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Regulation of Breathing Rate
The control centre for breathing rate is located in the brain. Breathing rate is generally controlled involuntarily and the area of the brain that is responsible for this is the respiratory centre located in the medulla. Breathing rate can however also be controlled voluntarily by the primary motor cortex.
The respiratory centre consists of an inspiratory and expiratory centre.
The inspiratory centre is responsible for sending messages via the phrenic nerve to the diaphragm and the thoracic nerves to the intercostal muscles instructing them to contract. This causes the chest wall to expand which lowers the pressure inside, causing air to be sucked in.
Stretch receptors (sensory receptors that detect mechanical pressure, distortion and stretch) located mainly in the smooth muscle of the bronchi and bronchioles detect that the lungs have expanded to their ideal maximum point and then send messages back to the control centre (Inspiratory centre). The expiratory centre then sends messages to the diaphragm and intercostal muscles instructing them to relax. This reduces the volume inside the chest, raising the pressure inside, causing the air to be blown out.
Homeostasis of Breathing Rate
It is the levels of carbon dixiode, rather than the levels of oxygen, in the blood that have the greatest influence over breathing rate.
Carbon dioxide is the waste product of aerobic cellular respiration. When carbon dixide levels rise e.g. during exercise, blood pH lowers. Chemoreceptors located in the medulla, aorta and carotid arteries detect the levels and send messages to the respiratory centre.
When carbon dioxide levels rise, the respiratory centre sends messages to the diaghragm and intercostal muscles to work harder and faster. This causes the breathing rate to increase, allowing the carbon dioxide to be blown off.
The chemoreceptors are much less sensitive to changes in oxygen levels, and there would need to be a marked fall in oxygen before this was detected.
Breathing Rate Abnormalities
The rate and depth of breathing can be modified by a number of factors including drugs such as morphine and emotions.
An increase in carbon dioxide levels result in hyperventilation. If carbon dioxide levels remain high this results in a decrease in blood pH causing a condition called acidosis. This affects the nervous system and can result in headaches, tiredness, weakness, confusion and eventually coma and death.
A decrease in carbon dioxide is more rare and is a result of hyperventilation due to anxiety, high fever or an overdose of aspirin. This causes an increase in blood pH and results in a condition called alkalosis. This makes the nervous system more irritable and can cause restlessness, muscle twithing, tingling and numbness of the fingers. Severe alkalosis can cause seizures and coma.