Respiratory System | EXCHANGE OF GASES

SEE PART 1 : Lists Of Respiratory Diseases | Respiratory Disorders

  • Alveolar air is separated from blood present in surrounding blood capillaries by very thin partition of 0.2 μm thickness. It is called respiratory membrane. The membrane consists of alveolar surfactant, alveolar epithelium, epithelial basement membrane, a thin interstitial space, capillary basement membrane and capillary endothelial membrane.
  • Diffusing capacity of a gas across a membrane is the volume of gas that diffuses per minute for a pressure difference of 1mmHg. The rate of diffusion of CO2 is 20 times faster than that of oxygen while oxygen diffuses faster ( twice) than nitrogen. Partial pressure of O2 in alveolar air (PO2) is about 100-104 mmHg while that of deoxygenated blood in alveolar
    capillary is 40mmHg. Therefore, oxygen diffuses into blood and combines with haemoglobin to form oxyhaemoglobin. Pressure of (PO2) oxygenated blood is 95 mmHg. PCO2 (pressure of carbon dioxide) of alveolar capillary blood is 46 mmHg, while in fresh alveolar air it is 40mmHg. As the diffusing capacity of CO2 is 20 times higher than that of O2, CO2 of blood rapidly passes out into alveolar air. Its partial pressure in oxygenated blood is 40 mmHg.

  • Gaseous exchange occurs again in the tissues cells and capillary blood through the interstitial fluid. Partial pressure of oxygen, PO2 in respiring cells is 20mmHg, tissue fluid is 40mmHg, while it is 95mmHg in capillary blood.
    Therefore, O2 diffuses from blood into
    tissue fluid and from there into cells. Blood leaving the tissue capillaries has PO2 of about 40mmHg. PCO2 of blood capillaries is 40mmHg, tissue fluid 45mmHg and that of cells 52mmHG. Therefore, carbon dioxide diffuses out of cells into tissue fluid and form tissue
    fluid into blood. Blood leaving the tissue capillaries has a PCO2 of 46mmHg.


    .Oxygen is carried by blood in two forms in solution and as oxyhaemoglobin as RBCs


    . Oxygen is soluble in plasma to a small extents under normal conditions of temperature and pressure. Hence most of it is carried by red blood cells. About
    3% of oxygen is transported by blood in dissolved form in plasma of blood.

    Example, out of about 4.6ml of oxygen entering each 100ml of blood in lungs only 0.17 ml travels in solution form in the plasma.


  • RBC contain a protein called haemoglobin. It has four polypeptide chains and four haem groups attached to it or 4 atom of iron in ferrous form (Fe2+), thus it can react with 4 molecules of oxygen to form Hb4O8. This is called oxyhaemoglobin.
    This combination process is called oxygenation.

    – On an average 15gm of haemoglobin (Hb) is present in 100ml of blood. 1gm of Hb combines with 1.34ml of O2. Thus 100ml of blood carries approximately 20ml of O2 (19.4 ml to be exact)

    – But when blood reaches the tissues, its O2 concentration reduces gradually to
    14.4 ml which is collected by veinules and vein. Thus 5ml of O2 is transported by 100ml of blood under normal condition.

    – Haemoglobin has higher affinity for oxygen and this affinity is increased by fall in PCO2 of blood.

    – At the alveoli, venous blood has low oxygen and is exposed to low PCO2 of
    alveolus, thus oxygen diffuses into red blood cells and form oxyhaemoglobin (
    bright red). As CO2 diffuses from blood to alveolus, blood PCO2 falls increasing
    further uptake of oxygen.

    – Oxyhaemoglobin remains unchanged till it reaches the tissues where it
    dissociates readily to release oxygen.


  • The percentage of haemoglobin that is bound with O2 is called percentage saturation of haemoglobin.
  • The relationship between the partial pressure of oxygen (PO2) and percentage of saturation of the haemoglobin with oxygen (O2) is graphically illustrated by a curve called oxygen haemoglobin dissociation curve.
  • Under normal conditions, the oxygen haemoglobin dissociation curve is sigmoid shaped or ‘S’ shaped. The lower part of the curve indicates dissociation of oxygen from haemoglobin.
    The upper part of the curve indicates the acceptance of oxygen by haemoglobin. When the partial pressure of oxygen is 255mmHg the haemoglobin gets saturated to about 50%. It means blood contains 50% oxygen. The partial pressure at which the haemoglobin saturation is 50% is called P50. At 40mmHg of partial pressure of oxygen the saturation is 75%. It becomes 95% when the partial pressure of oxygen is 100mmHg.

  • Haemoglobin does not take up oxygen are low PO2, but as oxygenation of pigment occurs, its affinity for more O2 increases. In haemoglobin where 4 sub-units are present, acquisition of one molecule of oxygen increases the affinity of neighbouring haems for oxygen. This is known as co-operativity between active sites.

    i. Temperature: At higher temperature haemoglobin gives up oxygen more readily and dissociation curve shits to the right. This is of physiological importance because increased temperature means higher metabolic rate or higher oxygen requirement.

    ii) pH: Increase in CO2 or other acids lower the pH of plasma and shifts the dissociation curve to the right. At higher CO2 concentration more O2 is given up at any oxygen pressure.

    iii) PCO2: CO2 lowers the oxygen affinity of haemoglobin even if the pH is kept constant. Oxygen dissociation curve shifts to the right and release more O2 with increase in PCO2

    iv) 2,3-diphosphoglyceric acid ( 2,3-DPG): It is present in the red blood cells of adult, formed from 3-phosphoglyceric acid. It competes for oxygen binding sites in haemoglobin molecule. As it binds to the β- chain of HbA, it causes right shift of
    dissociation curve resulting in higher P50.

    v) Lower CO2 concentration: lower body temperature lower 2,3-DPG lower the P50 and the curve moves to the left.


  • Shifting of the oxygen haemoglobin dissociation curve to the right by increasing carbon dioxide partial pressure is known as Bohr effect. It is named after Danish physiologist Christian Bohr.
  • The presence of carbon dioxide decreases the affinity of haemoglobin for oxygen and increases release of oxygen to the tissues.
  • The pH of the blood falls as its CO2 content increases so that when PCO2 rises the curve to the right and P50 rises.
  • In the tissue, PO2 is between 10 to 40 mmHg and PCO2 is high around 45mmHg. So, an active tissue will have high PCO2, low pH and raised temperature leading to the dissociation of oxygen. Oxygenated blood passing through inactive cells does not given up oxygen even if its PCO2 is low but active cells readily gives oxygen as PCO2 is very high.



    Because of its high solubility, about 7% carbon dioxide gets dissolved in the blood plasma and is carried in solution to lungs. Deoxygenated (venous) blood and oxygenated (arterial) blood carry about 2.7ml and 2.4ml of CO2 per 100ml of blood in dissolved state in plasma respectively.


    – The dissolved carbon dioxide in the blood reacts with water to form carbonic acid. This reaction is very slow in the blood plasma, but occurs very rapidly inside RBCs because a zinc containing enzyme, the carbonic anhydrase, present in RBCs, accelerates its rate about 5000 times.

    – Due to this, about, 70% of CO2, received by blood from the tissues, enters the
    RBCs where it reacts with water to form carbonic acid (H2CO3).

    – Carbonic anhydrase is exclusively found in R.B.Cs. All other tissue contains it in traces except stomach and pancreas in which have considerable amount. This enzyme not only speeds up the formation of carbonic acid (H2CO3) but also rapidly converts it back to carbon dioxide and water when blood reaches the lungs.

    – Almost as rapidly as formed all carbonic acid of RBCs dissociates into hydrogen (H+) and bicarbonate ions (HCO3-).

    – The most of bicarbonate ions (HCO3-) formed with RBCs diffuse out into blood plasma along the concentration gradient.

    – When the whole blood is saturated with carbon dioxide, the following changes are seen.

    (i) The bicarbonate content of plasma and corpuscles increase.

    (ii) The chloride content of plasma is diminished and that of the cells is increased.

    (iii) The total base (cations) of both plasma and corpuscles remain unchanged.

    (iv) The water content and the volume of corpuscles increase.

    – When carbon dioxide is removed from a sample of blood, reverse changes take place. From these observations, it is evident that, when carbon dioxide enters blood, chlorine from plasma enters the RBCs, while the base (NA) is left behind. When carbon dioxide escapes the plasma and combines with the base (Na) again. Due to this alternate movement of chlorine ions, this phenomenon is called chlorine shift or Hamburger phenomenon.


    – In addition to reacting with water, carbon dioxide also reacts directly with amine radicals (NH2) of haemoglobin to form an unstable compound carbamino-haemoglobin. This is a reversible reaction.

    – A small amount of carbon dioxide also reacts in this same way with the plasma proteins. About 23% CO2 is transported in combination with haemoglobin and plasma proteins.


  • Addition of hydrogen ions would make the blood acidic. However, most of the hydrogen ions are neutralized by combination with haemoglobin, which is negatively charged, forming acid haemoglobin. This reduces the acidity of the blood and also releases additional oxygen.
  • If the blood becomes too basic, acid haemoglobin dissociates, releasing hydrogen ions.

    HHb — H+ + Hb

    Thus, the haemoglobin also acts as buffer, a substance that keeps the pH from fluctuating.The haemoglobin of the foetus has a higher affinity for oxygen than the mother’s haemoglobin. After birth, the foetal haemoglobin is gradually replaced by adult haemoglobin.


    Please enter your comment!
    Please enter your name here