Gaseous Exchange In The Body

The exchange of Oxygen (O2) and Carbon Dioxide (CO2) between alveolar air and pulmonary blood occurs via passive diffusion. This is governed by the behaviour of gases as described by Dalton's Law and Henry's Law.

Gaseous exchange in the body occurs in two places:

  1. Between the air in the alveoli of the lungs and the blood in pulmonary capillaries (External respiration).
  2. Between the systemic capillaries and tissue cells (Internal respiration).

External Respiration

This process results in the conversion of deoxygenated blood from the right side of the heart to oxygenated blood returning to the left side of the heart. Gases are exchanged by diffusion according to the differences in their partial pressures.

The now deoxygenated blood returns to the heart and is pumped to the lungs where the process of external respiration begins again.

The partial pressure of O2 in alveolar air is 105mmHg, while the resting partial pressure of O2 in deoxygenated blood is ~40mmHg. Due to this difference O2 diffuses down its concentration gradient from the alveolar air into the deoxygenated blood until equilibrium is reached, the result being that the blood becomes oxygenated.

The partial pressure of CO2 in alveolar air is 40mmHg, while in deoxygenated blood it is 45 mmHg. CO2 therefore diffuses in the opposite direction to O2, again down its concentration gradient. The result being that CO2 is removed from the blood and exhaled.

The now deoxygenated blood returns to the heart and is pumped to the lungs where the process of external respiration begins again.

The rate of gas exchange during external respiration depends on several factors:

  • Partial pressure difference of the gases
  • Surface area available for gas exchange
  • Diffusion distance
  • Solubility and molecular weight of the gases

Internal Respiration

The process of internal respiration is much the same as external respiration, only the site in which it occurs is different. Internal respiration results in the conversion of oxygenated blood (from the capillaries) to deoxygenated blood. Once again the gases are exchanged in accordance with their partial pressures.

The partial pressure of O2 in capillary blood is ~100mmHg, whilst in the tissues it is ~ 40mmHg. Due to this difference O2 diffuses from the blood, through the interstitial fluid into the tissue cells until the partial pressure of O2 in the blood decreases to ~40mmHg.

CO2 again diffuses in the opposite direction. The partial pressure of CO2 in the tissue cells is 45mmHg, whilst in the blood it is 40mmHg. Therefore, CO2 diffuses from the tissue cells through the interstitial fluid into the blood until its partial pressure in the blood reaches ~45mmHg.

The deoxygenated blood returns to the heart from where it is pumped to the lungs and the process of external respiration begins again.

Bibliography
Tortora & Grabowski (2000) Principles Of Anatomy & Physiology. 9th Edn.New York. John Wiley & Sons Inc.

Gaseous Exchange

Erythrocytes (red blood cells) in the mammalian body are filled with a globular protein called Haemoglobin. This molecule consists of 4 polypeptide chains each with an Iron Haem group in the centre. This Haem group is very important for gaseous exchange.

Once the Erythrocyte, laden with Oxygen molecules, reaches some tissues which have an Oxygen deficit, the following reaction occurs:

CO2 + H2O <> H2CO3 <> HCO3- + H +

(<> = a reversible reaction)

This equation is catalysed by the highly efficient enzyme called Carbonic Anhydrase. The CO2 is absorbed into the red blood cell then in this equation it is reacted with water to make firstly to Hydrogen Carbonate and then to Hydrogen Carbonate- ions and H+ ions. The HCO3- ions are pumped out of the cell and are replaced with Cl- ions to keep the charges balanced.

The next stage in this process is as follows:

H+ + HbO8 <> HHb+ + 4O2

Here I have used Hb for Haemoglobin for simplicity because Haemoglobin is actually many thousands of molecules long.

This stage of the reaction involves the Hydrogen ions bonding with HbO8 (Oxygen bonded with Haemoglobin). The Hydrogen ions displace the Oxygen molecules from the Haem groups and allow the Oxygen molecules to come into solution in the red blood cell. These then diffuse through the phospholipid bilayer membrane of the red blood cell and move into the surrounding tissues where Oxygen is needed leaving the Hydrogen bonded with the Haemoglobin, acting as a spectator ion, as it is not actually used for anything.

Once this reaction has occurred there will be Oxygen in the tissues and the Carbon Dioxide which was in the tissues will now be carried back to the lungs to be exchanged for more Oxygen.

This reaction is entirely reversible and the opposite reaction happens at the lung to absorb new Oxygen.

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