Transport of Gas Definition

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

Both gasses must be delivered between the external and internal respiration sites in order for oxygen and carbon dioxide to be exchanged. Despite the fact that carbon dioxide is more soluble in blood than oxygen, both gasses require a specific transport system to carry the majority of gas molecules between the lungs as well as other tissues.

Oxygen Transport in the Blood

Despite the fact that oxygen is carried through the blood, it is not extremely soluble in liquids. Although a tiny amount of oxygen dissolves in the blood and travels through the bloodstream, it only accounts for roughly 1.5 percent of the overall amount. 

A specialized transport mechanism that depends on the erythrocyte—the red blood cell—transports the bulk of oxygen molecules from the lungs to the body’s tissues. Haemoglobin, a metalloprotein found in erythrocytes, is responsible for binding oxygen molecules to the erythrocyte. Haem is the iron-containing component of haemoglobin, and it is haem that binds oxygen.

Because each haemoglobin molecule includes four iron-containing Haem molecules, each haemoglobin molecule can carry up to four molecules of oxygen. As oxygen diffuses from the alveolus to the capillary over the respiratory membrane, it also diffuses into red blood cells and therefore is bound by haemoglobin. 

The ultimate product, oxyhaemoglobin (Hb–O₂), is generated when oxygen binds to haemoglobin, as shown in the following reversible chemical reaction. Oxyhaemoglobin is a brilliant red-colored molecule that helps to give oxygenated blood its vivid red color.

Function of Haemoglobin

Haemoglobin is made up of subunits, which is known as a quaternary structure in protein terms. Each of haemoglobin’s four subunits is structured in a ring-like pattern, with an Fe atom covalently bonded to the haem in the middle of each. 

When the first oxygen molecule binds, haemoglobin undergoes a conformational shift that makes it easier for the second oxygen molecule to bind. As every molecule of oxygen is bound, it makes it easier for the next molecule to bind, and so on, until all four-haem space is filled by oxygen. When the first oxygen molecule dissociates and is “dropped off” in the tissues, the second oxygen molecule dissociates more quickly.

When the first oxygen molecule dissociates and is “dropped off” in the tissues, the second oxygen molecule dissociates more quickly. Haemoglobin is said to be saturated when all four haem sites are occupied. 

Haemoglobin is said to be partly saturated when one to three haem sites are occupied. As a result, haemoglobin saturation refers to the percentage of accessible haem units that are linked to oxygen at any particular time in the blood as a whole. 

A 100 percent saturation of haemoglobin means that each haem unit in all of the body’s erythrocytes is bound to oxygen. Haemoglobin saturation ranges from 95 percent to 99 percent in a healthy person with normal haemoglobin levels.

Oxygen Dissociation From Haemoglobin

The binding of oxygen to it and disassociation from haem is complicated by partial pressure. A graph depicting the relationship between partial pressure and oxygen binding to haem and subsequent dissociation from haem is known as an oxygen–haemoglobin dissociation curve.

Keep in mind that gasses flow from a higher partial pressure area to a lower partial pressure area. Furthermore, as more oxygen molecules are bound, the affinity of an oxygen molecule for haem rises.

 As a result, as the partial pressure of oxygen rises, a proportionately greater number of oxygen molecules are bound by haem in the oxygen–haemoglobin saturation curve. The oxygen–haemoglobin saturation/dissociation curve, predictably, demonstrates that as lower the partial pressure of oxygen, the fewer oxygen molecules coupled to haem.

 As a consequence, the partial pressure of oxygen is crucial in determining the degree of oxygen binding to haem at the respiratory membrane’s site, as well as the degree of oxygen dissociation from haem at the location of body tissues.

The oxygen–haemoglobin saturation/dissociation curve’s mechanisms also function as automatic control mechanisms that govern the amount of oxygen given to various tissues throughout the body. Since some tissues have such a higher metabolic rate than in others, this is crucial. 

Highly active tissues, like muscle, consume oxygen quickly to make ATP, reducing the partial pressure of oxygen within tissue to around 20 mm Hg. Because the partial pressure of oxygen within capillaries is around 100 mm Hg, the distinction between the two is relatively large, around 80 mm Hg. 

As a result, more oxygen molecules separate from haemoglobin and enter the tissues. The opposite is true of tissues with lower metabolic rates, such as adipose (body fat).

Because these cells utilise less oxygen, the partial pressure of oxygen inside these tissues remain high, resulting in fewer oxygen molecules dissociation from haemoglobin and entering the interstitial fluid of the tissue. 

Despite the fact that venous blood is deoxygenated, oxygen is still linked to haemoglobin in red blood cells. This creates an oxygen reserve that is used when tissues require more oxygen unexpectedly.

Carbondioxide transport in Blood

 Three major methods transfer carbon dioxide. Because some carbon dioxide molecules dissolve in the blood, the initial pathway of carbon dioxide transport is through blood plasma. The second process is bicarbonate (HCO₃–) transport, which likewise dissolves in plasma. The third mode of carbon dioxide transfer is analogous to how erythrocytes carry oxygen.

Although carbon dioxide is not thought to be very soluble in blood, a tiny percentage of the carbon dioxide which diffuses into blood from the tissues dissolves in plasma (about 7 to 10%). The dissolved carbon dioxide then travels through the bloodstream until it reaches the pulmonary capillaries, where it diffuses through the respiratory membrane into the alveoli and is exhaled during pulmonary ventilation.

Carbaminohaemoglobin

Haemoglobin binds around 20% of carbon dioxide and transports it to the lungs. Carbon dioxide, unlike oxygen, does not link to iron; instead, it binds to amino acid moieties on the globin sections of haemoglobin to produce carbaminohemoglobin, which is formed when haemoglobin and carbon dioxide come together. 

When haemoglobin isn’t transferring oxygen, it takes on a bluish-purple hue, resulting in the darker maroon color associated with deoxygenated blood. This reversible reaction is represented by the following formula:

                                      CO₂ + Hb 🡨🡪 HbCO₂

The binding & dissociation of carbon dioxide to or from haemoglobin is reliant on the partial pressure of carbon dioxide, just like the transport of oxygen by haem. Because carbon dioxide is released from the lungs, the partial pressure of carbon dioxide in blood that leaves the lungs and reaches body tissues is lower than the partial pressure of carbon dioxide in the tissues. 

As a result of its higher partial pressure, carbon dioxide leaves the tissues, enters the circulation, and then travels into red blood cells, binding to haemoglobin. In contrast to the alveoli, the partial pressure of carbon dioxide in the pulmonary capillaries is high.

 As a reason, carbon dioxide easily separates from haemoglobin and diffuses into the air through the respiratory membrane. The affinity of haemoglobin for carbon dioxide is influenced by the oxygen saturation of haemoglobin and the partial pressure of oxygen in the blood, in addition to the partial pressure of carbon dioxide. 

The Haldane effect is a result of the link between the partial pressure of oxygen and haemoglobin’s affinity for carbon dioxide. Carbon dioxide does not readily attach to oxygen-saturated haemoglobin. Haemoglobin quickly binds to carbon dioxide when oxygen is not coupled to haem and the partial pressure of oxygen is low.

Conclusion

 Once oxygen has diffused across the alveoli, it enters the blood and is carried to the tissues, where it is unloaded, while carbon dioxide diffuses out of the bloodstream and into the alveoli, where it is evacuated from the body. Despite the fact that gas exchange is a continuous process, oxygen and carbon dioxide are carried in separate ways.