A level Biology exam revision resources written by A level Examiners


The resting breathing rate is maintained primarily by the medulla in the brain and by stretch receptors in the lungs.

The respiratory centre in the brain contains two sections an inspiratory centre and an expiratory centre. Nerves impulses are sent from the inspiratory centre of the respiratory centre in the medulla to the diaphragm and intercostals. These impulses (from respiratory centre) cause muscle to contract. Contraction of intercostal muscles and contraction of diaphragm muscles causes the ribs to move up and out and the diaphragm to move down. These two events increase the volume of thorax / lungs thereby decreasing pressure (which causes inspiration). As the lungs reach full inflation stretch receptors in the intercostal muscles are stimulated by increase in size of thorax/lungs. These receptors send impulses to the expiratory centre that inhibits the inspiratory centre therefore inspiratory impulses are inhibited so contraction of the diaphragm and intercostals ceases and elastic recoil of the lung tissue causes expiration.   Exam Advice

This process is summarised in the flow diagram, figure 1, below

Figure 1

The breathing rate during is affected by the amount of hydrogen ions (H+) generated by the increased rate of respiration associated with the muscle movements occurring.

Exercise causes an increase in the rate of respiration. This increases the carbon dioxide concentration (and therefore H+ concentration) of the blood. This increase is detected by chemoreceptors that are contained within the carotid body, aortic body and medulla. These receptor cells send more frequent nervous impulses to the respiratory centre in the brain when the H+ concentration is elevated. The rate and depth of breathing increases in response to the increased impulses. The higher the CO2 concentration (and therefore H+ concentration) the more frequently they send impulses to the respiratory centre.

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