Question

How do the Starling forces affect filtration or reabsorption?

A) Colloid osmotic pressure causes osmosis due to large protein present in the plasma that cannot cross the capillary membrane. This force draws fluid back into the capillary.

B) Osmotic force due to interstitial-fluid protein concentration occurs when there is a large concentration of Na+ in the interstitial space, as is the case in the nephron.

C) Capillary hydrostatic pressure causes filtration to occur across the entire length of the capillary.

D) Interstitial-fluid hydrostatic pressure is the pressure created when the fluid in the blood pushes against the inside of the capillary, causing increased reabsorption.

Answer 1

The rate of filtration / reabsorption at any point along a capillary depends on a balance of forces called "Starling forces" :

Fluid Movement = k {(Pc - Pi) - (Tc - Ti)}

where, k = Capillary filtration coefficient

Pc = Capillary hydrostatic pressure

Pi = Interstitial hydrostatic pressure

Tc = Capillary colloid osmotic pressure

Ti = Interstitial colloid osmotic pressure

(Pc - Pi) = Hydrostatic pressure gradient

(Tc - Ti) = Osmotic pressure

From this equation, filtration occurs when the value is +ve and absorption occurs when the value is -ve.

From this information we can conclude that options A, B and C are correct. Interstitial fluid hydrostatic pressure is the pressure created when the fluid in the interstitium pushes against the outside of the capillary, causing increased absorption into the capillary.