Breathing and Exchange of Gases

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.

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




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 Ofrom 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 Osince the solubility of CO2 is 20-25 times higher than that of O2.

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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 Ofrom alveoli to tissues and that of COfrom the tissues to alveoli.

gas exchange

Cross section of an alveolus (Source: Wikimedia Commons)

Transport Of Gases

The transport of Oand COhappens through blood. The RBCs in the blood carry 97% of Owhile 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

Obinds 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 Ocan bind to a molecule of haemoglobin. The partial pressure of Ois 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. 

gas exchange

Oxygen Dissociation Curve (Source: Wikimedia Commons)

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 Oto the tissues.

Transport Of Carbon Dioxide

Apart from O2, 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 +H2 ⇔   H2CO3   ⇔    HCO3 + H+

The partial pressure of COis high in the tissues due to catabolism. Here, COdiffuses 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 COto 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.

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