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In this diagram, D is apparently the point at which "the osmotic pressure into the capillaries is the greatest", but I cannot figure out why. Any help would be much appreciated!

enter image description here

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closed as off-topic by kmm, David, theforestecologist, Satwik Pasani, fileunderwater Apr 9 '18 at 10:41

This question appears to be off-topic. The users who voted to close gave this specific reason:

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    $\begingroup$ Homework questions are off-topic on Biology unless you have shown your attempt at an answer. For more information see our homework policy. $\endgroup$ – David Apr 2 '18 at 16:31
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Fluid movements across capillary wall is determined by 2 main factors.

  • Hydrostatic Pressure (Blood Pressure) - tends to push fluid out of the blood vessel
  • Osmotic Pressure - Tends to pull fluid back into the blood vessels mainly due to the presence of plasma proteins. (especially albumin)

Important points you should know

  • Hydrostatic pressure or blood pressure is the pressure exerted by blood on the capillary walls.

  • Osmotic pressure depends on the number of osmotically active, non diffusible particles in the solutions separated by the membrane.

  • The main substance responsible for the osmotic pressure between blood and tissue fluid are the plasma proteins. Especially albumin.

  • Plasma proteins are absent in tissue fluid.

Filtration of fluids across capillaries is described by Starling Forces. (Forces were introduced by an English physiologist Ernest Starling.)

There are four main forces

  1. Capillary Hydrostatic Pressure (Pc) - This forces fluid out through the capillary membrane.
  2. Interstitial Fluid Pressure (Pif) - Tends to force fluid from interstitial spaces across capillary membrane
  3. Capillary Plasma Colloid Osmotic Pressure (∏p) - Tends to cause osmosis of fluid inward through capillary membrane.
  4. Interstitial Fluid Colloid Osmotic Pressure (∏if) - Tends to cause osmosis of fluid outward through capillary membrane. -Negligible

enter image description here

According to Starling forces, Filtration = k (Pc - Pif) - (∏p - ∏if)

Filtration = k x Net Filtration pressure

Here k is the capillary filtration coefficient which is depend on permeability of capillary wall and surface area for filtration.

Look at the following diagram enter image description here

In your diagram A is the Arteriole end and D is the Venous end.

Now we can calculate Net Filtration Pressure at A and D.

At Arteriole end (A);

  • Outward forces are;

Capillary hydrostatic pressure (Pc) = 37 mm Hg

and

Interstitial Fluid Colloid Osmotic Pressure (∏if) = 0 mm Hg

So outwards force is 37 mm Hg.

Inward forces are;

Capillary Plasma Colloid Osmotic Pressure (∏p) = 25 mm Hg

and

Interstitial Fluid Pressure (Pif) = 1 mm Hg.

So inward force is 26 mm Hg.

Therefore net force is 37 - 26 = 11 mm Hg and it is outwards.


At Venous end (D);

Outwards forces;

Capillary hydrostatic pressure (Pc) = 17 mm Hg

and

Interstitial Fluid Colloid Osmotic Pressure (∏if) = 0 mm Hg

So outwards force is 17 mm Hg.

Inward forces are;

Capillary Plasma Colloid Osmotic Pressure (∏p) = 25 mm Hg

and

Interstitial Fluid Pressure (Pif) = 1 mm Hg.

So inward force is 26 mm Hg.

Therefore net force is 26 - 17 = 9 mm Hg and it is inwards.

So that's why

In this diagram, D is apparently the point at which "the osmotic pressure into the capillaries is the greatest"

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