Chapter Summary

Internal Affairs (104.0K)

Introduction to the Endocrine System

1. Hormones are regulatory molecules released into the blood by endocrine glands. The action of a hormone on target cells is dependent on its concentration and the specific receptor sites on cell membranes.
2. Hormones are classified chemically as steroids, proteins, and amines.
3. Negative feedback occurs when information concerning an imbalance in hormone concentration is fed back to an organ that acts to correct the imbalance.

Pituitary Gland

1. The pituitary gland (hypophysis) is divided into an anterior adenohypophysis and a posterior neurohypophysis.
1. In adults the adenohypophysis consists of a glandular pars distalis and a thin proximal extension called the pars tuberalis. A pars intermedia is present in the fetus but does not constitute a separate structure in the adult.
2. The neurohypophysis consists of the lobus nervosa and the infundibulum.

2. The anterior pituitary produces and secretes growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, prolactin, and melanocyte-stimulating hormone.
3. The posterior pituitary releases oxytocin and antidiuretic hormone.
4. Secretions of the anterior pituitary are controlled by hypothalamic hormones and regulated by the feedback of hormones from the target cells. Release of hormones from the posterior pituitary are controlled by the hypothalamo-hypophyseal nerve tract.

Thyroid and Parathyroid

1. The bilobed thyroid gland is located in the neck, just below the larynx. Four small parathyroid glands are embedded in its posterior surface.
2. Thyroid follicles secrete thyroxine and triiodothyronine, which increase the rate of protein synthesis and the rate of energy release from carbohydrates. They also regulate the rate of growth and the rate of maturation of the nervous system.
3. Parafollicular cells of the thyroid secrete the hormone calcitonin, which lowers blood calcium by inhibiting the release of calcium from bone tissue and stimulating the excretion of calcium by the kidneys.
4. Parathyroid hormone causes an increase in blood calcium and a decrease in blood phosphate levels. It acts on the large intestine, kidneys, and bones.

Pancreas

1. The pancreas is a mixed endocrine and exocrine gland, located in the abdominal cavity.
2. The pancreatic islets contain beta cells that secrete insulin and alpha cells that secrete glucagon.
1. Insulin lowers blood glucose and stimulates the production of glycogen, fat, and protein.
2. Glucagon raises blood glucose by stimulating the breakdown of liver glycogen.

Adrenal Glands

1. Each adrenal gland consists of an adrenal cortex and an adrenal medulla and is positioned along the superior border of a kidney.
2. Hormones of the adrenal cortex include mineralocorticoids, which regulate sodium reabsorption and potassium excretion; glucocorticoids, which influence metabolism by promoting vasoconstriction and resistance to stress; and gonadocorticoids, which supplement gonadal hormones.
3. Epinephrine and norepinephrine, secreted from the adrenal medulla, produce effects similar to those of the sympathetic division of the ANS.

Gonads and Other Endocrine Glands

1. Testes are the male gonads that produce the male sex hormone testosterone within the interstitial cells.
2. Ovaries are the female gonads that produce estrogens within the ovarian follicles and corpus luteum.
3. The pineal gland, located in the roof of the third ventricle of the brain, secretes melatonin, which seems to have an effect on the hypothalamus in the release of gonadotrophin.
4. The thymus is positioned behind the sternum within the mediastinum. It produces T cells that are important in maintaining body immunity.
5. Certain gastrointestinal cells secrete hormones that aid digestion.
6. The maternal placenta secretes hormones that maintain pregnancy.