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RESPIRATORY PHYSIOLOGY

Pulmonary Circulation

Distribution of Pulmonar>' Blood Flow    VenouS

Arterial    Alveolar pressure    pressure

pressure    (mm Hg)    (mm Hg)

Zonę 1. Alveolar pressure ex ceeds arterial pressure and there is no blood flow to this area. Occurs only abnormally when alveolar pressure is in-creased or arterial pressure is reduced

Zonę 2. Arterial pressure exceed$ alveolar pressure, and alveolar pressure exceeds venous pressure. Blood flow varies with difterence between arterial and alveolar pressure and is greater at bottom of zonę than al top

Zonę 3. Both arterial and venous pressures excced akeolar pressure. Blood flow depends on arterial-venous pressure difterence, which is constant throughout the zonę. Because arterial pressure increases down zonę, transmural pressure becomes greater, capillaries distend, and resistance to flow falls

Figurę 5.15 Pulmonary Circulation

Due to the effects of gravitv. blood tlow is not evenly distributed throughout the lung. As a result. the capillaries in the apex o! the lung are almost completely collapsed and blood tlow through them

is minimal. but still occurs. Row is absent if alveolar pressure is increased te.g., positive-pressure ventilation) or arterial pressure is decreased łe.g., hemorrhage).

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