HUMAN ENDOCRINE SYSTEM

Human Endocrine System

The human endocrine system modulates several processes of the body by the function of hormones. The endocrine system secretes hormones that control how bodily functions work. Thus, the human endocrine system watches over and coordinates all the systems of the body with the use of hormones.

Pituitary gland

The pituitary gland is located at the base of the human brain. The gland consists of two parts: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis).

The anterior lobe secretes at least seven hormones. One hormone, the human growth hormone (HGH), promotes body growth by accelerating protein synthesis. This hormone is also known as somatotropin. A deficiency of the hormone results in dwarfism; an oversecretion results in gigantism.

Another hormone of the anterior pituitary is prolactin, also called lactogenic hormone (LH). This hormone promotes breast development and milk secretion in females. A third hormone is thyroid-stimulating hormone (TSH). The function of TSH is to control secretions of hormones from the thyroid gland. A fourth hormone is adrenocorticotropic hormone (ACTH). This hormone controls the secretion of hormones from the adrenal glands.

There are three more hormones produced in the anterior lobe of the pituitary gland. The first is follicle-stimulating hormone (FSH). In females, FSH stimulates the development of a follicle, which contains the egg cell; in males, the hormone stimulates sperm production. The next hormone is luteinizing hormone (LH). In females, LH completes the maturation of the follicle and stimulates the formation of the corpus luteum, which temporarily secretes female hormones. In males, LH is interstitial cell-stimulating hormone (ICSH), which stimulates the production of male hormones in the testes. The final hormone is melanocyte-stimulating hormone (MSH), which stimulates production of the pigment melanin.

The posterior pituitary gland stores and then releases two hormones that are produced in the hypothalamus of the brain. The first hormone is antidiuretic hormone (ADH). This hormone stimulates water reabsorption in the kidneys. It is also called vasopressin. The second hormone is oxytocin, which stimulates contractions in the muscles of the uterus during birth.

Thyroid gland

The thyroid gland lies against the pharynx at the base of the neck. It consists of two lateral lobes connected by an isthmus. The gland produces thyroxine, a hormone that regulates the rate of metabolism in the body. It also produces a second hormone, calcitonin, which regulates the level of calcium in the blood.

Thyroxine production depends on the availability of iodine. A deficiency of iodine causes thyroid gland enlargement, a condition called goiter. An undersecretion of thyroxine during childhood development results in a condition known as cretinism (dwarfism with abnormal body proportions and possible mental retardation). In adults, an undersecretion can also result in myxedema (physical and mental sluggishness). Thyroxine oversecretion results in a high metabolic rate and Graves’ disease.

Parathyroid glands

The parathyroid glands are located on the posterior surfaces of the thyroid gland. They are tiny masses of glandular tissue that produce parathyroid hormone, also called parathormone. Parathyroid hormone regulates calcium metabolism in the body by increasing calcium reabsorption in the kidneys and by increasing the uptake of calcium from the digestive system.

Adrenal glands

The adrenal glands are two pyramid-shaped glands lying atop the kidneys. The adrenal glands consist of an outer portion, the adrenal cortex, and an inner portion, the medulla.

The adrenal cortex secretes a family of steroids called corticosteroids. The two main types of steroid hormones are mineralocorticoids and glucocorticoids. Mineralocorticoids such as aldosterone control mineral metabolism in the body. They accelerate mineral reabsorption in the kidneys. Mineralocorticoid secretion is regulated by ACTH from the pituitary gland. Glucocorticoids such as cortisol and cortisone control glucose metabolism and protein synthesis in the body. Glucocorticoids are also anti-inflammatory agents.

The adrenal medulla produces two hormones: epinephrine (adrenaline) and norepinephrine (noradrenaline). Epinephrine increases heart rate, blood pressure, and the blood supply to skeletal muscles. Epinephrine functions in stressful situations to promote the fight-or-flight response. Norepinephrine intensifies the effects of epinephrine. Both hormones prolong and intensify the effects of the sympathetic nervous system.

Pancreas

The pancreas is located just behind the stomach. Its endocrine portion consists of cell clusters called the islets of Langerhans.

The pancreas produces two hormones: insulin and glucagon. Insulin is a protein that promotes the passage of glucose molecules into the body cells and regulates glucose metabolism. In the absence of insulin, glucose is removed from the blood and excreted in the kidney, a condition called diabetes mellitus. Diabetes mellitus is characterized by glucose in the urine, heavy urination, excessive thirst, and a generally sluggish body metabolism.

The second pancreatic hormone, glucagon, stimulates the breakdown of glycogen to glucose in the liver. It also releases fat from the adipose tissue so the fat can be used for the production of carbohydrates.

Other endocrine glands

Among the other endocrine glands are the ovaries and testes. The ovaries secrete estrogens, which encourage the development of secondary female characteristics. The testes secrete androgens, which promote secondary male characteristics. Testosterone is an important androgen.

The pineal gland is a tiny gland in the midbrain. Its functions are largely unknown, but it seems to regulate mating behaviors and day-night cycles. The thymus gland is located in the neck tissues. It secretes thymosins, which influence the development of the T-lymphocytes of the immune system.

Prostaglandins are hormones secreted by various tissue cells. These hormones produce their effects on smooth muscles, on various glands, and in reproductive physiology. Erythropoietin is a hormone produced by the kidney cells. Erythropoietin functions in the production of red blood cells. Gastrin and secretin are hormones produced by digestive glands to influence digestive processes.