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Physiology

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Respiration

Question
29 out of 33
 

normally lungs kept dry by



A osmotic pressure in interstitium

B surfactant

C surface tension

D air water interphase

Ans. A osmotic pressure in interstitium

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

Pc = capillary hydrostatic pressure
πc = capillary osmotic pressure (due to plasma protein, oncotic pressure)
Pt = tissue hydrostatic pressure
πt = tissue osmotic pressure

II. Note: Hydrostatic pressure difference (Pc - Pt) tends to move fluid out of capillaries
Note: Osmotic pressure difference (πc - πt) tends to move fluid into capillaries

III. surface tension of fluid lining the alveoli is an inward force which pull fluid from alveolar wall by average pressure= -3 mmHg in normal lung , but without surfactant it's increased to -20 mmHg , thus decreased surfactant leads to pulmonary edema, but the main factor is low pressure in pulmonary capillaries and interstitium prevent filteration of fluid.

IV. Therefore both surfactant and osmotic pressure as answers are correct ,but better answer is pressure in interstitium.

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