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The Human Body:
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Gaseous Transport
Gas diffusion through liquids is determined by partial pressure and solubility of the gas. In the alveoli, the partial pressure of oxygen is 100 mmHg partial pressure of carbon dioxide is 40 mmHg. While in the arterial blood, the partial pressure of oxygen is 100 mmHg partial pressure of carbon dioxide is 40 mmHg. Oxygen is carried in two ways, about 1.5% is dissolved in plasma and 98.5% is bound to hemoglobin(Hb). The alveolar partial pressure of oxygen and oxygen solubility determine amount of oxygen dissolved in the plasma of pulmonary capillaries. At arterial partial pressure of oxygen of 100 mmHg, only 3ml of oxygen can dissolve in 1L of blood. (at rest, the cells consume 250ml of O2/min).
O2-Hb Saturation depends on blood partial pressure of oxygen. When partial pressure of oxygen is more than 40 mmHg, haemoglobin saturation remains at 75% (plateau), only 25% of oxygen released . At the partial pressure of oxygen less than 40 mmHg, oxygen is released increases significantly at tissue.
2,3-diphosphoglycerate, produced from red blood cell glycolysis, bind reversibly with haemoglobin, thus reduce Hb’s affinity to oxygen, just like carbon dioxide & H+ do on the curve. There is yet another type of oxygen transporter, myoglobin which works in oxygen-poor environment. This is especially important as fetal blood has a very low partial pressure of oxygen, about 30 mmHg.
Carbon dioxide is transported in the blood by being dissolved in plasma , that accounts for 10%. Those that are bound to haemoglobin is about 30%. Those as HCO3- in red blood cells is 60%. The carbonic anhydrase (in RBCs) as follows shows how carbon dioxide is taken in to be transported.
CO2 + H2O « H2CO3 « HCO3- + H+
carbonic acid bicarbonate
The respiratory Acid-Base Balance is important to maintain physiological balance. So, the ventilation rate is altered to keep neutral pH (7.4), by the following process,
CO2 + H2O « H2CO3 « HCO3- + H+
When pH drops, H2CO3 produced converted to carbon dioxide, and exhaled out of the lungs.
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