Effect of intrapulmonary hydrostatic pressure gradients on the pulmonary circulation area
The pulmonary artery and its arterial branches transport blood to the alveolar capillaries for gas exchange, and the pulmonary veins and then return the blood to the left atrium to be pumped by the left ventricle through the systemic circulation.
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,
The blood pressure in the legs of a standing person can be up to 90 mmHg greater than the same level of pressure in the heart. This difference is because the hydrostatic pressure is the weight of the blood itself in the blood vessels. Similar effects, but to a lesser extent, occur in the lungs. When an adult is upright, the lowest point in the normal lung is about 30 cm below the highest point, corresponding to a difference of 23 mmHg, of which about 15 mmHg is above the heart and 8 mmHg below. In addition, the pulmonary artery pressure in the upper part of the lung of a standing person is about 15 mmHg lower than the pulmonary artery pressure at the same level in the heart, and the pressure in the lowest part of the lung is about more than 8 mmHg.
Such different pressures have a profound effect on blood flow through different areas of the lungs. This effect is demonstrated by the lower curves in the figure, which depict blood flow per unit of lung parenchyma at different levels of the lung in an upright person. Note that in the standing position at rest, there is very little flow in the apex of the lung but about five times as much flow in the base. To help explain these differences, the lungs are often depicted as being divided into three areas as shown. Within each region, the pattern of bleeding is quite different.
Blood circulation in areas 1,2,3 of the lungs
The capillaries in the alveolar walls are being inflated by the blood pressure inside them but at the same time being compressed by the alveolar air pressure coming in from the outside. So, any time the alveolar air pressure becomes greater than the blood capillary pressure, the capillaries close and no blood circulates. Under otherwise normal and pathological conditions of the lungs, any one of three areas of pulmonary circulation can be found, as follows:
Zone 1: There is no blood in all parts of the cardiac cycle because local alveolar-capillary pressure in this region of the lung does not rise above alveolar air pressure during any part of the cardiac cycle.
Zone 2: zone of intermittent circulation only during peaks because systolic pressure is greater than alveolar pressure but diastolic pressure is lower than alveolar pressure.
Zone 3: Blood flows continuously because alveolar-capillary pressures remain greater than alveolar air pressure during the cardiac cycle.
Figure. Blood flow in the lungs of an upright person at rest and during exercise. Note that when the patient is at rest, blood flow at the top of the lungs is very low; Most of the flow passes through the base of the lung.
Figure. Mechanism of blood flow in the three pulmonary blood flow regions: zone 1, no flow - alveolar air pressure (PALV) is greater than arterial pressure; zone 2, intermittent flow - systolic arterial pressure rises above alveolar pressure, but diastolic arterial pressure falls below alveolar pressure; and zone 3, continuous flow - arterial pressure and pulmonary capillary pressure (Ppc) are always greater than alveolar air pressure.
Normally, the lungs have the only circulation in zones 2 and 3. Zone 2 (intermittent circulation) in the apex and zone 3 (continuous circulation) in all lower areas. For example, when a person is standing upright, the pulmonary artery pressure at the top of the lungs is about 15 mmHg above the same level at the heart. Therefore, the peak systolic blood pressure is only about 10 mmHg (25 mmHg at heart level minus 15 mmHg hydrostatic pressure difference). 10 mmHg peak pulmonary pressure is greater than alveolar zero pressure, so blood circulating through the apical capillaries during the cardiac cycle is not sufficient to push blood up to the 15-mmHg hydrostatic pressure gradient. diastole is required to induce diastolic capillary flow. Therefore, blood flow through the top of the lungs is not continuous. Area 2 circulation begins in the normal lung about 10 cm above the mid-cardiac level and extends from there to the top of the lung.
In the lower part of the lungs, from about 10 cm above heart level to the base of the lungs, pulmonary artery pressures during both systole and diastole remain greater than alveolar air pressures of 0. Therefore, circulation is Continuity occurs through alveolar capillaries or zone 3 blood circulation. Also when a person lies down, no part of the lung is more than a few centimetres above heart level. In this case, the blood circulation in a normal human is completely according to zone 3, including the apex of the lung.
Area 1 blood circulation occurs only under abnormal conditions. Zone 1 circulation means no blood at any time in the cardiac cycle, which occurs when either the pulmonary artery systolic pressure is too low or the alveolar pressure is too high to allow circulation. . For example, if an upright person breathes a positive barometric pressure so the interalveolar air pressure is at least 10 mmHg greater than normal but the pulmonary systolic pressure is normal, one would expect zone 1 blood circulation – no blood circulation – in the top of the lungs. Another example, where zone 1 circulation occurs in an upright person whose pulmonary systolic pressure is extremely low, as can occur after severe blood loss.
Exercise increases blood flow through all parts of the lungs. Recalling the figure, it was found that blood circulation in all parts of the lungs increased during exercise. A major reason for increased circulation is that pulmonary vascular pressure increases sufficiently during exercise to convert the apex of the lung from a zone 2 to a zone 3 pattern.
The increase in cardiac output during strenuous exercise is usually mediated by pulmonary circulation without a large increase in pulmonary artery pressure
During heavy exercise, blood circulation through the lungs can be increased four to seven times. This extra flow is regulated in the lungs in three ways: (1) by increasing the number of dilated capillaries, sometimes by a factor of three.; (2) by stretching all capillaries and increasing the flow rate through each capillary more than twofold; and (3) by increasing pulmonary artery pressure. Normally, the first two changes reduce pulmonary vascular resistance so much that pulmonary artery pressure increases very little, even during peak exercise. This effect is shown in Fig.
The ability of the lungs to upregulate blood flow during exercise without increasing the pulmonary artery pressure saves the energy of the right heart. This ability of the lungs also prevents a significant increase in pulmonary capillary pressure and the development of pulmonary oedema.
Figure. Effect on mean pulmonary artery pressure due to increased cardiac output during exercise.
Pulmonary circulation when left atrial pressure increases as a result of left heart failure
Left atrial pressure in a healthy person almost never rises above +6 mmHg, even during the most difficult exercise. Small changes in left atrial pressure have almost no effect on pulmonary function because these merely widen the pulmonary venules and open up more capillaries to allow blood to continue to flow with relative ease. equally from the pulmonary artery.
Although the left heart fails, blood begins to pool in the left atrium. As a result, left atrial pressure may randomly increase from its normal value of 1-5 mmHg up to 40-50 mmHg. The initial increase in atrial pressure up to about 7 mmHg has little effect on pulmonary circulatory function. However, when left atrial pressure increases greater than 7 or 8 mmHg, the left atrial pressure increases further leading to a steady increase in pulmonary artery pressure, thereby increasing the burden on the right heart simultaneously. Increasing left atrial pressure above 7 or 8 mmHg increases capillary pressure almost equally as much. When left atrial pressure rises above 30 mm Hg, causing a similar increase in capillary pressure, pulmonary oedema may develop, which is discussed later in this chapter.