TRANSPORT OF GASES CLASSIFICATION

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. Even though the gas exchange is a continuous process, oxygen and carbon dioxide are carried in separate ways.

HEMOGLOBIN:

Haemoglobin, or Hb, is a four-subunit protein present in red blood cells (erythrocytes) that consists of two alpha and two beta subunits. Each component is surrounded by an iron-containing core heme group that binds one oxygen molecule, allowing each haemoglobin molecule to bind four oxygen molecules. 

Molecules including more oxygen bonded to their heme groups have a richer colour. As a consequence, oxygenated arterial blood with four oxygen molecules in the Hb is brilliant red, while deoxygenated venous blood is a deeper red.

The second and third oxygen molecules bond to Hb more easily than the first. This is because haemoglobin’s shape, or structure, changes as oxygen connects to it. The fourth oxygen is then more difficult to bind.

The oxygen binding to haemoglobin can be displayed as a function of blood oxygen partial pressure (x-axis) vs relative Hb-oxygen saturation (y-axis). The oxygen dissociation curve that results is sigmoidal, or S-shaped. The haemoglobin becomes increasingly saturated with oxygen as the partial pressure of oxygen rises.

FACTORS AFFECTING OXYGEN-BINDING:

The amount of oxygen carried in the blood is determined by haemoglobin’s oxygen-carrying capacity. Other environmental conditions and disorders, in addition to PO2, can impact oxygen-carrying capacity & delivery.

Oxygen-carrying capacity is affected by carbon dioxide levels, blood pH, and body temperature. Carbon dioxide combines with water in the blood to create bicarbonate (HCO3) & hydrogen ions (H+). More H+ is created as the amount of carbon dioxide in the blood rises, and the pH falls. The affinity of haemoglobin for oxygen is reduced as carbon dioxide levels rise and pH levels fall.

 The dissociation of oxygen from the Hb molecule causes the oxygen dissociation curve to shift to the right. As a result, more oxygen is required to achieve the same amount of haemoglobin saturation as when the pH was higher. An increase in body temperature causes a similar change in the curve. Greater warmth, such as that caused by increased skeletal muscular activity, reduces haemoglobin’s affinity for oxygen.

Sickle cell anaemia and thalassemia cause the blood’s ability to supply oxygen to tissues as well as its oxygen-carrying capacity to be reduced. The shape of the red blood cell in sickle cell anaemia is crescent-shaped, elongated, & rigid, decreasing its ability to carry oxygen. Red blood cells cannot flow through capillaries in this state. When this happens, it hurts.

 Thalassemia is an uncommon hereditary condition caused by a deficiency in the alpha or beta subunits of the haemoglobin molecule. Patients with thalassemia produce a significant number of red blood cells, but their haemoglobin levels are lower than usual. As a result, the oxygen-carrying capacity is reduced.

TRANSPORT OF CO2 IN BLOOD:

Carbon dioxide molecules are delivered in the blood from body tissues to the lungs through one of three methods: direct dissolution, haemoglobin binding, or bicarbonate ion transport. A few factors influence carbon dioxide’s passage through the bloodstream.

Carbon dioxide, for starters, is more soluble in blood than oxygen. In the plasma, about 5% to 7% of all carbon dioxide is dissolved. In contrast, carbon dioxide can bind to protein or penetrate red blood cells & bind to haemoglobin. About 10% of the carbon dioxide is transported in this form. 

When carbon dioxide binds to haemoglobin, a molecule called carbaminohemoglobin is formed. The binding of carbon dioxide to haemoglobin is reversible. As a consequence, when carbon dioxide reaches the lungs, it may easily separate from haemoglobin and be expelled.

Third, the bicarbonate buffer system is responsible for transporting the vast majority of carbon dioxide molecules (85 percent). In this method, carbon dioxide diffuses into red blood cells. Carbonic anhydrase (CA), a red blood cell protein, converts carbon dioxide to carbonic acid quickly (H2CO3). 

Carbonic acid is an unstable intermediate molecule that quickly breaks down into hydrogen (H+) and carbon (HCO3) ions. Carbon dioxide can be taken into the bloodstream at a smaller concentration gradient because it is quickly converted to bicarbonate ions. As a result of this action, H+ ions are generated. If too much H+ is produced, the pH of the blood may shift. Haemoglobin, on either hand, binds to free H+ ions to keep pH variations to a minimum.

The bicarbonate ion is transferred back into the red blood cell in return for the chloride ion when the blood reaches the lungs. 

The carbonic acid intermediate is formed, which is then transformed back to carbon dioxide by CA’s enzymatic action. During exhalation, the carbon dioxide created is evacuated through the lungs. After dissociating from haemoglobin, the bicarbonate ion binds to the H+ ion.                  

                                            CO2 + H2O 🡨🡪 H2CO3 🡨🡪 HCO3 + H+

The bicarbonate buffer system has the advantage of allowing carbon dioxide to be “absorbed” into the bloodstream with little change in the pH of the system. This is critical since even a little alteration in the body’s total pH can cause serious harm or death. 

People may also travel and survive at high altitudes because of the presence of this bicarbonate buffer system: when the partial pressures of oxygen and carbon dioxide fluctuate at high altitudes, the bicarbonate buffer system adapts to regulate carbon dioxide while preserving the proper pH in the body.

CARBON MONOXIDE POISONING:

Other molecules, like carbon monoxide (CO), cannot readily bind with and dissociate from haemoglobin. Carbon monoxide binds to haemoglobin more strongly than oxygen. As a result, carbon monoxide preferentially binds to haemoglobin over oxygen when it is present. Only a tiny amount of oxygen is transported throughout the body due to oxygen’s inability to attach to haemoglobin.

Because carbon monoxide is a colourless and odourless gas, it is difficult to detect. Gas-powered cars and tools produce it. Carbon monoxide can lead to headaches, disorientation, and nausea, as well as brain damage and death in long-term exposure.

Carbon monoxide poisoning is usually treated by administering 100 percent (pure) oxygen. The separation of carbon monoxide from haemoglobin is accelerated when pure oxygen is administered.

CONCLUSION:

Haemoglobin is a protein made composed of two alpha and two beta subunits that surround an iron-containing heme group in red blood cells. The oxygen quickly binds to this heme group. As more oxygen molecules are attached to heme, the ability of oxygen to bind rises. 

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 dissolved in the bloodstream instantly, coupled to plasma proteins or haemoglobin, or converted to bicarbonate. The bicarbonate system is responsible for transporting the majority of carbon dioxide. Diffusion allows carbon dioxide to enter red blood cells.

In exchange for chloride, bicarbonate is transported back into red blood cells in the lungs. With the help of carbonic anhydrase, the H+ dissociates from haemoglobin and mixes with bicarbonate to generate carbonic acid, which is then converted back into carbon dioxide and water by carbonic anhydrase. Following that, the carbon dioxide is evacuated from the lungs.