liquid in the waste is gradually reabsorbed through the intestinal walls. Thus the waste is formed into fairly solid feces and pushed down into the rectum for eventual evacuation. This takes from 10 to 20 hours. The evacuation consists of bacteria, cells cast off from the intestines, some mucus, and such indigestible substances as cellulose. The normal dark brown color of the
feces is caused by bile pigments.