Kidneys and excretion

Excretion is removing the waste products of the body’s chemical reactions. The lungs remove carbon dioxide, and the kidneys remove urea, excess water and excess ions in urine. The kidneys filter the blood and then take back what the body needs, keeping the water and ion content of the blood in balance.

Key facts

  • Carbon dioxide comes from respiration and is breathed out. Urea is made in the liver when excess amino acids are broken down (deamination), and is removed by the kidneys.
  • Each kidney has about a million tiny tubes called nephrons. Blood enters the kidney in the renal artery and leaves in the renal vein. Urine flows down the ureter to the bladder and leaves through the urethra.
  • Filtration: in the glomerulus, blood at high pressure is filtered into Bowman’s capsule. Water, glucose, ions and urea pass through; blood cells and proteins are too big.
  • Selective reabsorption: along the tubule, all the glucose, and as much water and as many ions as the body needs, are taken back into the blood. Urea stays in the tubule and becomes part of the urine.
  • The water level of the blood is controlled by ADH, a hormone from the pituitary gland. Too little water: more ADH, so the collecting ducts reabsorb more water and the urine is concentrated. Too much water: less ADH, and the urine is dilute. This is negative feedback.
  • Kidney failure can be treated by dialysis or a transplant. In dialysis, blood flows past a partially permeable membrane with dialysis fluid that has the same glucose and ion concentrations as normal plasma but no urea, so urea diffuses out.
  • A transplanted kidney can be rejected by the immune system, so the donor must be a close tissue match and the patient takes drugs that suppress the immune system.

Worked examples

Example 1

Explain why there is no glucose in the urine of a healthy person, even though glucose is filtered out of the blood.

  1. Glucose is a small molecule, so it is filtered from the glomerulus into Bowman’s capsule.
  2. In the first coiled tubule, all the glucose is reabsorbed into the blood.
  3. So none is left by the time the filtrate becomes urine.

Answer: all the glucose is reabsorbed into the blood from the first part of the tubule

Example 2

The kidneys filter 180 dm³ of fluid a day and make 1.8 dm³ of urine. What percentage of the filtered fluid is reabsorbed?

  1. Work out how much is reabsorbed. 180 − 1.8 = 178.2 dm³
  2. Divide by the amount filtered and multiply by 100. 178.2 ÷ 180 × 100 = 99%

Answer: 99%

Example 3

After a long run on a hot day, a runner makes a small volume of dark urine. Explain why.

  1. The runner loses a lot of water in sweat, so the blood becomes too concentrated.
  2. The brain detects this, and the pituitary gland releases more ADH.
  3. ADH makes the collecting ducts more permeable, so more water is reabsorbed into the blood.
  4. So a small volume of concentrated urine is made.

Answer: water was lost in sweat, so the blood became concentrated; more ADH was released, so more water was reabsorbed in the collecting ducts