ANATOMY AND PHYSIOLOGY OF THE DIGESTIVE SYSTEM
Section I. ANATOMY OF THE DIGESTIVE SYSTEM
a. Food is Essential to Life. Food is necessary for the chemical reactions that
take place in every body cell; for example, formation of new enzymes, cell structures,
bone, and all other parts of the body that give the energy to supply the body's needs.
Most of the foods we eat are just too large to pass through the plasma membranes of
the cells. The process of breaking down food molecules for the body's cells to use is
called digestion, and the organs which work together to perform this function are termed
the digestive system.
b. Regulation of Food Intake. How much food we eat is regulated by two
sensations--hunger and appetite. When we crave food in general, we are experiencing
hunger, and when we want a specific food, the correct term is appetite. The stronger of
the two sensations is hunger which is accompanied by a stronger feeling of discomfort.
The hypothalamus is the control center for food intake. There are a cluster of nerve
cells in the lateral hypothalamus (the appetite center) which send impulses causing a
person to want to eat. Another cluster of nerve cells tell the person he has had enough.
These cells are located in the medial hypothalamus and called the satiety center. A
person's food intake must be regulated to prevent the digestive tract from becoming too
full. The upper digestive tract expands to let food enter the tract. Receptors in the walls
of the digestive tract are stimulated and send signals to the satiety center, signals that
tell the person he is full. He stops taking in food, and the contents of the digestive tract
c. Digestive Processes. Five basic activities help the digestive system prepare
for use by the cells. These activities are ingestion, peristalsis, digestion, absorption,
Ingestion. Taking into the body of food, drink, or medicines by mouth.
(2) Peristalsis. Alternating contraction and relaxation of the walls of a
tubular structure by which food is move along the digestive tract.
(3) Digestion. The processes by which food is broken down chemically and
mechanically for the body's use. In chemical digestion, catabolic reactions break down
protein, lipid, and large carbohydrate molecules we have eaten into smaller molecules
which can be used by the body's cells. Mechanical digestion refers to the various
movements which aid chemical digestion. Examples of such movements are the
chewing of food by teeth and the churning of food by the smooth muscles of the
stomach and the small intestine.