Digestion is the process of breaking large molecules into smaller molecules by
chemical and physical means so that the body can use them. The digestive tract,
which is also known as the alimentary canal, is a long tube extending from the
mouth to the anus. This tube is composed of the mouth, pharynx, esophagus,
stomach, small intestines, and large intestines. Along the alimentary canal are
various glands which secrete substances which contribute to the process of
digestion.
Digestion starts when food is chewed in the mouth. Chewing breaks up food so that
it has more exposed surface area. The tongue moves food under the teeth so that
it can be chewed. The tongue has taste buds which convey information to the brain
about the food. It also aids in swallowing food. The act of chewing is called
mastication.
Salivary glands supply the mouth with a liquid substance called saliva. Saliva
moistens and lubricates food so that it can be more easily chewed and swallowed.
Saliva also dissolves some of the food so that it can be tasted. Saliva has
a pH of 6.7, making it slightly acidic. Although saliva is 99% water, it also
contains a digestive enzyme called salivary amylase which helps to digest starch.
There are three pairs of salivary glands. The largest pair are the parotid glands
which lie just in front of, and slightly below the opening of the ears. The
parotid salivary glands become infected and enlarged when one has the mumps.
Below the parotid salivary glands near the angle of the lower jaw are the
submaxillary salivary glands. Under the sides of the tongue are the sublingual
salivary glands. If our body becomes dehydrated, the body draws water from the
salivary glands to replace that which is lost from the blood through sweating.
Therefore, when we are thirsty (dehydrated) our mouth is dry from the lack of
saliva. Mastication in the presence of saliva changes food into a ball of food
called a bolus.
The act of swallowing starts as a voluntary process involving muscles in the mouth
and pharynx, but becomes an involuntary process as the food comes under the
control of the smooth muscles of the esophagus. When we swallow, the bolus passes
from the mouth to the pharynx. The pharynx is a region shared by both the
digestive and respiratory systems. The uvula, the small flap of tissue which
hangs from the back of the mouth, prevents food from entering the nasal cavity.
Also in the pharynx, the epiglottis covers the windpipe so that the bolus cannot
enter the lungs. When swallowing, the part of the nervous system responsible for
breathing is inhibited, so that it is almost impossible to breath and swallow at
the same time. This also serves to prevent food from entering the respiratory
system.
The bolus is taken down the esophagus by a series of involuntary muscular waves known as peristalsis. Lining the esophagus are cells which produce mucus. The mucus lubricates the esophagus to allow the bolus to pass with less friction. The muscular contractions of peristalsis occur behind the bolus so that it is pushed toward the stomach. At the point where the esophagus meets the stomach is a value called the cardiac sphincter. When the peristaltic wave reaches the cardiac sphincter, the sphincter opens to allow the bolus to pass into the stomach. Then it closes. It is the cardiac sphincter which prevents food from coming back into the esophagus when one, for example, stands on their head.
The stomach lies to the left of and just below the diaphragm. The capacity of the average stomach is a little over a quart. The internal lining of the stomach is called the gastric mucosa. The gastric mucosa contains long tubular gastric glands that secrete the gastric juices used in digestion. About 35,000 of these glands line the normal stomach. The gastric glands have three different types of cells which secrete three different substances. The parietal cells secrete hydrochloric acid, the chief cells secrete pepsinogen, and the mucous neck cells secrete mucin.
Hydrochloric acid causes the acidity of gastric juice to be in the range of pH 1 to pH 3. The hydrochloric acid of the stomach practically guarantees that any bacteria which enters with the food is destroyed. The few that are not destroyed can in some cases cause food poisoning. Hydrochloric acid combines with pepsinogen to form the enzyme called pepsin. Pepsin digests only protein. Mucin is a secretion which protects the stomach from being dissolved by the hydrochloric acid. The presence of the acid producing bacteria H. pylori, increases the acidity of the stomach so that the stomach lining is dissolved. This condition is called a peptic ulcer. The quantity of gastric juices secreted to digest and average meal is a little over a half quart.
Within the walls of the stomach are muscular contractions and waves which move the food about. As in the esophagus, this muscular action is still referred to as peristalsis. This muscular action combined with the digestive action of gastric juices converts the food into a thick liquid called a chyme. Under certain conditions, the normal path of the peristaltic wave is reversed to produce vomiting. The region of the nervous system which produces vomiting is in a part of the brain stem called the medulla oblongata.
At the bottom of the stomach is the pyloric sphincter. When it opens it releases
the chyme into the first part of the small intestines called the duodenum, a
region which is wider that the rest of the small intestines. The liver and the
pancreas pour digestive juices into the duodenum. The digestive juice produced
by the liver is called bile. Bile is made in the liver from the worn out
hemoglobin of red blood cells. When bile is not needed for digestion, it is
stored in the gall bladder. Bile is a base which digests fats. The digestive
part of the pancreas produces pancreatic juice, which is a digestive enzyme which
digests protein, fat and carbohydrates. Another part of the pancreas produces
insulin, the hormone which transports sugar across the cell membrane. If insulin
is not produced, a disease called diabetes mellitus results. However, insulin is
not involved in digestion in the small intestines.
The digestion of the chyme is completed by the intestinal enzymes and the digested food is ready to be passed into the blood steams. As the peristaltic wave continues, the digested food and waste passes through the circular folds of the small intestines which have small finger-like projections called villi. The capillaries of the villi actually absorbs the digested food into the blood steam.
The waste products of digestion move on by way of peristaltic contraction into the large intestines, which is also called the colon. The large intestines remove water from waste and allows it to be reabsorbed into the body. No digestion occurs in the large intestines. In the large intestines are large colonies of bacteria which act on the undigested waste and convert these into gases, acids, vitamins, and waste. The reabsorption of water and the action of bacteria change the consistency of the intestinal contents from a liquid to a semisolid called feces. Feces are composed of bacteria, waste products brought by blood, products of the action of bacteria in the intestines, salts, mucus, and the indigestible components of food, such as cellulose. The indigestible parts of food, such as cellulose, are called fiber. Fiber stimulates the lining of the intestines to induce peristalsis and the opening of the anal sphincter which allows the elimination of feces from the body.
A part of the large intestines is a small projection called the appendix. It may have been useful when and if we were ever able to digest cellulose. In modern man, it is useless. However, it can occasionally become infected and inflamed by bacteria and rupture. The bacteria of the large intestines rush into the body cavity from a ruptured appendix. Since the body cavity has few blood vessels, white blood cells cannot be delivered quickly enough to prevent massive bacterial infection of the body cavity. The quicker this rupture can be closed, the better the chance to avoid massive infection and death. Severe inflammation and rupture of the appendix is known as appendicitis.
The small intestines is about 25 feet long. The large intestines is about 5 feet
long. The small intestines is called "small" because its diameter is smaller than
that of the large intestines. Conversely, the large intestines is called "large"
because its diameter is larger than that of the small intestines.