Please explain Starling Forces as it relates to filtration in the kidney.?

Question

Does this have to do with oncotic and hydrostatic pressure around the bowman's capsule? If so, what exactly is the oncotic and hydrostatic pressures?

Answer 1

Oncotic pressure is the osmotic force exerted by substances in fluid.  The oncotic pressure of the plasma is greater in the capillaries than in the interstitial fluid.  This is primarily due to plasma proteins that do not leave the plasma.

This would tend to push fluid from the capsule into the capillaries. But....

The hydrostatic pressure in the capillary is very high and overcomes the oncotic pressure.  This is because the renal artery which has high pressure passes through a short afferent arteriole into the glomerulus in addition to the fact that the efferent arteriole has a smaller diameter than the afferent arteriole.

So the blood "slams" into the glomerulus with a high hydrostatic pressure driving net fluid movement into bowman's capsule.

What results is a process called ultrafiltration.  

This is also due to the fenestrated endothelium in the capillaries here.  They are very leaky (gaps between cells).