SPECIFICATION:The function of the endocrine system: glands and hormones

The function of the endocrine system: glands and hormones.

A second system supplements the work of the nervous system in the body, the endocrine system. This is a network of glands throughout the body that manufacture and secrete chemical messengers known as hormones. It works alongside the nervous system to control vital functions in the body. It instructs glands to release hormones directly into the bloodstream via blood vessels. These hormones are carried towards target organs in the body.

Endocrine Glands

 These produce and secrete hormones, chemical substances that regulate the activity of cells or organs in the body. The major glands in the endocrine system are the pituitary gland, adrenal glands and reproductive organs (ovaries and testes). Each gland produces different hormones.

 Gland

Function

Pituitary

Often called the “master gland.” Some of the hormones released are important for regulating the endocrine system.

Adrenal

An important part of the fight-or-flight response as it facilitates the release of adrenaline.

Testes

They facilitate the release of testosterone (male hormone)

Ovaries

They facilitate the release of oestrogen and progesterone (female hormones).

 The endocrine system is regulated by feedback to ensure a stable concentration of hormones. For example, a signal is sent from the hypothalamus to the pituitary gland as a ‘releasing hormone’. This causes the pituitary to secrete a ‘stimulating hormone’ into the bloodstream. This hormone then signals the target gland (e.g. the adrenal glands) to secrete its hormone. As levels of this hormone rise in the bloodstream, the hypothalamus shuts down the secretion of the releasing hormone and the pituitary gland shuts down the secretion of the stimulating hormone. This slows down the secretion of the target gland’s hormone, resulting in a stable concentration of hormones circulating in the bloodstream.

 Hormones

Behaviour is thought to be influenced by hormones, and each hormone is thought to affect behaviour in a different way.

 Hormones are chemicals that circulate in the bloodstream and are carried to target sites throughout the body. Although hormones come into contact with most cells in the body, a given hormone usually affects only a limited number of cells, known as target cells. There have to be particular receptors for particular hormones. Cells that don’t have such a receptor cannot be influenced directly by that hormone. When enough receptor sites are stimulated, this results in a physiological reaction in the target cell.

 Timing of hormone release is critical for normal functioning, as are the levels of hormones released. Too much or too little at the wrong time can result in the dysfunction of bodily systems. E.g. too high a level of cortisol can lead to Cushing’s syndrome, characterised by high blood pressure and depression.

 Pituitary Gland

 The pituitary gland produces hormones whose primary function is to influence the release of hormones from other glands and, in so doing, regulate many of the body’s functions. The pituitary is controlled by the hypothalamus, a region of the brain just above the pituitary gland.

As the “master gland,” the pituitary produces hormones that travel in the bloodstream to their specific target. These hormones either directly cause changes in physiological processes in the body or stimulate other glands to produce other hormones. High levels of hormones produced in other endocrine glands can stop the hypothalamus and the pituitary from releasing more of their hormones to stop hormone levels from rising too high.

Hormones produced by the pituitary gland

The pituitary has two parts: the anterior (front) and the posterior (back). They each release different hormones that target different parts of the body. E.g. the anterior pituitary produces adrenocorticotropic (ACTH) as a response to stress. ACTH stimulates the adrenal glands to produce cortisol. The anterior also produces two other hormones important in controlling reproductive functioning: Luteinising hormone (LH) and follicle-stimulating hormone (FSH). In females, these hormones stimulate the ovaries to produce oestrogen and progesterone, and in males, they stimulate the testes to produce testosterone and sperm.

 The posterior pituitary releases oxytocin, which stimulates the contraction of the uterus in childbirth, and is important for mother-infant bonding. Research using mice has found that oxytocin is indispensable for healthy maintenance and repair and that it declines with age (Elabd et al 2014).

 The Adrenal Glands

 The two adrenal glands sit on top of the kidneys. Each adrenal gland has two parts. The outer part is the adrenal cortex, and the inner region is the adrenal medulla. The adrenal cortex and adrenal medulla have very different functions. One of the main distinctions is that the adrenal cortex releases hormones necessary for life, whereas the adrenal medulla releases hormones that do not.

Hormones produced by the adrenal glands

 The adrenal cortex produces cortisol – a stress hormone. It has a variety of functions such as cardiovascular and anti-inflammatory functions. If cortisol levels are low, the individual has low blood pressure, poor immune function and an inability to deal with stress. The adrenal cortex also produces aldosterone, which maintains blood volume and blood pressure.

 The adrenal medulla releases adrenaline and noradrenaline – hormones that prepare the body for fight or flight. Adrenaline helps the body respond to a stressful situation, e.g. increasing heart rate and blood flow to the muscles and brain. Noradrenaline constricts the blood vessels, causing blood pressure to increase.

 Ovaries

 Another huge part of the endocrine system is the ovaries. The two ovaries are part of the female reproductive system. Ovaries are responsible for the production of eggs and for the hormones of oestrogen and progesterone. Progesterone is more important in the post-ovulation phase of the menstrual cycle.

Testes

The testes are the male reproductive glands that produce the hormone testosterone. Testosterone causes the development of male characteristics such as the growth of facial hair, deepening of the voice and growth spurts. The hypothalamus and the pituitary gland control testosterone production. The hypothalamus instructs the pituitary gland on how much testosterone to produce, and the pituitary gland passes this message to the testes. Testosterone also plays a role in sex drive, sperm production and maintenance of muscle strength and is associated with overall health and well-being in men. Testosterone is not exclusively a male hormone. Women also have it, but in smaller amounts.