Which of the following organs regulates the levels of water, salts , hydrogen ions and urea in the mammalian blood?

Step 1: Analyze the question The question asks which organ controls the levels of water, salts, hydrogen ions (pH) and urea in mammalian blood. This is the process of osmoregulation and excretion, which also maintains homeostasis.

Step 2: Explain the concept The kidney is the main organ of osmoregulation and excretion in mammals. Each kidney contains millions of nephrons that do three things:

• Ultrafiltration at the Bowman's capsule: water, salts, glucose, urea and other small molecules leave the blood.
• Selective reabsorption: useful substances (all the glucose, most of the water and salts) return to the blood.
• Secretion and excretion: excess H⁺ ions, excess salts and urea are removed in the urine.

The kidney therefore adjusts the water content (with the help of ADH), the salt balance, the blood pH (by excreting H⁺) and the urea level.

Step 3: Check each option

• A. Liver: It forms urea from excess amino acids (deamination), but it does not regulate the level of urea in the blood by removing it. It does not control water or salts either.
• B. Kidney: It regulates all four: water, salts, H⁺ and urea.
• C. Bladder: It only stores urine temporarily.
• D. Colon: It reabsorbs water from undigested food, but it does not regulate blood urea, salts or pH.

Step 4: Answer The correct option is B. Kidney.

Exam tip: Remember the difference between the two organs:

• Liver = makes urea
• Kidney = removes urea and balances water, salts and pH

Whenever you see "regulates water, salts, pH" in a question, think kidney.