Fluid Physiology

4.3 Ultrafiltration in the Glomerulus

The situation in the glomerular capillaries is quite remarkable. In the rest of the body, the net excess of ultrafiltration over reabsorption is of the order of two to four liters a day. The net excess in the glomerular capillaries is known as the glomerular filtration rate (GFR) and is 180 litres/day.

The situation in the glomerulus
The filtration coefficient is high (mostly because of a high permeability but also because of a large surface area)
The reflection coefficient is high: about 1.0 (i.e. the filtrate is a true ultrafiltrate as the glomerular capillaries are essentially impermeable to protein (so oncotic pressure in the filtrate is zero)
The hydrostatic pressure in the capillaries is high and does not decrease much along the length of the capillary
Because of the large loss of fluid and the impermeability to protein, the oncotic pressure in the glomerular capillary increases along its length. (This increased oncotic pressure in important in the reabsorption from the proximal tubule into the peritubular capillaries)
There is a net outward filtration pressure often along the whole length of the capillary.

 

Typical values of Starling Forces in Glomerular Capillaries (mmHg)
  Aff. Arteriolar End Eff. Arteriolar End
Hydrostatic pressure in capillary (HPGC)  60  58
Hydrostatic pressure in Bowmanís capsule (HPBC) 15 15
Oncotic pressure in capillary (OPGC) 21 33
Oncotic pressure in Bowmanís capsule (OPBC) 0 0
Net Filtration Pressure 24 10

The flux equation discussed earlier simplifies to just 4 terms:

GFR = Kf x (HPGC - HPBC - OPGC)

The term that varies along the length of the glomerular capillary is OPGC. This is a quite a different situation to what occurs in most tissue capillary beds, where the change that occurs along the length of the capillary is a decrease in capillary hydrostatic pressure. The glomerulus is different because of the very large fluid loss that occurs.

The hydrostatic pressure in the glomerular capillaries is affected by the balance between afferent and efferent arteriolar constriction.