Easy explanation of the pulmonary circulation

1. PULMONARY CIRCULATION

2. LEARNING OUTCOMES
1. Organization of pulmonary vessels in
intrauterine life
2. Physiologic anatomy of the pulmonary
Circulatory system
3. Pressures in the pulmonary system
4. Interrelations between interstitial fluid
pressure and other pressures in the lung
5. Applied/Pulmonary Edema

4. Organization of pulmonary vessels in
intrauterine life
• In a fetus, the pulmonary circulation has a
high vascular resistance because the lungs are
partially collapsed.
• This high vascular resistance helps to shunt
blood from the right to the left atrium through
the foramen ovale, and from the pulmonary
artery to the aorta through the ductus
arteriosus

5. • After birth, the foramen ovale and ductus
arteriosus close, and the vascular resistance of
the pulmonary circulation falls sharply. This
fall in vascular resistance at birth is due to
(1) Opening of the vessels as a result of the
subatmospheric intrapulmonary pressure
and physical stretching of the lungs during
inspiration
(2) Dilation of the pulmonary arterioles in
response to increased alveolar PO2

6. Physiologic Anatomy of the Pulmonary
Circulatory System
• 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

7. 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.

9. 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 edema.

10. Pressures in the Pulmonary
System

11. Pressure Pulse Curve in the Right
Ventricle.
• The pressure pulse curves of the right
ventricle and pulmonary artery are shown in
the lower portion of Figure
• These curves are contrasted with the much
higher aortic pressure curve shown in the
upper portion of the figure.
• The systolic pressure in the right ventricle of
the normal human being averages about 25
mm Hg

13. Pressures in the Pulmonary Artery
• The systolic pulmonary arterial pressure
averages about 25 mm Hg in the normal
human being, the diastolic pulmonary arterial
pressure is about 8 mm Hg, and the mean
pulmonary arterial pressure is 15 mm Hg

15. Left Atrial and Pulmonary Venous
Pressures
• The mean pressure in the left atrium and the major
pulmonary veins averages about 2 mm Hg in the
recumbent human being, varying from as low as 1 mm
Hg to as high as 5 mm Hg.
• It usually is not feasible to measure a human being’s
left atrial pressure using a direct measuring device
because it is difficult to pass a catheter through the
heart chambers into the left atrium.
• However, the left atrial pressure can often be
estimated with moderate accuracy by measuring the
so-called pulmonary wedge pressure.

16. • This is achieved by inserting a catheter first
through a peripheral vein to the right atrium,
then through the right side of the heart and
through the pulmonary artery into one of the
small branches of the pulmonary artery, finally
pushing the catheter until it wedges tightly in the
small branch.
• The pressure measured through the catheter,
called the “wedge pressure,” is about 5 mm Hg.

17. Pulmonary Capillary Pressure
• The mean pulmonary capillary pressure is
about 7 mm Hg.

18. • In the adult, the right ventricle (like the left) has a
cardiac output of about 5.5 L per minute.
• The rate of blood flow through the pulmonary
circulation is thus equal to the flow rate through
the systemic circulation.
• Blood flow, is directly proportional to the
pressure difference between the two ends of a
vessel and inversely proportional to the vascular
resistance.

19. • The pulmonary circulation, in other words, is a
low-resistance, low-pressure pathway.
• The low pulmonary blood pressure produces
less filtration pressure than that produced in
the systemic capillaries, and thus affords
protection against pulmonary edema

20. Interrelations Between Interstitial
Fluid Pressure and Other
Pressures in the Lung.

22. • This filtration pressure causes a slight
continual flow of fluid from the pulmonary
capillaries into the interstitial spaces, and
except for a small amount that evaporates
• in the alveoli, this fluid is pumped back to the
circulation through the pulmonary lymphatic
system

23. FACTORS AFFECTING PULMONARY
CIRCULATION
• Pulmonary arterioles constrict when the
alveolar PO2 is low and dilate as the alveolar
PO2 is raised. This response is opposite to that of
systemic arterioles, which dilate in response to
low tissue PO2
• Dilation of the systemic arterioles when the PO2
is low helps to supply more blood and oxygen to
the tissues; constriction of the pulmonary
arterioles when the alveolar PO2 is low helps to
decrease blood flow to alveoli that are
inadequately ventilated.

24. • Constriction of the pulmonary arterioles where
the alveolar PO2 is low and their dilation where
the alveolar PO2 is high helps to match
ventilation to perfusion (the term perfusion refers
to blood flow).
• If this did not occur, blood from poorly ventilated
alveoli would mix with blood from well-ventilated
alveoli, and the blood leaving the lungs would
have a lowered PO2 as a result of this dilution
effect

25. APPLIED

26. PULMONARY EDEMA
• Pulmonary edema occurs in the same way
that edema occurs elsewhere in the body. Any
factor that causes the pulmonary interstitial
fluid pressure to rise from the negative range
into the positive range will cause rapid filling
of the pulmonary interstitial spaces and
alveoli with large amounts of free fluid.

27. CAUSES
1. Left-sided heart failure or mitral valve disease, with
consequent great increases in pulmonary venous
pressure and pulmonary capillary pressure and
flooding of the interstitial spaces and alveoli.
2. Damage to the pulmonary blood capillary membranes
caused by infections such as pneumonia or by
breathing noxious substances such as chlorine gas or
sulfur dioxide gas. Each of these causes rapid leakage
of both plasma proteins and fluid out of the capillaries
and into both the lung interstitial spaces and the
alveoli.

28. Safety Factor in Chronic Conditions
• When the pulmonary capillary pressure remains
elevated chronically (for at least 2 weeks), the
lungs become even more resistant to pulmonary
edema because the lymph vessels expand greatly,
increasing their capability of carrying fluid away
from the interstitial spaces perhaps as much as
10-fold.
• Therefore, in patients with chronic mitral
stenosis, pulmonary capillary pressures of 40 to
45 mm Hg have been measured without the
development of lethal pulmonary edema.