MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 3b Renal System Osmoregulation

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This deck focuses on 3b Renal System Osmoregulation, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

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

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

Flashcard 2: What is the filtration barrier of the glomerulus composed of?

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

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

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

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

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

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

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

Flashcard 6: Which blood vessel directly drains blood away from the glomerulus?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Flashcard 14: Which blood vessel directly delivers blood into the glomerular capillaries?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Answer: 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.

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

Answer: 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.

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

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