Which organ in the human body primarily filters blood and removes wastes and excess water?

Introduction / Context:
The human body constantly produces metabolic wastes that must be removed to maintain a stable internal environment. Several organs play roles in processing blood, but one organ system is primarily responsible for filtering blood to form urine and regulate fluid and electrolyte balance. This question tests whether you can identify that main blood filtering organ.

Given Data / Assumptions:

• The options include lungs, heart, liver, and kidneys.
• The word filter here refers to removing wastes and excess water from the bloodstream.
• We assume normal, healthy human physiology.

Concept / Approach:
The kidneys, part of the urinary system, receive a large portion of the cardiac output and filter blood through millions of nephrons. They remove urea, creatinine, and other wastes, regulate water and salt balance, and help maintain acid base balance. The liver also processes toxins and metabolites but is not the primary organ that produces urine by filtering blood through nephrons. The heart pumps blood and the lungs handle gas exchange, not waste filtration in the same sense. The correct choice must reflect the organ with specialized blood filtering units.

Step-by-Step Solution:
Step 1: Evaluate option A, lungs. Lungs exchange oxygen and carbon dioxide between blood and air; they do not filter metabolic wastes into urine.Step 2: Evaluate option B, heart. The heart pumps blood around the body but does not filter wastes from the blood.Step 3: Evaluate option C, liver. The liver detoxifies many substances and processes nutrients, but it does not form urine and is not the primary blood filter in the context of excreting wastes and water.Step 4: Evaluate option D, kidneys. The kidneys contain nephrons that filter blood plasma, reabsorb needed substances, and excrete waste as urine, matching the description in the question.Step 5: Conclude that the kidneys are the primary blood filtering organs, so option D is correct.

Verification / Alternative check:
A quick verification method is to think about which organ fails in kidney disease. When kidneys fail, toxins and fluid build up in the body, and dialysis machines are used to artificially filter the blood. This confirms that the kidneys are the organs that normally perform this filtering function.

Why Other Options Are Wrong:
Option A is wrong because lungs manage gas exchange rather than excreting nitrogenous wastes or controlling water balance. Option B is wrong because the heart moves blood but does not selectively remove wastes from it. Option C is wrong because, although the liver detoxifies and processes chemicals, it does not filter blood into urine in the way the kidneys do.

Common Pitfalls:
Students sometimes confuse the liver and kidneys because both deal with toxins and blood. Remember that the liver chemically modifies substances and produces bile, whereas the kidneys physically filter blood through nephrons and produce urine. Keeping these roles distinct will help you answer similar questions quickly.

Final Answer:
Kidneys.