Respiratory system
All living things require energy. Most cells obtain energy by the process of cellular respiration which involves the reaction of oxygen with glucose sugar to release energy, as well as carbon dioxide as a waste product. (see Plant systems B: Photosynthesis, nutrition and respiration). The function of the respiratory system is to facilitate the intake and absorption of oxygen, so that it is available to the cells for cellular respiration in which energy will be released. Worms absorb oxygen through their moist skin. Fish use their blood-rich gill plates to absorb oxygen from the surrounding water. Large endothermic (warm-blooded) animals such as land-dwelling humans breathe air (which is approximately 20% oxygen) through internal specialised organs called lungs. The lungs of humans are situated in the pleural cavity (chest area), surrounded on the sides by the ribs and below by the diaphragm.
Each alveolus has a capillary network covering it which provides an enormous surface area for oxygen to enter the blood from the lungs and carbon dioxide to leave. There are approximately 300 million alveoli in each lung, with a combined surface area for exchange of about the size of a tennis court. The oxygen combines with the red pigment haemoglobin carried by the red
blood cells, while the carbon dioxide is carried in solution. Breathing When the intercostals and the diaphragm relax, the chest volume decreases and air is forced out removing waste carbon dioxide (expiration).
Breathing is an involuntary action under the control of the respiratory centre found in the base of the brain (medulla). The medulla sends rhythmic impulses to the intercostal and diaphragm muscles to maintain breathing, although we can temporarily over-ride this process voluntarily. The medulla is sensitive to carbon dioxide levels in the blood, so the breathing rate can be altered to accommodate varying needs during exercise or rest.
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