The pancreas, thymus, gonads, and pineal gland

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Anatomy of other endocrine glands

Additional endocrine glands include the pancreas, thymus gland, gonads, and pineal gland. The following sections describe these glands.

Pancreas: The pancreas is located inferior and posterior to the stomach. It has both endocrine and exocrine functions. As already discussed, an endocrine gland releases hormones into the blood, whereas an exocrine gland releases secretions through ducts. The pancreas releases digestive enzymes into the small intestine through pancreatic ducts.

The pancreas contains pancreatic islets or islets of Langerhans, which produce hormones. The pancreas produces several hormones:

Glucagon: Secreted by the alpha islet cells. It raises the blood glucose level in two different ways. It stimulates the stored glycogen in the liver to be converted into glucose. It also stimulates amino acids in the liver to be converted into glucose (gluconeogenesis). The glucose is absorbed by the bloodstream, which increases the blood glucose level.
Insulin: Secreted by the beta islet cells. Normally after a meal, the blood glucose level is elevated. This triggers the pancreas to release insulin. Insulin helps move the blood glucose from the blood to the cells. This lowers the blood glucose level. This is an example of a negative feedback loop.
Somatostatin: Secreted by the delta islet cells; it regulates the other pancreatic hormones and inhibits the secretion of growth hormones.
Ghrelin (GHRL): Secreted by the epsilon islet cells; it signals hunger to the brain before meals and helps regulate food intake, body weight, and glucose metabolism.

Watch out: it’s easy to mix up the alpha and beta islet cells. Alpha cells release glucagon, which raises blood glucose. Beta cells release insulin, which lowers it.

Thymus gland

The thymus gland is in the mediastinum behind the sternum (breastbone) and secretes thymosin and thymopoietin hormones. These hormones stimulate the production and maturity of T cells, a type of lymphocyte (white blood cell). T cells have an important role in immunity.

Gonads

The gonads are considered the primary sex organs. The male gonads are the testes, and the female gonads are the ovaries. Both types of gonads produce hormones:

Testes: Secrete testosterone, which stimulates the development of male secondary sexual characteristics (e.g., voice changes, growth of facial and pubic hair). Testosterone also promotes sperm production and muscle development.
Ovaries: Produce estrogen and progesterone. Estrogen stimulates the development of breasts and other female secondary sexual characteristics. Progesterone helps maintain a pregnancy. Both estrogen and progesterone are important in the menstrual cycle.

Pineal gland

The pineal gland is located deep within the brain and secretes the hormone melatonin. Melatonin helps regulate waking and sleeping patterns and may affect seasonal reactions to the availability of sunlight.

Glands and organs of the endocrine system labeled — The endocrine system

Next | 20.3 Endocrine physiology and pituitary gland disorders

The pancreas, thymus, gonads, and pineal gland

Anatomy of other endocrine glands

Additional endocrine glands include the pancreas, thymus gland, gonads, and pineal gland. The following sections describe these glands.

The pancreas contains pancreatic islets or islets of Langerhans, which produce hormones. The pancreas produces several hormones:

Glucagon: Secreted by the alpha islet cells. It raises the blood glucose level in two different ways. It stimulates the stored glycogen in the liver to be converted into glucose. It also stimulates amino acids in the liver to be converted into glucose (gluconeogenesis). The glucose is absorbed by the bloodstream, which increases the blood glucose level.
Insulin: Secreted by the beta islet cells. Normally after a meal, the blood glucose level is elevated. This triggers the pancreas to release insulin. Insulin helps move the blood glucose from the blood to the cells. This lowers the blood glucose level. This is an example of a negative feedback loop.
Somatostatin: Secreted by the delta islet cells; it regulates the other pancreatic hormones and inhibits the secretion of growth hormones.
Ghrelin (GHRL): Secreted by the epsilon islet cells; it signals hunger to the brain before meals and helps regulate food intake, body weight, and glucose metabolism.

Watch out: it’s easy to mix up the alpha and beta islet cells. Alpha cells release glucagon, which raises blood glucose. Beta cells release insulin, which lowers it.

Thymus gland

Gonads

The gonads are considered the primary sex organs. The male gonads are the testes, and the female gonads are the ovaries. Both types of gonads produce hormones:

Testes: Secrete testosterone, which stimulates the development of male secondary sexual characteristics (e.g., voice changes, growth of facial and pubic hair). Testosterone also promotes sperm production and muscle development.
Ovaries: Produce estrogen and progesterone. Estrogen stimulates the development of breasts and other female secondary sexual characteristics. Progesterone helps maintain a pregnancy. Both estrogen and progesterone are important in the menstrual cycle.

Pineal gland

Anatomy of other endocrine glands

Additional endocrine glands include the pancreas, thymus gland, gonads, and pineal gland. The following sections describe these glands.

The pancreas contains pancreatic islets or islets of Langerhans, which produce hormones. The pancreas produces several hormones:

Glucagon: Secreted by the alpha islet cells. It raises the blood glucose level in two different ways. It stimulates the stored glycogen in the liver to be converted into glucose. It also stimulates amino acids in the liver to be converted into glucose (gluconeogenesis). The glucose is absorbed by the bloodstream, which increases the blood glucose level.
Insulin: Secreted by the beta islet cells. Normally after a meal, the blood glucose level is elevated. This triggers the pancreas to release insulin. Insulin helps move the blood glucose from the blood to the cells. This lowers the blood glucose level. This is an example of a negative feedback loop.
Somatostatin: Secreted by the delta islet cells; it regulates the other pancreatic hormones and inhibits the secretion of growth hormones.
Ghrelin (GHRL): Secreted by the epsilon islet cells; it signals hunger to the brain before meals and helps regulate food intake, body weight, and glucose metabolism.

Watch out: it’s easy to mix up the alpha and beta islet cells. Alpha cells release glucagon, which raises blood glucose. Beta cells release insulin, which lowers it.

Thymus gland

Gonads

The gonads are considered the primary sex organs. The male gonads are the testes, and the female gonads are the ovaries. Both types of gonads produce hormones:

Testes: Secrete testosterone, which stimulates the development of male secondary sexual characteristics (e.g., voice changes, growth of facial and pubic hair). Testosterone also promotes sperm production and muscle development.
Ovaries: Produce estrogen and progesterone. Estrogen stimulates the development of breasts and other female secondary sexual characteristics. Progesterone helps maintain a pregnancy. Both estrogen and progesterone are important in the menstrual cycle.