The rhythmic contractions of the respiratory muscles that drive breathing are generated by neurons in the brainstem. Breathing is regulated predominantly in response to the PCO2 of the blood. Respiratory chemosensors are cells on the ventral surface of the medulla that are sensitive to the blood PCO2, and the carotid and aortic bodies on the large arteries leaving the heart that are sensitive to the blood PO2 or to a decrease in the blood they are receiving.
learning outcomes
You should be able to:
Explain the roles of the dorsal and ventral groups of respiratory neurons in the medulla.
Apply knowledge about the physiological processes underlying breathing to explain the body’s response to intense exercise and to decreases in PO2.
Describe where chemosensitive cells are located and how they participate in negative feedback control of breathing.
In terms of respiratory motor neurons and respiratory muscle groups, what changes occur in transitioning from breathing at rest to breathing that uses more of the lung’s vital capacity?
During tidal breathing, a regular cyclical pattern of firing of respiratory motor neurons in the dorsal respiratory group of the medulla drives muscle contractions of the diaphragm. The need for increased respiratory gas exchange is first met with an increase in the rate of that oscillatory pattern of firing in the dorsal respiratory group, but as the need increases, the ventral respiratory group is recruited and those neurons drive contractions of the intercostal muscles, increasing the volume of air exchanged with each breath.
When you go rapidly to high altitude, you may experience a breathing pattern in which periods of no breathing alternate with periods of fast breathing. Explain that response in terms of diffusion gradients for O2 and CO2 and the feedback signals regulating breathing.
When you go to high altitude, the diffusion gradient for O2 decreases as atmospheric pressure decreases. However, the diffusion gradient for CO2 does not change; ambient CO2 is close to 0 at sea level and also at high altitude. Therefore breathing to satisfy O2 needs results in a greater loss of CO2 than at sea level. Since CO2 is the major stimulus for respiration, respiration slows or ceases until the PCO2 returns to a level that stimulates breathing.
Cells in the carotid and aortic bodies have high metabolic rates, and they are sensitive either to a decrease in blood flow or to a decrease in the PO2 of the blood. How are these two facts related?
Because the carotid and aortic bodies have high metabolic rates, they are compromised by a decrease in their O2 supply, whether it is because of decreased blood flow or decreased PCO2 in the blood.