Page 5 - 86 human physiology part-2

Chapter 12

about food. Saliva moistens the food while the teeth chew it up and make it easier to swallow.
Amylase, which is the digestive enzyme found in saliva, starts to break down starch into simpler sugars
before the food even leaves the mouth. The nervous pathway involved in salivary excretion requires
stimulation of receptors in the mouth, sensory impulses to the brain stem, and parasympathetic
impulses to salivary glands.

Swallowing your food happens when the muscles in your tongue and mouth move the food into
your pharynx. The pharynx, which is the passageway for food and air, is about five inches (5") long. A
small flap of skin called the epiglottis closes over the pharynx to prevent food from entering the trachea
and thus choking. For swallowing to happen correctly a combination of 25 muscles must all work
together at the same time. Salivary glands also produce an estimated three liters of saliva per day.

Esophageal Sphincter

After passing through the throat, the food moves down a muscular tube in the chest called the
esophagus. Peristalsis (involuntary wavelike muscle contractions along the G.I. tract) moves the food
from the esophagus and pushes it down into the stomach. At the end of the esophagus there is a
sphincter that allows food into the stomach then closes back up so the food cannot travel back up into
the esophagus.

Stomach

The stomach is a thick walled organ that lies of the left side of the diaphragm. It stores food and
acid for digestion. Stomach muscles mix up the food with enzymes and acids to make smaller
digestible pieces. Acid is needed for digestion in the stomach and is secreted by chief cells. The gastric
glands begin secreting before food enters the stomach due to the parasympathetic impulses of the vagas
nerve. The stomach lining has glands that produce up to three quarts of this digestive fluid daily. The
secretion of gastric juices occurs in three phases: cephalic, gastric, and intestinal. The cephalic phase is
activated by the smell and taste of food and swallowing. The gastric phase is activated by the chemical
effects of food and the distension of the stomach. The intestinal phase blocks the effect of the cephalic
and gastric phases. Gastric juice also contains an enzyme named pepsin, which digests proteins,
hydrochloric acid and mucus. Hydrochloric acid causes the stomach to maintain a pH of about 2, which
helps kill off bacteria that comes into the digestive system via food.

Water, alcohol, salt, and simple sugars can be absorbed directly through the stomach wall.
However, most substances in our food need a little more digestion and must travel into the intestines
before they can be absorbed. When the stomach is empty it is about the size of one fifth of a cup of
fluid. When stretched and expanded, it can hold up to eight cups of food after a big meal.

Once mixed with digestive juices in the stomach the food is called chyme. The pyloric sphincter, a
walnut shaped muscular tube at the stomach outlet, keeps chyme in the stomach until it reaches the
right consistency to pass into the small intestine. The food leaves the stomach in small squirts rather
than all at once.

Food that has not been digested then travels from the small intestine to the large intestine. When
the food reaches the large intestine, the work to absorb the nutrients is nearly done. The main function
of the large intestine is to excrete the fluid from the undigested food and produce solid waste and
finally to be excreted through the anus.

220 | Human Physiology