The endocrine system: glands, locations and hormones

The endocrine system is the collection of glands and hormone-producing tissues that release chemical messengers into the blood without using ducts. Its structures are distributed throughout the body rather than joined into a continuous organ system. Their anatomy includes specialised glands, regions of the brain and hormone-producing tissue within other organs.

What makes a gland endocrine rather than exocrine?

A gland is a structure containing cells specialised to produce and release substances. A hormone is a chemical messenger carried in the blood to cells with suitable receptors, the molecules that recognise that messenger. Endocrine glands release their products into surrounding tissue fluid, from which they enter nearby small blood vessels.

Endocrine glands have no ducts. A duct is a passage that carries a gland's secretion to a surface or into a cavity. Most multicellular exocrine glands use ducts: salivary glands, for example, deliver saliva into the mouth rather than releasing it into the blood.

Some organs contain both types of glandular tissue. The distinction depends on where the secretion goes, not simply on the organ's name or position. Endocrine tissue also occurs in organs whose main anatomical role belongs to another body system.

Where are the hypothalamus, pituitary gland and pineal body?

The hypothalamus is a region at the base of the brain, beneath the thalamus, a deeper brain region. It forms part of the floor of the third ventricle, a fluid-filled cavity within the brain. It connects nervous activity with hormone secretion through the pituitary gland.

The pituitary gland lies in the hypophyseal fossa, a depression in the sphenoid bone at the skull base. It connects to the hypothalamus by the infundibulum, or pituitary stalk. Its anterior part is glandular tissue; its posterior part contains nerve fibres extending from the hypothalamus.

Hypothalamic releasing and inhibiting hormones reach the anterior pituitary through a portal circulation: blood passes through a small-vessel network in the hypothalamic region and then another in the pituitary. The main anterior pituitary hormones are:

• Growth hormone, which supports body growth.
• Prolactin, which supports milk production.
• Thyroid-stimulating hormone, which stimulates thyroid hormone secretion.
• Adrenocorticotrophic hormone, which stimulates the adrenal cortex, principally its zona fasciculata and zona reticularis.
• Follicle-stimulating hormone and luteinising hormone, which regulate reproductive gland activity.

The hypothalamus also produces antidiuretic hormone, which promotes water retention by the kidneys, and oxytocin, which acts on the uterus and milk-ejection mechanisms. These travel along nerve fibres and are released from the posterior pituitary.

The pineal body lies near the roof of the third ventricle, behind the thalamus. It produces melatonin, a hormone associated with the body's daily timing rhythms.

Where are the thyroid gland and parathyroid glands?

The thyroid gland lies in the anterior neck, below the larynx, or voice box. Its right and left lobes lie beside the upper trachea, the airway leading towards the lungs. An isthmus, a bridge of glandular tissue, usually joins the lobes across the front of the trachea.

Thyroid follicles are microscopic rounded units containing stored material used to make thyroid hormones. They produce thyroxine and triiodothyronine, which influence the rate of chemical activity in tissues. Other thyroid cells produce calcitonin, which contributes to calcium regulation.

The parathyroid glands are small, separate glands usually situated on the posterior surface of the thyroid gland. Their position can vary, so they should not be understood as fixed subdivisions of the thyroid itself. They produce parathyroid hormone, which helps maintain calcium levels in the blood.

Where is the thymus, and why is it included?

The thymus lies mainly behind the sternum, or breastbone, in the superior mediastinum and the anterior part of the inferior mediastinum. The mediastinum is the central compartment of the thorax between the lungs. The thymus may extend upwards into the lower neck.

The thymus is primarily a lymphatic organ, involved in the maturation of T lymphocytes, a type of immune cell. It also produces signalling substances traditionally grouped as thymic hormones, including thymosin and thymopoietin.

It is more prominent in childhood and undergoes progressive involution, especially after puberty, with increasing replacement by fatty tissue while some functional thymic tissue persists. Its inclusion in endocrine summaries reflects its hormone-producing role, rather than making it exclusively an endocrine gland.

Where are the adrenal glands, and how do cortex and medulla differ?

An adrenal gland lies near the superomedial aspect of each kidney, within the surrounding fatty tissue. The right adrenal gland is usually pyramidal, while the left adrenal gland is crescent-shaped. Both lie behind the peritoneum, the membrane associated with the abdominal cavity.

Each gland has an outer cortex and an inner medulla. These regions differ in their tissue organisation and hormone production.

The cortex contains zones arranged from outside to inside:

• The zona glomerulosa produces mineralocorticoids, principally aldosterone, which influences sodium and potassium handling by the kidneys.
• The zona fasciculata produces glucocorticoids, principally cortisol, which influences nutrient processing and responses to stress.
• The zona reticularis produces adrenal androgens, hormones contributing to sex-related characteristics.

The medulla contains specialised cells that release adrenaline and noradrenaline into the blood. These belong to the catecholamine group of chemical messengers. Medullary hormone secretion is stimulated directly by preganglionic sympathetic nerve fibres, whereas cortical secretion is regulated mainly by hormonal signals and blood potassium levels. Sympathetic nerves are part of the nervous system concerned with involuntary responses to increased demands.

How does the pancreas combine endocrine and exocrine functions?

The pancreas lies across the upper abdomen, behind the stomach. Its head sits within the curve of the duodenum, the first part of the small intestine, while its tail extends towards the spleen.

Most pancreatic tissue is exocrine. It consists of acini, small clusters of secretory cells, connected to a branching duct system. Their digestive secretions pass through pancreatic ducts into the duodenum.

Pancreatic islets are clusters of endocrine cells scattered among this exocrine tissue. They release hormones into nearby blood vessels rather than into pancreatic ducts. Their principal hormones include insulin, which promotes glucose uptake and storage, glucagon, which promotes glucose availability, and somatostatin, which inhibits several secretory activities.

The pancreas therefore provides a clear comparison: its exocrine products enter the intestinal cavity through ducts, while its endocrine products enter the blood.

Which reproductive organs produce hormones?

The gonads are the reproductive organs that produce reproductive cells and hormones: the ovaries and testes. The left ovary, like the right ovary, usually lies beside the uterus in the pelvic cavity. Ovarian follicles, structures containing developing egg cells, produce oestrogens.

After ovulation, the release of an egg, the remaining follicular tissue forms the corpus luteum. This temporary structure produces progesterone as well as oestrogens. The ovaries also produce inhibin, a hormone that reduces follicle-stimulating hormone secretion.

The testis lies in the scrotum, the external sac beneath the pelvis. Its seminiferous tubules are coiled channels where sperm develop. Between these tubules, Leydig cells produce testosterone; supporting Sertoli cells within the tubules produce inhibin.

These organs illustrate how endocrine tissue can occupy a specific cell population within a structure that also has a reproductive role.

What are the main endocrine locations and hormones?

This table brings together the principal locations and hormone groups described above. It distinguishes hormone production from posterior pituitary release.

In the 3D model beside this article, rotate and zoom to compare gland positions, then select and isolate a structure to examine its shape.