The endocrine system is a complex network of glands and organs. It uses hormones to control and coordinate your body's metabolism, energy level, reproduction, growth and development, and
response to injury, stress, and mood. The following are integral parts of the endocrine system:
Hypothalamus. The hypothalamus is located at the base of the brain, near the optic chiasm where the optic nerves behind each eye cross and meet. The hypothalamus secretes hormones that stimulate or suppress the release of hormones in the pituitary gland, in addition to controlling water balance, sleep, temperature, appetite, and blood pressure.
Pineal body. The pineal body is located below the corpus callosum, in the middle of the brain. It produces the hormone melatonin, which helps the body know when it's time to sleep.
Pituitary. The pituitary gland is located below the brain. Usually no larger than a pea, the gland controls many functions of the other endocrine glands.
Thyroid and parathyroid. The thyroid gland and parathyroid glands are located in front of the neck, below the larynx (voice box). The thyroid plays an important role in the body's metabolism. The parathyroid glands play an important role in the regulation of the body's calcium balance.
Thymus. The thymus is located in the upper part of the chest and produces white blood cells that fight infections and destroy abnormal cells.
Adrenal gland. An adrenal gland is located on top of each kidney. Like many glands, the adrenal glands work hand-in-hand with the hypothalamus and pituitary gland. The adrenal glands make and release corticosteroid hormones and epinephrine that maintain blood pressure and regulate metabolism.
Pancreas. The pancreas is located across the back of the abdomen, behind the stomach. The pancreas plays a role in digestion, as well as hormone production. Hormones produced by the pancreas include insulin and glucagon, which regulate levels of blood sugar.
Ovary. A woman's ovaries are located on both sides of the uterus, below the opening of the fallopian tubes (tubes that extend from the uterus to the ovaries). In addition to containing the egg cells necessary for reproduction, the ovaries also produce estrogen and progesterone.
Testis. A man's testes are located in a pouch that hangs suspended outside the male body. The testes produce testosterone and sperm.
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Why is it important for life?
The endocrine system controls growth and development during childhood, regulation of bodily functions in adulthood, and the reproductive process.
The endocrine system is important for the control and regulation of all the major functions and processes of the body:
- Energy control
- Reproduction
- Immunity
- Behaviour (e.g. fight or flight response)
- Growth and development
Hormones interact to maintain the above functions and help to regulate our responses to disease, our ability to reproduce and even influence our relationships and behaviour (such as mother:child bonding).
As can be seen, many of the hormones act on tissues and organs at several sites throughout the body. The target cells in these tissues or organs contain specialised structures (receptors) to which only a specific hormone can bind. The response that occurs in the cell will depend on the receptor and cell type, and the effects of other hormones to which that cell may also be exposed. Also, a hormone that stimulates the activity of one cell type may suppress that of a different cell type.
A similar, but not identical, endocrine system to that of humans is found in nearly all vertebrates including other mammals, fish, amphibians, reptiles and birds, although the precise structures and roles of the various organs and hormones differ between different groups, particularly in relation to the different life cycle and development stages in different species. Invertebrates such as molluscs, crustacea and insects also have endocrine systems that control a similar range of body functions although these have evolved along markedly different lines to those of vertebrates.
The main glands and hormones of mammals are:
| Hypothalamus (specialised region of the brain) | Releasing or inhibiting hormones (e.g. growth hormone releasing hormone (GHRH), somatostatin, corticotropin releasing hormone (CRH), arginine vasopressin (AVP), gonadotropin releasing hormone (GnRH) & thyrotropin releasing hormone (TRH) | Anterior pituitary | Controls production/release of other hormones |
| Antidiuretic hormone (ADH) | Kidney | Controls water loss | |
| Oxytocin (OT) -released via the posterior pituitary (neurohypothysis) | Uterus, Mammary glands | ||
| Anterior pituitary [adenohypophysis] | Luteinizing hormone (LH) | Gonads | Control of ovarian oestrus cycle & Leydig cells in testis Control of ovarian oestrus cycle & Sertoli cells in testis Promotes milk production |
| Follicle stimulating hormone (FSH) | Gonads | ||
| Prolactin | Breast | ||
| Thyroid stimulating hormone (TSH) | Thyroid glands | Stimulates T4 production/release Affects growth and metabolism | |
| Growth hormone (GH) | Generalised | ||
| Adrenocorticotropic hormone (ACTH) | Adrenal glands | Stimulates adrenal cortex | |
| Thyroid | Thyroxine (T4) [active form is tri-iodothyronine T3] | Many tissues | Control of general metabolic rate, also important for development and reproductive functions |
| Adrenal [suprarenal] cortex | Glucocorticoids (e.g. cortisol & corticosterone | Many tissues | Diverse effects on inflammation and protein synthesis |
| Mineralocorticoids (e.g. aldosterone) | Many tissues | Salt balance | |
| Adrenal [suprarenal] medulla | Adrenaline (Epinephrine) | Many tissues | Glycogenolysis, lipid mobilisation, smooth muscle contraction, cardiac function |
| Noradrenaline (Norepinephrine) | Many tissues | Lipid mobilisation, arteriole contraction | |
| Pancreas | Insulin | Many tissues including liver, adipose tissue & muscle | Facilitates utilisation of glucose by cells & prevents excessive glycogen breakdown in liver & muscle |
| Glucagon | Many tissues including liver, adipose tissue & muscle | Prevents hypoglycaemia through action on carbohydrate, fat & protein metabolism | |
| Gonads | Sex steroids (e.g. oestradiol, testosterone, progesterone) | Brain, gonads, accessory sex organs | Sexual development & maintainance of reproductive function and behaviour |