Double Award - Science for GCSE

Chapter 8

Hormones and control

1.	Hormones in our bodies are produced by glands and then released (secreted) into the bloodstream. Hormones are called chemical messengers. They control our growth, our sexual characteristics and how our digested food is used. The diagram below shows the positions of our glands, the hormones they release and the effects of these hormones.

2.	Controlling glucose levels in the blood Cells will only metabolise effectively if the glucose concentration stays fairly steady. Our bodies control the level of glucose in the blood with insulin and glucagon. If the glucose concentration in the blood is too high, the pancreas releases insulin into the bloodstream. This causes the liver to convert glucose to glycogen which is then stored in the liver. If the glucose concentration in the blood is too low, the release of insulin stops. If the glucose concentration in the blood is too low, the pancreas releases glucagon. The glucagon causes the liver to convert glycogen into glucose and release it into the blood. This control of insulin and glucagon output, and hence glucose levels, by feeding back information to the pancreas is called feedback control. It is an example of homeostasis. Some people are unable to control glucose levels in the blood. They are called diabetics and the condition is known as diabetes. Diabetics need to eat regular meals, with little or no sugary foods. Some diabetics have to inject insulin before a meal.
3.	Controlling our body temperature Our body temperature should stay close to 37 °C for our cells and overall metabolism to work effectively. To do this, we may need to gain heat or lose heat. Temperature receptors in the skin send impulses to the brain which then controls the responses of the skin. There are four ways in which the skin can respond and try to keep our body temperature constant. Blood capillaries in the skin dilate (expand) when we are hot. This cools us down as we lose more heat. When we are cold, the skin capillaries contract to reduce the loss of heat. Shivering causes muscles to contract. This stimulates respiration and heat is given out. Sweating causes cooling. As sweat evaporates, it takes heat from the skin. Hairs stand up on the skin in cold weather. These raised hairs trap more air next to the skin, which acts as an insulator and helps to keep us warm. Keeping our temperature and other body conditions in a steady state is another example of homeostasis.
4.	Controlling the water and salt content of our bodies After our cells have used nutrients in the blood, waste and unwanted materials have to be removed. This is done by the kidneys which filter out: The kidneys receive a plentiful supply of blood via the renal arteries. As branches of the renal arteries pass into the kidneys, they divide into a network of capillaries. These capillaries are wrapped around thousands of looped tubules. As blood flows through the capillaries, urea, salts and water diffuse into the tubules forming urine. Urine flows into the tubules, down the ureter and collects in the bladder. Urine is excreted through the urethra.
5.	Drugs Drugs are chemicals which affect our metabolism and behaviour. There are four types of drugs which affect our central nervous system and the way we feel and behave: Sedatives, like alcohol, slow us down. Stimulants, like caffeine in coffee, speed us up. Painkillers, like aspirin, reduce the feeling of pain. Hallucinogens, like cannabis, cause weird and unusual sensations.
6.	Controlling the growth of plants Plants respond to three main stimuli – light, gravity and moisture. These stimuli result in the release of plant hormones which control cell growth and activity in a similar way to hormones in animals. The responses of plants are known as tropisms. Plant shoots lean towards the light - phototropism. Plant roots grow down because of gravity - geotropism. Plant growth hormones are called auxins. Some auxins which increase growth are produced in those parts of a plant which receive less light so they: promote the growth of roots; promote growth on the shaded side of a plant so that it bends towards the light.