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                                              P              =              partial pressure


                                             I               =              Inspired air

                                             E              =              Expired air

                                             A             =              Alveolar

                                             a              =              arterial

                                             v              =              venous

                                             v              =              mixed venous

                                             B              =              Barometric

                                             F              =              Fractional percentage

PIO2 means partial pressure of O2 in inspired air

Partial Pressure of O2 / CO2 at Different Sites (in mmHg)


























* The partial pressure of blood leaving the pulmonary capillaries is 97 mmHg but it falls to 95 mmHg in the systemic arterial blood because of physiologic shunt. The physiologic shunt is due to a part of the bronchial blood flow and a part of the coronary blood flow which bypasses the pulmonary capillaries

AO2 is given by alveolar gas equation PAO2 = FiO2 x (PB - PH2O) - PaCO2





=   PO2 of the alveolar air


=   Fraction of O2 in the air (Eg 20% in atm or inspired air & 16% in expired)


=   Barometric pressure (760mmHg)


=   Water vapour pressure (47mm H2O)


=   Partial pressure of CO2 ( 40 mm Hg)


=   Respiratory quotient

O2 transport

Most of the O2 in the blood is carried along with Hb (99%) Each Hb carries 4 molecules of O2. Hb exhibits the ‘relaxed’ and the ‘tense’ state. When Hb takes up O2 the beta chains move closer and the haem enters the relaxed state. The relaxed state favours binding of O2 while the tense state decrease binding.

Oxygen – Hb dissociation curve (O-HDC)

This is a plot of the partial pressure of O2 and the % saturation of Hb with O2 It is normally sigmoid-shaped.

If the OHDC is shifted to the right, it means that the affinity of Hb for O2 has become less (which favours O2 delivery).

Shift to the right is caused by:

  1. pH
  2. in temperature
  3. in 2,3 – DPG

P50 = The partial pressure of oxygen as which Hb is 50% separated. It value = 26 mmHg. (or 3.45 Kpa). When the OHDC shifts to the right, P50 increases.  (1Kpa = 7.5 mmHg)


Factors affecting 2,3 DPG:

  1. pH Q (acidosis inhibits red cell glycolysis, the 2,3-BPG concentration falls when the pH is low).
  2. Thyroid hormones, growth hormone, and androgensQ increase the concentration of 2,3-BPG and the P50.
  3. ExerciseQ has been reported to produce an increase in 2,3-BPG
  4. Ascent to high altitudeQ triggers a substantial rise in 2,3-BPG concentration in red cells,
  5. The affinity of fetal hemoglobin (hemoglobin F) for O2. The cause of this greater affinity is the poor binding of 2,3-BPG by the polypeptide chains that replace chains in fetal hemoglobinQ.
  6. Red cell 2,3-BPG concentration is increased in anemiaQ and in a variety of diseases in which there is chronic hypoxia
  7. In bank blood that is stored, the 2,3-BPG level falls. This decrease, is less if the blood is stored in citrate–phosphate–dextrose solutionQ rather than the usual acid–citrate–dextrose solution.
  8. Inosine increases 2,3 DPG in red blood cells. Q
  9. HbF binds 2,3 – DPG poorly; hence it has a greater affinity for O

Effect of pH on O-HDC

1.       pH

  1. Direct effect : shift to right
  2. By decreasing 2,3 – DPG, shift to left

2.       pH

  1. Direct effect : shift to left
  2. By increasing 2,3 – DPG, shift to right

Bohr Effect

There is a decrease in O2 affinity for Hb with a decrease in pH

  1. Calculation of O2 carrying capacity of blood
  1. 1 gm of Hb, when fully saturated, contains 1.34 ml of O2. At a pO2 of 40 mmHg (as exists in Venous blood), Hb is only 75% saturated. Therefore, 1 gm of Hb would carry 1.34 X 75 / 100 mL of O2 at pO2 of 40 mmHg
  2. The amount of dissolved O2 in plasma is 0.003/ dL / mmHg of pO2. At a pO2 of 40mmHg, the amount of dissolved O2 is 0.003 X 40 = 0.12 mL of O2 / dL.


This is present in skeletal muscles. 1 mole (OMDC) cule of myoglobin binds / 1 molecule of O2. The shape of O2 myoglobin dissociation curve is a rectangular hyperbola. It lies to the left of the O2 – Hb dissociation curve.


1. : O – H DC

2. : O – M DC

  1. CO2 transport
  1. Different ways in which CO2 is transported in :
    1. Plasma
    2. In dissolved form of
    3. As carbamino compound with plasma proteins
    4. Getting hydrated CO2 + H2O                 H2 CO3           H+ + HCO3- The H+ gets buffered with plasma proteins. Since there is no carbonic anhydrase in plasma, the process of hydration is slow.
  2. RBC
    1. In dissolved form
    2. As carbamino compound with Hb
    3. Getting hydrated

CO2 + H2O                   H2 CO3          H+ + HCO3- The H+ gets buffered by Hb; 70% the HCO3- enters plasma and Cl- enters RBC (chloride shift)

(Since there is carbonic anhydrase in RBC, the process of hydration is rapid.)

  1. It is clear from the above that for each CO2 molecule that goes into RBC, there is either one HCO3- or one Cl- inside the RBC; the chloride content of the venous blood RBC is more than that of arterial blood RBC. Therefore, there is an increase in the volume of RBC in venous blood and hence the haematocrit of venous blood is more.
  2. Out of the 49ml of CO2 / dL in arterial blood, 2.6 mL is dissolved 2.6 mL of CO2 exists as carbamino compound and 43.8 mL is transported as HCO3-
  3. Haldane effect: loading of O2 causes unloading of CO2.
  4. Chloride shift: in venous blood increase in Cl- inside RBCs due to exchange with bicarbonate ions, which are formed due to increase CO2 levels. RBC volume & PCV increases in venous blood.

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