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

Analyze the question: The question asks which organ regulates the levels of water, salts, hydrogen ions (pH), and urea in a mammal's body. This is the process of osmoregulation and excretion.

Explain the concept: The kidney is the main organ of homeostasis for the body's fluid and chemical balance. It filters the blood and adjusts what is removed and what is kept:

• Water: controlled by ADH (antidiuretic hormone), which changes how much water is reabsorbed in the collecting duct.
• Salts (ions): reabsorbed or excreted in the nephron, with aldosterone helping to control sodium.
• Hydrogen ions (pH): excreted in the urine or retained to keep blood pH stable.
• Urea: a nitrogenous waste filtered from the blood and excreted in the urine.

Step-by-step elimination:

• A. Kidney: regulates all four substances. ✔
• B. Colon: absorbs water from undigested food and forms faeces. It does not regulate salts, hydrogen ions, or urea in the blood.
• C. Bladder: only stores urine temporarily. It does no regulation.
• D. Liver: makes urea from excess amino acids (deamination), but it does not remove urea from the body or regulate the water, salt, and pH balance. So the liver produces urea, while the kidney regulates its level by excreting it.

Answer: The correct option is A. Kidney.

Exam tip: Remember "the Liver makes urea, the Kidney removes it." Whenever you see water, salts, pH, and urea regulation together in a question, think kidney. Keep going, you're doing well! 💪