Excretory System Essentials: 20 High-Yield Q&As for Physiology NEET PG 2025

Q1. What is the Oxygen extraction ratio?

Ans. The ratio between O2 consumption and supply is known as the oxygen extraction ratio. In the case of cortical nephron, oxygen consumption and supply both are higher but O2 supply is

more than that of oxygen consumption.

Q2. Explain the Counter-current system of the kidney? 

Ans. It is present in juxta-medullary nephrons and consists of two parts: countercurrent multiplier and

Exchanger. Countercurrent multiplication occurs at the level of the loop of Henle of juxta-medullary nephrons. Counter-current exchange occurs at the level of the peritubular capillary loop of the juxtamedullary loop. It is also called the vasa recta.

Q3. Explain the Mechanism of Autoregulation?

Ans. There are 2 mechanisms

Myogenic mechanism: It is mediated through calcium. It occurs at the level of smooth muscles of afferent arterioles. ↑BP causes the individual smooth muscle present on afferent arteriole to stretch. Smooth muscle membrane contains stretch-sensitive Ca++ channels that open because of mechanical stretch. When calcium reaches the cytoplasm of cells, there is contraction of smooth muscles leading to constriction of afferent arterioles.

Q4. What is the site of the mechanism of action of furosemide?

Ans. The site of the mechanism of action is TAL. Because it is the TAL segment of the Nephron which has the NKCC transporter.

Q5. Macula densa cell is in which part of the Nephron?

Ans. Macula densa cell is a part of the TAL segment of the Nephron.

Q6. What is the name of the universal transporter?

Ans. The universal transporter is sodium and potassium ATPase on the cell's basolateral membrane.

Q7. Maximum reabsorption of Mg++ occurs at

Ans. TAL through passive paracellular reabsorption.

Q8. Maximum reabsorption of Ca++ occurs at 

Ans. PCT

Q9. Magnesium is reabsorbed passively through 

Ans. Paracellular pathway.

Q10. Magnesium is absorbed actively through 

Ans. TRPM 6/7 channel that uses a transcellular route.

Q11. A person is taking potassium-containing salt in large amounts in his diet. What is the source of urinary potassium in this patient?

Ans. Source is secretion. It is important to remember that potassium is the only electrolyte that can be secreted as well as reabsorbed into the body. The portion from which it is getting secreted is the latter part of the DCT and the cortical collecting duct. The same part can also be reabsorbed.

Q12. What is the amount of tonicity contributed by sodium chloride and urea?

Ans. 50-60% of tonicity is contributed by sodium chloride and urea contributes 40-50% of tonicity.

Q13. What is the maximum tonicity developed in the medullary interstitium?

Ans. The maximum tonicity that can develop upon medullary interstitium taken in general is 1200-1400 mOsm/L. Therefore, it can be assumed that sodium chloride provides 600 mOsm and urea gives another 600 mOsm. Sodium provides 300 mOsm and chloride gives another 300 mOsm. Hence, urea is the single most important substance responsible for medullary hypertonicity.

Q14. Which among the following is the most important substance for medullary hypertonicity?

A. NaCl

B. Urea

Ans. As NaCl contributes 50-60% tonicity, it is the most important substance for medullary hypertonicity.

Q15 . Which among the following is the most important substance for medullary hypertonicity?

A. Na

B. Cl

C. Urea

Ans. Urea is the most important substance for medullary hypertonicity.

Q16. Which segment of the nephron plays the least important role in the countercurrent multiplier system

Ans. ATS

MCQs 

Q17. Normal range of urine osmolality (mosm/l):

A. 50-1400

B. 100-1400

C. 200-1000

D. 300-1200

Ans. 50-1400

Q18. The prime driving force for counter current multiplier system is:

A. Reabsorption of Na+ in thick ascending limb

B. Action of ADH via aquaporin channels

C. Urea recycling

D. Medullary hyperosmolarity

Ans. Reabsorption of Na+ in thick ascending limb

Q19. Maximum osmotic gradient is found in:

A. Outer medulla

B. Inner medulla

C. Outer cortex

D. Inner cortex

Ans. Inner medulla

Q20. Which of the following is not a component of the counter current multiplier mechanism?

A. Thick ascending loop of Henle

B. Collecting duct

C. Vasa recta

D. Thin descending loop of Henle

Ans.  Vasa recta

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