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 liters/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 (ie 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 & the impermeability to protein, the oncotic pressure in the 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 (PGC)

 60

 58

Hydrostatic pr. in Bowman’s capsule (PBC)

15

15

Oncotic pressure in capillary (pGC)

21

33

Oncotic pressure in Bowman’s capsule (pBC)

0

0

Net Filtration Pressure

24

10

The flux equation discussed earlier simplifies to the following:

GFR = Kf x (PGC - PBC - p GC)

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

  

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