The adrenal cortex also produces sex hormones, some with male characteristics (ANDROGENS), others with female characteristics (ESTROGENS). These hormones appear in different concentrations in both men and women.
The adrenal medulla secretes EPINEPHRINE (ADRENALIN) in the presence of emotional crises, hypoglycemia (low blood sugar), or low blood pressure. Epinephrine constricts the peripheral vascular system and dilates the blood vessels to the skeletal muscles. Heart rate, respiration rate and depth, blood pressure, blood sugar levels, and metabolism are all increased by epinephrine. It also stimulates the production of other adrenal cortical hormones.
NOREPINEPHRINE is also produced in the adrenal medulla. It is a chemical precursor to epinephrine. Its effects are similar to those of epinephrine, but its action differs.
Despite these marked influences, the medullary tissue of the adrenal gland is not essential to life, because its various functions can be assumed by other regulatory mechanisms.
The male gonads secrete the hormone TESTOSTERONE, which influences the development and maintenance of the accessory organs and the secondary sex characteristics of the male.
The female gonads, the OVARIES, produce ESTROGEN and PROGESTERONE. Estrogen influences the development and maintenance of the female accessory organs and the secondary sex characteristics and promotes changes in the mucous lining of the uterus (endometrium) during the menstrual cycle. Progesterone prepares the uterus for the reception and development of the fertilized ovum and maintains the lining during pregnancy. It is used in many birth control pills.
The islands of Langerhans in the pancreas contain two types of endocrine cells, alpha and beta. The alpha cells secrete glucagon, which causes a temporary rise in blood sugar levels. The beta cells secrete insulin, which is essential for carbohydrate metabolism. Insulin lowers blood sugar levels by increasing tissue utilization of glucose and stimulating the formation and storage of glycogen in the liver. Together, glucagon and insulin act to regulate sugar metabolism in the body.
When the islet cells are destroyed or stop functioning, the sugar absorbed from the intestine remains in the blood and is excreted by the kidneys into the urine. It is not used by the body or stored. This condition is called diabetes mellitus, or sugar diabetes. Insulin is given to patients having this disease as part of their ongoing treatment.
Figure 3-52.The digestive system.
The digestive system (fig. 3-52) consists of the alimentary tractmouth, pharynx, esophagus, stomach, intestines, and certain accessory organs of digestion. As food passes through the 9-meterlong alimentary tract, digestion and absorption occur, and eventually waste material is eliminated. Secretions of the accessory organs assist in preparing food for absorption and use by the tissues of the body (table 3-2).
Digestion is both mechanical and chemical. Mechanical digestion occurs when food is chewed, swallowed, and churned by peristalsis. Waste is evacuated when the bowels move. Chemical digestion consists of changing the various foods, with 3-42