Urine Formation

Urine Formation

Excretion of urine involves the following processes:
1. Filtration of fluid from glomerular capillaries to renal tubule called glomerular filtration.
2. As glomerular filtrate flows along renal tubule, volume of filtrate is reduced and its component is altered by:
A. Movement of fluid and solute from tubule back to blood.
B. Tubular secretion: movement in opposite direction.
Glomerular filtration:
Glomerular filtrate is an ultrafiltrate of plasma with comp. identical to plasma with almost no protein > (0.03).
GFR: volume of fluid filter from glomeruli to the Bowmans capsule/ time (125 ml/min.) (180 L/day).
Fraction of renal plasma flow that is filtered is 0.2. about (20%) of plasma flowing through kidney is filtered by glomerular capillaries.


The GFR is determined by:
1. Net filtration pressure: Sum of hydrostatic colloid osmotic forces act on capillary membrane.
2. KF (capillary filtration coefficient) determined by permeability and filtering surface area of capillary 12.5 ml/min./mmHg.
GFR = NFR × KF.
Net filtration pressure (= NFR).
1. Hydrostatic pressure inside glomerular capillaries is 45 mmHg and promote filtration.
2. Hydrostatic pressure in Bowman’s capsule is 10 mmHg and opposes filtration.
3. Colloid osmotic pressure of glomerular capillaries unfiltered plasma protein = 25 mmHg.
4. Colloid osmotic pressure of protein in Bowman’s capsule near zero. No effect.
Effective EFP = 45 – 25 – 10
= 10 mmHg × 12.5 ml/min/mmHg
GFR = 125 ml/min.