The Endocrine System Anatomy and Physiology

The endocrine system contains 9 major glands and organs that produce, store and secrete hormones.

What is Endocrine system?

The glands of the endocrine system produce hormones—chemical messengers, often carried in the blood—that act in a slower, more prolonged, and more generalized way. Both the autonomic nervous system and the endocrine system are governed by the hypothalamus in the brain. The pituitary gland produces hormones that affect other endocrine glands, which sometimes form discrete organs. There are also hormone-producing cells in the tissues of many other organs.

Endocrine System Anatomy and physiology:

The endocrine system consists of three major components:

• Glands, which are specialized cell clusters or organs
• Hormones, which are chemical substances secreted by glands in response to stimulation
• Receptors, which are protein molecules that trigger specific physiologic changes in a target cell in response to hormonal stimulation.

Endocrine disorders:

Endocrine disorders alter a patient’s health and self-image. These disorders may affect the patient’s growth and development, reproductive system, energy level, metabolic rate, or ability to adapt to stress. Some disorders, such as Cushing’s syndrome and goiter, profoundly alter the body. Others, such as diabetes mellitus, require the patient to follow a stringent drug regimen and meal plan.

Glands of the Endocrine System:

Endocrine glands secrete hormones directly into the bloodstream to regulate body function. The major glands of the endocrine system are:

• Pituitary gland
• Thyroid gland
• Parathyroid glands
• Adrenal glands
• Pancreas
• Thymus
• Pineal gland
• Gonads

The Pituitary Gland:

The tiny pituitary gland, at the base of the brain, secretes hormones that stimulate other glands to produce their own hormones. It is often called the master gland because of its wide-ranging influences, but the real master is the hypothalamus, linking the endocrine and nervous systems.

Hormone Controllers:

The pituitary gland consists of two anatomically and functionally different parts: an anterior lobe and a posterior lobe. The anterior lobe forms the bulk of the pituitary, and consists of glandular tissue that manufactures hormones. The posterior pituitary is really part of the brain and is derived from hypothalamic tissue. It does not make hormones itself, but stores and releases hormones produced by the hypothalamus.

The two lobes link to the hypothalamus differently. The anterior lobe is linked by a system of interconnected blood vessels called a portal system. In a portal system, blood from arteries and veins connects directly rather than traveling through the heart first. This system allows hormones from the hypothalamus to be delivered to the anterior pituitary rapidly. The posterior lobe is linked to the hypothalamus by a nerve bundle, the hormone-producing neurons of which originate in the hypothalamus. The axons of these neurons extend into the posterior lobe and carry their hormones there for storage. Nerve signals from these neurons prompt release of their hormones “on demand.”

Anterior Lobe Hormones:

Seven hormones are produced in the anterior pituitary. Four of these, known as tropic hormones, target other glands, prompting them to release their hormones. They are thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). The others—growth hormone (GH), prolactin, and melanocyte-stimulating hormone (MSH)—act directly on target organs.

The release of hormones from the anterior pituitary is regulated by the hypothalamus, which secretes releasing or inhibiting hormones. Although different hormones from the hypothalamus reach the anterior lobe, secretory cells recognize those directed at them and secrete or release their specific hormones accordingly. The hormones are secreted into capillaries that drain into veins and into the general circulation to reach their target organs.

Posterior Lobe Hormones:

Two hormones—oxytocin and antidiuretic hormone (ADH)—are stored in the posterior lobe of the pituitary gland. These hormones are not made in the gland but by the cell bodies of neurons located in two different areas of the hypothalamus. After production, the hormones are packaged in tiny sacs and transported down the axons (nerve fibers) of the neurons to the axon terminals, where they are stored until needed. Nerve impulses from the same hypothalamic neurons where they were produced trigger the release of the hormones into capillaries.

From the capillaries, they pass into veins for distribution to their target cells. Oxytocin and ADH are almost identical in structure: each is made of nine amino acids, only two of which differ between them. However, each has a different effect. Oxytocin stimulates smooth muscle to contract, especially that of the uterus, cervix, and breast. ADH influences the balance of water in the body.

Hormone Producers:

The thyroid, parathyroid, adrenal glands, and pineal gland are all organs of the endocrine system that exclusively produce hormones. Other organs and tissues also considered part of the endocrine system.

Thyroid Gland:

The butterfly-shaped thyroid gland is composed mainly of spherical sacs called follicles, the walls of which produce two important hormones, T3 (triiodothyronine) and T4 (thyroxine), collectively known as thyroid hormone (TH). Almost every cell in the body has receptors for TH, and it has widespread effects in the body. The thyroid gland is unusual among endocrine glands as it can store large quantities of hormones—maintaining about 100 days’ supply of TH.

The thyroid gland also produces calcitonin from parafollicular cells located between the follicles. An important effect of this hormone is to inhibit the loss of calcium from bones into the blood. It is most important in childhood, when skeletal growth is rapid.

Thyroid hormone regulation:  Thyrotropin-releasing hormone (TRH) from the hypothalamus and thyroid-stimulating hormone (TSH) from the anterior pituitary stimulate the production and release of thyroid hormones (TH). Blood levels of TH feed back to the pituitary and hypothalamus to stimulate or inhibit activity.

Parathyroid Glands:

The four tiny parathyroid glands at the back of the thyroid gland produce parathyroid hormone (PTH), the major regulator of calcium levels in blood. The correct balance of calcium is essential for many functions, including muscle contractions and the transmission of nerve impulses, so it needs to be controlled precisely. When blood calcium levels fall too low, PTH stimulates the release of stored calcium from bone into the blood and reduces calcium loss from the kidneys into urine.

It indirectly increases the absorption of calcium from ingested food in the small intestine. In order for the intestine to absorb calcium, vitamin D is needed, but the ingested form is inactive: PTH stimulates the kidneys to convert vitamin D from its precursor form into its active form, calcitriol.

Effects of parathyroid hormone

Parathyroid hormone acts on the bone, kidneys, and (indirectly) the small intestine in order to increase the amount of calcium in the blood.

Parathyroid hormone has a relatively short life span in the blood stream, its levels falling by 50 percent every 4 minutes.

Adrenal Glands:

The outer and inner regions of the adrenal glands differ from each other in structure, and each produces different hormones. The outer adrenal cortex is glandular tissue, while the inner medulla is part of the sympathetic nervous system and contains bundles of nerve fibers. The adrenal cortex produces three groups of hormones: mineralocorticoids, corticosteroids, and androgens. An important mineralocorticoid is aldosterone, which regulates the sodium–potassium balance in the body and helps adjust blood pressure and volume. The main glucocorticosteroid is cortisol, which controls the body’s use of fat, protein, carbohydrates, and minerals.

It also helps the body to resist stress, including from exercise, infection, extreme temperatures, and bleeding. The androgens produced by the adrenals are relatively weak in their effects, compared with those produced by the ovaries and testes during late puberty and adulthood. However, they probably play a role in the appearance of underarm and pubic hair in both sexes. In adult women, they are linked to the sex drive. The adrenal medulla produces epinephrine and norepinephrine. In stressful situations, when the sympathetic nervous system becomes activated, the hypothalamus stimulates the adrenal medulla to secrete these hormones, which augment the stress response.

Pineal Gland:

The tiny pinecone-shaped pineal gland is located near the center of the brain, behind the thalamus. It secretes the hormone melatonin, which is involved in the body’s sleep–wake cycle. Pineal activity lessens in bright light, so melatonin levels are low during the day. They rise at night, increasing about tenfold, making us sleepy. Bright light does not directly affect the pineal gland; instead, input from the visual pathways stimulates the suprachiasmatic nucleus (part of the hypothalamus), which sends signals to the pineal gland via nerve connections near the spinal cord. The suprachiasmatic nucleus also controls other diurnal biological rhythms, such as body temperature and appetite, and it is likely that melatonin cycles influence these processes. Melatonin is also an antioxidant and may protect against damage from free radicals in the body. In animals that breed seasonally, melatonin inhibits reproductive function but it is not known whether melatonin affects reproduction in humans.

Melatonin levels

The level of circulating melatonin rises at night or when it is dark, creating a daily rhythm of rising and falling hormone levels.

Pancreas:

The pancreas is a dual-purpose gland with both digestive and endocrine functions. The bulk of the gland consists of acinar cells, which produce enzymes used in digestion.  Scattered among these cells are about a million pancreatic islets, or islets of Langerhans, cell clusters that produce pancreatic hormones. There are four different types of hormone-producing cell. Beta cells make insulin, which enhances transport of glucose into cells, where it is used for energy or converted into glycogen for storage. In this way, beta cells lower blood glucose levels. Alpha cells secrete glucagon, which has the opposite effect of insulin, stimulating release of glucose from the liver and raising blood glucose levels. Somatostatin, secreted by delta cells, regulates alpha and beta cells. There are only a few F cells. They secrete pancreatic peptide, which inhibits secretion of bile and pancreatic digestive enzymes.

Blood sugar regulation:

The body needs to regulate blood glucose levels so that cells receive enough energy to meet their needs. The main source of fuel is glucose, which is carried in the blood stream—any excess glucose is stored in liver, muscle, and fat cells. The pancreatic hormones insulin and glucagon prompt storage or release of glucose from cells, keeping blood levels stable.

O varies and Testes:

The female ovaries and male testes, also known as gonads, produce eggs and sperm respectively. They also produce sex hormones, the most important of which are estrogens and progesterone in females, and testosterone in males. Release of these sex hormones is stimulated by folliclestimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. Before puberty, FSH and LH are almost absent from the blood stream, but during puberty they begin to rise, causing the ovaries and testes to increase hormone production. As a result, secondary sexual characteristics develop and the body is prepared for reproductive functions. The hormone inhibin inhibits release of FSH and LH. In males it regulates sperm production and in females it plays a role in the menstrual cycle. The ovaries also produce relaxin, which prepares the body for childbirth.

Hormone-producing cells In the testes, interstitial cells (dark circles) secrete testosterone. In the ovaries, granulosa cells (dark purple dots), shown here surrounding an egg follicle, produce estrogen.

Hormonal Action

The hormones that raise or lower blood pressure become effective over a period of several hours. Their effects may last for days.