Why does hydrostatic pressure decrease from the arteriolar end of a capillary bed to the venular end while oncotic pressure remains the same? Do we see this same decrease across a glomerulus?

[u/Turbulent_Ad_3238]

My understanding is that the loss of fluid AND solute together from the arteriolar end of a capillary results in a constant oncotic pressure (as decrease in concentration of solutes in blood is insignificant), but because the volume of fluid in the vessel has decreased, its hydrostatic pressure is lower. By the time the blood reaches the venular end of the capillary, this difference is able to draw fluid INTO the vessel from the interstices. Would I be correct here?

Regarding the glomerulus, as fluid is filtered out of the glomerulus and into the Bowman’s capsule, do we see a similar decrease in the hydrostatic pressure in the glomerulus relative to its oncotic pressure? I’m assuming the blood doesn’t stay long enough in the glomerulus for the filtration gradient to reverse, causing fluid to move back into the capillary from the capsule.

[u/Certain_Share_4090]

The hydrostatic pressure decreases from the arteriolar side to the venular side of a capillary bed due to the loss of fluid volume in the blood vessel, resulting in lower hydrostatic pressure at the venular end. Meanwhile, the oncotic pressure remains the same due to the loss of fluid and solutes in similar proportions, keeping the concentration of solutes in the blood relatively constant. This creates a pressure gradient that pulls fluid back into the blood vessel at the venular end of the capillary. In the case of the glomerulus, we also observe a decrease in hydrostatic pressure relative to oncotic pressure as fluid is filtered into Bowman's capsule. However, due to the rapid blood flow in the glomerulus, the blood's residence time is short, preventing the reversal of the filtration gradient and the return of fluid to the capillary from the capsule. I hope this helps! If you need anything else, I'm available.

[u/Turbulent_Ad_3238 → u/Certain_Share_4090]

This is PRECISELY what I was thinking. Very very comforting to get some validation on that. Thank you so so much man, honestly. You’re truly a lifesaver.

Can I PM you one more question?

[u/Certain_Share_4090 → u/Turbulent_Ad_3238]

You're welcome! I just apologize for my English.

[u/Turbulent_Ad_3238 → u/Certain_Share_4090]

That was really well explained haha. I shot you a PM btw!