Higher animals, including humans, have developed the ability to regulate their internal environment, independent of wide fluctuations in external factors, in the form of endocrine systems. An endocrine system consists of (1) an endocrine gland that secretes a hormone, (2) the hormone itself, and (3) a target tissue that responds to the hormone. The classical definition of a hormone is “a chemical substance produced by a ductless endocrine gland and secreted into the blood, which carries it to a specific target organ to produce an effect.” In addition to the humoral communication regulated by endocrine systems, the nervous system also regulates overall bodily functions. The two systems are intimately interconnected and normally work in close concert. One direct point at which the two systems interface involves “neuroendocrine” cells, which are special types of neurons capable of secreting humoral substances (hormones) in response to synaptic input (neurotransmitters).
The hormone-producing glands of humans include the pituitary (hypophysis), the thyroid and parathyroids, the adrenals, the gonads, and the pancreas. The mechanisms by which endocrine glands synthesize, store, and secrete hormones depend on the chemical properties of the hormone. There are primarily three chemical classes of hormones: amino acid derivatives, peptides and proteins, and steroids. The amino acid derivatives include the catecholamines, epinephrine, and norepinephrine (produced in the adrenal medulla), and the thyroid hormones, triiodothyronine (T3), and thyroxine (T4). A large number of hormones are peptides or proteins, such as the neurohormones of the posterior pituitary, tropic hormones of the anterior pituitary, and pancreatic hormones. Steroids are produced by the adrenal cortex, the testes, the ovaries, and in pregnancy, the placenta. These hormones are derivatives of cholesterol. In general, designation of a hormone as hydrophilic (peptides and proteins, catecholamines) or hydrophobic (thyroid hormones, vitamin D, steroids) provides information useful in understanding the synthesis, secretion, transport, and target cell mechanism of action of that particular hormone (Table 20-1).
Table 20-1Groups of Hormones ||Download (.pdf) Table 20-1 Groups of Hormones
|ROLE ||HYDROPHILIC ||HYDROPHOBIC (LIPOPHILIC) |
|Endocrine gland cellular storage ||Yes ||No |
|Secretion ||Secretory vesicles ||Passive diffusion |
|Transport in blood ||Free* ||Protein bound |
|Target organ receptors ||Plasma membrane ||Nuclear |
Circulating hormone levels are influenced by the rate of secretion from the endocrine gland, rate of metabolism, and method by which hormone is transported in the blood. The rate of secretion is regulated in the endocrine gland by a variety of physiological factors. The rate of hormonal metabolism or degradation in the bloodstream determines its half-life (t1/2) in blood. The cells of the endocrine gland typically contain only a small store of synthesized hormone, although one noted exception is the thyroid that stores hormone for several months. Therefore, in response to a specific stimulus, the cells synthesize and secrete hormones directly into the bloodstream where they are ...