The followings are the functions of the kidneys, except

Analyze the question: We need the one option that is NOT a function of the kidneys.

Concept: The kidneys are the main excretory and osmoregulatory organs. Their key functions are:

• Removing nitrogenous wastes (urea, uric acid, creatinine) from the blood
• Osmoregulation, which means controlling the water content and salt balance of the blood
• Helping maintain blood pH by excreting H⁺ ions and reabsorbing bicarbonate
• Maintaining homeostasis in general (and producing hormones such as erythropoietin)

Carbon dioxide is different. It is produced during cellular respiration, carried in the blood, and excreted mainly by the lungs. The skin is not a major route for it either.

Step-by-step check of the options:

• A. Regulation of water content in the blood. This is a kidney function (osmoregulation, controlled by ADH). ✔ Not the answer.
• B. Maintenance of blood pH and homeostasis. This is a kidney function. ✔ Not the answer.
• C. Excretion of CO₂. This is done by the lungs, not the kidneys. ✘ This is the exception.
• D. Removal of waste products from the blood. This is a kidney function (urea and others). ✔ Not the answer.

Correct answer: C

Exam tip: Link each organ to its main excretory product:

• Kidneys → urea, excess water and salts
• Lungs → CO₂ and water vapour
• Skin → sweat (water, salts, a little urea)
• Liver → bile pigments (and it makes urea)

When you see "except" questions, test each option on its own and eliminate the true statements.