Transport of gases(oxygen and Carbon-dioxide) in Respiration

Transport of gases

1. Transport of oxygen

Molecular oxygen is carried in blood in two ways: bound to haemoglobin within the red blood cells and dissolved in plasma. Oxygen is poorly soluble in water, so only 3% of the oxygen is transported in the dissolved form. 97% of oxygen binds with haemoglobin in a reversible manner to form oxyhaemoglobin (HbO₂). The rate at which haemoglobin binds with O₂ is regulated by the partial pressure of O₂. Each haemoglobin carries maximum of four molecules of oxygen. In the alveoli high pO₂, low pCO₂, low temperature and less H⁺ concentration, favours the formation of oxyhaemoglobin, whereas in the tissues low pO₂, high pCO₂, high H⁺ and high temperature favours the dissociation of oxygen from oxyhaemoglobin.

A sigmoid curve (S–shaped) is obtained whenpercentagesaturationofhaemoglobin with oxygen is plotted against pO₂. This curve is called oxygenhaemoglobin dissociation curve (Figure 6.7). This S–shaped curve has a steep slope for pO₂ values between 10 and 50mmHg and then flattens between 70 and 100 mm Hg.

Under normal physiological conditions, every 100mL of oxygenated blood can deliver about 5mL of O₂ to the tissues.

2. Transport of Carbon–dioxide

Blood transports CO₂ from the tissue cells to the lungs in three ways

i. Dissolved in plasma About 7 – 10% of CO₂ is transported in a dissolved form in the plasma.

ii.Bound to haemoglobin About 20 – 25% of dissolved CO₂ is bound and carried in the RBCs as carbaminohaemoglobin (Hb CO₂)

iii.   As bicarbonate ions in plasma about 70% of CO₂ is transported as bicarbonate ions This is influenced by pCO₂ and the degree of haemoglobin oxygenation. RBCs contain a high concentration of the enzyme, carbonic anhydras, Whereas small amounts of carbonic anhydrase is present in the plasma.

At the tissues the pCO₂ is high due to catabolism and diffuses into the blood to form HCC₃– and H+ ions. When CO₂ diffuses into the RBCs, it combines with water forming carbonic acid (H₂CC₃) catalyzed by carbonic anhydrase. Carbonic acid is unstable and dissociates into hydrogen and bicarbonate ions.

Carbonic anhydrase facilitates the reaction in both directions.

The HCC₃^– moves quickly from the RBCs into the plasma, where it is carried to the lungs. At the alveolar site where pCO₂ is low, the reaction is reversed leading to the formation of CO₂ and water. Thus CO₂ trapped as HCC₃^– at the tissue level it is transported to the alveoli and released out as CO₂. Every 100mL of deoxygenated blood delivers 4mL of CO₂ to the alveoli for elimination.

Bohr effect and Haldane effect

Increase in PCO₂ and decrease in pH decrease the affinity of haemoglobin for oxygen and shifts the oxyhaemoglobin dissociation curve to the right and facilitates unloading of oxygen from hemoglobin in the tissue. This effect of pCO₂ and pH on the oxyhaemoglobin dissociation curve is called the Bohr small effect.

The Haldane effect, on the other hand describes how oxygen concentrations determines hemoglobin’s affinity for carbon dioxide. The amount of carbon dioxide transported in blood is remarkably affected by the degree oxygenation of the blood. The lower the partial pressure of. O₂ lower is the affinity of haemoglobin saturation with oxygen hence more CO₂ is carried in the blood. This phenomenon is called Haldane effect. This effects CO₂ exchanges in both the tissues and lungs.

In the lungs the process is reversed as the blood moves through the pulmonary capillaries, its PCO₂ declines from 45mm Hg to 40mm Hg. For this to occur carbondioxide is freed from HCC₃- ions and Cl- ions moves in to the plasma and reenters the RBC and binds with H+ to form carbonic acid which dissociates in to CO₂ and water. This CO₂ diffuses along its partial gradient from the blood to the alveoli (Figure 6.8).