Kidney: Isotonic filtrate?

[Dirtybird]

Reviewing the kaplan book, they talk about how hydrostatic is higher in glomerulus but is opposed by oncotic pressure. Hydrostatic pressure wins out and net flow of fluid into forming the filtrate.

So they say the opposing oncotic pressure is due to the higher osmolarity of the blood compared to that of Bowman's space. Then the next paragraph they seem to directly contradict this and say that filtrate is isotonic to blood.

What am I missing here? How does it go from hypotonic to isotonic?

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[lpp06]

It should probably read more like "nearly isotonic". Protein makes up less than 5% of total blood volume and the filtrate will contain its own proteins (not from the blood), that will make up some of the difference in tonicity.

I wouldn't put too much thought into though - the main take aways are that 1) Glomerular Filtration is the result of hydrostatic pressure in the capillaries exceeding Colloid Osmotic Pressure and hydrostatic pressure of Bowman's Capsule and 2) the resulting filtrate that is passed onto the PCT is isomolar to plasma for ions, glucose, and other key molecules involved in secretion/absorption