What does the term "transport maximum (Tm)" refer to?
The volume of urine produced per hour.
The pressure at which urine is excreted.
The amount of ATP needed for filtration.
The point at which increasing concentration does not increase transport rate.
The Correct Answer is D
A. The volume of urine produced per hour: This describes the rate of diuresis or urine flow rate, which is a measure of total renal output. It is influenced by GFR and tubular reabsorption but is not a fixed maximum for specific solute transport. It varies based on hydration and hormones.
B. The pressure at which urine is excreted: Excretion pressure involves the hydrostatic forces within the collecting system and the contraction of the detrusor muscle. This is a mechanical aspect of micturition and is unrelated to the biochemical saturation of tubular carrier proteins. It does not define transport limits.
C. The amount of ATP needed for filtration: Filtration is a passive process driven by blood pressure and does not directly consume ATP. While active transport requires energy, the term Tm specifically refers to the capacity of the transporters rather than the total energy expenditure of the kidney.
D. The point at which increasing concentration does not increase transport rate: This represents the saturation point of all available carrier proteins for a specific solute within the renal tubules. At this limit, the transport system is operating at its peak velocity. Any additional filtered load will exceed the Tm and bypass reabsorption.
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Related Questions
Correct Answer is D
Explanation
A. Water follows sodium, increasing blood volume: This describes the mechanism of sodium reabsorption, not the effect of Atrial Natriuretic Peptide (ANP). If water followed sodium back into the blood, volume would expand. ANP is secreted to correct fluid overload, not to promote it further through retention.
B. Blood pressure increases due to water retention: This statement describes the physiological response to the renin-angiotensin-aldosterone system or ADH. Retention of water increases systemic venous return and cardiac output, elevating blood pressure. ANP works as an antagonist to these systems to lower pressure.
C. Water is retained to maintain homeostasis: Homeostasis in a state of fluid excess requires the excretion of water, not its retention. If the body retained water when blood volume was already high, it would exacerbate cardiac strain. ANP ensures that excess fluid is removed to restore balance.
D. Water is excreted, reducing blood volume and pressure: ANP inhibits sodium reabsorption in the collecting ducts, causing sodium to remain in the filtrate. Due to osmotic gradients, water stays within the tubule and is excreted as urine. This diuresis effectively lowers systemic blood volume and arterial pressure.
Correct Answer is A
Explanation
A. True: Anatomically, the proximal convoluted tubule is significantly longer and more tortuous than the distal segment. This increased length provides a greater surface area for the massive reabsorption of filtrate components. The DCT is shorter and has a less developed brush border.
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