Hormones as messengers
The endocrine system uses hormones, chemicals released into the blood by glands and carried to target cells that have the matching receptor. Compared with nerve signals, hormonal effects are slower to start but usually last longer.
Hormones act in two main ways. Peptide and amine hormones (such as insulin) are water-soluble and bind receptors on the cell surface, triggering second messengers inside. Steroid hormones (such as cortisol) and thyroid hormones are lipid-soluble: they pass through the membrane and bind receptors inside the cell, changing which genes are active.
Major glands
- Hypothalamus: links nervous and endocrine systems; controls the pituitary.
- Anterior pituitary: growth hormone, TSH, ACTH, prolactin and others.
- Posterior pituitary: releases ADH (increases water reabsorption in the kidneys) and oxytocin; both are made in the hypothalamus.
- Thyroid: thyroxine and triiodothyronine, which set the basal metabolic rate; calcitonin.
- Parathyroid glands: parathyroid hormone, which raises blood calcium.
- Adrenal glands: the cortex makes cortisol (long-term stress response, raises blood glucose); the medulla makes epinephrine.
- Pancreatic islets: insulin from beta cells, glucagon from alpha cells.
- Pineal gland: melatonin, which follows the sleep and wake rhythm.
Control by feedback
Most hormones are controlled by negative feedback. The hypothalamus releases a hormone that makes the anterior pituitary release TSH, which makes the thyroid release thyroid hormones. When thyroid hormone levels are high, the hypothalamus and pituitary reduce their output, so levels come back down.
Worked walk-through: blood glucose after a meal
Imagine blood glucose rises after a carbohydrate-rich lunch.
- Receptors: beta cells in the pancreatic islets sense the higher glucose.
- Control: they release insulin into the blood.
- Effectors: insulin helps muscle and fat cells take up glucose and prompts the liver to store it as glycogen.
- Result: blood glucose falls toward the set point, and insulin release slows.
Hours later, glucose drifts below the set point. Alpha cells release glucagon, which signals the liver to break glycogen down and release glucose. This pair of opposing hormones keeps glucose within a narrow range. This is a clear example of two hormones regulating one variable in both directions.