Learning Objectives:
1. Describe the 4 functions of the pulmonary circulation.
2. Describe the normal anatomical shunts that add venous blood to arterialized blood
leaving the left ventricle.
3. Contrast the systemic and pulmonary circulations with respect to normal pressures,
resistance to blood flow, and vascular response to hypoxia.
4. Describe how pulmonary vascular resistance changes with alterations in pulmonary
arterial pressure, lung volume, and alveolar hypoxia.
5. Describe the potential causes of pulmonary edema and pleural effusion.
6. Describe the cause of ventilation – perfusion mismatch in normal lungs.
I. Pulmonary Circulation has 4 functions: blood filter,
metabolic organ, blood reservoir, and gas exchange.
1. The lungs filter the blood and protect against clots (thrombi) and air or fat globules
(emboli).
2. An important metabolic function of the lung is
conversion of angiotensin I to angiotensin II by
angiotensin converting enzyme (ACE) located in
endothelial cells.
3. The pulmonary circulation is an important reservoir
having about 500 ml of blood or 10% of total blood
volume. This stabilizes left ventricle stroke volume when going from supine to
standing.
· The pulmonary circulation is a high flow, low resistance, low pressure system.
· Arteries and veins are thin-walled and more compliant than systemic vessels.
· Vascular resistance is controlled mainly by local factors e.g. when alveoli have
low O2, the vessels serving those alveoli constrict. This response to low oxygen
is opposite to that in the systemic circulation but is also beneficial by
redirecting blood flow to areas of the lung that are better ventilated.
4. Gas exchange is the main business of the lungs. Efficient gas exchange across the
air-blood barrier depends on a good matching of air flow (ventilation) and blood
flow (perfusion). Gravity creates uneven matching of ventilation and perfusion.
Both air flow and blood flow are higher at the base of the upright lung, but blood
flow is more affected by gravity than air flow. The angiogram above shows the
distended blood vessels at the base of the lungs.
II. Shunts
A shunt refers to blood that somehow bypasses the normal flow route. They can occur
from arterial into venous blood (a left to right shunt) or from venous to arterial blood (a right to
left shunt). Shunts can be both anatomical and physiological (e.g., blood that perfuses areas of
the lung with low or no ventilation).
Normal Anatomical Shunts
1. Bronchial circulation supplies the connective tissue, septa, and large and small
bronchi. Bronchial veins drain into the pulmonary veins producing a shunt.
2. Coronary circulation supplies the heart muscle. Thebesian veins (venae cordis
minimae) empty into the cardiac chambers which, on the left side of the heart, create
a shunt.
· These normal shunts are about 1 to 2 per cent of the total cardiac output.
· The human fetus has additional normal shunts; i.e. the foramen ovale and the ductus
arteriosus.
III. Pulmonary Pressures and Volumes
· The pressures in the various parts of the
pulmonary circulation and systemic circulation
are shown in the figure to the right.
· The mean driving pressure in the pulmonary
system is about 10 mm Hg, (mean PA = 15 minus
mean LA = 5) compared with a driving pressure
of about 91 mm Hg in the systemic circulation
(93 – 2).
· Since the flows are equal, the vascular resistance
of pulmonary circulation is about 9 times less
than that of the systemic circulation.
A. Left Atrial Pressure is Measured via a Pulmonary Artery Catheter
Pulmonary artery balloon catheters (Swan-Ganz) can be used to measure
pulmonary artery and left atrial pressures. When the balloon is inflated blood flow
stops. Since there is no flow, there is no
pressure difference between the tip of the
catheter and the left atrium. Therefore the
pressure measured at the tip of the catheter
equals left atrial pressure. Elevated left atrial
pressure can be due to mitral stenosis and can
cause pulmonary edema.
B. Pulmonary Blood Volumes
· The pulmonary circulation has less smooth muscle and is much more compliant than
the systemic circulation.
· Consequently, the total volume changes readily with changes in intrathoracic
pressures e.g.....................................
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Dr Khaled Abdelfattah
الأحد نوفمبر 28, 2010 7:46 pm