pulmonary blood flow
Dr.Wael Abd Elfattah
Lecturer of chest disease
faculty of medicine - Ain Shams univeristy
Ibn al-Nafis (1213–1288) was an Arab physician who made several important
contributions to the early knowledge of the pulmonary circulation. He was the
first person to challenge the long-held contention of the Galen School that blood
could pass through the cardiac interventricular septum, and in keeping with this
he believed that all the blood that reached the left ventricle passed through the
lung
What is pulmonary circulation?
• Pulmonary circulation is the movement of blood from
the heart, to the lungs, and back to the heart again.
• This is just one phase of the overall circulatory system.
• It carries waste-rich blood and oxygen depleted blood
away from the heart and to the lungs and returns
oxygenated blood back to the heart.
• Deoxygenated blood exits the heart through the
pulmonary arteries and enters the lungs and
oxygenated blood comes back through pulmonary
veins. The blood moves from right ventricle of the
heart to the lungs back to the left atrium.
What is the importance and functions
of pulmonary circulation?
•
•
•
•
•
•
Gas exghange.
Blood reservoir.
Metabolic function.
Filtration of thrombi.
Defensive function.
Drug administration.
What is difference between pulmonary
and systemic circulation?
Pulmonary vessels
pressure
Resistance
Pulmonary circulation
systemic circulation
Thinner ( 30% )
less elastic and smooth
muscle fibers
Thicker
More elastic and smooth
muscle fibers
25/8 mmHg
120/80mmHg
1.8 mmHg/L/min
(About 10% of systemic)
18 mmHg/L/min
What are characteristics of pulmonary
circulation?
• low pressure
• low resistance
• High capacitance
What are factors affecting pulmonary
blood flow?
• Neural control:
-Sympathetic stimulation VC of pulm. vessels
-Parasympathetic stimulation VD of pulm. vessels
• Chemical control:
-Mediators → VC:catecholamines-angiotensin ǁThromboxanA2- PGF2α
-Mediators → VD: Histamin- acetylecholine- PGI2
• Blood gases control:
- Hypoxia and hypercapnia VC of pulm. vessels,
unlike most of the vessels in the body !!!
What are other factors affecting
pulmonary blood flow?
• Gravity.
• Respiratory movement.
• Cardiac output.
What are factors affecting fluid movement
across pulmonary capillary?
• The Starling equation can be applied to the
pulmonary microcirculation in the same way as any
other capillary bed.
fluid movement = k × (ΔP – Δπ)
• Hydrostatic pressure gradient
• Oncotic pressure gradient
• capillary permeability co-effiecent
Typical values for the Starling’s Forces in Pulmonary
Capillaries
Capillary hydrostatic pressure (Pc) is 13 mmHg (arteriolar end) to
6 mmHg (venous end) but variable because of the hydrostatic
effects of gravity esp in the erect lung.
• Interstitial hydrostatic pressure (Pi) - Variable but ranges from
zero to slightly negative.
• Capillary oncotic pressure = 25 mmHg (Same as in systemic
capillaries)
• Interstitial oncotic pressure = 17 mmHg (This is estimated from
measurements on lung lymph)
The net force favors a small continuous leak of fluid out of capillaries into
interstitial space.This fluid travels through the lung lymphatic drainage.
Safety Factors Preventing Pulmonary Oedema
• Increased lymph flow: Increased fluid filtration causes increased
lymph flow which tends to remove the fluid.
• Decrease in interstitial oncotic pressure (oncotic buffering
mechanism): When filtration increases, the albumin loss in the
filtrate decreases. This combined with the increased lymph flow
washes the albumin out of the interstitium and interstitial oncotic
pressure decreases. This protection does not work if the capillary
membrane is damaged eg by septic mediators.
• High interstitial compliance: A large volume of fluid can accumulate
in the gel of the interstitium without much pressure rise.
• Surfactant prevent alveolar collapse.
 These safety mechanisms are quite effective especially in
preventing pulmonary oedema associated with rises in capillary
hydrostatic pressure. It has been estimated that the capillary
hydrostatic pressure can rise to three times normal before alveolar
flooding occurs. Surfactant assists in the prevention of alveolar
flooding also.
Pulmonary edema
• Def: It is extra vascular accumulation of fluid in
the lung.( frist in the interstitium and later in the
alveoli)
• Causes:
• Increased capillary Hydrostatic pressure
• Decreased interstitial Hydrostatic pressure
• capillary permeability
• Lymphatic obstruction
• Oncotic pressure gradient
• Other conditions.
Ventilation
Definition:
• The movement of air between the atmosphere and alveoli
and the distribution of air within the lungs to maintain
appropriate concentrations of oxygen and carbon dioxide in
the blood.
• In normal resting individuals, alveolar ventilation is about
4.0 L/min and pulmonary blood flow is about 5.0 L/min.
• In normal individuals, alveolar ventilation and blood flow
are each distributed uniformly to the gas-exchanging units
• Healthy subjects seated in the upright posture breathing
quietly from FRC show differences in regional ventilation.
• Ventilation is directed preferentially to the bases of the
lungs.
Explanation for Differences in Regional Ventilation
• In the upright lung intrapleural pressure varies from the top to the
base of the lungs.
• the intrapleural pressure at the apex of the lung is about – 8 cm
H2O and at the base about – 1.5 cm H2O. This means that the
alveoli at the apex are exposed to a greater distending pressure (PAPpl= 0 - -8= 8 cm H2O) compared to those at the base (PA-Ppl= 0 - 1.5= 1.5 cmH2O). It is this difference in initial volume that results in
the preferential distribution of ventilation to the alveoli at the base
of the lungs.
• The alveoli at the base of the lung have a smaller initial and being
relatively more compliant, the alveoli at the base fill to a greater
extend for a given change in intrapleural pressure during inspiration
compared to the alveoli at the apex.
• Conversely, the alveoli at the apex are exposed to a larger
distending pressure, being relatively less compliant and fill less
during inspiration.
Distribution of Perfusion in the Normal Upright Lung
• Pulmonary blood flow is preferentially directed to the base of the
lungs. This distribution is dependent on three relative pressures:
alveolar pressure, pulmonary arterial pressure and pulmonary
venous pressure.
• Three functional zones based on these pressure relationships where
first suggested by John West and characterized in experiments on
excised, perfused upright animal lungs under low arterial perfusion
pressures.
• The most apical region, zone 1:
where PA>Pa>Pv, alveolar pressure exceeds vascular pressures
resulting in capillary collapse and no blood flow. The alveoli in this
zone do not participate in gas exchange and are part of the lung’s
alveolar dead space.
• In healthy subjects under normal perfusion pressures, zone I is not
present because arterial pressures is just sufficient to raise blood
to the top of the lung and exceed alveolar pressure.
• Zone I may be present if pulmonary arterial pressure is
reduced (followingsevere hemorrhage) or if alveolar
pressure is raised (during positive pressure ventilation).
• In Zone 2: where Pa>PA>Pv, there is intermittent
blood flow only during the pulmonary arterial pressure
peaks because the systolic pressure is then greater
than the alveolar air pressure, but the diastolic
pressure is less than the alveolar air pressure
• InZone 3: where Pa>Pv>PA, there is continuous blood
flow because the alveolar capillary pressure remains
greater than alveolar air pressure during the entire
cardiac cycle
• Normally, the lungs have only zones 2 and 3 blood
flow—zone 2 (intermittent flow) in the apices, and
zone 3 (continuous flow) in all the lower areas.
Ventilation Perfusion Ratio
• Both airflow and blood flow increase down the lung, but the
differences in perfusion are greater than the differences in
ventilation. Blood flow shows about a 5-fold difference
between the top and bottom of the lung, while ventilation
shows about a 2-fold difference. Blood flow is proportionately
greater than ventilation at the base, reflecting a lower V/Q
ratio and ventilation is proportionately greater than blood
flow at the apex, demonstrating a higher V/Q ratio
• The alveolar partial pressure of oxygen and carbon dioxide are
determined by the ratio of ventilation to perfusion.
TYPES OF V/Q RELATIONSHIP
• IDEAL V/Q or V/Q OF 1
• An ideal is a condition where in the respiratory
unit receives equal amount of ventilation and
perfusion. In this situation gas exchange between
the alveolar gas and pulmonary capillary blood is
optimal that is why it is known as an ideal .
• For example, if a respiratory unit receives 3 lpm
of ventilation and 3 lpm of blood flow, then the
relationship is 3/3 or 1.
IDEAL V/Q or V/Q OF 1
LOW V/Q or V/Q<1
• This condition exists when the respiratory unit receives
inappropriate low ventilation relative to its perfusion
such as at lung base.
• This may be caused by a low respiratory unit
ventilation due to low compliance such
as
in
pulmonary fibrosis and lack of surfactant or due to high
airway resistance found
in asthma and chronic
obstructive pulmonary disease.
• For example, a respiratory unit receives 2 lpm of
ventilation and 3 lpm of blood flow, then the
relationship is 2/3 or V/Q less than 1.
HIGH V/Q or V/Q>1
• This condition exists when the respiratory unit
receives inappropriate low perfusion relative
to its ventilation such as lung apical part.
• Hypotensive states or a partial obstruction of
pulmonary blood vessels present in
pulmonary embolism may be responsible for a
high V/Q.
• For example, a respiratory unit receives 3 lpm
of ventilation and 2 lpm of blood flow, then
the relationship is 3/2 or V/Q greater than 1.
PO2-PCO2 , V/Q diagram
• The upper regions of the lungs with higher V/Q ratio have
relatively higher PO2s and lower PCO2s compared to the
lower regions.
• At first glance this may seem to indicate that there is greater
gas exchange at the upper regions of the lungs but recall that
there is greater ventilation and perfusion to the bases of the
lungs and therefore greater gas exchange at the base of the
lungs.
• Overall this arrangement results in adequate gas exchange but
if disease states affect either the distribution of ventilation or
perfusion, there will be an increase in V/Q mismatching and
gas exchange worsens
ZERO V/Q
• This conditon exists when the respiratory unit
receives no ventilation but perfusion remains
normal.(venous to arterial shunt)
• This may be caused by alveolar flooding found in
pneumonia and ARDS, complete obstruction of
the airway, and extrinsic compresion of alveoli
present in compression atelectasis due to
hydrothorax or pneumothorax.
• For example, a respiratory unit receives 0 lpm of
ventilation and 3 lpm of blood flow, then the
relationship is 0/3 or V/Q = 0.
V/Q of Infinity
• This condition exists when the respiratory unit
receives no perfusion but ventilation remains
normal. ( alveolar dead space)
• This may be found when there is a complete
obstruction of the pulmonary blood vessels.
• For example, a respiratory unit receives 3 lpm
of ventilation and 0 lpm of blood flow, then
the relationship is 3/0 or V/Q of infinity.
Compensating mechanisms matching
ventilation and perfusion
• Alveolar hypoventilation.
• Decreased pulmonary perfusion
32
33
36