The pressures in the pulmonary circulation are remarkably low. The mean pressure in the main pulmonary artery is only about 15 mm Hg; the systolic and diastolic pressures are about 25 and 8 mm Hg, respectively (Figure 4-1)- The pressure is therefore very pulsatile. By contrast, the mean pressure in the aorta is about 100 mm Hg—about six times more than in the pulmonary artery. The pressures in the right and left atriums are not very dissimilar—about 2 and 5 mm Hg, respectively. Thus, the pressure differences from inlet to outlet of the pulmonary and systemic systems are about (15 — 5) = 10 and (100 — 2) = 98 mm Hg, respectively—a factor of 10.
In keeping with these low pressures, the walls of the pulmonary aitery and its branches are remarkably thin and contain relatively little smooth muscle (they
re easily mistaken for veins). This is in striking contrast to the systemic circula-\ n where the arteries generally have thick walls and the arterioles in particular ive abundant smooth muscle.
The reasons for these differences become clcar when the functions of the two . :rculations are compared. The systemic circulation regulates the supply of blood • various organs, including those which may be far above the level of the heart "he upstretched arm, for example). By contrast, the lung is required to accept the whole of the cardiac output at all times. It is rarely concerned with directing -. od from one region to another (an exception is localised alveolar hypoxia, see below), and its arterial pressure is therefore as low as is consistent with lifting blood to the top of the lung. This keeps the vvork'of the right heart as small as is r'easible for efficient gas exchange to occur in the lung.
The pressure within the pulmonary capillaries is uncertain. The best evidence -j^gests that it lies about halfway between pulmonary arterial and venous prestare, and probably that much of the pressure drop occurs within the capillary bed .Tself. Certainly the distribution of pressures along the pulmonary circulation is :.ir more symmetrical rhan in its systemic counterpart, where most of the pressure vrop is just upstream of the capillaries (Figure 4-1). In addition, the pressure within the pulmonary capillaries varies considerably throughout the lung because f hydrostatic effects (see below).
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