In Chapter 15, we pointed out that 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 difference 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 upright adult, the lowest point in the lungs is normally about 30 cm below the highest point, which 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 effect is demonstrated by the lower curve in Figure 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, the lung is often described as being divided into three zones, as shown in Figure 2. In each zone, the patterns of blood f low are quite different.

Fig1.  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  f low is through the bottom of the lung. 

Fig2. 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 f low—arterial pressure and pulmonary capillary pressure (Ppc) remain  greater than alveolar air pressure at all times. 

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 are com pressed 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 f low, 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

Normally, the lungs have only zones 2 and 3 blood f low—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 f low 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 f low begins in 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, continuous flow occurs 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 f low, including the lung apices.

Zone 1 Blood Flow Occurs Only Under Abnormal Conditions. Zone 1 blood flow, which means no blood f low 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.

Exercise Increases Blood Flow Through All Parts of the Lungs. Referring again to Figure 1, one sees that the blood flow in all parts of the lung increases during exercise. A major reason for increased blood flow 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.

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 may increase 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. Normally, the first two changes decrease pulmonary vascular resistance so much that the pulmonary arterial pressure rises very little, even during maximum exercise. This effect is shown in Figure 3.

Fig3. Effect on mean pulmonary arterial pressure caused by  increasing the cardiac output during 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 and the development of pulmonary edema.

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 little effect on pulmonary circulatory function. However, when the left atrial pressure rises to greater than 7 or 8 mm Hg, further increases in left atrial pressure 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 capillary pressure almost equally as much. When the left atrial pressure rises above 30 mm Hg, causing similar increases in capillary pressure, pulmonary edema is likely to develop, as we discuss later in the chapter.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الفكرية يصدر عددين جديدين من نشرتي الكفيل والخميس

متحف الكفيل يستعرض حاملَ شموعٍ يعود إلى العصر القاجاري

ذوو شهداء مدرسة ميناب الإيرانية يتشرفون بزيارة مرقد أبي الفضل العباس (عليه السلام)

قسم الصناعات ينجز 75‎%‎ من أعمال إنشاء سور حديدي في شركة عين الحياة

المزيد

الآخبار الصحية

السجائر الإلكترونية والإقلاع عن التدخين.. نتائج متباينة تثير الجدل العلمي

مصر تحقق إنجازاً دولياً جديداً في منظمة الصحة العالمية

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عامل خطر في بداية الحمل يؤخر تطور الكلام والمهارات الحركية لدى الرضع

المزيد

الآخبار التكنلوجية

أحلامك ليست عشوائية.. إليك ما يفعله دماغك أثناء نومك

روسيا.. العلماء يضاعفون الاستقرار الحراري للألواح الشمسية ثلاث مرات

علماء يكشفون سر سرعة الدلافين في السباحة

سلمي أو دموي.. هكذا تنتقل السلطة في مجتمع الجرذان

المزيد

مواضيع ذات صلة

Pulmonary Capillary Dynamics

Blood Flow Through the Lungs and its Distribution

Conversion of Prothrombin to Thrombin

Formation of the Platelet Plug

Hemoglobin Formation

Erythropoietin Regulates Red Blood Cell Production

Stages of Differentiation of Red Blood Cells

Genesis of Blood Cells

Role of the Nervous System in Rapid Control of Arterial Pressure

Special Features of Nervous Control of Arterial Pressure

The Lymphatic System Plays A Key Role in Controlling Interstitial Fluid Protein Concentration, Volume, and Pressure

Formation of Lymph