The endocrine system

The body’s two main control mechanisms, which regulate all aspects of physical life, are the nervous system and the endocrine system. The endocrine system consists of eight principal ductless glands that release chemicals called hormones directly into the blood. These glands are the anterior pituitary; the posterior pituitary; the thyroid; the parathyroid; the islets of Langerhans in the pancreas; the adrenal cortex; the adrenal medulla; and the gonads— ovaries in a female, testes in a male.

Endocrine glands and their hormones are shown below. The anterior pituitary influences the thyroid, adrenal medulla, and sex glands through thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

Hormones at work

Hormones comprising several dozen different chemicals travel through the bloodstream and influence different kinds of “target” cells, modifying their activity in a variety of ways.
Each target cell has at least one type of receptor in its membrane, and each kind of receptor receives one type of hormone, which fits it rather as a key fits into a lock. For example, a hormone may cause the target cell to produce the messenger compound cyclic adenosine monophosphate, which affects such things as protein manufacture. These proteins then go on to regulate specific cellular functions.

The action of endocrine glands

Endocrine glands are specialized chemical factories that produce hormones that perform a special task or group of tasks.

The thyroid and parathyroid glands lie in the front of the neck. The thyroid’s main task is the control of energy metabolism. The thyroid uses almost all the iodine inside the body, principally to manufacture iodine-rich thyroxine, a hormone essential for growth and for regulating the body’s basal metabolic rate. The thyroid gland also produces calcitonin, which encourages the deposition of calcium in bone. The four parathyroid glands produce parathormone, which raises the calcium level in the blood.

The pancreas a large gland opening into the small intestine principally produces digestive enzymes, although its cell clumps called the islets of Langerhans produce the hormones insulin and glucagon, which regulate carbohydrate metabolism. Insulin is necessary for cells to take up glucose, which is then used for energy in the cell. Insulin also stops the liver from producing unwanted glucose, and prevents adipose tissue from releasing glycerol and fatty acids. Glucagon counters the effects of insulin by boosting the release of glucose as this is required. These hormones all act together to maintain the level of glucose in the body.

The adrenal glands, which lie one above each kidney, in fact each contain two glands. The outer part, the adrenal cortex, produces steroid hormones, including aldosterone and hydrocortisone. Aldosterone plays a major role in regulating salt balance in the body. Hydrocortisone is involved in metabolizing amino acids, fat, and glucose, and it helps to provide the raw materials and energy required for building and repairing tissues. As well as the testes and ovaries, the adrenal cortex synthesizes the male sex hormones (androgens) and the female sex hormones (estrogens), the balance of which in individuals of either sex determines secondary sexual characteristics.
The inner part of each adrenal gland, the adrenal medulla, produces epinephrine and norepinephrine hormones that prepare the body for “fight or flight” in situations of stress which between them increase heart rate, channel blood to muscles, and release glucose. These activities prepare the body for sudden action in an emergency.

The male testes and female ovaries yield testosterone and estrogen, respectively, which have fundamental effects on sexual development and reproductive activity.
The pituitary gland is the most important endocrine gland; among its functions is the control of other endocrine glands.

Endocrine feedback between the thyroid and parathyroid glands controls calcium metabolism. Low blood calcium levels stimulate the parathyroid glands, causing them to release parathormone, which decreases the deposition of calcium in bone, decreases calcium excretion by the kidneys, and increases intestinal calcium absorption, thus raising blood calcium levels. This stimulates the thyroid to produce calcitonin, which has the reverse effect.

Controlling the endocrine system

Too much or too little of any hormone may cause deformity, disease, or death. The pituitary gland and the hypothalamus of the brain control which hormones are released, in what quantity, and when.

The pituitary is a gland about the size of a pea, suspended in a bone cavity below the brain and above the nasal cavity. This tiny unit releases more than two dozen hormones that regulate the activity and hormone output of almost all the other endocrine glands.

The pituitary’s anterior and posterior parts work separately. The anterior lobe (front part) releases trophic (nourishing) hormones, each of which triggers the production and release of a special hormone in a “target” gland. Thyrotropic, or thyroid-stimulating, hormone (TSH) stimulates the thyroid gland to produce thyroxine. Adrenocorticotropic hormone (ACTH) encourages hormone output from the cortex of each adrenal gland. Follicle stimulating hormone (FSH) and luteinizing hormone (LH) act upon the ovaries and testes. The anterior pituitary also produces growth hormone (GH), which indirectly affects cartilage in ways that are essential to growth and broadly influences metabolism. Changes in the anterior pituitary’s output of gonadotropic hormones initiate sexual maturity.

The posterior lobe (rear part) of the pituitary stores and releases two hormones manufactured in the adjacent hypothalamus, a complex region of the brain located immediately above the pituitary and beneath the thalamus. Vasopressin, or antidiuretic hormone (ADH), controls the amount of water excreted with urinary toxins and so helps to maintain the water balance of the body. Oxytocin causes the uterus to contract during childbirth and stimulates the release of milk during nursing.

The pituitary gland knows when to liberate hormones stimulating target glands or muscles by a process called “feedback”—by means of which the controller is itself controlled. For example, when the thyroid gland releases thyroxine into the bloodstream, its presence inhibits the pituitary’s production of TSH. This reduction in TSH results in a reduction in the thyroid’s output of thyroxine. As this reduces, the pituitary responds by producing more TSH, so that overall, a balanced output of thyroxine is maintained.

This example illustrates the principle, but oversimplifies the mechanism of endocrine feedback. Endocrine feedback mechanisms depend to a great extent on the hypothalamus, chief coordinator of the endocrine and nervous systems. This part of the brain combines hormonal feedback information from the blood with nerve impulses received from brain centers, including those controlling body rhythms. The hypothalamus responds by sending amino acid chains called peptides to the pituitary, and it is primarily these that stimulate or inhibit pituitary hormone output. Thus, the peptide called thyrotropin-releasing hormone (TRH) stimulates manufacture of thyrotropin (thyrotropic hormone, or TSH) in the pituitary, and so indirectly affects the release of thyroxine from the thyroid gland; in this way, the central nervous system exerts control over the simplified feedback system described above.