Question

A patient has the following Starling forces measured at the capillary level: capillary hydrostatic pressure = 18 mmHg, capillary oncotic pressure = 27 mmHg, interstitial oncotic pressure = 7 mmHg. If there is no net movement of fluid across the capillary wall, what is the value of the interstitial hydrostatic pressure?

• A. 0 mmHg
• B. +1 mmHg
• C. +2 mmHg
• D. $-2$ mmHg

Hint:

For no net capillary fluid movement: outward forces = inward forces.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to calculate a specific Starling force using the Starling equation under the condition of zero net fluid filtration.
Step 2: Key Formula or Approach:
The Starling equation for net fluid filtration (\(J_v\)) is: \[ J_v = K_f \times [ (P_c - P_i) - (\pi_c - \pi_i) ] \] Where:
\(P_c\) = Capillary Hydrostatic Pressure = 18 mmHg
\(P_i\) = Interstitial Hydrostatic Pressure (Unknown)
\(\pi_c\) = Capillary Oncotic Pressure = 27 mmHg
\(\pi_i\) = Interstitial Oncotic Pressure = 7 mmHg
If no net movement occurs, \(J_v = 0\).
Step 3: Detailed Explanation:

Setting the equation to zero:
\[ (P_c - P_i) - (\pi_c - \pi_i) = 0 \]

Substitute the known values:
\[ (18 - P_i) - (27 - 7) = 0 \]
\[ (18 - P_i) - 20 = 0 \]

Solve for \(P_i\):
\[ 18 - P_i = 20 \]
\[ -P_i = 20 - 18 \]
\[ -P_i = 2 \implies P_i = -2 \text{ mmHg} \]

A negative interstitial hydrostatic pressure is a physiologically normal finding in many tissues, as it helps "pull" fluid slightly from the capillaries or supports lymphatic drainage.

Step 4: Final Answer:
The interstitial hydrostatic pressure must be -2 mmHg to balance the other Starling forces and result in zero net fluid movement.