exist in the kidney
s, where they are surrounded by Bowman's Capsule
. They are very tightly packed
, and are under high pressure.
Only about 10 percent of the glomerular capillary is permeable to plasma --- the rest is not used for filtration. The average adult has about the equivalent of 10 US-letter sized sheets of paper worth of glomerular capillary surface area, but only 1 of those sheets would be the area used for filtration.
Blood is fed into the glomerular capillary by an afferent arteriole, and drained by an efferent arteriole. Note that the draining vasculature is not referred to as a venule. This is mostly because the kidney only filters the blood, and does not consume much oxygen or dump waste products into the blood like the rest of the body.
Note that unless the pressure in the glomerular capillary is maintained higher than that in Bowman's Capsule, filtration will not occur. The kidney is part of several systems that maintain the blood pressure in order to ensure that the glomerular capillaries have a high enough pressure supplied. In some cases, this can lead to hypertension in the rest of the body.
These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano)