RESPIRATION PHYSIOLOGY: PULMONARY CIRCULATIION -- Capillary Fluid Exchange

CAPILLARY FLUID EXCHANGE

A. Fluid Balance

The low intracapillary hydrostatic pressure, a consequence of the low pulmonary arterial pressure, means that normally the osmotic force tending to move fluid into pulmonary capillaries is greater than the hydrostatic pressure moving fluid out. (Note: transcapillary osmotic -- or oncotic -- pressure is due to plasma proteins. Pulmonary capillary walls are relatively impermeable to plasma proteins, so π_c is greater than π_t)

Thus, interstitial space is small and the alveoli are "moist" but not "wet".

P_c = capillary hydrostatic pressure
π_c = capillary osmotic pressure (due to plasma protein, oncotic pressure)
P_t = tissue hydrostatic pressure
π_t = tissue osmotic pressure

Note: Hydrostatic pressure difference (P_c - P_t) tends to move fluid out of capillaries
Note: Osmotic pressure difference (π_c - π_t) tends to move fluid into capillaries

Note contrast with systemic circulation, where ( P_c - P_t ) ≈ ( π_c - π_t ) (see figure =>)

Question: If a patient aspirates water into the lungs, what will happen to the water?

B. Pulmonary Edema

1. Low pulmonary capillary hydrostatic pressure provides a "safety factor" against pulmonary edema

2. Disturbances of pulmonary capillary fluid balance (examples)

a. pulmonary venous congestion ( P_c) ↑ (e.g. left heart failure)
b. pulmonary permeability increase (π_t ↑(e.g. pulmonary inflammation or damage)

Question: Why might lying down increase pulmonary edema in someone with left heart failure?

3. Results of increasing fluid movement into tissue

a. initial: excess filtered fluid taken up by lymphatics (no edema)
b. next: excess fluid accumulates in interstitial space (tissue edema)
c. finally: excess fluid enters alveoli (alveolar edema)

Note effect of pulmonary edema on reduced pulmonary diffusing capacity