Other Hormone-Producing Structures

Many tissues within the body release hormones including the placenta, kidneys, digestive system, and adipose tissue.

Learning Objective

Differentiate among the non-glandular, hormone-producing structures of the endocrine system

Key Points

There are several other organs in the body that secrete hormones although they are generally not thought of as being part of the endocrine system.
The kidneys secrete many hormones that help maintain blood pressure and volume, including erthyropoietin, calcitriol, and atrial natriuretic hormone, as well as several proteins and enzymes.
The heart secretes atrial natriuretic hormone to reduce blood pressure.
Many hormones are released in the digestive system to help in the absorption and break down of food, including gastrin, secretin, and cholecystokinin.
Adipose tissue, or fat, is a major endocrine organ and produces hormones such as leptin and estrogen.
The placenta secretes several important hormones in pregnancy, including human chorionic gonadotropin, human placental lactogen, estrogen, and progesterone.

Full Text

There are several other organs in the body that secrete hormones although they are generally not thought of as being part of the endocrine system.

The Kidneys

The kidneys secrete a variety of hormones, including erythropoietin, and the enzyme renin. Erythropoietin is released in response to hypoxia in the renal circulation. It stimulates erythropoiesis (production of red blood cells) in the bone marrow. Part of the renin-angiotensin-aldosterone system, renin is an enzyme involved in the regulation of aldosterone levels.

The Heart

Atrial natriuretic hormone (ANH) is a powerful vasodilator and a protein hormone secreted by heart muscle cells. It is involved in the homeostatic control of body water, sodium, potassium, and fat.

ANH is released by muscle cells in the upper chambers (atria) of the heart in response to high blood pressure. ANH acts to reduce the water, sodium, and adipose loads on the circulatory system to reduce blood pressure.

Stomach and Small Intestine

There are at least five hormones that aid and regulate the digestive system in mammals.

Gastrin is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid. Secretion of gastrin is stimulated by food arriving in the stomach. The secretion is inhibited by low pH.
Secretin is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the secretion of bile in the liver. This hormone responds to the acidity of the chyme.
Cholecystokinin (CCK) is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. This hormone is secreted in response to fat in chyme.
Gastric inhibitory peptide (GIP) is in the duodenum and decreases the stomach churning, which in turn slows the emptying in the stomach. Another function is to induce insulin secretion.
Motilin is in the duodenum and increases the migrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin.

Adipose Tissue

Adipose, or fat, tissue is loose connective tissue composed of adipocytes. Its main role is to store energy in the form of lipids, although it also cushions and insulates the body.

Adipose tissue has in recent years been recognized as a major endocrine organ, as it produces hormones such as:

Leptin, which targets the hypothalamus and is important in regulating food intake.
Estrogen, which plays a key role in sexual function.
Resistin, which targets several tissues with unknown function.

The Placenta

The placenta endocrine function in humans, aside from serving as the conduit for oxygen and nutrients for the fetus, secretes hormones that are important during pregnancy, such as human chorionic gonadotropin, human placental lactogen, estrogen, and progesterone.

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