Lecture-3 Pulmonary Circulation
Physiologic Anatomy of the Pulmonary Circulatory SystemThere are 2 type of vessels supply blood to the lungs one is pulmonary vessels (gas exchange) and other is bronchial vessels (nutrition).
Pulmonary Vessels: The pulmonary artery extends only 5 cm 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 is thin, with a wall thickness one third that of the aorta. The pulmonary arterial branches are very short, and all the pulmonary arteries, even the smaller arteries and arterioles, have larger diameters than their counterpart systemic arteries. This, 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 are also short. They immediately empty their effluent blood into the left atrium, to be pumped by the left heart through the systemic circulation.
Bronchial Vessels: Blood also flows to the lungs through small bronchial arteries that originate from the systemic circulation; it is about 1 to 2 per cent 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 connective tissue of lungs, septa, and large and small bronchi. After this bronchial and arterial blood has passed 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 the left ventricular output are about 1 to 2 per cent greater than the right ventricular output.
Lymphatics: Lymph vessels are beginning in the connective tissue spaces that surround the terminal bronchioles, coursing to the hilum of the lung, and thence mainly into the right thoracic lymph duct.
Pressures in the Pulmonary System
●The systolic pressure in the right ventricle of the normal human being 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.●The systolic pulmonary arterial pressure averages about 25 mm Hg in the normal human being, the diastolic pulmonary arterial pressure is about 8 mm Hg, and the mean pulmonary arterial pressure is 15 mm Hg.
●The mean pulmonary capillary pressure is about 7 mm Hg.
●The mean pressure in the left atrium and the major pulmonary veins averages about 2 mm Hg in the recumbent human being, varying from as low as 1 mm Hg to as high as 5 mm Hg.
Blood Volume of the Lungs
The blood volume of the lungs is about 450 ml, about 9 % of the total blood volume of the entire circulatory system. Approximately 70 ml of this pulmonary blood volume is in the pulmonary capillaries, and the remainder is divided about equally between the pulmonary arteries and the veins.Lungs as a Blood Reservoir: Under various physiological and pathological conditions, the quantity of blood in the lungs can vary from as little as one half normal (hemorrhage) up to twice normal.
Effect of diminished alveolar oxygen on local alveolar blood flowautomatic control of pulmonary blood flow distribution: When O2 concentration drops to 70% of normal (below 73mmHg), the pulmonary blood vessels constrict (opposite to other systemic vessels) and this are important to shift the blood to more aerated areas.
Effect of hydrostatic pressure (weight of blood) gradients in the lungs on regional pulmonary blood flow: In the normal, upright adult, the lowest point in the lungs is about 30 cm below the highest point. This represents a 23 mm Hg pressure difference, about 15 mm Hg of which is above the heart and 8 below. That is, the pulmonary arterial pressure in the uppermost portion of the lung of a standing person is about 15 mm Hg less than the pulmonary arterial pressure at the level of the heart (25 -15=10 mm Hg), and the pressure in the lowest portion of the lungs is about 8 mm Hg greater(25 +8=33 mm Hg). This explained that in the standing position at rest, there is little flow in the top of the lung but about five times as much flows in the bottom.
Zones 1, 2, and 3 of pulmonary blood flow (fig 7)
The capillaries in the alveolar walls are distended by the blood pressure inside them, but at the same time, they are compressed by the alveolar air pressure on their outsides.Under different normal and pathological lung conditions, one may find any one of three possible zones of pulmonary blood flow, as follows:
Zone 1: No blood flow when the lung alveolar air pressure becomes greater than the capillary blood pressure, the capillaries close and there is no blood flow (occur in pathological condition only).
Zone 2: Intermittent blood flow only during the pulmonary arterial pressure peaks because the systolic pressure is then greater than the alveolar air pressure, but the diastolic pressure is less than the alveolar air pressure
Zone 3: Continuous blood flow because the alveolar capillary pressure remains greater than alveolar air pressure during the entire cardiac cycle
Normally, the lungs have only zones 2 and 3 blood flowzone 2 (intermittent flow) in the apices, and zone 3 (continuous flow) in all the lower areas. During exercise the pulmonary vascular pressures increase enough to convert the lung apices from a zone 2 pattern into a zone 3 pattern of flow.
Heavy exercise effect on pulmonary blood flow and arterial pressure
During heavy exercise, blood flow through the lungs increases fourfold to sevenfold. This extra flow is accommodated in the lungs in three ways: (1) by increasing the number of open capillaries (2) by distending all the capillaries and increasing the rate of flow through each capillary and (3) by increasing the pulmonary arterial pressure. In the normal person, the first two changes decrease pulmonary vascular resistance so much that the pulmonary arterial pressure rises very little, even during maximum exercise.Function of the pulmonary circulation when the left atrial pressure rises as a result of left-sided heart failure: when the left atrial pressure rises to greater than 7 or 8 mm Hg cause almost equally great increases in pulmonary arterial pressure and then increased load on the right heart and pulmonary edema.
Pulmonary Capillary Dynamics
The capillary blood flows in the alveolar walls as a sheet of flow, rather than in individual capillaries. The pulmonary capillary pressure is about 7 mm Hg. When the cardiac output is normal, the blood passes through the pulmonary capillaries in about 0.8 second. When the cardiac output increases, this can shorten to as little as 0.3 second.Capillary exchange of fluid in the lungs, and pulmonary interstitial fluid dynamics
The dynamics of fluid exchange across the lung capillary membranes are the same as for peripheral tissues but there are important differences, as follows:
1-The pulmonary capillary pressure is lower than peripheral tissues capillary pressure.
2-The interstitial fluid pressure in the lung is slightly more negative than that in the peripheral subcutaneous tissue.
3-The pulmonary capillaries are relatively leaky to protein molecules, so that the colloid osmotic pressure of the pulmonary interstitial fluid is higher than the peripheral tissues.
4. The alveolar walls are extremely thin which explain dumping of fluid from the interstitial spaces into the alveoli in abnormal condition.