The endocrine system produces hormones that are instrumental in maintaining homeostasis and regulating reproduction and development. A hormone is a chemical messenger produced by a cell that effects specific change in the cellular activity of other cells (target cells). Unlike exocrine glands (which produce substances such as saliva, milk, stomach acid, and digestive enzymes), endocrine glands do not secrete substances into ducts (tubes). Instead, endocrine glands secrete their hormones directly into the surrounding extracellular space. The hormones then diffuse into nearby capillaries and are transported throughout the body in the blood.
The endocrine and nervous systems often work toward the same goal—both influence other cells with chemicals (hormones and neurotransmitters). However, they attain their goals differently. Neurotransmitters act immediately (within milliseconds) on adjacent muscles, glands, or other nervous cells, and their effect is short‐lived. In contrast, hormones take longer to produce their intended effect (anywhere from seconds to days); may affect any cell, nearby or distant; and produce effects that last as long as they remain in the blood (up to several hours).
Hormones can be chemically classified into four groups:
- Amino acid‐derived hormones are modified amino acids.
- Polypeptide and protein hormones are chains of amino acids of less than or more than about 100 amino acids, respectively. Some protein hormones are actually glycoproteins, containing glucose or other carbohydrate groups.
- Steroid hormones are lipids that are synthesized from cholesterol. Steroids are characterized by four interlocking carbohydrate rings.
- Eicosanoids are lipids that are synthesized from the fatty acid chains of phospholipids found in the plasma membrane.
Mechanisms of hormone action
Hormones circulating in the blood diffuse into the interstitial fluids surrounding the cell. Cells with specific receptors for a hormone respond with an action that is appropriate for the cell. Because of the specificity of hormone and target cell, the effects produced by a single hormone may vary among different kinds of target cells.
Hormones activate target cells by one of two methods, depending on the chemical nature of the hormone:
- Lipid‐soluble hormones (steroid hormones and hormones of the thyroid gland) diffuse through the cell membranes of target cells. The lipid‐soluble hormone then binds to a receptor protein that in turn activates a DNA segment that turns on specific genes. The proteins produced as a result of the transcription of the genes and subsequent translation of mRNA act as enzymes that regulate specific physiological cell activity.
- Water‐soluble hormones (polypeptide, protein, and most amino acid hormones) bind to a receptor protein on the plasma membrane of the cell. The receptor protein in turn stimulates the production of one of the following second messengers:
- Cyclic AMP (cAMP) is produced when the receptor protein activates another membrane‐bound protein called a G protein. The G protein activates adenylate cyclase, the enzyme that catalyzes the production of cAMP from ATP. Cyclic AMP then triggers an enzyme that generates specific cellular changes.
- Inositol triphosphate (IP 3) is produced from membrane phospholipids. IP 3 in turn triggers the release of Ca 2+ from the endoplasmic reticulum, which then activates enzymes that generate cellular changes.
Control of hormone production
Endocrine glands release hormones in response to one (or more) of the following stimuli:
- Hormones from other endocrine glands
- Chemical characteristics of the blood (other than hormones)
- Neural stimulation
Most hormone production is regulated by a negative feedback system. The nervous system and certain endocrine tissues monitor various internal conditions of the body. If action is necessary to maintain homeostasis, hormones are released, either directly by an endocrine gland or indirectly via the action of the hypothalamus of the brain, which stimulates other endocrine glands to release hormones. The hormones activate target cells, which initiate physiological changes that adjust body conditions. When normal conditions have been restored, the corrective action (the production of hormones) is discontinued. Thus, in negative feedback, when the original (abnormal) condition has been repaired, or negated, corrective actions decrease or are discontinued. For example, the amount of glucose in the blood regulates the secretion of insulin and glucagons through negative feedback.
The production of some hormones is regulated by positive feedback. In such a system, hormones cause a condition to intensify (rather than decrease). As the condition intensifies, hormone production increases. Such positive feedback is uncommon but does occur during childbirth (hormone levels build with increasingly intense labor contractions) and lactation (where hormone levels increase in response to nursing, which causes milk production to increase).