Transport of Gases

Transport of Gases

The breathing process, or gas exchange, is the most important action in the lungs. The purpose of respiration is to deliver oxygen to body cells and remove carbon dioxide from the body, which is a byproduct of cellular respiration.

Both gases must be delivered between the exterior and interior respiratory sites to exchange oxygen and carbon dioxide. Despite the fact that carbon dioxide is more soluble in blood than oxygen, both gases follow a particular transport system termed as transport of gases.

Once oxygen gets diffused all across alveoli, it travels through the bloodstream and is carried to the tissues, where it gets discharged. Conversely, carbon dioxide is released out of the bloodstream and into the alveoli, from where it is expelled from the body. 

Transport of Gas Definition

When gas travels through the air sacs of the lungs by convection or bulk flow and, subsequently by molecular diffusion, into the alveoli and pulmonary capillaries, it is known as ventilation or transport of gases. Transport of gases occurs throughout the body via the bloodstream, controlled by the cardiovascular system (heart and blood vessels).

Transport of Gases Classification

Transport of gases takes place in two ways:

• Transport of oxygen in the blood

The blood itself absorbs only about a quarter of the oxygen it carries. The majority of oxygen, i.e. 98.5 per cent, is transported to the tissues by a protein called haemoglobin.

Erythrocytes are the primary carriers of oxygen in the blood. Haemoglobin, a metalloprotein made up of four components with a ring-like shape, is found in these cells. One atom of iron is linked to a heme molecule in each subunit. Heme absorbs oxygen and can connect up to four oxygen molecules per haemoglobin molecule. Haemoglobin is said to be saturated if all heme units in the bloodstream are coupled to oxygen.

When just a few heme units are coupled to oxygen, haemoglobin is partly saturated. Haemoglobin binds to oxygen more rapidly when the partial pressure of oxygen rises. At the same time, once one oxygen molecule is bound by haemoglobin, other gaseous oxygen attaches to haemoglobin more rapidly. Temperature, pH, partial pressure of carbon dioxide, and 2,3-bisphosphoglycerate content are all variables that can promote or hinder haemoglobin and oxygen binding.

Because fetal haemoglobin differs from adult haemoglobin in structure, foetal haemoglobin has a higher oxygen affinity than adult haemoglobin. This is how the transport of oxygen takes place.

When oxygen joins the haemoglobin molecule, it forms oxyhemoglobin, which is red in colour. Haemoglobin that has not been coupled to oxygen has a blue-purple tint. Oxyhemoglobin is abundant in oxygenated blood moving through the systemic arteries. Most of the oxygen in the blood is released into systemic capillaries when it travels through the tissues.

As a result, the amount of oxyhemoglobin in the deoxygenated blood flowing through the venous system is substantially lower. The redder the fluid is, the more oxyhemoglobin is detectable in the blood.

Consequently, oxygenated blood is significantly redder than deoxygenated blood, resulting from the transport of gases.

• Transport of Carbon dioxide in the blood

The transport of carbon dioxide in the blood is a complex process.

Carbon dioxide is carried through the bloodstream in three ways:

1. As dissolved carbon dioxide; 
2. As bicarbonate; or 
3. As carbaminohemoglobin. 

Bicarbonate, which is generated in erythrocytes, facilitates the transport of carbon dioxide. With the help of an enzyme called carbonic anhydrase, carbon dioxide binds with water for this conversion. Carbonic acid is formed due to this reaction, which naturally disintegrates into bicarbonate and hydrogen ions. The chloride shift occurs when bicarbonate accumulates in erythrocytes and is exchanged for chloride ions across the membrane into the plasma. Bicarbonate re-enters erythrocytes in return for chloride ions at the pulmonary capillaries, and the interaction with carbonic anhydrase is repeated, regenerating carbon dioxide and water.

The carbon dioxide then disperses out of the erythrocyte and, later, into the atmosphere via the respiratory membrane. Carbaminohemoglobin is formed when a transitional amount of carbon dioxide is bonded directly to haemoglobin.

The bicarbonate system transports the vast bulk of carbon dioxide. Carbon dioxide enters red blood cells through diffusion. Carbonic anhydrase transforms carbon dioxide into carbonic acid (H2CO3), which is then converted into bicarbonate (HCO3-) and hydrogen (H+).

Conclusion

In simple terms, the transport of gases can be summarised as:

Haemoglobin is a protein composed of two alpha and two beta components that enclose an iron-containing heme group in red blood cells. The oxygen quickly binds to this heme group. As more oxygen molecules get bonded to heme, the ability of oxygen to attach increases. The binding ability of oxygen and its capacity to dissociate from haemoglobin can be affected by disease states and changing environments in the body.

Carbon dioxide can be transported via the bloodstream in three different ways. It can be immediately dissolved in the blood, linked to plasma or haemoglobin, or transformed to bicarbonate. Bicarbonate reaches the blood plasma after leaving the red blood cells. Bicarbonate is carried back into red blood cells in the lungs in return for chloride. With the support of carbonic anhydrase, the H+ dissociates from haemoglobin and mixes with bicarbonate to generate carbonic acid, which is then reconverted into carbon dioxide and water by carbonic anhydrase. After that, carbon dioxide is expelled from the lungs. 

This is the complete transport of gases that takes place in the human body.