Blood Supply

The cardiovascular system can be divided into two components (as shown in Fig. 4): the pulmonary circulation and the systemic circulation. The pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood from the lungs to the left atrium. Emerging from the right ventricle is the main pulmonary artery, which branches to form smaller pulmonary and intrapulmonary arteries. This pulmonary arterial tree undergoes further rapid subdivision in parallel with the pulmonary tree to form pulmonary capillaries, a fine network of blood vessels present in intimate contact with the alveolus. The capillaries drain into postcapillary venules that unite to form small veins and, distally, larger veins. These drain into the pulmonary vein, which returns blood from the lungs to the left atrium. The systemic circulation carries oxygenated blood from the left ventricle to the tissues of the body and returns deoxygenated blood from the body to the right atrium. Arteries and arterioles carry blood to capillary networks within tissues, and venules and veins return blood from tissues to the heart. There are several important differences between the systemic and pulmonary circulations.

oxygen systemic circulation systemic circulation

Figure 4 Cardiovascular system in the body. Oxygen diffuses from the oxygen-rich environment of the alveolus into the deoxygenated blood of the pulmonary circulation. The newly oxygenated blood returns by the pulmonary vein to the left atrium and ventricle; contraction of the latter provides the driving force for circulation of oxygen-rich blood to the organs and tissues of the body. Cells of the body use oxygen for energy-producing processes that result in the formation of carbon dioxide. Depleted of oxygen and richer in carbon dioxide, blood leaving the tissues returns by the venous circulation to the right atrium. The right ventricle pumps the carbon dioxide-rich blood into the pulmonary circulation from whence the carbon dioxide may diffuse into the alveolus. Arrows indicate the direction of blood flow.

Figure 4 Cardiovascular system in the body. Oxygen diffuses from the oxygen-rich environment of the alveolus into the deoxygenated blood of the pulmonary circulation. The newly oxygenated blood returns by the pulmonary vein to the left atrium and ventricle; contraction of the latter provides the driving force for circulation of oxygen-rich blood to the organs and tissues of the body. Cells of the body use oxygen for energy-producing processes that result in the formation of carbon dioxide. Depleted of oxygen and richer in carbon dioxide, blood leaving the tissues returns by the venous circulation to the right atrium. The right ventricle pumps the carbon dioxide-rich blood into the pulmonary circulation from whence the carbon dioxide may diffuse into the alveolus. Arrows indicate the direction of blood flow.

First, blood pressure is lower in the pulmonary circulation than in the systemic circulation; for example, mean pressures in the pulmonary artery (emerging from the right ventricle) and aorta (artery emerging from the left ventricle) are 15 and 100mmHg, respectively. As a result, the walls of the various pulmonary arterial vessels are thinner and are invested with less smooth muscle than their systemic counterparts, presumably as a consequence of the differences in stress borne by the vessel walls. Secondly, the pulmonary vessels are subject to different transmural pressures than are systemic vessels, such as those caused by changes in pressure in the alveolus and thoracic cavity, together with the stresses applied to the vessel wall through movement of adherent lung tissue. For example, during inspiration, alveoli expand and tend to compress the pulmonary capillaries, which are located in the walls of the alveoli. At the same time, large vessels are subject to distention caused by the negative intrapleural pressure during inspiration.

Blood vessels supplying the conducting or central airways (i.e., the bronchial circulation) are part of the systemic circulation. By contrast, the blood supply to airways of the respiratory zone involve the pulmonary circulation. The separation of these vascular networks can be almost complete. For example,

Figure 5 Diagram of staining of the bronchial circulation in the cat. The bronchial and pulmonary circulations of the cat were perfused separately with aerated physiological salt solution containing bovine serum albumin (4% wt/vol) maintained at 37°C. Perfusates from the bronchial and pulmonary circulations were collected from cannulae positioned in the right and left ventricles, respectively. Infusion of Evans blue dye (30mg/Kg) into the systemic circulation resulted in deep blue staining of the central airways (black) with no staining of the parenchymal tissues (dotted). Further, 75-80% of the dye was collected from the cannula from the right heart.

Figure 5 Diagram of staining of the bronchial circulation in the cat. The bronchial and pulmonary circulations of the cat were perfused separately with aerated physiological salt solution containing bovine serum albumin (4% wt/vol) maintained at 37°C. Perfusates from the bronchial and pulmonary circulations were collected from cannulae positioned in the right and left ventricles, respectively. Infusion of Evans blue dye (30mg/Kg) into the systemic circulation resulted in deep blue staining of the central airways (black) with no staining of the parenchymal tissues (dotted). Further, 75-80% of the dye was collected from the cannula from the right heart.

in the cat, with separately perfused systemic (bronchial) and pulmonary circulations, infusion of Evans blue dye into the systemic circulation results in staining of the central airways (distal trachea to fourth-generation intralobar bronchus) and no staining of parenchymal tissue (Fig. 5). From 75% to 80% of the dye returns to the right ventricle, a result consistent with this circulations being part of the systemic circulation. The remainder of the dye returns to the left ventricle. This result, taken together with the absence of staining of parenchymal tissue, suggests that part of the bronchial circulation drains into the venous return of the pulmonary circulation. Recent studies demonstrate the ability to independently perfuse these two circulations in larger animals [41]. In studying the absorption of an aerosol from the central versus the peripheral airways, it is therefore prudent to be cognizant of the nature of the circulation serving the site of expected deposition and sample from the appropriate sites, namely, right ventricle or left ventricle.

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