Endocrine system

Endocrine system

The Endocrine system

The endocrine system works closely with the nervous system to control and coordinate the body’s activities. It consists of the Endocrine glands and the hormones. Hormones are often referred to as ‘chemical messengers’, which the glands secrete and/or store. They communicate between:

  1. Two endocrine glands to stimulate one gland to release a hormone.
  2. An endocrine gland and a target organ.

Each gland is rich in capillaries, which cluster around the glands so that hormones can be easily secreted and pass into the bloodstream. Hormones attach to plasma proteins in the blood and are transported around the body to their target organs. Hormones may affect a number of target organs.

Hormones are secreted by the hormone glands into the blood stream; from there they are circulated around the body affecting the different organs. These organs are known as target organs.

The endocrine system – supply and demand

The amount of hormone released by an endocrine gland is determined by the need for the hormone at any given time. The body is normally regulated so that hormones are not over or under produced (homeostasis). There are times when the regulating mechanism does not operate properly and hormonal levels may become too high, too low or not secreted at all. When this happens endocrine disorders occur.

 

Some hormones are associated with long-term changes, for instance the growth hormone, as this takes place over years. Others have fast changes, such as the adrenaline, which prepares the body very quickly for sudden stress.

Endocrine glands and their functions

  1. The pituitary gland is situated in the brain and is known as the master gland, as it acts on other endocrine glands (tropic hormones). It secretes follicle stimulating hormone (FSH), luteinizing hormone (LH), which controls reproduction, and anti-diuretic hormone (ADH), which affects the water balance.
  2. The thyroid gland is situated in the neck, on either side of the trachea, and secretes thyroxine, which controls the metabolism.
  3. The parathyroid gland is made up of four small glands which are situated on the posterior of the thyroid gland. It secretes the para hormone, which controls the blood calcium levels.
  4. The pancreas secretes insulin from the pancreatic islets of langerhans. This controls the blood sugar level.
  5. The adrenal glands consist of two triangle shapes that lie on top of the kidney. They are made of two parts:
  6. medulla. This secretes adrenalin, which is released at times of sudden stress.
  7. cortex. This secretes:
  • gluccorticoids, which reduces stress responses such as inflammation
  • aldosterone, which controls the level of potassium and sodium in the blood. This hormone can also cause excess oedema (water retention)
  • corticosteroids (oestogens and androgens) which controls the function of the sex organs and the development of sex characteristics at puberty.
  1. There are two of these and they are situated below the kidneys. They secrete the hormones oestrogen and progesterone, which control female reproductive events such as puberty, menstruation, pregnancy and the menopause. This gland also influences the female shape, such as fat being stored in the breasts, hips and thighs.
  2. Testes glands are situated in the groin of a male and are contained in a sac named the scrotum. These glands secrete testosterone, the function of which is to control the sex characteristics in puberty as well as produce sperm. It also causes facial and body hair to develop and muscular development to take place, which will influence the body shape.

The Endocrine glands

Pituitary gland

Often referred to as the master gland, as it produces hormones that control the function of other endocrine glands. This gland is the size of a pea and sits in a hollow in the base of the skull, beneath the brain and behind the nose. Attached to the hypothalamus which controls its activity.
It has two parts:

Anterior – connects to the brain by blood vessels and consists of gland cells.

Posterior – part of brain and secretes directly into the bloodstream under the command of the brain.

The thyroid gland

Located in the front of the neck just below the larynx. Has a distinct butterfly shape with two lobes – one on each side of the trachea. Reliant on supplies of iodine in the diet to function effectively.

Parathyroid

Located in the front of the neck, just below the larynx and just behind the thyroid gland. There are four of these small pea-sized glands – two glands lie behind each wing of the thyroid.

The adrenals (suprarenal) glands

There is one adrenal gland on the top of each kidney. Each gland has two parts: • outer adrenal cortex • inner adrenal medulla.

Pineal gland

Located deep in the middle of the brain where the two halves meet. It sits above the thalamus.

Thymus

Located behind the sternum, between the lungs. Predominantly active during puberty then atrophies. Helps the body against autoimmunity and has a vital role in the lymphatic system.

Pancreas (Islets of Langerhans)

The pancreas is a large gland, about 15cm long, sitting alongside the stomach and small bowel. Both an exocrine and endocrine gland, it also plays an important role in digestion. The Islets of Langerhans contain endocrine tissue. These are clusters of cells making up 1–2% of the pancreas.

Ovaries

Small oval organs about 4cm in size. They lie on either side of the uterus, held in place by ligaments – they are not attached to the fallopian tubes. They produce ova and are part of the female reproductive system.

Testes

The two male gonads lie in the scrotal sacs – one each side of the penis. They form part of the male reproductive system.