Respiratory Physiology - Pulmonary Blood FLow Flashcards
Mean pressures in pulmonary and systemic vascular systems
What contributes to the vast drop in pressure within the systemic vascular system from heart to capillaries
Muscular arterioles
Definition of vascular resistance
Pressure drop / flow
Pulmonary vascular resistance much lower than systemic as flow is the same but drop in pressure is much less
Pressure in pulmonary capillaries
Very low and also largely dependent and affected by alveolar pressure
If alveolar pressure increases above pulmonary capillary pressure then they collapse
Hence why too much PEEP is a problem
Pressures in small pulmonary veins and arteries
Actively pulled open by radial traction of lung parenchyma surrounding them
Definition of vascular resistance compared with electrical resistance
Effect of changing arterial or venous pressure (with the other kept constant) on pulmonary vascular resistance
Why does changing arterial or venous pressure alter pulmonary vascular resistance
Recruitment of collapsed capillaries
Distension of recruited capillaries
Effect of increasing lung volume on pulmonary vascular resistance
Smaller lung volumes - extra alveolar vessels smaller thus increasing vascular resistance
Parabolic graph shape as when volume continues to increase, capillaries become distorted and very thin, thus increasing vascular resistance
Fick principle
Volume of oxygen consumption measured at level of mouth will be equal to amount of O2 removed by pulmonary blood flow
Amount of O2 removed by pulmonary blood flow is difference between O2 concentration of incoming mixed venous blood and concentration of O2 in arterial blood
Q = flow
Dot above letter = per unit time
Distribution of blood flow in upright lung
Blood flow greatest at bottom and least at apices as blood flow working against gravity
Effects of posture on pulmonary blood flow distribution
Distance = distance rom bottom to top of lung
Three zone model of blood flow distribution
These are metaphorical zone situations depending on clinical picture
When are zone 1 situations seen?
Zone 1 not seen in normal healthy conditions
Pulmonary artery pressure just high enough to be greater than alveolar pressure to maintain blood flow to apex / top of lung according to gravity and patient position
Can be seen in hypovolaemic patients (decreased arterial pressure) or positive pressure ventilation (increase alveolar pressure)
What determines blood flow in zone 2 situations
Capillaries are collapsible so rather than being arterial - venous pressure, blood flow is determined by arterial - alveolar pressure
How does alveolar pressure change across the lung?
Alveolar pressure is the same throughout the lung
Causes of uneven pulmonary blood flow
1) Gravity
2) Random variations in blood vessel resistance
3) Proximal regions of an acinus receive more blood flow than distal
4) Some regions of lung have intrinsically higher vascular resistance (in some animals)
Effect on mean pulmonary artery pressure of breathing hypoxic gas
Animal studies show breathing 10% O2 causes increase in mean pulmonary artery pressure
Therefore increases pulmonary vascular resistance (as blood flow remains unchanged)
Hypoxic pulmonary vasoconstriction
As alveolar PO2 decreases, Blood flow decreases
Subsequently increases pulmonary vascular resistance
How does hypoxic pulmonary vasoconstriction occur?
Exact mechanism unclear
But alveolar gas is very close to arterial wall (as pulmonary arteries are surrounded by alveoli)
Possible evolutionary theory for why hypoxic vasoconstriction occurs
Need rapid switch from placenta for gas exchange to lungs at birth
Substance metabolism by the lung processes
Biological activation
Biological inactivation
Not effected
Metabolised and released into circulation
Secreted
Biological activation by the lung example
Angiotensin I converted to vasoconstrictor angiotensin II via ACE
Biological inactivation by the lung examples
Bradykinin
Serotonin
Prostaglandins E1, E2 and F2 alpha
Noradrenaline partially inactivated
Not affected by the lung examples
Adrenaline
Prostaglandins A1 and A2
Angiotensin II
Vasopressin
Metabolised and released by the lung examples
Arachidonic acid metabolites - the leukotrienes
Prostaglandins
Secreted by the lung examples
Immunoglobulins - in particular IgA secreted in bronchial mucus
Location of ACE
Surface of endothelial cells of vessels
