Controlling body temperature
- Body temperature is monitored by the thermoregulatory centre in the brain, which has receptors sensitive to the temperature of the blood. Skin receptors also send information.
- If you are too hot: sweat glands release sweat, which evaporates and cools the skin; blood vessels near the skin surface dilate (vasodilation), so more heat is lost.
- If you are too cold: blood vessels near the skin constrict (vasoconstriction), sweating stops, and muscles contract rapidly (shivering) to release heat from respiration.
Water and waste
- Water leaves the body from the lungs when we breathe out, from the skin in sweat, and in urine. Ions and urea are also lost in sweat.
- We have no control over water, ion and urea loss by the lungs and skin, so the kidneys control the balance.
- Excess amino acids are broken down in the liver (deamination), forming ammonia, which is converted to urea for safe excretion.
How the kidneys work
- The kidneys make urine by filtering the blood.
- Useful substances, including all the glucose, some ions and water, are selectively reabsorbed into the blood.
- Urine contains urea, excess water and excess ions. It is stored in the bladder.
ADH (Higher)
- The water level in the blood is controlled by ADH, a hormone released by the pituitary gland.
- When the blood is too concentrated, more ADH is released. ADH makes the kidney tubules reabsorb more water, so less, more concentrated urine is produced.
- This is a negative feedback system.
Kidney failure
- Kidney failure can be treated by dialysis or a kidney transplant.
- In dialysis, the blood flows past a partially permeable membrane surrounded by dialysis fluid; urea and excess ions diffuse out, but useful substances stay. It must be done several times a week.
- A transplant is a long-term cure but there are not enough donors, and anti-rejection drugs must be taken for life.
Key terms
- Thermoregulatory centre
- The part of the brain that monitors and controls body temperature.
- Vasodilation
- The widening of blood vessels near the skin surface to lose heat.
- Vasoconstriction
- The narrowing of blood vessels near the skin surface to reduce heat loss.
- Shivering
- Rapid muscle contractions that release heat from respiration.
- Deamination
- The breakdown of excess amino acids in the liver, forming ammonia.
- Urea
- A waste substance made from ammonia in the liver and excreted in urine.
- Selective reabsorption
- Taking useful substances, such as glucose, back into the blood in the kidneys.
- ADH
- A hormone from the pituitary gland that makes the kidneys reabsorb more water.
- Dialysis
- A treatment for kidney failure that removes urea and excess ions from the blood.