Detailed and comprehensive notes on control of breathing rate (Edexcel biology A). [“A-Level Biology: Edexcel A Year 1 & 2 Complete Revision & Practice” (CGP, ISBN: 2986), “Salters-Nuffield AS/A level Biology Student Book 1” (Pearson, ISBN: 1007) and “Salters-Nuffield A level Biology Stud...
Breathing Rate
7.9 ii) Understand how variations in ventilation and cardiac output enable rapid delivery of oxygen to
tissues and the removal of carbon dioxide from them, including how the heart rate and ventilation
rate are controlled and the roles of the cardiovascular control centre and the ventilation centre in the
medulla oblongata.
Control of Breathing Rate
Inhalation:
• The inspiratory centre in the medulla
oblongata sends nerve impulses to the
intercostal muscles and diaphragm to
make them contract.
• This increases the volume of the lungs,
which lowers the pressure.
• The inspiratory centre also sends
impulses to inhibit the action of the
expiratory centre.
• Air enters the lungs due to the pressure
difference between the lungs and the
air outside.
Exhalation:
• As the lungs inflate, stretch receptors in the lungs are stimulated.
• Stretch receptors send nerve impulses back to the medulla oblongata. These impulses inhibit the
action of the inspiratory centre.
• The expiratory centre then sends nerve impulses to the diaphragm and intercostal muscles to relax.
• This causes the lungs to deflate, expelling air.
• As the lungs deflate, the stretch receptors become inactive. The inspiratory centre is no longer
inhibited and the cycle starts again.
Increasing Breathing Rate
• During exercise, the level of carbon dioxide in the blood increases.
• Carbon dioxide dissolves in water to form carbonic acid, decreasing the pH of the blood.
• CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
• Chemoreceptors are sensitive to changes in pH and are found in the medulla oblongata, aortic bodies
and carotid bodies.
• When chemoreceptors detect a decrease in blood pH, they send nerve impulses to the medulla
oblongata, which sends more frequent nerve impulses to the intercostal muscles and diaphragm.
• This increases the rate and depth of breathing.
• This causes gaseous exchange to speed up – CO2 levels drop and extra O2 is supplied for muscles;
the pH returns to normal and breathing rate decreases.
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