5. Blood Supply, Gas Exchange, Ventilation and Perfusion Flashcards
What is the bronchial circulation (nutritive) supplied by?
The bronchial arteries arising from the systemic circulation
What is the role of the bronchial circulation?
To supply oxygenated blood to airway smooth muscle, nerves and lung tissue
What does the pulmonary circulation (gas exchange) consist of?
Left and right pulmonary arteries originating from the right ventricle
What does the pulmonary circulation do?
- Carries entire cardiac output from RV. -Unique system
- Supplies the dense capillary network surrounding the alveoli and returns oxygenated blood to the left atrium via the pulmonary vein
Describe the flow and pressure of the pulmonary circulation.
-High flow, low pressure
How does air diffuse across membranes?
Down partial pressure gradients
What does A stand for?
alveolar
What does a stand for?
arterial blood
What does ṽ stand for?
mixed venous blood (e.g. in pulmonary artery)
What does PaO2 mean?
partial pressure of oxygen in arterial blood
What does PACO2 mean?
partial pressure of carbon dioxide in alveolar air
The rate of diffusion across the membrane is:
- Directly proportional to the partial pressure gradient
- Directly proportional to gas solubility
- Directly proportional to the available surface area
- Inversely proportional to the thickness of the membrane
- Most rapid over short distances
What can affect gas exchange?
Various pathologies
Describe the membrane of the alveoli?
- Large surface area
- Thin membrane
- Creates short diffusion distance
Diffusion: emphysema
Destruction of alveoli reduces surface area for gas exchange
Diffusion: fibrotic lung disease
Thickened alveolar membrane slows gas exchange. Loss of lung compliance may decrease alveolar ventilation
Diffusion: pulmonary oedema
Fluid in interstitial space increases diffusion distance. Arterial PCO2 may be normal due to higher CO2 solubility in water.
Diffusion: asthma
Increased airway resistance decreases airway ventilation
What is the ventilation-perfusion relationship?
They ideally match each other
What is ventilation in alveoli matched to through?
pulmonary capillaries
What is the distribution of blood flow in the lungs influenced by?
Hydrostatic (blood) pressure (Pa) and alveolar pressure.
What is blood flow inversely proportional to?
Vascular resistance and declines with height across the lung
Why is blood flow high at the base of the lungs?
Arterial pressure exceeds alveolar pressure and vascular resistance is therefore low.
Why s blood flow low at the apex of the heart?
Arterial pressure is less than alveolar pressure. This compresses the arterioles and vascular resistance is increased
What effect does gravity have on ratio of ventilation to perfusion.
When in the upright position the ratio within the lung changes from the base to the apex due to gravity
What is a perfectly matched ratio?
Ventilation: perfusion = 1.0
What are mismatched ratios?
- Ventilation> perfusion> 1.0
- Ventilation< perfusion <1.0
Where does the majority of mismatch take place?
Apex
What happens if ventilation decreases in a group of alveoli?
PCO2 increases and PO2 decreases. Blood flowing past those alveoli does not get oxygenated
Shunt
Passage of blood through areas of the lung that are poorly ventilated
What is shunt the opposite of?
Alveolar dead space
What does alveolar dead space refer to?
Alveoli that are ventilated but not perfused
What does anatomical dead space refer to?
Air in the conducting zone of the respiratory tract unable to participate in gas exchange as walls of airways in this region are too thick.
Physiological dead space
Alveolar DS + Anatomical DS
What tries to keep ventilation and perfusion matched?
Local control mechanisms
What happens when there is decreased tissue PO2?
Causes the arterioles of under ventilated alveoli to constrict, diverting blood to better ventilated alveoli.
What can cause mild bronchial dilation?
Increased PCO2
What is constriction in response to hypoxia particular to?
Pulmonary vessels (systemic vessels dilate)
When does autoregulation take place?
when alveolar dead space occurs
In what ay is autoregulation opposite to shunt?
- Increase in alveolar PO2 causes pulmonary vasoldilation
- Decreases in alveolar PCO2 causes bronchial constriction
What does autoregulation ultimately lead to?
Increases in perfusion and to a lesser extent a decrease in ventilation bringing the ratio back towards 1.
