Ventilation and perfusion Flashcards
What is the usual value of tidal volume?
500mls
How do you calculate minute ventilation?
Tidal volume x respiratory rate
What is the typical value of minute ventilation?
6-8 L/min at rest
Define anatomical dead space.
Volume of air in the conducting airways
What is the typical value of anatomical dead space?
150mls
Define physiological dead space.
The total volume of air that does not participate in gas exchange
Give the calculation for physiological dead space.
Physiological dead space = anatomical dead space + alveolar dead space
Define hyperventilation.
An increase in the rate and depth of breathing
Describe how increased alveolar ventilation affects the partial pressure of CO2 in the alveoli and how this would affect diffusion of CO2 from the blood.
When alveolar ventilation increases, it helps remove CO2 from the alveoli and brings in more oxygen. Removing CO2 reduces the partial pressure of CO2 in the alveoli. This lowers alveolar pCO2 which increased the pressure gradient between the alveoli and the blood. With a greater pressure gradient, CO2 will diffuse more easily from the blood into the alveoli to be exhaled.
What are some common causes of low V/Q ratio?
Airway obstruction such as asthma or pneumonia
What could be a cause of high V/Q ratio?
Pulmonary embolism
The respiratory control centre is in which area of the brain?
Medulla
The brainstem responds to changes in the lungs via which 2 mechanisms?
Change in lung volume via mechanoreceptors
Change in blood gas composition via chemoreceptors
The central chemoreceptors are located in which area of the medulla?
Ventral surface of the medulla
What is the function of the central chemoreceptors in relation to CO2?
Maintain pCO2 within normal range
What 2 things do the central chemoreceptors respond to and how?
Directly to changes in pH of CSF
Indirectly to changes in arterial pCO2
Which 2 areas are the peripheral chemoreceptors located?
Carotid bodies – at the bifurcation of the common carotid artery
Aortic bodies – in the aortic arch
What 3 things do the peripheral chemoreceptors respond to?
Arterial pO2
Arterial pCO2
Arterial H+ and thus pH
The peripheral chemoreceptors respond dramatically when pO2 is below what value?
Respond dramatically when pO2 <8kPa
How do the peripheral chemoreceptors signal for a change in ventilation?
Signal to the respiratory control centre to modulate ventilation via the vagus nerve and glossopharyngeal nerve
Which 2 mechanisms do the lungs use to prevent a large increase in pressure when blood flow increases?
Capillary distension
Capillary recruitment – previously closed capillaries open up to accommodate more blood flow
Name a local lung mediator which causes vasoconstriction.
Thromboxane A2
Name a local lung mediator which causes vasodilation.
Prostacyclin
What effect does O2 have on pulmonary arteriole smooth muscle?
Relaxes pulmonary arteriole smooth muscle
What is the normal V/Q value?
0.8
Describe what a shunt is and what value V/Q ratio this would lead to.
An area of lung with perfusion but no ventilation, resulting in a V/Q ratio of zero
Describe what dead space is and what value V/Q ratio this would lead to.
An area of lung with ventilation but no perfusion, resulting in an infinite V/Q ratio
In which area of the lung is perfusion greater, apex or base? Why?
Base, due to the vessels at the base having less gravity to work against
In which area of the lung is ventilation greater, apex or base? Why?
Base
Due to gravity, the intrapleural pressure at the apex of the lung is more negative compared to the base. This means the alveoli at the apex are more expanded at rest. In contrast, alveoli at the base are relatively compressed. Since the alveoli at the apex are already more expanded, they are less compliant and don’t expand as much as we inhale. The alveoli at the base, being less expanded to start with, are more compliant and can expand more with each breath. As a result, a larger proportion of inhaled air goes to the base of the lungs, making ventilation greater there
Are gravitational effects greater on ventilation or perfusion?
Perfusion
How does V/Q ratio change as you move from the apex to the base of the lung?
V/Q is high at the apex of the lung and low at the base of the lung
Describe why other areas of the lung can’t compensate when certain areas of the lung have a low V/Q ratio, for example in pneumonia?
In areas with a normal V/Q ratio, the haemoglobin quickly becomes saturated with oxygen, meaning increased ventilation will not increase the oxygen content of the blood
