PULMONARY BLOOD VESSELS
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
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
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
Definition
Physiological
shunt is defined as a diversion through which the venous blood is mixed
with arterial blood.
Components
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.
CHARACTERISTIC FEATURES OF PULMONARY
BLOOD VESSELS
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.
PULMONARY BLOOD FLOW
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 PRESSURE
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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