Lecture 21 Flashcards
What regions is ventilation enhanced in?
High lung compliance.
Low resistance to air-flow.
What is the effect of regional variation of compliance on the regional variation of V/Q?
For a given increment of pressure applied at the base of the ling, get a larger effect (change in volume) than at the top of the lung. This is due to gravity. Thus ventilation is greater at the base of the lung.
Describe the effect of regional variation of compliance on the regional variation of V/Q except with low lung volumes?
The ventilation/blood flow ratio becomes zero.
How is Q measured?
Using the “Fick-principle”. the rate of consumption of oxygen is the difference between the rate at which we inspire oxygen and expire oxygen. the rate of oxygen consumed by the tissues is given by the rate of blood flow times the difference in concentration of oxygen (difference of arterial and venous blood).
What is the effect of regional variation of blood-flow on the regional variation of V/Q?
From the base of the lung to the apex, there is higher blood flow at the base. The pleural apex doesn’t get as much apex or blood flow.
Explain why there is greater blood flow at the base of the lung than the apex?
Pressure in the pulmonary artery driving blood through ventilation. The pressure in the alveolus affects the potency of the capillary. if the pressure gets high enough (by raising the outflow) you will compress the vessel until flow is pinched off. there will be a point at which the flow goes to zero. This will be greater at the base of the lung. Fluid flow in any vessel is diminished if the external pressure exceeds the fluid pressure in the vessel. The apex are pinching off the capillaries, but allowing the capillaries to expand in the base. This allows for variation.
Describe regional variation of the ventilation-perfusion ratio?
Blood flow changes more rapidly than ventilation. The ratio is good at the apex, yet ventilation and blood flow isn’t. Blood flow decreases more from base to apex than ventilation does. Hence the ventilation-perfusion ratio increases from base to apex.
Describe the consequences of regional variation of the V/Q ratio?
Coming to the alveolus is blood. Mixed venous blood is being returned, at rest there is a partial pressure of 40mmHg and CO2 45mmHg. The difference in partial pressure between the alveolus and the capillary, which allows for oxygen to enter the capillary (gradient - high to low). The difference in carbon dioxide partial pressure which allows carbon dioxide to go from blood to lung.
What happens in a ventilation block?
When you block ventilation, the pressure in the lung for oxygen will hover around 40-45mmHg. Can’t change as there is no access to the outside air. As time goes on, oxygen still diffuses out, and carbon dioxide diffuses in. because ventilation is zero the ratio will be zero.
What happens when you have a capillary block?
A block to the blood flow, you keep pumping air in and out of the alveolus. You’re just moving air in and out of the lung but it is not diffusing out of the lung into the capillary wall. because blood flow is zero (yet good ventilation) the ratio will increase.
Describe redress of regional V/Q inequalities by ‘Local’ mechanisms?
Known as hypoxic pulmonary vasoconstriction. What happens is that as a consequence of poor ventilation, a capillary becomes hypoxic, it has undergone vasoconstriction. Overall shunting of blood (capillaries are parallel) to another capillary past an alveolus which is well ventilated. You can get an increase of pulmonary arterial resistance, leading to increase of pulmonary arterial pressure, causing and increased pressure-volume work by the right heart. If this continues to happen for a long time, there will be hypertrophy.
What happens when you increase alveolar partial pressure of oxygen?
As you increase the alveolar PO2 the blood flow increases. If you want an extreme PO2 you get even more blood flow.
