Key Points

• Peptide and protein hormones are usually stored in membrane-bound vesicles and secreted by exocytosis triggered by an increase in cytosolic calcium.

• Steroid hormones are derivatives of cholesterol and are generally not stored, but are secreted as they are synthesized.

• After secretion, hormones distribute throughout extracellular fluid and are rapidly cleared from the blood.

• Many hormones circulate bound specifically and reversibly to large carrier proteins. Protein binding protects against loss of hormone by the kidney, slows the rate of hormone degradation by decreasing cellular uptake, buffers changes in free hormone concentrations, and in certain cases may facilitate delivery to target cells.

• Hormones are rapidly degraded in the liver and kidney as well as by their target tissues.

• Hormone levels in blood are not static, but vary widely in accordance with physiologic demand. Hormone secretion may be episodic, pulsatile, or follow a daily rhythm.

• Measurement of hormones, although difficult because of their low concentrations, is essential for diagnostic, therapeutic, and research purposes. Immunoassays take advantage of the exquisite sensitivity and specificity of antibody antigen reactions to recognize and quantitate hormones in biologic fluids.

• Modulation of responding systems: Hormones regulate both the sensitivity and the capacity of target tissues to respond either to themselves or to other hormones. Sensitivity is the acuity of a cell's ability to recognize and respond to a

Essential Medical Physiology, Third Edition

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