Hormones are defined as “chemicals produced in endocrine glands that are secreted into the bloodstream and cause an effect on target tissues elsewhere in the body”. They play a wide variety of roles in the healthy functioning and development of the body.
The iGCSE specification only really mentions a small number of hormones so these are the ones I will focus on in this post.
ADH (anti-diuretic hormone)
ADH is secreted into the blood by an endocrine gland at the base of the brain called the Pituitary Gland. The stimulus for the release of ADH into the blood comes from the hypothalamus (a region of brain right next to the pituitary gland) when it detects that the blood plasma is becoming too concentrated. This might be caused by the body becoming dehydrated due to sweating. ADH travels round the body in the blood until it reaches its target tissue which are the cells that line the collecting ducts in the nephrons in the kidney. ADH increases the permeability of the connecting duct walls to water, thus meaning more water is reabsorbed by osmosis from the urine in the collecting duct and back into the blood. This results in a small volume of concentrated urine being produced.
Adrenaline is secreted into the blood by the adrenal glands in situations of danger or stress.. The adrenals are found just above the two kidneys on the back of the body wall. Adrenaline secretion is controlled by nerve cells that come from the central nervous system. Adrenaline is often described as the “fight or flight” hormone as its effects are to prepare the body to defend itself or run away from danger. There are receptors for adrenaline in many target tissues in the body but some of the most significant effects of adrenaline are:
- affects the pacemaker cells in the heart causing an increase in heart rate
- shifts the pattern of blood flow into muscles, skin and away from the intestines and other internal organs
- decreases peristalsis in the gut
- causes pupils to dilate in the eye
- increases breathing rate in the lungs
- promotes the passing of urine from the bladder
Insulin is a hormone made in the islets of Langerhans in the pancreas. It plays a vital role in the homeostatic control of the blood sugar concentration. The pancreas will secrete insulin into the blood when the blood glucose concentration gets too high. There are many cells in the body with insulin receptors but the main target tissue for insulin is the liver.
Insulin causes the liver (and muscle) cells to take glucose out of the blood and convert it into the storage polysaccharide glycogen. This results in a lowering of the blood glucose concentration: a good example of the importance of the principle of negative feedback in homeostasis
Testosterone is a steroid hormone made by cells in the testes of males. It is the main hormone of puberty in males resulting in the growth of the reproductive organs at puberty as well as the secondary sexual characteristics (pitch of voice lowering, muscle growth stimulated, body hair grows etc.)
Oestrogen is a steroid hormone made by the cells in the ovary that surround the developing egg cell in the first half of the menstrual cycle. In puberty it causes the development of the female secondary sexual characteristics (breast growth, change in body shape, pubic hair etc.) but in the menstrual cycle, oestrogen has a variety of important effects. It stimulates the rebuilding of the uterine endometrium (or lining) to prepare the uterus for the implantation of an embryo. Oestrogen also affects the pituitary gland and can cause the spike in LH concentrations that trigger ovulation on day 14 of the cycle.
Progesterone is also made in the ovary but at a different time in the menstrual cycle. It is secreted by cells in the corpus luteum, a structure found from day 14 onwards after the egg has been released in ovulation. Progesterone has two main target tissues: it maintains the thickened lining of the endometrium in the uterus ready for implantation. Progesterone also causes the pituitary gland to stop secreting the hormones FSH and LH so a new cycle is never started. It is for this reason that progesterone can be used in women as a contraceptive pill.