Carbon dioxide

Biology

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars or fats with oxygen as part of their metabolism, in a process known as cellular respiration. This includes all plants, animals, many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues to the lungs where it is exhaled.

Related Topics:
Sugar - Fat - Oxygen - Metabolism - Cellular respiration - Animal - Fungi - Bacteria - Blood - Lung

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Carbon dioxide content in fresh air is approximately 0.04%, in exhaled air ca. 4.5%. When inhaled in high concentrations (about 5% by volume), it is toxic to humans and other animals.

Related Topics:
Air - Toxic

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Hemoglobin, the main oxygen-carrying molecule in red blood cells, can carry both oxygen and carbon dioxide, although in quite different ways. The decreased binding to oxygen in the blood due to increased carbon dioxide levels is known as the Haldane Effect and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO2 or a lower pH will cause offloading of oxygen from hemoglobin. This is known as the Bohr Effect.

Related Topics:
Hemoglobin - Red blood cell - Haldane Effect - Bohr Effect

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According to a study by the USDA http://itest.slu.edu/articles/90s/hannan.html, an average person's respiration generates approximately 450 liters (roughly 900 grams) of carbon dioxide per day.

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CO2 is carried in blood in three different ways. Most of it (about 80-90%) is converted to bicarbonate ions HCO3−, by the enzyme carbonic anhydrase in the red blood cells. 5-10% is dissolved in the plasma and 5-10% is bound to hemoglobin as carbamino compounds. The exact percentages vary depending whether it is arterial or venous blood.

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The CO2 bound to hemoglobin does not bind to the same site as oxygen; rather it combines with the N-terminal groups on the four globin chains. However, due to allosteric effects on the hemoglobin molecule, the binding of CO2 does decrease the amount of oxygen that is bound for a given partial pressure of oxygen.

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Carbon dioxide may be one of the mediators of local autoregulation of blood supply. If it is high, the capillaries expand to allow a greater blood flow to that tissue.

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Bicarbonate ions are crucial for regulating blood pH. As breathing rate influences the level of CO2 in blood, too slow or shallow breathing causes respiratory acidosis, while too rapid breathing, hyperventilation, leads to respiratory alkalosis.

Related Topics:
Respiratory acidosis - Hyperventilation - Respiratory alkalosis

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It is interesting to note that although it is oxygen that is required by the body for metabolism, it is not low oxygen levels that stimulate breathing, but is instead higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (eg. pure nitrogen) leads to loss of consciousness without subjective breathing problems. This is especially perilous for high-altitude fighter pilots, and is also the reason why the instructions in commercial airplanes for case of loss of cabin pressure stress that one should apply the oxygen mask to oneself before helping others - otherwise one risks going unconscious without being aware of the imminent peril.

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Plants remove carbon dioxide from the atmosphere by photosynthesis, which uses light energy to produce organic plant materials by combining carbon dioxide and water. This releases free oxygen gas. Sometimes carbon dioxide gas is pumped into greenhouses to promote plant growth. Plants also emit CO2 during respiration; but on balance they are net sinks of CO2.

Related Topics:
Photosynthesis - Water - Greenhouse

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OSHA limits carbon dioxide concentration in the workplace to 0.5% for prolonged periods, or to 3% for brief exposures (up to ten minutes). OSHA considers concentrations exceeding 4% as "immediately dangerous to life and health." People who breathe 5% carbon dioxide for more than half an hour show signs of acute hypercapnia, while breathing 7–10% carbon dioxide can produce unconsciousness in only a few minutes. Carbon dioxide, either as a gas or as dry ice, should be handled only in well ventilated areas.

Related Topics:
OSHA - Immediately dangerous to life and health - Hypercapnia

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