Pulmonary blood flow and its distribution: regulation of pulmonary blood flow distribution
Pulmonary blood flow is essentially equal to cardiac output. Therefore, the factors that mainly control cardiac output are peripheral as well as pulmonary blood flow control.
The lungs have two circulations, high pressure, low flow, and low pressure, high flow. High-pressure, low-flow circulation supplies arterial blood to the trachea, bronchial tree (including terminal bronchioles), the supporting tissues of the lungs, and the outer coat (periosteum) of the arteries. and pulmonary veins. The bronchial artery, a branch of the thoracic aorta, supplies blood to most of these aortic systems at a pressure only slightly lower than the aortic pressure. Low pressure, high circulation delivers venous blood from all parts of the body to the alveolar capillaries where oxygen (O2) is added and carbon dioxide (CO2) is removed. The pulmonary artery (receiving blood from the right ventricle) and its arterial branches transport blood to the alveolar capillaries for gas exchange,
Pulmonary blood flow is essentially equal to cardiac output. Therefore, the factors that mainly control cardiac output are peripheral as well as pulmonary blood flow control. In most conditions, the pulmonary vessels that act like tubes can expand with increasing pressure and narrow with decreasing pressure. For fully oxygenated blood to occur, blood must be distributed to segments of the lungs where the alveoli are best oxygenated. This allocation is done by the following mechanism.
Alveolar hypoxia reduces alveolar local blood flow and modulates pulmonary blood flow distribution. When the concentration of O2 in the alveolar air falls below normal, especially when it falls below 70% of normal (ie, less than 73 mmHg PO2), the adjacent blood vessels atrophy and the resistance of the blood vessels increases more than 5 times at the lowest O2 concentration. This effect is in contrast to the effect observed in systemic vessels that dilate rather than constricting at low O2 concentrations. Although the mechanisms that promote pulmonary vasoconstriction in hypoxia are not completely understood, low O2 levels may stimulate the production of vasoconstrictors or reduce the release of a vasodilator such as NO from the tissues. lung.
Some studies have suggested that hypoxia may directly induce vasoconstriction by inhibiting the sensitive potassium ion channel in the membrane of pulmonary vascular smooth muscle cells. With local low oxygen pressure, these channels are blocked leading to membrane depolarization and calcium channel activation causing an increase in calcium ion concentration. The increase in calcium concentration causes constriction of small arteries and arterialise.
The increase in pulmonary vascular resistance as a result of low O2 levels has an important function in the distribution of blood flow to the most affected site. That is, if some of the alveoli are poorly ventilated and have low O2 levels, local blood vessels atrophy. This constriction results in better blood flow through other areas of the lungs, thus providing an automatic control system to distribute blood flow across lung regions in proportion to the O2 pressure in the alveoli. its.