Factors affecting breathing rate, and how?  

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The main factors affecting breathing rate are the levels of carbon dioxide and oxygen in the blood, and the blood's pH. The circulatory system has many, many small capillaries that are next to the alveoli in the lungs. Oxygen and carbon dioxide move from the area where the concentration of each is higher to the area where each is lower. When a breath is taken the air breathed in is usually higher in oxygen, and lower in carbon dioxide, than that being returned via the veins from the body. The gases diffuse across the cell membranes; oxygen is then taken by the arteries to the body's cells and carbon dioxide breathed out. Oxygen is required by various process in the body and carbon dioxide produced and eliminated. When an animal is at rest it needs relatively less oxygen than if it is active. When it is moving faster more carbon dioxide is produced and more oxygen needed and its respiratory rate increases. Sensors in the respiratory center in the brain and in the carotid artery and aorta sense the oxygen and carbon dioxide levels and the resuling pH, and send impulses to the muscles involved in breathing to either increase or decrease breathing rate.

This is why when someone hyperventilates it is helpful to have the person to breathe into a bag. When the person "can't catch their breath", they do not need more oxygen--they need to increase the level of blood carbon dioxide so the the stimulus to take another breath is restored.

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