High-pressure, Low-flow
Circulation
▪ supplies systemic arterial blood
▪ Trachea
▪ bronchial tree (including the terminal bronchioles)
▪ supporting tissues of the lung
▪ outer coats (adventitia) of the pulmonary arteries
and veins
▪ BRONCHIAL ARTERIES - supply most of this
systemic arterial blood at a pressure that is only
slightly lower than the aortic pressure
Low-pressure, High-flow
Circulation
▪ supplies venous blood from all parts of the body to
the alveolar capillaries where oxygen (O2) is added
and carbon dioxide (CO2) is removed
▪ pulmonary artery (receives blood from the right
ventricle) and its arterial branches carry blood to
the alveolar capillaries for gas exchange
▪ pulmonary veins - return the blood to the left
atrium to be pumped by the left ventricle though
the systemic circulation
Right Ventricle
▪ systolic pressure -
about 25 mm Hg
▪ diastolic pressure averages about 0 to 1
mm Hg
▪ values are only one fifth
those for the left
ventricle
25
Pulmonary Arteries
▪ Systolic pressure is essentially equal to the pressure in the
right ventricle
▪ Closure of pulmonary valves at the end of systole →
▪ ventricular pressure falls precipitously
▪ pulmonary arterial pressure falls more slowly as blood
flows through the capillaries of the lungs
▪ systolic pulmonary arterial pressure - averages about 25
mm Hg in the
▪ diastolic pulmonary arterial pressure - about 8 mm Hg
▪ mean pulmonary arterial pressure is 15 mm Hg
Pulmonary Capillaries
▪ mean pulmonary
capillary pressure about 7 mm Hg
Left Atrium and Pulmonary Veins
▪ mean pulmonary venous & left atrial pressure -
averages about 2 mm Hg in the recumbent human
being (1 mm Hg - 5 mm Hg)
▪ estimated with moderate accuracy by measuring
the so-called pulmonary wedge pressure
▪ “wedge pressure,” = 5 mm Hg (usually only 2 to 3
mm Hg greater than the left atrial pressure)
measurement 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
▪ 9% of the total blood volume
▪ 450 mL
▪ ~70 mL of this is in the pulmonary capillaries
▪ remainder equally divided between the pulmonary
arteries and the veins
▪ cardiac output of the right heart = cardiac output of
the left heart
▪ characterized by much lower pressures and
resistances
▪ Pulmonary blood flow is directly proportional to
the pressure gradient between the pulmonary
artery and the left atrium and is inversely
proportional to the resistance of the pulmonary
vasculature
(Q = ΔP/R)
Regulation of Pulmonary Blood
Flow
▪ regulated primarily by altering the resistance of
the arterioles
▪ changes in resistance are accomplished by
changes in the tone of arteriolar smooth
muscle
▪ mediated by local vasoactive substances, especially
O2
Hypoxic vasoconstriction
▪ partial pressure of O2 in alveolar gas (PAO2) - major factor
regulating pulmonary blood flow
PAO2 produce pulmonary vasoconstriction
▪ adaptive mechanism reducing pulmonary blood flow to poorly
ventilated areas where the blood flow would be “wasted”
▪ PAO2 <70 mm Hg:
-hypoxia causes depolarization of vascular smooth muscle
cells; depolarization opens voltage-gated Ca2+ channels, leading to
Ca2+ entry into the cell and contraction
-Inhibition of NO synthase
Bridge to Anatomy:
▪ Fetal circulation
▪ another example of global hypoxic
vasoconstriction
▪ PAO2 is much lower in the fetus than in
the mother → producing vasoconstriction
in the fetal lungs-->increases pulmonary
vascular resistance --> dec pulmonary
blood flow to approximately 15% of the
cardiac output
▪ At birth, the neonate’s first breath
increases PAO2 to 100 mm Hg -->
hypoxic vasoconstriction is reduced->pulmonary vascular resistance
decreases--> pulmonary blood flow
increases and eventually equals cardiac
output of the left side of the heart (as in
the adult)
Other vasoactive substances
▪ Thromboxane A2 - arachidonic acid metabolism
(cyclooxygenase pathway) in macrophages, leukocytes, and
endothelial cell
▪ produced in response to certain types of lung injury
▪ a powerful local vasoconstrictor of both arterioles and veins
▪ Prostacyclin (prostaglandin I2) - arachidonic acid
metabolism (cyclooxygenase pathway)
▪ a potent local vasodilator
▪ produced by lung endothelial cells
▪ leukotrienes - arachidonic acid metabolism (lipoxygenase
pathway)
▪ cause airway constriction
▪ When a person is supine, blood flow is nearly
uniform throughout the lung.
▪ When a person is standing, blood flow is unevenly
distributed because of the effect of gravity.
▪ Blood flow is lowest at the apex of the lung (zone 1)
and highest at the base of the lung (zone 3).
▪ Alveolar pressure > arterial pressure > venous pressure
▪ No blood flow during all portions of the cardiac cycle
▪ The high alveolar pressure may compress the capillaries and
reduce blood flow in zone 1
▪ Seen in hemorrhage or if alveolar pressure is increased
because of positive pressure ventilation
▪ Arterial pressure > alveolar pressure > venous pressure
▪ Intermittent blood flow
▪ Moving down the lung, arterial pressure progressively increases
because of gravitational effects on arterial pressure.
▪ Arterial pressure is greater than alveolar pressure in zone 2, and
blood flow is driven by the difference between arterial pressure
and alveolar pressure.
▪ Arterial pressure > venous pressure > alveolar pressure
▪ Continuous blood flow
▪ arterial pressure is highest because of gravitational effects,
and venous pressure finally increases to the point where it
exceeds alveolar pressure
▪ In zone 3, blood flow is driven by the difference between
arterial and venous pressures, as in most vascular beds.
▪ Pulmonary Capillary Pressure - 7 mm Hg
▪ blood passes through the pulmonary capillaries in
about 0.8s
▪ Increase cardiac output - 0.3s
▪ additional capillaries open up to accommodate the
increased blood flow
Fluid Dynamics
1. The pulmonary capillary pressure is low, about 7 mm Hg, in
comparison with a considerably higher functional capillary
pressure in the peripheral tissues of about 17 mm Hg.
2. The interstitial fluid pressure in the lung is slightly more
negative than that in peripheral subcutaneous tissue. (−5 mm
Hg, −8 mm Hg.)
3. The colloid osmotic pressure of the pulmonary interstitial fluid
is about 14 mm Hg, in comparison with less than half this value
in the peripheral tissues.
4. The alveolar walls are extremely thin, and the alveolar
epithelium covering the alveolar surfaces is so weak that it can be
ruptured by any positive pressure in the interstitial spaces
greater than alveolar air pressure (>0 mm Hg), which allows
dumping of fluid from the interstitial spaces into the alveoli.
PULMONARY EDEMA
Any factor that increases
fluid filtration out of the
pulmonary capillaries or
that impedes pulmonary
lymphatic function and
causes the pulmonary
interstitial fluid
pressure to rise from the
negative range into the
positive range
Most Common Causes Of
Pulmonary Edema
LEFT-SIDED HEART
FAILURE OR MITRAL
VALVE DISEASE
Most Common Causes Of
Pulmonary Edema
INFECTIONS AND
NOXIOUS SUBSTANCES
“potential space”
Pleural Fluid volume –
10-15 ml (0.2/kg)
−7 mm Hg
• accumulation of large amounts
of free fluid in the pleural
space
• “edema of the pleural cavity”
(1) blockage of lymphatic
drainage from the pleural cavity
(2) cardiac failure
of fluid into the pleural cavity;
(3) greatly reduced plasma
colloid osmotic pressure
(4) infection or any other cause
of inflammation