Pulmonary blood vessels include pulmonary artery, which carries deoxygenated blood to alveoli of lungs and bronchial artery, which supply oxygenated blood to other structures of lungs (see below).


Pulmonary artery supplies deoxygenated blood pumped from right ventricle to alveoli of lungs (pulmonary circulation). After leaving the right ventricle, this artery divides into right and left branches. Each branch enters the corresponding lung along with primary bronchus. After entering the lung, branch of the pulmonary artery divides into small vessels and finally forms the capillary plexus that is in intimate relationship to alveoli. Capillary plexus is solely concerned with alveolar gas exchange. Oxygenated blood from the alveoli is carried to left atrium by one pulmonary vein from each side.


Bronchial artery arises from descending thoracic aorta. It supplies arterial blood to bronchi, connective tissue and other structures of lung stroma, visceral pleura and pulmonary lymph nodes. Venous blood from these structures is drained by two bronchial veins from each side. Bronchial veins from right side drain into azygos vein and the left bronchial veins drain into superior hemiazygos or left superior intercostal veins. However, the blood from distal portion of bronchial circulation is drained directly into the tributaries of pulmonary veins.



Physiological shunt is defined as a diversion through which the venous blood is mixed with arterial blood.


Physiological shunt has two components:

1. Flow of deoxygenated blood from bronchial circulation into pulmonary veins without being oxygenated

makes up part of normal physiological shunt

2. Flow of deoxygenated blood from thebesian veins into cardiac chambers directly

Venous Admixture and Wasted Blood

Physiological shunt results in venous admixture. Venous admixture refers to mixing of deoxygenated

blood with oxygenated blood. Fraction of venous blood, which is not fully oxygenated is generally considered as wasted blood.

Normal Shunt Level and its Variations

Normal physiological shunt of venous blood to the left side of heart is 1% to 2% of cardiac output. In normal persons, it may increase up to 5% of cardiac output, which may be due to mismatching of ventilationperfusion ratio within physiological limits. Pathological increase in the shunt occurs in several

conditions such as acute pulmonary infections and bronchiectasis (permanent dilatation of bronchi due

to chronic pulmonary infections and inflammatory processes).

Physiological Shunt Vs Physiological

Dead Space

Physiological shunt is analogous to physiological dead space. Physiological shunt includes

wasted blood and physiological dead space includes wasted air. Both wasted blood and wasted air exist on either side of alveolar membrane and both affect the ventilation-perfusion ratio.


Following are the characteristic features of pulmonary blood vessels:

1. Pulmonary artery has a thin wall. Its thickness is only about one third of thickness of the systemic

aortic wall. Wall of other pulmonary blood vessels is also thin.

2. Pulmonary blood vessels are highly elastic and more distensible

3. Smooth muscle coat is not well developed in the pulmonary blood vessels

4. True arterioles have less smooth muscle fibers

5. Pulmonary capillaries are larger than systemic capillaries. Pulmonary capillaries are also dense

and have multiple anastomosis, so, each alveolus occupies a capillary basket.

6. Vascular resistance in pulmonary circulation is very less; it is only one tenth of systemic circulation

7. Pulmonary vascular system is a low pressure system. Pulmonary arterial pressure and pulmonary

capillary pressure are very low (see below).

8. Pulmonary artery carries deoxygenated blood from heart to lungs and pulmonary veins carry

oxygenated blood from lungs to heart

9. Physiological shunt is present.


Lungs receive the whole amount of blood that is pumped out from right ventricle. Output of blood per minute is same in both right and left ventricle. It is about 5 liter. Thus, the lungs accommodate amount of blood, which is equal to amount of blood accommodated by all other parts of the body.


Pulmonary blood vessels are more distensible than systemic blood vessels. So the blood pressure is less

in pulmonary blood vessels. Thus, the entire pulmonary vascular system is a low pressure bed.

Pulmonary Arterial Pressure

Systolic pressure : 25 mm Hg

Diastolic pressure : 10 mm Hg

Mean arterial pressure : 15 mm Hg.

Pulmonary Capillary Pressure

Pulmonary capillary pressure is about 7 mm Hg. This pressure is sufficient for exchange of gases between alveoli and blood.

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