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How does the brain know how fast or slow you should breathe?

Since breathing is a subconscious action, there must be some physiological process that governs it within the brain. It is however possible to control your breathing consciously e.g. hold your breath, but the longer you overide your subconcious drive, the harder it becomes to resist the urge to breathe. The subconscious control responsible for this urge to breathe is known as the respiratory centre.

Sensory messages reach the control centre from chemoreceptors. These chemoreceptors are responsible for detecting the amount of CO2 and O2 in our blood stream.

Central chemoreceptors are located in the medulla oblongata (back of the brain). The central chemoreceptors are the most important when it comes to stimulating breathing; their stimulus is hydrogen ion (H+) concentration or carbon dioxide concentration.

The peripheral chemoreceptors are located in the walls of major arteries. Since the peripheral chemoreceptors are not in the brain, they have to relay any messages via two cranial nerves of the peripheral nervous system.

Peripheral chemoreceptors are sensitive to the same stimulus as the central receptors, as well as oxygen concentration. Peripheral chemoreceptors are again subdivided further in to two groups. These are the aortic body (found in clusters in the inside wall of the aorta –artery near the heart) and carotid bodies (inside wall of both carotid arteries – in the neck). Sensory impulses bound for the breathing centre from the aortic body are sent via the vagus nerve, whereas those from the carotid bodies travel via the left and right glossopharyngeal nerves (both sides of neck).

In summary, you have special sensors in your brain and major arteties which specialise in monitoring carbon dioxide levels (and oxygen to a lesser extent). These sensors are co-ordinated by the respiratory centre in the back of your brain, which tells you when to breathe without you even knowing.

Bibliography
Tortora Grabowski (2000) Principles of Anatomy and Physiology (9th Edition) John Wiley & Sons, Inc.

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