Excretion and the kidneyIB MYP Biology: Revision notes

Excretion is the removal of the waste products of metabolic reactions from the body. These wastes would be harmful if they built up.

Three organs excrete wastes:

• Lungs: carbon dioxide (and water vapour) from respiration, which leave in the breath.
• Kidneys: urea, extra water and salts, which leave in urine.
• Skin: sweat, which contains water, salts and a small amount of urea.

Our bodies cannot store amino acids. Excess amino acids are broken down in the liver, and the nitrogen-containing part is made into urea.

Urea is poisonous if it builds up. It dissolves in the blood plasma and is carried to the kidneys, which remove it from the blood.

So: the liver makes urea, and the kidneys remove it.

The kidneys are a pair of organs that clean the blood. At a simple level:

1. Blood flows into the kidney, where a filter lets small molecules (water, urea, salts, glucose) out of the blood. Large molecules such as proteins and blood cells stay in the blood.
2. All the glucose and as much water and salts as the body needs are taken back into the blood (reabsorbed).
3. What remains, urea, extra water and extra salts, is urine. It flows to the bladder, where it is stored until it is passed out.

Healthy urine therefore contains urea, water and salts, but no protein or glucose.

The body must keep the amount of water in the blood steady. This is water balance: the water we gain (from drinks and food) must match the water we lose (in urine, sweat and breath).

• Hot day or exercise: we sweat more, so the kidneys reabsorb more water. We produce a small volume of dark, concentrated urine.
• Drinking a lot of water: the kidneys reabsorb less water. We produce a large volume of pale, dilute urine.

Worked example: a runner loses 1.5 dm³ in sweat. Her blood has less water, so her kidneys save water, and her urine is dark and small in volume.