Exchange and Transport of Gases in Lungs

Gas exchange is the process that occurs between oxygen and carbon dioxide. Oxygen is passed from the lungs to the bloodstream and carbon dioxide is eliminated from the bloodstream to the lungs. Exchange of Gas takes place in lungs between the alveoli and capillaries which are tiny blood vessels, placed at the walls of alveoli. The rate of diffusion depends on the thickness of the biological membrane which forms the boundary between the external environment and organisms. Let’s learn more about how this gas exchange and transport take place.

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Exchange Of Gases

Although the primary sites of gas exchange are the alveoli, exchange of O₂ and CO₂ also happens between blood and tissues. Gas exchange at these sites happens by simple diffusion based on a concentration/pressure gradient. The rate of diffusion depends not only on the solubility of gases but also on the thickness of the membranes involved in gas exchange.

Partial pressure is the pressure that comes from an individual gas in a mixture of gases. It is represented as pO₂ for oxygen and pCO₂ for carbon dioxide. The table below shows the partial pressures of these gases in the atmospheric air and different sites of gas exchange.

Respiratory Gas	Atmospheric air	Alveoli	Blood (Deoxygenated)	Blood (Oxygenated)	Tissues
O2	159	104	40	95	40
CO2	0.3	40	45	40	45

The data in the above table clearly shows that there is a concentration gradient for O₂ from the alveoli to blood and blood to tissues. Similarly, CO₂ has a concentration gradient in the opposite direction i.e. from tissues to blood and blood to the alveoli. The amount of CO₂ that can diffuse through a membrane is much higher than that of O₂ since the solubility of CO₂ is 20-25 times higher than that of O₂.

Browse more Topics under Breathing And Exchange Of Gases

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Diffusion Membrane

The diffusion membrane has three layers –

• The thin squamous epithelium of the alveoli.
• The endothelium containing alveolar capillaries.
• The basement substance between the epithelium and endothelium.

The total thickness of the diffusion membrane is less than a millimeter. All these factors in our body make it easy for the diffusion of O₂ from alveoli to tissues and that of CO₂ from the tissues to alveoli.

Transport Of Gases

The transport of O₂ and CO₂ happens through blood. The RBCs in the blood carry 97% of O₂ while plasma carries the remaining 3% of O₂ in a dissolved state. In case of CO₂, the RBCs carry about 20-25%, plasma carries 7% of CO₂ in a dissolved state while the remaining 70% is carried as bicarbonate.

Transport Of Oxygen

O₂ binds reversibly to haemoglobin in blood to give oxyhaemoglobin. Haemoglobin is a red coloured pigment in the RBCs that contains iron. A maximum of 4 molecules of O₂ can bind to a molecule of haemoglobin. The partial pressure of O₂ is the primary factor that affects this binding. Other factors that can influence this binding are partial pressure of CO₂, temperature and hydrogen ion concentration.

When we plot the percent of saturation of haemoglobin with O2 against pO2, we get a sigmoid curve. This curve is the ‘Oxygen Dissociation Curve’ and is useful in studying the effects of H+ concentration, pCO2 etc, on the binding of O2 with haemoglobin. 

In the alveoli, there is high pO₂, low pCO₂, low temperature, and low H⁺ concentration. These conditions are favourable for the formation of oxyhemoglobin. However, in the tissues, there is low pO₂, high pCO₂, high temperature, and high H⁺ concentration which is favourable for the dissociation of O₂ from oxyhemoglobin.

This shows that O₂ binds to haemoglobin on the lung surface and gets dissociated in the tissues. Under normal physiological conditions, every 100 ml of oxygenated blood delivers about 5 ml of O₂ to the tissues.

Transport Of Carbon Dioxide

Apart from O_2, haemoglobin also carries about 20-25% of CO₂, as carbamino-haemoglobin. This binding is primarily related to the partial pressure of CO₂, however, pO₂ is also a major factor that affects this binding. In the tissues, when the pCO₂ is high and pO₂ is low, more binding of CO₂ takes place. On the other hand, when pCO₂ is low and pO₂ is high in the alveoli, CO₂ dissociates from carbamino-haemoglobin formed in the tissues and is released in the alveoli.

The enzyme – carbonic anhydrase is present in high concentration in the RBCs. Small amounts of this enzyme are also present in plasma. This enzyme catalyzes the following reaction in both directions:

CO₂ +H₂O   ⇔   H₂CO₃   ⇔    HCO₃^– + H⁺

The partial pressure of CO₂ is high in the tissues due to catabolism. Here, CO₂ diffuses into the blood (plasma and RBCs) and gives rise to HCO₃^– and H⁺. The reaction takes place in the opposite direction in the alveoli where the pCO₂ is low, leading to the formation of CO₂ and H₂O. Thus, at the tissue level, CO₂ is trapped as bicarbonate, transported to the alveoli and released as CO₂. Under normal physiological conditions, every 100 ml of deoxygenated blood delivers about 4 ml of CO₂ to the alveoli.

Solved Example For You

Q: Which of the following factors is not favourable for oxyhaemoglobin formation in the alveoli?

1. Low pCO₂
2. High pO₂
3. High H⁺ concentration.
4. Low temperature

Solution: The answer is ‘c’. Formation of oxyhemoglobin requires low H⁺ concentration.