Which organ removes the largest quantity of water from the blood?

1. Analyze the question The question asks which organ removes the largest quantity of water from the blood. This is about excretion and osmoregulation, meaning how the body gets rid of water and waste.

2. Explain the concept Water leaves the body through four main routes:

• Lungs: water vapour in exhaled air (about 300-400 ml/day)
• Skin: sweat (about 400-500 ml/day normally, though it can rise sharply in hot weather or during exercise)
• Intestine: water in faeces (about 100-200 ml/day)
• Kidney: water in urine (about 1,000-1,500 ml/day)

The kidneys filter the blood and produce urine. They also control water balance: when you drink a lot of water, they excrete more, and when you are dehydrated, they conserve it (under the influence of the hormone ADH).

3. Step-by-step solution

• Step 1: Compare the usual daily volume of water removed by each organ.
• Step 2: The lungs, skin and intestine each remove a few hundred millilitres under normal conditions.
• Step 3: The kidneys remove the most, about 1-1.5 litres per day as urine, by filtering blood in the nephrons.
• Step 4: Therefore, the kidney removes the largest quantity of water from the blood.

4. Answer The correct option is C. Kidney.

5. Exam tip Remember: "Kidney = main water regulator." If a question asks about the major organ of water loss or osmoregulation, think kidney first. Skin only overtakes it in unusual conditions such as heavy sweating, and the lungs and intestine are minor routes for water loss.