In the human body, which of the following is the main function performed by the kidneys?

Introduction / Context:
The human body has specialized organs for digestion, respiration, circulation, and excretion. The kidneys are part of the excretory system and have very specific roles that are often tested in exams. This question asks you to identify the main function of the kidneys among the given options.

Given Data / Assumptions:

• The options describe excretion, respiration, digestion, and transportation.
• We assume normal anatomical positions and functions of human organs.
• The focus is on the principal role of kidneys, not on minor or indirect effects.

Concept / Approach:
Kidneys filter blood to remove metabolic waste products such as urea, creatinine, and excess ions. They form urine, regulate water and electrolyte balance, and help maintain acid base balance and blood pressure. Respiration is primarily the function of lungs. Digestion is mainly performed by organs of the digestive system such as stomach and intestines. Transportation of substances around the body is the responsibility of the circulatory system, especially the heart and blood vessels. Therefore, the correct answer must highlight excretion and fluid regulation as kidney functions.

Step-by-Step Solution:
Step 1: Evaluate option A, excretion of metabolic wastes and regulation of fluid balance. This accurately describes the kidneys’ role in filtering blood and forming urine.Step 2: Evaluate option B, respiration. This is the job of lungs, which handle gas exchange, not kidneys.Step 3: Evaluate option C, digestion. The digestive system, especially the stomach and intestines, breaks down food, not the kidneys.Step 4: Evaluate option D, transportation. Transport of oxygen and nutrients is mainly done by the heart and blood vessels, not by kidneys.Step 5: Conclude that option A is the only correct description of kidney function.

Verification / Alternative check:
To verify, think of what happens in kidney failure: waste products build up in the blood, fluid balance is disturbed, and dialysis is required to artificially remove wastes and excess water. These clinical facts underline that excretion and fluid regulation are primary kidney functions. No similar role in digestion or respiration is attributed to kidneys.

Why Other Options Are Wrong:
Option B is wrong because lungs are the organs responsible for gas exchange. Option C is wrong because digestion involves the breakdown of food in the gastrointestinal tract, not in the kidneys. Option D is wrong because, while kidneys influence blood composition, they are not the main organs responsible for moving blood or substances around the body; that is the role of the heart and circulatory system.

Common Pitfalls:
Students sometimes confuse excretion with elimination of undigested food. Remember that excretion refers to removal of metabolic wastes produced inside the body, primarily through urine, sweat, and exhaled air, while egestion refers to removal of undigested food via the digestive tract. Keeping these terms clear will help you correctly assign functions to organs like kidneys, lungs, and intestines.

Final Answer:
Excretion of metabolic wastes and regulation of fluid balance.