Principal cells in the collecting ducts are responsible for the regulation of
Sodium and potassium
Glucose and urea
Bicarbonate and chloride
Calcium and phosphate
The Correct Answer is A
A. Sodium and potassium: These cells represent the primary site for aldosterone-mediated electrolyte regulation in the distal nephron. They utilize apical sodium channels and potassium channels to facilitate sodium reabsorption and potassium secretion. This mechanism is critical for maintaining systemic fluid balance and normokalemia.
B. Glucose and urea: Glucose reabsorption occurs almost exclusively in the proximal convoluted tubule via specialized sodium-glucose cotransporters. While the medullary collecting ducts are permeable to urea under certain conditions, principal cells do not regulate its transport. Their metabolic machinery is specialized for ion and water homeostasis.
C. Bicarbonate and chloride: Acid-base balance and bicarbonate transport are the primary functions of intercalated cells, which are distinct from principal cells. While chloride often follows sodium passively, principal cells do not actively regulate its concentrations. Intercalated cells manage the secretion of hydrogen and bicarbonate ions.
D. Calcium and phosphate: The regulation of these minerals occurs primarily in the proximal tubule and the distal convoluted tubule under parathyroid hormone influence. Principal cells lack the specific receptors and transporters required for significant calcium or phosphate handling. Their role is restricted to water and monovalent cation transport.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is D
Explanation
A. Due to water secretion from the collecting ducts: Water is reabsorbed from the collecting ducts into the interstitium, which would actually dilute the medullary gradient if not for countercurrent mechanisms. Water is never actively secreted into the medullary space. Such a process would prevent urine concentration.
B. Due to protein digestion by nephron enzymes: The medullary gradient is established by inorganic ions and small organic molecules, not protein catabolism. Proteins are largely excluded from the filtrate. The energy for the hypertonic state comes from active transport of sodium in the thick ascending limb.
C. Due to passive filtration of all solutes: Filtration is a cortical process occurring in the glomeruli. The medullary gradient requires active energy expenditure to move solutes against a concentration gradient. Passive filtration alone would result in an isotonic interstitium similar to plasma concentrations.
D. Due to ion reabsorption in the ascending limb and urea recycling: The NKCC2 transporters in the thick ascending limb move solutes into the interstitium while excluding water. This, combined with the recycling of urea from the collecting ducts, creates a high osmotic pressure. This gradient is essential for water recovery.
Correct Answer is B
Explanation
A. It inhibits bladder contraction during filling: Inhibition of the voiding reflex is managed by the storage center and higher cortical structures. The pontine micturition center (PMC) acts as the "on switch" for urination. It receives signals to initiate the voiding cycle once a threshold is reached.
B. It coordinates relaxation of the urethral sphincter and contraction of the detrusor muscle during voiding: The PMC, or Barrington’s nucleus, acts as the primary relay for the micturition reflex. It sends descending signals that simultaneously activate parasympathetic outflow and inhibit sympathetic/somatic tone. This ensures the bladder contracts while the outflow tract remains open.
C. It stimulates sympathetic activity to keep the bladder relaxed: Sympathetic dominance is a feature of the storage phase, controlled by the pontine storage center and spinal centers. The PMC is specifically designed to suppress sympathetic activity to allow micturition. It opposes the storage mechanisms to facilitate bladder emptying.
D. It regulates sodium reabsorption in the kidney: The PMC is a neurological control center in the brainstem dedicated to the lower urinary tract. It has no direct involvement in the biochemical or hormonal processes of the kidney. Sodium regulation is managed by the JGA and endocrine axes.
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