KEY POINTS

General Concepts

1. Hormones are chemicals produced by specific tissues that are transported by the vascular system to affect other tissues at low concentrations.

2. The endocrine and nervous systems are integrated in their control of physiological processes.

Synthesis of Hormones

1. Protein hormones are initially synthesized as preprohormones and then cleaved in the rough endoplasmic reticulum to form prohormones and in the Golgi apparatus to form the active hormones, which are stored in granules before being released by e?ocytosis.

2. Steroids are synthesized from cholesterol, which is synthesized by the liver; steroids are not stored but are released as they are synthesized.

Transport of Hormones in the Blood

1. Protein hormones are hydrophilic and carried in the plasma in dissolved form.

2. Steroids and thyroid hormones are lipophilic and carried in plasma in association with both specific and nonspecific binding proteins; the amount of unbound, active hormone is relatively small.

Hormone-Cell Interaction

1. Protein hormones have specific receptors on target tissue plasma membranes, whereas steroids have specific receptors within the cytoplasm or nucleus.

Postreceptor Cell Responses

1. Steroids interact directly with the cell nucleus through the formation of a complex with its cytosolic receptor, whereas protein hormones need a messenger because they cannot enter the cell.

Metabolism of Hormones

1. Steroid hormones are metabolized by conjugation with sulfates and glucuronides, which makes steroids water soluble.

Feedback Control Mechanisms

1. The most important feedback control for hormones is the negative-feedback system, in which increased hormone concentrations result in less production of the hormone, usually through an interaction with the hypothalamus or pituitary gland.

2. Endocrine secretory patterns can be influenced by factors such as sleep or light and can produce circadian rhythms.

The Hypothalamus

1. The hypothalamus coordinates the activity of the pituitary gland through the secretion of peptides and amines.

The Pituitary Gland

1. The neurohypophysis has cell bodies that originate in the hypothalamus, with cell endings that secrete oxytocin and vasopressin.

2. Oxytocin and vasopressin are synthesized in cell bodies within the hypothalamus and are carried by axon flow to the posterior lobe, where they are released.

3. The main effects of oxytocin are on the contraction of smooth muscle (mammary gland and uterus); the effects of vasopressin are primarily on the conservation of water (antidiuresis) and secondarily on blood pressure.

4. Plasma osmolality controls the secretion of vasopressin.

5. The anterior pituitary produces growth hormone, prolactin, thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone, and corticotropin.

6. Adenohypophyseal activity is controlled by hypothalamic releasing hormones, which are released into the portal system, which in turn connects the median eminence of the hypothalamus and the anterior pituitary gland.