Describe the role of carbon dioxide in the control of alveolar ventilation.
Better answers considered the role of CO2 in the control of alveolar ventilation in terms of sensors, central processing and effectors - with an emphasis on sensors. Features of central and peripheral chemoreceptors should have been described in detail. The PCO2/ventilation response curve is best described using a graph, with key features of the curve identified (including gradient and axes). Various factors affecting the gradient of this curve and how CO2 affects the response to hypoxic drive should be described.
In brief summary:
- Increasing PaCO2 causes an increase in minute ventilation.
- This is mediated by peripheral chemoreceptors over the timescale of seconds, and by central chemoreceptors over minutes.
- Peripheral chemoreceptors are the carotid glomus cells, which sense PaCO2 as well as PaO2, pH, temperature and blood pressure
- Central chemoreceptor areas are found in multiple areas of the brain, but are generally said to concentrate in the ventral medulla
- The relationship between PaCO2 is fairly linear in the range of 45-80 mmHg; the rate of minute volume increases by 2-5L/min per every 1mm Hg of CO2 increase.
Effects of physiological and pathological changes on the ventilatory response to CO2 can be graphically represented thus:
- The CO2/ventilation response curve is shifted to the left by metabolic acidosis and hypoxia
- Sleep, sedation, anaesthesia and opiates shift the curve to the right and decrease the slope of the curve (i.e. the increase in minute ventilation is reduced per unit rise of CO2)
- Age decreases the ventilatory response to CO2
- A high level of physical fitness also diminishes hypercapnic respiratory drive
- The response to raised PaCO2 is rapid; about 75% of the maximum minute volume change is achieved over minutes
- At a stable metabolic rate and with minimal inspired CO2 the relationship between minute volume and PaCO2 is described by a hyperbolic curve.
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