Question
As the glomerular filtrate first enters the renal capsular (Bowman’s) space, what is its tonicity relative to blood plasma?
| A. |
Isotonic
|
| B. |
Hypotonic
|
| C. |
Hypertonic
|
| D. |
Hyposmotic
|
Show Answer
|
Correct Answer » A
Explanation
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|
Filtration at the glomerulus is a passive process driven by hydrostatic pressure. The filtration barrier allows:
-
Water
-
Electrolytes
-
Glucose
-
Amino acids
-
Urea
to pass freely.
However, it restricts:
-
Cells
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Large plasma proteins
Because small solutes pass freely in the same proportion as plasma water, the initial glomerular filtrate has nearly the same osmolarity as plasma.
Therefore, the filtrate in Bowman’s space is isotonic with plasma.
Isotonic means having the same osmotic concentration as plasma.
At the level of Bowman’s capsule:
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No selective reabsorption has occurred yet.
-
No active transport has modified the filtrate.
-
Filtration is size-selective but not concentration-selective for small solutes.
Thus, osmolarity remains approximately 300 mOsm/L, same as plasma.
