The Lungs
The pulmonary circulation
The pulmonary circulation takes blood from the right side of the heart through the lungs to the
left side of the heart Blood leaves the right ventricle in the pulmonary trunk and is distributed to
the lungs by the pulmonary arteries. Within the lung the blood supply is segmental. Blood is
collected into two pairs of pulmonary veins and returned to the left atrium.
The pulmonary trunk
The pulmonary trunk
arises from above the infundibulum of the right ventricle. It passes
upwards, backwards and to the right to bifurcate under the arch of the aorta. A set of semilunar
valves prevents regurgitation into the right ventricle. The pulmonary trunk at its division forms
part of the left border of the cardiac image on a PA x-ray. Since the trunk arose by subdivision of
the primitive truncus arteriosis, it is enclosed in a common serous pericardial sheath
with the
aorta. The vessel is not as thick as the aorta, being required to contain pressures of only about
15mmHg.
The pulmonary arteries
The pulmonary arteries
pass
right and left from the bifurcation of the trunk, to reach the hilum of
the lung. The right pulmonary artery passes to the right lung below the arch of the aorta. At the
hilum of the lung it divides into an upper branch accompanying the right upper lobe bronchus, and
a lower branch accompanying the middle and lower lobe bronchi. The left pulmonary artery is
shorter than the right and provides two branches at the hilum of the lung to the upper and lower
lobes. The left pulmonary artery is connected to the aortic arch by the ligamentum arteriosum,
the obliterated ductus arteriosus.
The pulmonary veins
On the right side the veins of the
upper and middle lobe join to form an upper vein. A lower vein
is formed from the segmental veins of the lower lobe. On the left each lobe contributes one vein.
At the hilum of the lungs the upper pulmonary vein is anterior and inferior to the artery. The
primary bronchi are posterior to the arteries and veins.
Blood distribution
In the lung the distribution of blood closely matches the ventilation of the segments. Blood passes
predominantly to the base of the lung under the influence of gravity. The distribution of blood
can be described as three zones: the basal zone is normally well perfused, the middle zone is
perfused intermittently as pulmonary vessels open and close, and the upper zone is rarely perfused
except when the demand is greatest. Pulmonary arterial pressure must be increased to perfuse the
middle and upper zones.
At the alveolar level, capillaries form a mesh over each alveolus, with only a basement membrane
between the endothelium and epithelium. Blood can be shunted from the arterial side to the
venous side, bypassing unventilated alveoli. The endothelium of the alveolar capillaries is
specialised in that a decrease in oxygen tension induces a vasoconstriction (the opposite of the
effect in the systemic circulation). Hypoxia-induced vasoconstriction aids in the regulation of
ventilation/perfusion matching in normal subjects. However this reflex also contributes to the
pulmonary hypertension occurring in patients with hypoxia related abnormalities such as chronic
obstructive lung disease, cystic fibrosis, chronic bronchitis and emphysema. The pulmonary
endothelium is also specialised in that there is a high level of angiotensin converting enzyme
activity expressed.
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