Pulmonary Circulation

It is the system of circulation connecting the heart and lungs, which comprises the widely interconnected blood vascular system and lymphatics. They perform certain unique functions, like exchanging gases in the lungs apart from acting as a reservoir for blood storage.

1. Arterial circuit: Arterial circuit begins with the pulmonary artery arising from the right ventricle and then divides into the right and left branches, followed by further divisions into many branches to form an extensive network of small arteries and arterioles, which finally ramifies into capillaries. The pulmonary arteries are much wider (hav­ing a large diameter) and thinner, which confers them with enhanced distensibility and compliance (approximately 7 mL/mmHg) features. It is advantageous for accommo­dating a larger blood volume in a limited space.

2. Venous circuit: It begins with the finest venules that merge into smaller veins and eventually join the main pulmonary veins, which drain blood into the left atrium. Compared to the systemic veins, the pulmonary veins are thinner with enhanced distensibility, like the arteries, making them more compliant that enables them to accom­modate more blood.

3. Lymphatics: The lymphatics originate near the terminal bronchioles and help maintain an excess fluid accumula­tion near alveoli that may otherwise affect the function of gas exchange and finally drain into the mediastinal lymphatics that ultimately terminate near the right lymphatic duct.

The pleural space between the two pleural membranes is also supplied by a similar lymphatic system that aids in pleural fluid drainage and provides a frictionless viscous environment for lung movement during respiration. The lym­phatic system also contributes to a highly negative (approx. —4 to —7 mmHg) pleural pressure that prevents alveolar collapse.

Bronchial Vessels Small bronchial arteries branch away from the systemic circulation and supply oxygenated blood to the lungs. Although this constitutes only a tiny fraction (1-2%) of cardiac output, it plays an essential role in supply­ing oxygenated blood to various lungs, including the connec­tive tissues and bronchi. It later drains into the left atrium via the pulmonary veins. It contributes to the slightly increased (1-2%) inflow and output of the left atrium and left ventricle, respectively, in comparison to the output of the right ventricle.

7.5.1 Pressures in the Pulmonary Artery

The average systolic, diastolic, and mean pulmonary arterial pressure is about 25, 8, and 15 mmHg, respectively, in humans, while the mean pulmonary capillary pressure is 7 mmHg. A mean pressure of 2 mmHg exists in the left atrium and the major pulmonary veins. Pulmonary arterial pressure is only about one-sixth of systemic arterial pressure.

The capillary and venous pressures do not vary much in the two circulations. Thus, there is only a minute fall of pressure along the pulmonary arterioles and, therefore, a reduced potential for active regulation of the distribution of the pul­monary blood flow. It is also responsible for arterial pressure wave’s minimal dampening and pronounced pulsatile nature of the capillary blood flow in lungs.

7.5.2 Blood Volume of the Lungs

At any given point of time in physiological conditions, only about 9% of total blood volume is present in the lungs, which can increase by four- to sevenfold during heavy exercise. The lungs have the capacity to increase the flow by (1) threefold increasing the number of open capillaries, (2) twofold disten­sion of capillaries to increase the flow rate of individual capillary and (3) increasing the pulmonary arterial pressure.

Constriction of blood vessels takes place when the oxygen concentration in the alveolar air falls about 70% below nor­mal (73 mmHg PO₂), a response strikingly different from systemic vessels, which dilate in low oxygen concentrations. The systemic vessels act oppositely by dilating in low concentrations of oxygen. This feature of pulmonary vascular resistance in response to low oxygen is practically important since this promotes the distribution of blood to areas where there is better alveolar oxygen pressure. Table 7.2 demonstrates the partial pressure of various gases, atmo­spheric, humidified, alveolar and expired air.

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