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The Endocrine System

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.

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:
  1. medulla. This secretes adrenalin, which is released at times of sudden stress.
  2. cortex. This secretes:
  • glucocorticoids, 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 (oestrogens and androgens) which controls the function of the sex organs and the development of sex characteristics at puberty.

  1. Ovaries. 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.

Puberty

Menstruation in females starts at puberty. This is controlled by the hormones oestrogen and progesterone and also by the follicle stimulating and luteinizing hormones which are released from the pituitary gland, bringing about changes in the ovaries and the uterus.

It is at this stage in a female that the hormones oestrogen and progesterone become active. This is when the sex characteristics of the body begin to change. Oestrogen causes the breasts to become bigger and the vulva and vagina to develop.

Pregnancy

The hormone known as the pregnancy hormone is progesterone. This develops the placenta and prepares the breasts to produce milk after the birth.

Menopause

At this stage of life the menstrual cycle ceases, the ovaries become inactive and the production of oestrogen and progesterone stops.

The breasts

The breasts lie over the pectoral and serratus muscles. The breasts are attached to a layer of connective tissue which runs through the breast tissue, attaching the skin to the connective tissue layer that covers the muscles. It is the hormone oestrogen that is responsible for them growing at puberty.

The breasts are also known as the mammary glands and their main function is to produce milk after pregnancy. Milk is produced by the hormone prolactin and is passed through ducts to the nipple. The milk is then released during breast feeding, as the hormone oxytocin is produced as the baby starts to suckle.

The glandular tissue that the breasts are composed of is very similar to the tissue found in the sweat glands.

The cells that secrete the milk are supported by connective tissue and are divided into lobules that are separated by fat. It is the fat that determines the size of the breast.

The breasts contain many lymphatic vessels. Drainage takes place mainly at the axilla glands, which are situated under the arm. The main blood vessels are the subclavian arteries and axillary.

There are lots of nerves, many of which are found around the nipple. When breast feeding, these nerves are stimulated and the flow of oxytocin is stimulated which will promote the flow of milk.

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