Renal System and Osmoregulation (3B) - Bio & Biochem

1. State the formula for renal clearance of a substance $x$.

   $C_x=\frac{U_x\times V}{P_x}$. Calculates volume of plasma cleared of $x$ per unit time, equaling GFR for freely filtered, non-reabsorbed/secreted substances.

2. What is the filtration barrier of the glomerulus composed of?

   Fenestrated endothelium, GBM, podocyte slit diaphragms. Barrier selectively filters blood by size and charge, preventing large proteins from entering Bowman's space.

3. What is the key mechanism that creates the corticomedullary osmotic gradient?

   Countercurrent multiplication in the loop of Henle. Differential permeability and active transport in loop segments amplify osmotic gradient for urine concentration.

4. Which segment actively reabsorbs Na$^+$, K$^+$, and Cl$^-$ and is impermeable to water?

   Thick ascending limb (NKCC2); diluting segment. Actively transports ions out via NKCC2, creating dilute filtrate and contributing to medullary gradient.

5. Identify the cell type in the collecting duct that secretes H$^+$ to acidify urine.

   Type A (alpha) intercalated cell. Uses H$^+$-ATPase to excrete acid, maintaining blood pH during acidosis.

6. Which blood vessel directly drains blood away from the glomerulus?

   Efferent arteriole. Carries filtered blood from glomerulus to peritubular capillaries for reabsorption and secretion.

7. Which hormone increases water permeability in the collecting duct via aquaporin-2 insertion?

   ADH (vasopressin). Binds V2 receptors to insert aquaporins, allowing water reabsorption down osmotic gradient.

8. Identify the cell type in the collecting duct that secretes HCO$_3^-$ during alkalosis.

   Type B (beta) intercalated cell. Secretes bicarbonate via pendrin exchanger to correct alkalosis and reclaim H$^+$.

9. What is the relationship between plasma osmolarity and ADH release?

   Increased plasma osmolarity increases ADH release. Hypothalamic osmoreceptors detect high osmolarity, stimulating posterior pituitary to release ADH for water retention.

10. Which nephron type (cortical or juxtamedullary) is most important for concentrating urine?

    Juxtamedullary nephrons. Long loops extend deep into medulla, enabling greater water reabsorption via countercurrent system.

11. What is the macula densa and what does it sense?

    DCT cells sensing tubular NaCl delivery (flow-related). Specialized cells at DCT end detect NaCl levels to regulate GFR via tubuloglomerular feedback.

12. What happens to glucose in the nephron under normal plasma glucose levels?

    Completely reabsorbed in the PCT (below transport maximum). Glucose is actively reabsorbed via SGLT transporters in PCT until saturation, preventing glucosuria.

13. Which enzyme produced by juxtaglomerular cells initiates the RAAS cascade?

    Renin. Released in response to low BP or Na, cleaves angiotensinogen to activate RAAS for homeostasis.

14. Which blood vessel directly delivers blood into the glomerular capillaries?

    Afferent arteriole. Delivers oxygenated blood under pressure to glomerulus for filtration into Bowman's capsule.

15. Which conclusion follows if $C_x>\text{GFR}$ for a solute $x$?

    Net secretion of $x$. Indicates tubular secretion adds more $x$ to urine beyond filtered amount.

16. What is the primary role of the vasa recta in the kidney medulla?

    Countercurrent exchange to preserve the medullary gradient. Prevents dissipation of hyperosmotic medullary interstitium by equilibrating with surrounding fluid.

17. What is the main function of the distal convoluted tubule (early DCT)?

    NaCl reabsorption (NCC) with low water permeability. Further dilutes urine by reabsorbing NaCl without water, regulated by thiazide-sensitive cotransporter.

18. Which segment is permeable to water but not to NaCl: thin descending limb or thick ascending limb?

    Thin descending limb. Allows water efflux into hyperosmotic medulla, concentrating filtrate without solute movement.

19. Which hormone increases Na$^+$ reabsorption and K$^+$ secretion in principal cells?

    Aldosterone. Stimulates ENaC and Na/K-ATPase for Na$^+$ retention and ROMK for K$^+$ excretion in late DCT/CD.

20. What fraction of filtered Na$^+$ and water is reabsorbed in the PCT (approximate)?

    About $65\%$. PCT reabsorbs majority of filtered load iso-osmotically via active transport and osmosis.

21. What are the three major processes that determine urine composition in the nephron?

    Filtration, reabsorption, secretion. These processes filter plasma at glomerulus, reclaim essentials along nephron, and add wastes to form concentrated urine.

22. What is the correct order of filtrate flow from Bowman's space to the renal pelvis?

    PCT → loop of Henle → DCT → collecting duct → papillary duct → calyx → pelvis. Filtrate progresses from proximal tubule through loop for concentration, distal tubule for fine-tuning, and collecting system for final urine drainage.

23. What is the main function of the proximal convoluted tubule (PCT)?

    Bulk reabsorption of solute and water; secretion of some wastes. Performs iso-osmotic reabsorption of most filtered solutes and water while secreting H$^+$ and organics.

24. Which conclusion follows if $C_x<\text{GFR}$ for a solute $x$?

    Net reabsorption of $x$. Indicates tubular reabsorption removes some filtered $x$, reducing excretion below filtered load.

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