Of the approximately 180 L of fluid filtered by the kidneys each day, less than
The Correct Answer is {"dropdown-group-1":"B"}
A. 10%: If 10% of the 180 L filtrate became urine, a person would excrete 18 L per day. This volume would lead to rapid and fatal hypovolemic shock due to massive fluid loss. Renal physiology is designed to conserve the vast majority of filtered water and solutes.
B. 1%: Normal daily urine output averages 1 to 2 L, which represents approximately 0.5% to 1% of the total glomerular filtration rate. The kidneys reabsorb over 99% of the filtered load through the renal tubules. This efficiency is critical for maintaining systemic fluid and electrolyte homeostasis.
C. 2%: A 2% excretion rate would result in nearly 3.6 L of urine daily, which exceeds the typical physiological range for a healthy adult. While possible under high fluid intake, it is not the standard baseline for filtration efficiency. The kidney's reabsorptive capacity is generally more robust.
D. 5%: This figure suggests an excretion of 9 L daily, which is characteristic of pathological states like diabetes insipidus. In a healthy state, the tubular system prevents such excessive loss through active and passive reabsorption. The standard percentage of filtrate converted to urine remains much lower.
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Correct Answer is A
Explanation
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.
Correct Answer is A
Explanation
A. Glomerulus: This is the only site in the nephron where blood is filtered across a specialized capillary membrane. The high hydrostatic pressure forces water and small solutes into Bowman's space. All subsequent segments of the nephron are involved in modification rather than filtration.
B. Distal convoluted tubule: This segment is responsible for the regulated reabsorption of sodium and calcium and the secretion of potassium. It does not possess a filtration barrier or the necessary pressure gradients to filter blood. It processes filtrate that has already been significantly modified.
C. Collecting duct: The collecting duct is the final site for water reabsorption and acid-base regulation before urine enters the calyces. It is located deep within the medulla where filtration is impossible due to the absence of glomerular tufts. It serves a conducting and concentrating role.
D. Nephron loop: The loop of Henle is specialized for creating an osmotic gradient through countercurrent multiplication. It reabsorbs water in the descending limb and solutes in the ascending limb. It does not interface with the systemic arterial supply to perform filtration.
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