GLOSSARY

Endocrine secretion is the process of making, storing, and releasing hormones from endocrine cells into the blood; its pattern and amount are regulated by hormonal, neural, and humoral (changes in blood chemistry) stimuli.

Types of Stimuli

• Hormonal: one hormone stimulates the secretion of another (such as TSH → T4).
• Neural: nerve fibres directly stimulate secretion (such as sympathetic → adrenaline from the adrenal medulla).
• Humoral (non-hormonal): a change in the concentration of a substance in the blood (such as glucose → insulin; calcium → PTH; osmolality → ADH).

Modes of Secretion

• Peptide hormones: made as prohormones, stored in vesicles, and released rapidly by exocytosis (calcium-dependent).
• Steroid hormones: not stored; made on demand and diffuse out through the membrane, so secretion rate = synthesis rate.

Temporal Patterns of Secretion