Pulmonary Vessels. 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,
which is similar to that of the entire systemic arterial tree. This large compliance
allows the pulmonary arteries to accommodate the stroke volume output of the
right ventricle.
The pulmonary veins, like the pulmonary arteries, are also short. They immediately
empty their effluent blood into the left atrium, to be pumped by the left heart
through the systemic circulation.
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.
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 edemaThe blood volume of the lungs is about 450 milliliters,
about 9 per cent of the total blood volume of the entire
circulatory system. Approximately 70 milliliters of this
pulmonary blood volume is in the pulmonary capillaries,
and the remainder is divided about equally
between the pulmonary arteries and the veins.
Lungs as a Blood Reservoir. Under various physiological
and pathological conditions, the quantity of blood in
the lungs can vary from as little as one half normal up
to twice normal. For instance, when a person blows out
air so hard that high pressure is built up in the lungs—
such as when blowing a trumpet—as much as 250 milliliters
of blood can be expelled from the pulmonary
circulatory system into the systemic circulation.
Also, loss of blood from the systemic circulation by
hemorrhage can be partly compensated for by theautomatic shift of blood from the lungs into the systemic
vessels.
Shift of Blood Between the Pulmonary and Systemic
Circulatory Systems as a Result of Cardiac Pathology.
Failure of the left side of the heart or increased resistance
to blood flow through the mitral valve as a result
of mitral stenosis or mitral regurgitation causes blood
to dam up in the pulmonary circulation, sometimes
increasing the pulmonary blood volume as much as
100 per cent and causing large increases in the pulmonary
vascular pressures. Because the volume of the
systemic circulation is about nine times that of the pulmonary
system, a shift of blood from one system to the
other affects the pulmonary system greatly but usually
has only mild systemic circulatory effects.