Pulmonary Circulation

Easy explanation of the pulmonary circulation

1. PULMONARY CIRCULATION

2. LEARNING OUTCOMES Organization of pulmonary vessels in intrauterine life Physiologic anatomy of the pulmonary Circulatory system Pressures in the pulmonary system Interrelations between interstitial fluid pressure and other pressures in the lung Applied/Pulmonary Edema

3. Organization of pulmonary vessels in intrauterine life In a fetus, the pulmonary circulation has a high vascular resistance because the lungs are partially collapsed. This high vascular resistance helps to shunt blood from the right to the left atrium through the foramen ovale , and from the pulmonary artery to the aorta through the ductus arteriosus

4. After birth, the foramen ovale and ductus arteriosus close , and the vascular resistance of the pulmonary circulation falls sharply. This fall in vascular resistance at birth is due to Opening of the vessels as a result of the subatmospheric intrapulmonary pressure and physical stretching of the lungs during inspiration Dilation of the pulmonary arterioles in response to increased alveolar PO2

5. Physiologic Anatomy of the Pulmonary Circulatory System 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

6. Bronchial Vessels Blood also flows to the lungs through small bronchial arteries that originate from the systemic circulation, amounting to 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 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 per cent greater than the right ventricular output.

7. 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 thence 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.

8. Pressures in the Pulmonary System

9. Pressure Pulse Curve in the Right Ventricle. The pressure pulse curves of the right ventricle and pulmonary artery are shown in the lower portion of Figure These curves are contrasted with the much higher aortic pressure curve shown in the upper portion of the figure. The systolic pressure in the right ventricle of the normal human being averages about 25 mm Hg

10. Pressures in the Pulmonary Artery T he 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

11. Left Atrial and Pulmonary Venous Pressures 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. It usually is not feasible to measure a human being’s left atrial pressure using a direct measuring device because it is difficult to pass a catheter through the heart chambers into the left atrium. However, the left atrial pressure can often be estimated with moderate accuracy by measuring the so-called pulmonary wedge pressure.

12. This is achieved by inserting a catheter first through a peripheral vein to the right atrium, then through the right side of the heart and through the pulmonary artery into one of the small branches of the pulmonary artery, finally pushing the catheter until it wedges tightly in the small branch. The pressure measured through the catheter, called the “wedge pressure,” is about 5 mm Hg.

13. Pulmonary Capillary Pressure The mean pulmonary capillary pressure is about 7 mm Hg.

14. In the adult, the right ventricle (like the left) has a cardiac output of about 5.5 L per minute. The rate of blood flow through the pulmonary circulation is thus equal to the flow rate through the systemic circulation. Blood flow, is directly proportional to the pressure difference between the two ends of a vessel and inversely proportional to the vascular resistance .

15. The pulmonary circulation, in other words, is a low-resistance, low-pressure pathway. The low pulmonary blood pressure produces less filtration pressure than that produced in the systemic capillaries, and thus affords protection against pulmonary edema

16. Interrelations Between Interstitial Fluid Pressure and Other Pressures in the Lung.

17. 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

18. FACTORS AFFECTING PULMONARY CIRCULATION Pulmonary arterioles constrict when the alveolar PO2 is low and dilate as the alveolar PO2 is raised. This response is opposite to that of systemic arterioles, which dilate in response to low tissue PO2 Dilation of the systemic arterioles when the PO2 is low helps to supply more blood and oxygen to the tissues; constriction of the pulmonary arterioles when the alveolar PO2 is low helps to decrease blood flow to alveoli that are inadequately ventilated.

19. Constriction of the pulmonary arterioles where the alveolar PO2 is low and their dilation where the alveolar PO2 is high helps to match ventilation to perfusion (the term perfusion refers to blood flow). If this did not occur, blood from poorly ventilated alveoli would mix with blood from well-ventilated alveoli, and the blood leaving the lungs would have a lowered PO2 as a result of this dilution effect

20. APPLIED

21. PULMONARY EDEMA Pulmonary edema occurs in the same way that edema occurs elsewhere in the body. Any factor that causes the pulmonary interstitial fluid pressure to rise from the negative range into the positive range will cause rapid filling of the pulmonary interstitial spaces and alveoli with large amounts of free fluid.

22. CAUSES 1. Left-sided heart failure or mitral valve disease, with consequent great increases in pulmonary venous pressure and pulmonary capillary pressure and flooding of the interstitial spaces and alveoli. 2. Damage to the pulmonary blood capillary membranes caused by infections such as pneumonia or by breathing noxious substances such as chlorine gas or sulfur dioxide gas. Each of these causes rapid leakage of both plasma proteins and fluid out of the capillaries and into both the lung interstitial spaces and the alveoli.

23. Safety Factor in Chronic Conditions When the pulmonary capillary pressure remains elevated chronically (for at least 2 weeks), the lungs become even more resistant to pulmonary edema because the lymph vessels expand greatly, increasing their capability of carrying fluid away from the interstitial spaces perhaps as much as 10-fold. Therefore, in patients with chronic mitral stenosis , pulmonary capillary pressures of 40 to 45 mm Hg have been measured without the development of lethal pulmonary edema.