Ventilation-perfusion Relationships Flashcards
What is hypoxaemia ?
Any situation where there is an abnormally low partial pressure of oxygen in the arteries
What is ventilation ?
It is the movement of air from the atmosphere through the ventilatory system
What are the 4 basic causes of hypoxaemia ?
Hypoventilation
Diffusion limitation
Shunt
Ventilation - perfusion inequality
What determines the partial pressure of oxygen in the alveoli ?
Rate of removal of oxygen by the blood
Rate of replenishment of oxygen by alveolar ventilation
What is hypoventilation ?
Situation where ventilation falls below the demands of the respiring tissue
What are the causes of hypoventilation ?
- drugs that depress central respiratory centres= opiates and barbiturates
- damage to the chest wall
- paralysis of respiratory muscles
When does the diffusion limit become larger ?
During exercise or as a result of thickening of the blood-gas barrier
What is shunted blood ?
Blood entering arterial system without going through the ventilated areas of the lung
What are some of the causes of shunted blood ?
Abnormal vascular connection between small pulmonary artery and vein - pulmonary arteriovenous fistula
Cardiac atrial septal defect so venous and arterial blood mix
What happens to the resistance in the pulmonary circulation when pulmonary arterial pressure rises and why ?
It decreases
1- recruitment causes capillaries which were shut to open and conduct more blood
2- distension causes the capillary lumen radius to increase which decreases resistance
What is the relationship between the distribution of blood flow though an upright human lung ?
Flow decreases almost linearly from base to apex
Pressure gradient difference of about 23 mmHg which means there is more recruitment and distension at the base
What happens to blood flow throughout the lungs during exercise ?
Increases throughout lungs and the regional differences are reduced
What are the 3 main forces that affect blood flow through the lungs ?
- hydrostatic pressure
- arteriovenous pressure difference
- alveolar pressure
Explain zone 1 of the flow zones in an upright lung…
The capillaries are squashed flat causing no flow - doesn’t occur under normal conditions
PA> Pa > Pv
Explain zone 2 of the flow zones in an upright lung…
Flow determined by difference between arterial pressure and alveolar pressure
Pa> pA>Pv
