Biology Kidney Question

[vivatix]

I am confused on how to interpret this figure.

I know that lumen side inside some tubule in the nephron, the tubule is lines with the tubular cell and is surrounded by extracellular fluid. Where is the filtrate? I would guess that the filtrate is inside the lumen but since it is showing bicarbonate ions being sent out into the extracellular fluid space, I am confused.

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[popopopop]

If your blood is acidic, then the kidneys will attempt to absorb bicarbonate and excrete H+ into the filtrate (urine/tubule lumen/glomerular filtrate). Answer 'a' is probably right. I think a "trick" to this question is the naming of the urine/filtrate. Correct me if I'm wrong!