Question

Give the formula for net filtration pressure; then include typical valves (at the arterial end of a capillary bed) for all variables and include units.

Answer 1

Filtration and reabsorption across microvascular endothelium
The rate of filtration and absorption at any point along the capillary wall depends on the balance of forces known as Starling forces
. According to the Starling hypothesis, the filtration absorption is expressed as
K(Pc+πi) - (Pi + πc),

K= the permeability-surface area coefficient.
Pe the hydrostatic capillary pressure,
Pi= the hydrostatic interstitial pressure,
πc=oncotic pressure of blood and
πi=oncotic pressure of the interstitium.

thus, Pc-Pi, represents the hydrostatic pressure gradient and
πc-πi , represents the oncotic pressure gradient.
Starling forces are defined:

1. Hydrostatic capillary pressure (Pc) tends to force the fluid out through the capillary membrane. The values of hydrostatic capillary pressure in most of the tissues are:
• At the arterial end = 30-40 mm Hg,
• At the venous end = 10-15 mm Hg and
• In the middle = 25 mm Hg.

2. Hydrostatic interstitial pressure (Pi) tends to force fluid inward through the capillary membrane. It is about -2 mm Hg in the subcutaneous tissue.

3. Oncotic pressure of blood or plasma colloid osmotic pressure (pc) results from the osmotic pressure of plasma proteins. It tends to pull fluid inward through the capillary membrane. It is about 25-27 mm Hg.

4. Oncotic pressure of the interstitium (ni) is due to the presence of proteins in the interstitial space. It tends fluid out of the capillary membrane. The effective oncotic pull pressure in the interstitium is estimated to range between 2 and 10 mm Hg (average 3 mm Hg).

Calculation of net filtration at the capillaries

From the above description, the net forces acting on the fluid at the arteriolar and venous end of a typical muscle capillary can be calculated

Table Calculation of net Capillared Filtration force on - At na arteriolarend At Venous end o forces tending to move e fluid outward (MM HG) (mm Hg). 35 10 a Pc b) Ti Total outward force (Pc +T;) 38 • forces tending to move Fluid inward.. a) TC 25 25 -2 23 Total inward force (T P:) 23 0 Filterina Force An no 38-23 13-22 = 15z le • - Net Filtering force of whole capillary 15-10 - 5 mm Hg

• Thus at arteriolar end, the filtering force of 15 mm Hg moves the fluid out of capillary (filtration) into the interstitium, while at venous end the filtering force of-10mm Hg moves the fluid into the capillary (reabsorption) from the interstitium.

-The net filtering force for the whole capillary is 5 mm Hg.

In the example given above we have studied the balance of the Starling forces at the arteriolar and venous end of the capillary only. However, over the length of the capillary, the hydrostatic pressure gradually declines to zero near the middle of capillary. From here the inward forces become dominant and reabsorption process starts to reach its maximum at the venular end.

Refer to diagram for more information given below.

Arteriole Venule Interstitial space Capillary Po 35 II Filtration Reabsorption Oncotic P (TTC) = 25 Interstitial P (pi) = -2