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Learning Objectives
The student will be able to quantify and contrast the pulmonary and systemic circulations with respect to pressures, flow rates, and vascular resistances.
The student will be able to describe the several vascular perfusion zones of lungs and the forces affecting flow in each.
The student will be able to use Starling’s law of the capillary to explain how interstitial and alveolar edemas occur.
The student will be able to distinguish features and mechanisms underlying cardiogenic, noncardiogenic, and high altitude forms of pulmonary edema.
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The rate at which the right ventricle pumps blood, its cardiac output (Q̇) (L/min), must equal that of the left ventricle to avoid an imbalanced distribution of blood volume within the systemic and pulmonary circulations. However, normal pulmonary arterial blood pressure (PPA) averages just 12-16 mm Hg, or about one-eighth of the average blood pressure in the dorsal aorta. Because Ohm’s law states that flow = ΔP/R (Chap. 6), total pulmonary vascular resistance (PVR) normally is also a small fraction of vascular resistance in the much larger systemic circulation. (Diseases for which PVR is significantly increased will be presented elsewhere in this book.) This chapter will focus on factors that affect PVR and thus PPA, realizing that increases in PPA will increase the work required of the right ventricle and the gradient for fluid constituents of the blood to leak out of alveolar capillaries into the surrounding interstitium and airspaces.
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PRESSURES AND RESISTANCES TO FLOW IN THE PULMONARY VASCULATURE
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The basic features of the adult pulmonary circulation are shown in Fig. 7.1, which emphasizes its parallel nature to systemic flow patterns. A subject’s average PPA is most usually equated to their diastolic pressure (DP) plus one-third of the difference between systolic pressure (SP) and the DP:
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mean PPA = DP + [(SP − DP)/3]
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Pulmonary arterial blood flows through the alveolar microvasculature to drain as pulmonary venous blood into the left heart at a left atrial filling pressure = 4-6 mm Hg. Assuming a resting value for Q̇ = 6 L/min and re-arranging Ohm’s law (Q̇ = ΔP/R), normal resting PVR = (16 mm Hg – 4 mm Hg)/6 L, or about 2 mm Hg/L of blood flow. Contrast this value for PVR of 2 mm Hg/L in the low-resistance pulmonary circulation with a resting value for systemic vascular resistance (SVR). This SVR is calculated using the same resting Q̇ and rearrangement of Ohm’s law while estimating mean arterial and venous pressures to ...