Pulmonary Circulation

Effect of lung volume on pulmonary vascular resistance

At high lung volumes the lung tissue is more stretched and the pleural pressure is more negative. At high lung volumes the alveolus is more expanded, this will compress the alveolar vessels and increase their resistance. At low lung volumes the lung tissue is less stretched and the pleural pressure is less negative. At these low lung volumes the alveolus is less expanded, which will decompress the alveolar vessels and decrease their resistance. Hypoxemia or alveolar hypoxia results in pulmonary vasoconstriction. This effect is multiplied in face of hypercapnia or low blood pH. Regional hypoxia shunts the blood to where there is air. Generalized hypoxia is dangerous as vessels constrict and less gas exchange occurs. Hypoxic vasoconstriction Important role in matching the regional perfusion of alveolar blood vessels to the regional alveolar ventilation. This prevents "wasted blood flow" to areas of the lung that are not adequately ventilated and matches perfusion to ventilation. If generalized hypoxia occurs such as at high altitude or in the case of severe COPD, vasoconstriction occurs throughout the lung. This produces a significant increase in pulmonary vascular resistance and pulmonary blood pressures. Chronically this can lead to pulmonary hypertension and right heart hypertrophy or failure. May be additional complication of aortic valve stenosis. Blood flow is lower in the area of the apex. Exercise increases blood flow

Blood Flow through the lungs and its distribution

Blood Flow Through the Lungs and Its Distribution. The blood flow through the lungs is essentially equal to the cardiac output. Therefore, the factors that control cardiac output-mainly peripheral factors also control pulmonary blood flow. Under most conditions, the pulmonary vessels act as passive, distensible tubes that enlarge with increasing pressure and narrow with decreasing pressure. For adequate aeration of the blood to occur, it is important for the blood to be distributed to those segments of the lungs where the alveoli are best oxygenated. This is achieved by the following mechanism. Decreased Alveolar Oxygen Reduces Local Alveolar Blood Flow and Regulates Pulmonary Blood Flow Distribution. When the concentration of oxygen in the air of the alveoli decreases below normal, especially when it falls below 70 percent of normal (below 73 mm Hg Po2), the adjacent blood vessels constrict, with the vascular resistance increasing more than fivefold at extremely low oxygen levels. This is opposite to the effect observed in systemic vessels, which dilate rather than constrict in response to low oxygen. It is believed that the low oxygen concentration causes some yet undiscovered vasoconstrictor substance to be released from the lung tissue; this substance promotes constriction of the small arteries and arterioles. It has been suggested that this vasoconstrictor might be secreted by the alveolar epithelial cells when they become hypoxic. This effect of low oxygen on pulmonary vascular resistance has an important function: to distribute blood flow where it is most effective. That is, if some alveoli are poorly ventilated so that their oxygen concentration becomes low, the local vessels constrict. This causes the blood to flow through other areas of the lungs that are better aerated, thus providing an automatic control system for distributing blood flow to the pulmonary areas in proportion to their alveolar oxygen pressures.

Blood Volume of the lungs

Blood Volume of the Lungs. The blood volume of the lungs is about 450 milliliters, about 9 percent of the total blood volume of the entire circulatory system. Approximately 70 milliliters of this pulmonary blood volume is in the pulmonary capillaries, and the remainder is divided about equally between the pulmonary arteries and the veins. The Lungs Serve 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 up to twice normal. For instance, when a person blows out air so hard that high pressure is built up in the lungs-such as when blowing a trumpet-as much as 250 milliliters of blood can be expelled from the pulmonary circulatory system into the systemic circulation. Also, loss of blood from the systemic circulation by hemorrhage can be partly compensated for by the automatic shift of blood from the lungs into the systemic vessels. Cardiac Pathology May Shift Blood from the Systemic Circulation to the Pulmonary Circulation. Failure of the left side of the heart or increased resistance to blood flow through the mitral valve as a result of mitral stenosis or mitral regurgitation causes blood to dam up in the pulmonary circulation, sometimes increasing the pulmonary blood volume as much as 100 percent and causing large increases in the pulmonary vascular pressures. Because the volume of the systemic circulation is about nine times that of the pulmonary system, a shift of blood from one system to the other affects the pulmonary system greatly but usually has only mild systemic circulatory effects.

Function when left atrial pressure rises

Function of the Pulmonary Circulation When the Left Atrial Pressure Rises as a Result of Left-Sided Heart Failure. The left atrial pressure in a healthy person almost never rises above +6 mm Hg, even during the most strenuous exercise. These small changes in left atrial pressure have virtually no effect on pulmonary circulatory function because this merely expands the pulmonary venules and opens up more capillaries so that blood continues to flow with almost equal ease from the pulmonary arteries. When the left side of the heart fails, however, blood begins to dam up in the left atrium. As a result, the left atrial pressure can rise on occasion from its normal value of 1 to 5 mm Hg all the way up to 40 to 50 mm Hg. The initial rise in atrial pressure, up to about 7 mm Hg, has very little effect on pulmonary circulatory function. But when the left atrial pressure rises to greater than 7 or 8 mm Hg, further increases in left atrial pressure above these levels cause almost equally great increases in pulmonary arterial pressure, thus causing a concomitant increased load on the right heart. Any increase in left atrial pressure above 7 or 8 mm Hg increases the capillary pressure almost equally as much. When the left atrial pressure has risen above 30 mm Hg, causing similar increases in capillary pressure, pulmonary edema is likely to develop.

Increased CO during exercise...

Increased Cardiac Output During Heavy Exercise Is Normally Accommodated by the Pulmonary Circulation Without Large Increases in Pulmonary Artery 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, sometimes as much as threefold; (2) by distending all the capillaries and increasing the rate of flow through each capillary more than twofold; 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. The ability of the lungs to accommodate greatly increased blood flow during exercise without increasing the pulmonary arterial pressure conserves the energy of the right side of the heart. This ability also prevents a significant rise in pulmonary capillary pressure, thus also preventing the development of pulmonary edema.

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Lymphatics Lymph vessels are present in all the supportive tissues of the lung, beginning in the connective tissue spaces that surround the terminal bronchioles, coursing to the hilum of the lung, and then mainly into the right thoracic lymph duct. Particulate matter entering the alveoli is partly removed by way of these channels, and plasma protein leaking from the lung capillaries is also removed from the lung tissues, thereby helping to prevent pulmonary edema. 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. During systole, the pressure in the pulmonary artery is essentially equal to the pressure in the right ventricle. However, after the pulmonary valve closes at the end of systole, the ventricular pressure falls precipitously, whereas the pulmonary arterial pressure falls more slowly as blood flows through the capillaries of the lungs. 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 importance of this low capillary pressure is discussed in detail later. 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.

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Normally, the lungs have only zones 2 and 3 blood flow-zone 2 (intermittent flow) in the apices and zone 3 (continuous flow) in all the lower areas. For example, when a person is in the upright position, the pulmonary arterial pressure at the lung apex is about 15 mm Hg less than the pressure at the level of the heart. Therefore, the apical systolic pressure is only 10 mm Hg (25 mm Hg at heart level minus 15 mm Hg hydrostatic pressure difference). This 10 mm Hg apical blood pressure is greater than the zero alveolar air pressure, so blood flows through the pulmonary apical capillaries during cardiac systole. Conversely, during diastole, the 8 mm Hg diastolic pressure at the level of the heart is not sufficient to push the blood up the 15 mm Hg hydrostatic pressure gradient required to cause diastolic capillary flow. Therefore, blood flow through the apical part of the lung is intermittent, with flow during systole but cessation of flow during diastole; this is called zone 2 blood flow. Zone 2 blood flow begins in the normal lungs about 10 cm above the midlevel of the heart and extends from there to the top of the lungs. In the lower regions of the lungs, from about 10 cm above the level of the heart all the way to the bottom of the lungs, the pulmonary arterial pressure during both systole and diastole remains greater than the zero alveolar air pressure. Therefore, there is continuous flow through the alveolar capillaries, or zone 3 blood flow. Also, when a person is lying down, no part of the lung is more than a few centimeters above the level of the heart. In this case, blood flow in a normal person is entirely zone 3 blood flow, including the lung apices.

Interrelations between interstitial fluid pressure and other pressures in the lung

Note the balance of forces at the blood capillary membrane, as follows: Forces tending to cause movement of fluid outward from the capillaries and into the pulmonary interstitium (mmHg): Capillary pressure 7 Interstitial fluid colloid osmotic pressure 14 Negative interstitial fluid pressure 8 TOTAL OUTWARD FORCE 29 Forces tending to cause absorption of fluid into the capillaries: Plasma colloid osmotic pressure 28 TOTAL INWARD FORCE 28 Thus, the normal outward forces are slightly greater than the inward forces, providing a mean filtration pressure at the pulmonary capillary membrane; this can be calculated as follows: Total outward force +29 Total inward force -28 MEAN FILTRATION PRESSURE +1 This filtration pressure causes a slight continual flow of fluid from the pulmonary capillaries into the interstitial spaces, and except for a small amount that evaporates in the alveoli, this fluid is pumped back to the circulation through the pulmonary lymphatic system.

Pulmonary Capillaries

Pulmonary Capillary Dynamics. Exchange of gases between the alveolar air and the pulmonary capillary blood is discussed in the next chapter. However, it is important for us to note here that the alveolar walls are lined with so many capillaries that, in most places, the capillaries almost touch one another side by side. Therefore, it is often said that the capillary blood flows in the alveolar walls as a "sheet of flow," rather than in individual capillaries. Pulmonary Capillary Pressure. No direct measurements of pulmonary capillary pressure have ever been made. However, "isogravimetric" measurement of pulmonary capillary pressure has given a value of 7 mm Hg. This is probably nearly correct because the mean left atrial pressure is about 2 mm Hg and the mean pulmonary arterial pressure is only 15 mm Hg, so the mean pulmonary capillary pressure must lie somewhere between these two values. Length of Time Blood Stays in the Pulmonary Capillaries. From histological study of the total cross-sectional area of all the pulmonary capillaries, it can be calculated that when the cardiac output is normal, 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. The shortening would be much greater were it not for the fact that additional capillaries, which normally are collapsed, open up to accommodate the increased blood flow. Thus, in only a fraction of a second, blood passing through the alveolar capillaries becomes oxygenated and loses its excess carbon dioxide.

Physiologic Anatomy of the pulmonary circulatory system

Pulmonary Vessels. The pulmonary artery extends only 5 centimeters 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, like the pulmonary arteries, are also short. They immediately empty their effluent blood into the left atrium. Bronchial Vessels. Blood also flows to the lungs through small bronchial arteries that originate from the systemic circulation, amounting to about 1 to 2 percent 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 supporting tissues of the lungs, including the connective tissue, 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 percent greater than that of the right ventricular output.

Effects of Pulmonary Arterial or Venous Pressure on Pulmonary Vascular Resistance

Pulmonary vascular resistance is only about 1/10 that of the systemic circulation. The rise in cardiac output and the increased demand for gas exchange under higher metabolic demand - the pulmonary vasculature decreases its resistance. Potential zones of blood flow in lung: Can be relatively closed, able to open (in cardiac systole), or open. Influences of pulmonary artery pressure on pulmonary vascular resistance Changes in Pulmonary Vascular Resistance with cardiac output. When cardiac output rises the pulmonary pressures will also increase. The increase in pulmonary pressures will have 2 effects: 1. Open previously closed pulmonary capillaries 2. Distend the pulmonary vessels. Both of these effects reduce pulmonary vascular resistance. The first by opening parallel pathways and the second by increasing the diameter of the pulmonary vessels thus decreasing their resistance. This allows for increased gas exchange and cardiac output, increasing the surface area for gas exchange all while preventing huge increases in pulmonary capillary pressure.

Effect of hydrostatic pressure gradients in the lungs on regional pulmonary blood flow

The blood pressure in the foot of a standing person can be as much as 90 mm Hg greater than the pressure at the level of the heart. This is caused by hydrostatic pressure-that is, by the weight of the blood itself in the blood vessels. The same effect, but to a lesser degree, occurs in the lungs. 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, and the pressure in the lowest portion of the lungs is about 8 mm Hg greater. Such pressure differences have profound effects on blood flow through the different areas of the lungs. This is demonstrated by the lower curve in Fig. 1, which depicts blood flow per unit of lung tissue at different levels of the lung in the upright person. Note that in the standing position at rest, there is little flow in the top of the lung but about five times as much flow in the bottom. To help explain these differences, one often describes the lung as being divided into three zones, as shown in Fig. 2. In each zone, the patterns of blood flow are quite different. Figure 1. Blood flow at different levels in the lung of an upright person at rest and during exercise. Note that when the person is at rest, the blood flow is very low at the top of the lungs; most of the flow is through the bottom of the lung.

Zones 1, 2, and 3 of pulmonary blood flow

The capillaries in the alveolar walls are distended by the blood pressure inside them, but simultaneously they are compressed by the alveolar air pressure on their outsides. Therefore, any time the lung alveolar air pressure becomes greater than the capillary blood pressure, the capillaries close and there is no blood flow. Under different normal and pathological lung conditions, one may find any one of three possible zones (patterns) of pulmonary blood flow, as follows: Zone 1: No blood flow during all portions of the cardiac cycle because the local alveolar capillary pressure in that area of the lung never rises higher than the alveolar air pressure during any part of the cardiac cycle Zone 2: Intermittent blood flow only during the peaks of pulmonary arterial pressure 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 Figure 2. Mechanics of blood flow in the three blood flow zones of the lung: zone 1, no flow-alveolar air pressure (PALV) is greater than arterial pressure; zone 2, intermittent flow-systolic arterial pressure rises higher than alveolar air pressure, but diastolic arterial pressure falls below alveolar air pressure; and zone 3, continuous flow-arterial pressure and pulmonary capillary pressure (Ppc) remain greater than alveolar air pressure at all times.

Capillary exchange of fluid in the lungs and pulmonary intersitial fluid dynamics

The dynamics of fluid exchange across the lung capillary membranes are qualitatively the same as for peripheral tissues. However, quantitatively, there are important differences, as follows: 1. The pulmonary capillary pressure is low, about 7 mm Hg, in comparison with a considerably higher functional capillary pressure in the peripheral tissues of about 17 mm Hg. 2. The interstitial fluid pressure in the lung is slightly more negative than that in the peripheral subcutaneous tissue. (This has been measured in two ways: by a micropipette inserted into the pulmonary interstitium, giving a value of about -5 mm Hg, and by measuring the absorption pressure of fluid from the alveoli, giving a value of about -8 mm Hg.) 3. The pulmonary capillaries are relatively leaky to protein molecules, so the colloid osmotic pressure of the pulmonary interstitial fluid is about 14 mm Hg, in comparison with less than half this value in the peripheral tissues. 4. The alveolar walls are extremely thin, and the alveolar epithelium covering the alveolar surfaces is so weak that it can be ruptured by any positive pressure in the interstitial spaces greater than alveolar air pressure (>0 mm Hg), which allows dumping of fluid from the interstitial spaces into the alveoli.

Intro

The lung has two circulations: (1) A high-pressure, low-flow circulation supplies systemic arterial blood to the trachea, the bronchial tree including the terminal bronchioles, the supporting tissues of the lung, and the outer coats (adventia) of the pulmonary arteries and veins. The bronchial arteries, which are branches of the thoracic aorta, supply most of this systemic arterial blood at a pressure that is only slightly lower than the aortic pressure. (2) A low-pressure, high-flow circulation that supplies venous blood from all parts of the body to the alveolar capillaries where oxygen is added and carbon dioxide is removed. The pulmonary artery, which receives blood from the right ventricle, and its arterial branches carry blood to the alveolar capillaries for gas exchange and the pulmonary veins then return the blood to the left atrium to be pumped by the left ventricle though the systemic circulation.

Pulmonary Circulation

The primary function of the pulmonary circulation is to bring the systemic venous blood into close contact with the alveoli to allow gas exchange to occur. Secondary roles: blood reservoir - 40% of lungs weight is due to the ~500 ml of blood; blood filter - dissipation of small emboli by pulmonary endothelium, fibrinolysis, absorption of air emboli; metabolic activity - activation/inactivation of vasoactive substances. The pulmonary circulation is a low pressure, low resistance, highly compliant circulation. Pulmonary vessels are thinner walled and have less smooth muscle and elastic components that their systemic counterparts. Pulmonary capillaries differ from systemic capillaries. The pulmonary capillaries are exceptionally thin walled. Pulmonary Circulation: Mean Pressures (mmHg). Pulmonary artery 15, beginning of capillary 12, end of capillary 9, left atrium 8. Net driving pressure 15-8=7 Systemic circulation: mean pressures (mmHg). Aorta 95, beginning of capillary 35, end of capillary 15, right atrium 2. Net driving pressure 95-2=93 Pulmonary fluid exchange: Normal Starling forces. Pc 8-10 mm Hg, πc-25 mmHg

Negative Pulmonary Interstitial Pressure and the Mechanism for Keeping the Alveoli "Dry."

What keeps the alveoli from filling with fluid under normal conditions? One's first inclination is to think that the alveolar epithelium is strong enough and continuous enough to keep fluid from leaking out of the interstitial spaces into the alveoli. This is not true because experiments have shown that there are always openings between the alveolar epithelial cells through which even large protein molecules, as well as water and electrolytes, can pass. However, if one remembers that the pulmonary capillaries and the pulmonary lymphatic system normally maintain a slight negative pressure in the interstitial spaces, it is clear that whenever extra fluid appears in the alveoli, it will simply be sucked mechanically into the lung interstitium through the small openings between the alveolar epithelial cells. Then the excess fluid is either carried away through the pulmonary lymphatics or absorbed into the pulmonary capillaries. Thus, under normal conditions, the alveoli are kept "dry," except for a small amount of fluid that seeps from the epithelium onto the lining surfaces of the alveoli to keep them moist.

Zone 1

Zone 1 Blood Flow Occurs Only Under Abnormal Conditions. Zone 1 blood flow, which means no blood flow at any time during the cardiac cycle, occurs when either the pulmonary systolic arterial pressure is too low or the alveolar pressure is too high to allow flow. For instance, if an upright person is breathing against a positive air pressure so that the intra-alveolar air pressure is at least 10 mm Hg greater than normal but the pulmonary systolic blood pressure is normal, one would expect zone 1 blood flow-no blood flow-in the lung apices. Another instance in which zone 1 blood flow occurs is in an upright person whose pulmonary systolic arterial pressure is exceedingly low, as might occur after severe blood loss. Effect of Exercise on Blood Flow Through the Different Parts of the Lungs Referring again to Fig. 1, one sees that the blood flow in all parts of the lung increases during exercise. The increase in flow in the top of the lung may be 700 to 800 percent, whereas the increase in the lower part of the lung may be no more than 200 to 300 percent. The reason for these differences is that the pulmonary vascular pressures rise enough during exercise to convert the lung apices from a zone 2 pattern into a zone 3 pattern of flow.