3 Endocrine System Physiology

Hormones and target-cell signaling

The endocrine system coordinates body functions by releasing hormones, which are chemical messengers, into the blood or surrounding fluid. A hormone may circulate widely, but it changes the activity only of cells with the appropriate receptor. The response depends on the target cell and can include changes in enzyme activity, membrane transport, gene expression, growth, or secretion.

Hormones help regulate metabolism, growth, reproduction, and the balance of water, electrolytes, and nutrients. Their chemical class helps determine how they travel in blood and how target cells respond.

Peptide and protein hormones, as well as catecholamines such as epinephrine, generally bind to receptors on the cell surface and activate intracellular signaling pathways. Steroid hormones are derived from cholesterol and typically cross the cell membrane to bind receptors inside the cell, changing gene activity. Thyroid hormones are derived from amino acids and also act through intracellular receptors.

Endocrine organs and their roles

Endocrine glands are ductless: they release hormones into surrounding fluid, from which the hormones enter the circulation. Several organs also have endocrine functions even though hormone secretion is not their only role.

The hypothalamus and pituitary form a major control hub. The hypothalamus communicates with the anterior pituitary through releasing or inhibiting hormones carried in a specialized portal blood system. The posterior pituitary stores and releases ADH and oxytocin made by hypothalamic neurons. Pituitary hormones can then stimulate other glands: TSH stimulates the thyroid, and ACTH stimulates the adrenal cortex.

Major endocrine organs and representative functions include:

• Hypothalamus and pituitary: Link nervous-system information to hormonal control and regulate other glands, growth, reproduction, milk production, water balance, and childbirth. The pituitary hormones listed include TSH, ACTH, GH, FSH, LH, and prolactin; the posterior pituitary releases ADH and oxytocin.

• Thyroid: T3 and T4 regulate metabolic activity and contribute to growth and development. Calcitonin can lower blood calcium.

• Parathyroids: Parathyroid hormone (PTH) raises blood calcium when it falls below its regulated range.

• Adrenal glands: The cortex releases cortisol and aldosterone, supporting longer-term stress responses and fuel and salt-water balance. The medulla releases epinephrine and norepinephrine in rapid fight-or-flight responses.

• Pancreatic islets: Insulin promotes blood-glucose uptake and storage; glucagon helps raise blood glucose during fasting.

• Pineal gland and gonads: Melatonin contributes to sleep timing; estrogens, progesterone, and testosterone contribute to reproductive function.

How hormone secretion is regulated

Hormone secretion responds to feedback and to signals that indicate what the body needs. Negative feedback is the most common pattern: a response reduces the original stimulus and limits further hormone release. As thyroid hormone levels rise, for example, they suppress hypothalamic and pituitary signals that would otherwise stimulate the thyroid.

Signals that initiate secretion include:

• Humoral stimuli are changes in blood chemistry. High blood glucose stimulates pancreatic beta cells to release insulin, while falling glucose favors glucagon release. Increased blood osmolarity promotes ADH release, helping the kidneys conserve water.

• Hormonal stimuli occur when one hormone prompts another gland to release a hormone. The hypothalamus–pituitary–thyroid pathway and hypothalamus–pituitary–adrenal pathway are examples.

• Neural stimuli occur when nerve activity triggers secretion. During acute stress, sympathetic signals stimulate the adrenal medulla to release epinephrine and norepinephrine.

Positive feedback is less common. During labor, oxytocin strengthens uterine contractions, and contraction-related signals promote further oxytocin release. This cycle ends after delivery, when the initiating stimulus is removed.

Coordination across body systems

Endocrine signaling works alongside the nervous system. Neural signals can produce rapid, brief responses, while hormones often act more slowly and sustain effects for longer. This distinction is not absolute: neural activity can trigger a fast hormonal response, as when sympathetic nerves stimulate the adrenal medulla to release hormones.

Hormones coordinate activity across organs. When blood pressure or volume falls, the kidneys release renin, initiating the renin–angiotensin–aldosterone system. Angiotensin II narrows blood vessels and promotes signals that conserve sodium and water; aldosterone and ADH help the kidneys retain fluid. These changes support blood volume and pressure.

Blood-glucose regulation also involves multiple organs. The pancreas responds to changes in blood glucose, while the liver, skeletal muscle, and other target tissues adjust glucose uptake, use, or storage. Gut hormones can help regulate insulin secretion after food intake, and reproductive hormones interact with pituitary signals to coordinate gonadal function.

The endocrine system also influences digestive and reproductive activity, growth, and responses to stress. Because many hormones affect multiple tissues, homeostasis depends on coordinated signals rather than on any single gland acting alone. Together, endocrine signaling and other body systems help keep internal conditions within functional ranges.