Hormones are chemical messengers which are released into the blood from a gland and travel to a target organ. They usually give a fairly general reaction, the reaction is long lasting and takes a while to come about, which contrasts with nerves which give an immediate reaction, take no time to travel and effect a specific area or group of cells (e.g. a muscle) and the reaction itself is very quick. The glands which secret hormones are called endocrine glands
- The pituitary gland gives of ADH, LH and FSH and is seen as the centre of hormone activity. It also creates gonad stimulating hormone, thyroid-stimulating hormone and growth hormone. It is seen as a gland which secrets hormones which lead to other glands secreting hormones. (e.g. gonad stimulating glands make the sex organs release sex hormones, and thyroid stimulating hormone stimulates the thyroid to create thyroxin)
- The pancreas controls the blood-sugar levels with the hormones insulin (too much sugar) and glucagon ( not enough sugar)
- The ovaries create oestrogen which i. Repairs the lining of the womb and ii. Promotes secondary sexual characteristics such as extra hair, change in body and the production of eggs.
- The testes create testosterone which promote secondary sexual characteristics such as hair growth, change in body proportions and production of sperm
- The adrenal gland (which sits just on top of the kidneys) creates adrenalin. If we feel threatened or scared this releases adrenalin which starts a flight or flight reaction. This includes increased blood sugar levels, heart rate, and breathing rate, and blood is diverted from the skin to muscles. Adrenaline is a weird hormone because it is very fast acting.
- The thyroid gland creates thyroxin which controls metabolism
The menstrual cycle
The menstrual cycle has four stages
- Day 1 – The uterus lining breaks down for about four days, and menstruation occurs
- Day 4 – 14 the uterus lining is built up by oestrogen
- Day 14 – LH (which is stimulated by oestrogen going to the pituitary gland) is secreted from the pituitary gland and causes ovulation at day 14)
- Day 14 – 28 – Progesterone and Oestrogen maintainthe uterus lining until day 28. If no sperm fertilizes the egg by that point then the uterus begins to brake down and the cycle starts again.
Note; the egg matures in a follicle (which is like an envelope) after ovulation the follicle becomes the corpus luteum which has an important job (see later)
Birth control with hormones
FSH stimulates the ovaries to produce oestrogen; the oestrogen causes LH to be made which sets off ovulation. For this reason FSH is given to woman to stimulate egg production. However, there is a danger that if the dosage is too high there could be multiple pregnancies.
Oestrogen is used in “the pill” to stop egg production. This is because while oestrogen is present the hormone FSH won’t be created, and therefore after a while of having oestrogen at constantly high levels egg production stops and stays stopped.
How the hormones interact
FSH (which is high at day 1) stimulates the ovaries to produce oestrogen (the oestrogen then stops FSH production). This in turn causes LH to be produced, which then causes ovulation at day 14. Meanwhile progesterone levels are increasing because progesterone is created by the follicle, and as it matures levels increase. After ovulation, the corpus luteum (as it is now called) continues to make progesterone to stop the lining braking down, and to stop another egg being released. If the woman becomes pregnant then the corpus luteum remains and continues doing this, if not it degenerates with the lining of the womb. Progesterone maintains the lining. At this point oestrogen levels are falling, and FSH levels are increasing again. If the egg is not fertilized then progesterone levels drop and menstruation occurs. Then oestrogen begins again and everything starts again. If the egg is fertilized then progesterone levels stay high, so oestrogen levels never increase.
The hormones in menstruation
Insulin and diabetes
Eating carbohydrate-full food gives a lot of glucose into the body. Doing extreme exercise uses more glucose, so naturally levels fall faster. However, there must be a way to control the amount of glucose in the blood without having to do exercise, and there is, and it uses the pancreas and liver.
If the blood contains too much glucose then these high levels of blood sugar are detected by the pancreas and a substance called insulin is injected into the blood. The insulin and glucose are absorbed by the liver, where the insulin converts the glucose into glycogen (an insoluble substance) which can then be stored in the liver.
If, later on the blood which passes through the pancreas has too little glucose then the pancreas secretes glucagon, which enters the liver and turns glycogen back into glucose. Intelligent.
In diabetes the pancreas doesn’t produce insulin, meaning blood sugar levels increase to levels where you go into a coma and exploded. This can be controlled by not eating carbohydrate rich meals and by excising after meals to transform glucose into sweet, sweet energy.
Another way to control this (and a much more practical one, who wants to run after a meal?) is to inject insulin created by genetic engineering (this will be discussed later). This is very effective and easy. The danger of this is that if a diabetic takes too much insulin then their blood glucose levels drop too low, and they can become ill, sweating and becoming weak. To rectify this, the person should eat a couple of lumps of sugar.
Homeostasis is the maintenance of constant internal conditions. It is often controlled be negative feedback There are six different things which need to be kept constant
- Removal of carbon dioxide
- Removal of urea
- (Of these two we don’t need any of them, they are constantly being produced by the body and we need to constantly get rid of them.)
- Ion content
- Water Content
- Sugar content
- Amount of thyroxin
- (We need these four in certain amounts, and too much can be dangerous.)
The hypothalamus (or thermoregulatory gland) is located just above the pituitary gland. It contains sensors which monitor blood temperature and water content, and send nerve impulses to the pituitary gland. It also monitors carbon dioxide which is removed by the lungs in exhaled air. The kidneys control water content (via ADH) and ion content of the blood. The pituitary gland produces ADH for the kidneys, while the skin helps control the body temperature by sweating and raising hairs. The muscles can shiver to increase temperature. Finally the liver and pancreas work together to keep the blood sugar levels correct.
The pituitary gland secrets thyroid stimulating hormone, and the more secreted the more thyroxin is made. However, if too much thyroxin is made then it goes back to the pituitary gland and tells it to stop making so much thyroid stimulating hormone.
Here are the other comprehensive GCSE Biology notes by Prometheus:
- The Nervous System and the Eye
- The Digestive System
- The Skin
- The Kidneys
- The Circulatory System and Blood
- Evolution and Selective Breeding
- Genetics - Variation, Mitosis and Meiosis
- Hereditary Traits, Sex Determination and Mutations
- Disease and Drugs
- Plant Biology
- Population Size
- The Greenhouse Effect and Pollution Problems
- The Carbon and Nitrogen Cycles
- Suitable for: GCSE Biology.
- Written by: Prometheus along with all the other notes linked to above.
- From this thread