Ventilation and Perfusion Matching Flashcards
What must be closely macthed for effective gas exchange?
ventilation and perfusion
What is ventilation?
the change in volume through the respiratory cycle
What type of alveolus will have less ventilation?
an alveolus that is already open and extended at the start of the cycle
Where does more gas go in the lung?
More gas goes to the base of the lung than the apex.
Why does the base of the lung change volume more than the apex?
- Lower ribs are more curved and mobile than upper, the action of the diaphragm expands the lower lobes more than the upper, partly because upper lobes are attached to main bronchi and upper airways, so less easily stretched than tissue of lower lobes, and compliance alters across lung – recall larger compliance means a larger change in volume – lower lobes have greater compliance
- Small role due to gravity - apex has more negatuve ontrapleural pressure so the alveoli are extended already
What two circuits nourish the lung tissue?
pulmonary and bronchial
Where do the bronchial arteries branch from?
the descending aorta
What is the function of the bronchial circulation
to supply oxygen to lung parenchyma, airway smooth muscle, pulmonary arteries and veins and pleura. Additional function is warming of inspired air
What is flow through the pulmonary artery considered to be equal to?
cardiac output
What is the volume of the pulmonary capillary network at rest
~100ml
What is the volume of stroke volume?
~70ml
What type of system is the pulmonary circulation?
low pressure, low resistance system
What is the mean arterial pressure in the pulmonary circulation and what does it need to be sufficient to do?
~15mmHg
pressure only needs to be sufficient to lift blood to the apex of the lung
How thick are the walls of the pulmonary arteries?
thin (mistaken for systemic veins)
What are pulmonary capillaries in effect surrounded by?
surrounded by air - can collapse or be extended according to balance of blood pressure and alveolar pressure
What is transmural pressure?
the balance between alveolar and blood pressure which determines the blood flow through the pulmonary capillaries
Why is blood floe low at the apex and high at the base of the lung?
- due to the hydrostatic pressure differnce between the base and apex of the lung (~23mmHg)
- pressure in capillaries is lower at the apex than the base of the lung
When will blood only flow in the lungs?
if blood pressure is greater than alveolar pressure
Draw a diagram and graph showing the blood flow in each zone of the lung and the relative size of the capillary in that section
Describe the nature of the capillaries in each zone of the lungs and relate how this would affect blood flow
- zone 1 = closed (low)
- zone 2 = open/closed (medium)
- zone 3 = open (higher)
What is the consequence of changes in hydrostatic pressure at the apex of the lung? And when would this occur?
alveolar pressure > blood hydrostatic pressure - the capillary will be closed
occurs with low BP or raised alveolar pressure
What occurs in zone one if capillaries are closed? And when does this not tend to exist?
if capillaries are closed then they are ventilated but not perfused and this is considered alveolar dead space
Doesnt usually exist in normal people
under what conditions is there not normally a zone 1 and why?
Normal conditions
there is sufficient pressure to perfuse the apices
Describe the pressure and flow in zone 2
- alveolar pressure is lower than systolic arterial pressure, but may be higher than diastolic arterial pressure and venous pressure
- flow is determined by the arterial-alveolar pressure difference
- blood pressure must be greater than alveolar for flow to occur
