Lecture Nine- Friday, January 22 n, 2010
IV Pulmonary blood flow and ventilation-perfusion ratio
A. Pulmonary Circulation and Blood Pressure
In the pulmonary capillaries, blood is spread out in a multitude of thin-walled vessels
which have a surface area of approximately 100 m , or 40 times the body surface
area. The pulmonary circulation differs in many ways from the systemic one. Blood
pressure in the pulmonary circulation is lower than in the systemic circulation, and
the walls of the pulmonary capillaries are thinner than those of similar vessels in the
systemic circulation. Normally, the right ventricle develops a pressure of about 25
mmHg during its systole, and this is transmitted to the pulmonary arteries (Figure
16). When systole ends, right ventricle pressure falls to atmospheric (taken to be 0).
Since the pulmonary valves are now closed, blood pressure in the pulmonary
circulation decreases gradually during diastole to a low of about 8 mmHg as blood
flows through the pulmonary capillaries (Figure 16). The mean pulmonary arterial
pressure is about 15 mmHg while the left atrial pressure is about 5 mmHg (Figure
see picture in slides
B. Vascular Resistance
Change of volume per unit time= flow. Blood flow depends on vascular pressure and
resistance, i.e. flow=pressure/resistance. There is a total pressure drop from
pulmonary artery to left atrium of about 10 mm Hg, compared to ~100mmHg for the
systemic artery to right atrium (Figure 16). Therefore, the pulmonary resistance is
only 1/10 that of the systemic circulation. The low vascular resistance in the
pulmonary circulation relies on the thin walls of the vascular system. The low
vascular resistance and high compliance of the pulmonary circulation allows the lung
to accept the whole cardiac output at all times.
C. Accommodation of Pulmonary Blood Vessels
The pulmonary circulation has the capacity to accommodate two- to three-fold
increases in cardiac output with little change in pulmonary arterial pressure. The
increase in blood flow with little changes in driving pressure indicates that as
pulmonary blood flow increases, pulmonary resistance falls. This fall in vascular
resistance results from an increasing cross-sectional area of the vascular bed. Blood
vessels already perfused may increase their caliber (distension), and previously
closed vessels may open as the cardiac output rises (recruitment) (figure 17).
see picture in slides
•Drugs (serotonin, histamine, norepinephrine) which cause the contraction of smooth
muscle increase pulmonary vascular resistance in the larger pulmonary arteries.
•Drugs (acetylcholine, isoproteranol) which can relax smooth muscle may decrease
pulmonary vascular resistance.
•There is a reflex vasoconstriction is regions of the lungs that are poorly oxygenated.
•Nitric oxide produced by endothelial cells relaxes vascular smooth muscle leading to
D. Effects of Gravity on pulmonary blood flow
Pulmonary blood flow is affected by gravity and it differs with body posture. In the
upright position, blood flow increases almost linearly from top to bottom of the