Ok so the point that is confusing me, which neither Kaplan nor EK went into detail about, is GFR. Can you explain what happens with decreased gfr step by step? All I know is that you get less urine output and more reabsorption.
With a damaged kidney, as it is in this case, how does a decreased gfr translate into fluid accumulating in the interstitium.
Is it because solute concentrations in the interstitium increase relative to the capillary, thereby increasing the fluid left in the interstitium? This doesn't really make sense though, since the capillaries would have a higher oncotic pressure.
Or is it because, like sodium, when urea is getting reabsorbed in larger quantities, water follows it. And if water is following the urea then the hydrostatic pressure (due to blood pressure) will increase? This would make sense to me in the context of an edema
You might be overthinking this a bit, but I'll try to explain from what I can remember, bear in mind I am a paramedic.
The key to this passage is that the patient requires dialysis, most likely indicating CKD5, which approximates to a <15 ml/min/1.73^2 GFR. This results in very little water/sodium/other electrolytes etc. being absorbed or filtered into bowmans capsule, which in turn means the water etc. continues into the renal vein and remains in circulation. While it is true that your absolute solute concentrations are increasing in the intravascular space, you are also retaining a tremendous amount of water, keeping the relative concentrations of solute about the same. With fluid overload, your vascular system starts to get leaky, its like a cup that has been overfilled and now fluid is spilling over the brim. It has to go somewhere, hence the third-spacing, generally this means the feet and ankles start to swell first due to gravity (pitting edema is actually kind of interesting to see first hand), if clearance still isn't happening, this worsens, and eventually you start to see more fluid shift in other areas of the body, eventually leading to pulmonary edema, especially if the patient is lying supine. It isn't that the water is following the urea or sodium, but rather, the water just never gets cleared in the first place and then has to go somewhere.
Someone more versed in patho could probably give you a more scientific answer, but I hope this helps.
