Pulmonary Circulation, Pulmonary Edema, and Pleural Fluid

Pulmonary Vessels

The pulmonary artery extends only 5 centimeters beyond the apex of the right ventricle and then divides into right and left main branches that supply blood to the two respective lungs.

The pulmonary artery has a wall thickness one-third that of the aorta. The pulmonary arterial branches are short, and all the pulmonary arteries, even the smaller arteries and arterioles, have larger diameters than their counterpart systemic arteries. This feature, combined with the fact that the vessels are thin and distensible, gives the pulmonary arterial tree a large compliance , averaging almost 7 ml/mm Hg, which is similar to that of the entire systemic arterial tree. This large compliance allows the pulmonary arteries to accommodate the stroke volume output of the right ventricle.

The pulmonary veins, like the pulmonary arteries, are also short. They immediately empty their effluent blood into the left atrium.

Bronchial Vessels

Blood also flows to the lungs through small bronchial arteries that originate from the systemic circulation, amounting to 1% to 2% of the total cardiac output. This bronchial arterial blood is oxygenated blood, in contrast to the partially deoxygenated blood in the pulmonary arteries. It supplies the supporting tissues of the lungs, including the connective tissue, septa, and large and small bronchi. After this bronchial and arterial blood passes through the supporting tissues, it empties into the pulmonary veins and enters the left atrium , rather than passing back to the right atrium. Therefore, the flow into the left atrium and left ventricular output are about 1% to 2% greater than that of the right ventricular output.

Lymphatics

Lymph vessels are present in all the supportive tissues of the lung, beginning in the connective tissue spaces that surround the terminal bronchioles, coursing to the hilum of the lung, and then mainly into the right thoracic lymph duct . Particulate matter entering the alveoli is partly removed by these lymph vessels, and plasma protein leaking from the lung capillaries is also removed from the lung tissues, thereby helping to prevent pulmonary edema.

Pressures in the Right Ventricle

The pressure pulse curves of the right ventricle and pulmonary artery are shown in the lower portion of Figure 39-1 . These curves are contrasted with the much higher aortic pressure curve shown in the upper portion of the figure. The normal systolic pressure in the right ventricle averages about 25 mm Hg, and the diastolic pressure averages about 0 to 1 mm Hg, values that are only one-fifth those for the left ventricle.

Pressures in the Pulmonary Artery

During systole , the pressure in the pulmonary artery is essentially equal to the pressure in the right ventricle, as also shown in Figure 39-1 . However, after the pulmonary valve closes at the end of systole, the ventricular pressure falls precipitously, whereas the pulmonary arterial pressure falls more slowly as blood flows through the lungs.

As shown in Figure 39-2 , the systolic pulmonary arterial pressure normally averages about 25 mm Hg in the human being, the diastolic pulmonary arterial pressure is about 8 mm Hg, and the mean pulmonary arterial pressure is 15 mm Hg.

Pulmonary Capillary Pressure

The mean pulmonary capillary pressure, as diagrammed in Figure 39-2 , is about 7 mm Hg. The importance of this low capillary pressure is discussed in detail later in the chapter in relation to fluid exchange functions of the pulmonary capillaries.

Left Atrial and Pulmonary Venous Pressures

The mean pressure in the left atrium and major pulmonary veins averages about 2 mm Hg in the recumbent person, varying from as low as 1 mm Hg to as high as 5 mm Hg. It usually is not feasible to measure someone’s left atrial pressure using a direct measuring device because it is difficult to pass a catheter through the heart chambers into the left atrium. However, the left atrial pressure can be estimated with moderate accuracy by measuring the so-called pulmonary wedge pressure . This is measured by inserting a catheter first through a peripheral vein to the right atrium, then through the right side of the heart and through the pulmonary artery into one of the small branches of the pulmonary artery, and finally pushing the catheter until it wedges tightly in the small branch .

The pressure measured through the catheter, called the “wedge pressure,” is about 5 mm Hg. Because all blood flow has been stopped in the small wedged artery, and because the blood vessels extending beyond this artery make a direct connection with the pulmonary capillaries, this wedge pressure is usually only 2 to 3 mm Hg higher than the left atrial pressure. When the left atrial pressure rises to high values, the pulmonary wedge pressure also rises. Therefore, wedge pressure measurements can be used to estimate changes in pulmonary capillary pressure and left atrial pressure in patients with congestive heart failure.

Lungs Serve as a Blood Reservoir

Under various physiological and pathological conditions, the quantity of blood in the lungs can vary from as little as half-normal up to twice normal. For example, when a person blows out air so hard that high pressure is built up in the lungs, such as when blowing a trumpet, as much as 250 ml of blood can be expelled from the pulmonary circulatory system into the systemic circulation. Also, loss of blood from the systemic circulation by hemorrhage can be partly compensated for by the automatic shift of blood from the lungs into the systemic vessels.