If sodium concentration is too high in the filtrate, this implies that GFR is too ______ and causes the ______ of the afferent arteriole.[Fill in the blank]

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BlackTom AI · Accepted Answer

Let’s break down what the question is asking and evaluate each choice in turn, focusing on how tubular sodium concentration relates to glomerular filtration rate (GFR) and the reflexes of the afferent arteriole.
Option a: Slow, dilation. If the filtrate’s sodium concentration is high, that suggests the kidneys are not reabsorbing enough sodium, which would be consistent with a high GFR rather than a slow one. A slow GFR would typically reduce sodium delivery to the distal tubule, not increase it. Additionally, dilation of the afferent arteriole would increase GFR, not align with the scenario where high Na+ in filtrate implies something opposite. Therefore this option mismatches both the GFR direction and the expected vascular response.
Option b: Slow, constriction. A slow GFR would typically reduce the amount of filtrate and sodium delivered downstream, not produce high sodium concentration in the filtrate. Constriction of the afferent arteriole would decrease GFR further, which again does not fit the situation of high filtrate sodium implying a too-fast GFR that triggers a protective response. Thus this combination is not consistent with the physiological sequence described.
Option c: Fast, dilation. If GFR were fast, more filtrate is produced and more sodium tends to remain in the filtrate before full reabsorption, but the expected reflex to a high Na+ load in the filtrate is not dilation of the afferent arteriole. Dilation would tend to increase GFR further, creating a positive feedback rather than the protective mechanism. This makes the option biologically unlikely in this context.
Option d: Fast, constriction. A high sodium concentration in the filtrate indicates that the nephron is filtering rapidly (high GFR), delivering more Na+ downstream than can be reabsorbed efficiently. The kidney must reduce GFR to protect the filtration barrier, which is achieved by constricting the afferent arteriole. This matches the tubuloglomerular feedback concept: increased NaCl delivery to the macula densa signals for afferent arteriolar constriction to lower GFR. Therefore, this option correctly ties a high filtrate Na+ to a too-fast GFR and to constriction of the afferent arteriole.
In summary, each option was evaluated against the physiological logic of GFR, tubular reabsorption, and the efferent/afferent arteriolar responses. The only combination that aligns with the regulatory mechanism described is the fast GFR paired with afferent arteriolar constriction, making option d the correct physiological pairing for this scenario.

If sodium concentration is too high in the filtrate, this implies that GFR is too and causes the of the afferent arteriole.[Fill in the blank]

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