Oxygen Transport

17.3.1.1 In the Lung

If there is a mixture of gases above a liquid, the amount of each gas dissolved in the liquid will depend directly upon the partial pressure of the gas, i.e., upon the pressure exerted by the given gas.

The partial pressure of a gas in a mixture can be calcu­lated if the total pressure and the percentage composition of the mixture are known.

Partial pressure (P.P) of a gas in a mixture of gases =

% of a gas in a mixture ? Total pressure of the mixture.

17.3.1.2 InspiredAir

Barometric pressure of dry atmospheric air (at sea level) = 760 mm Hg.

N.B.: Total pressure of alveolar air = atmospheric pres­sure (760 mm Hg) - pressure of water vapour present in the respiratory tract (47 mm Hg) = 713 mm Hg.

17.3.1.4 Venous Blood

• CO₂ pressure is high → CO₂ moves from venous blood to alveolar air (6 volumes % given up by blood).

• O₂ pressure is high → O₂ moves from alveolar air to blood (about 6.5% of volume % taken by blood).

17.3.1.5 Arterial Blood

17.3.1.6 In the Tissues

% of gas the

tissues P.P (mm Hg.)

OXYGEN 0-40

CARBON DIOXIDE 50-60

• O₂ pressure is low → O₂ moves from arterial blood to tissues (6.5 volumes% given up by blood).

• CO₂ pressure is high → CO₂ moves from tissues to blood (6 volumes % taken up by blood).

N.B.: Concentrations of the gases and therefore the pres­sures exerted by them vary in the tissues depending on the metabolic activity of the particular tissue at any one time.

17.3.1.7 ExpiredAir

Expired air is laden with water vapour exerting a pressure of 47 mm Hg.

In conclusion:

The diffusion of a gas across membranes occurs from the higher tension to the lower tension. Therefore, O₂ passes from alveoli to venous blood and from arterial blood to tis­sues. On the other hand, CO₂ passes from tissues to venous blood and from venous blood to alveoli, as seen in the fol­lowing table and figure.

Significance:

• This amount is not enough at all for tissue oxygen­ation, which is 250 ml/minute at rest.

• O₂ in physical solution is very important in deter­mining the direction and rate of diffusion of O₂ from or to the blood.

N.B.:* O₂ content of blood varies between zero and 20 ml depending on:

17.3.1.8 Carriage and Transfer of O₂ by Blood

(I) In physical solution: (II) In chemical combination:

17.3.1.8.1 In Physical Solution

• About 3% of O₂ in the lung (alveolar air) is car­ried this way; 0.3 ml/100 ml blood is dissolved in plasma.

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17.3.1.9 O₂ - Dissociation Curve

• It is the relationship between O₂ tension and O₂% saturation of Hb.

The curve takes an S-shape represented by:

• Upper (horizontal) segment where affinity of Hb towards high O₂ tension is high.

• Lower (vertical) segment where affinity of Hb towards low O₂ tension is low

17.3.1.10 Physiological Significance of the S-shape of O₂ - Dissociation Curve

17.3.1.11 Causes of S-shape of O₂ Dissociation Curve of Blood Hb

1. Saturation of blood Hb occurs in steps, forming intermediate compounds:

(Hb4)O2 → (Hb4)O4 → (Hb4)O6 → (Hb4)O8

(N.B.: Hb is made up of 4 subunits)

2. If all Hb is (Hb₄)O₂, the curve will be a rectangu­lar hyperbola (like that of myo Hb).

17.3.1.12 Factors Affecting O₂ Dissociation Curve

17.3.1.13 O₂ Dissociation Curve of

Myoglobin "(Hb₄) O₂"

Differences between:

Blood Hb Muscle Hb

It has four iron atoms, each It has one iron atom which can combine can combine with one O₂ with one O₂ molecule → (Hb₁)O₂

molecule (2 oxygen atoms)

→ (Hb₄)O_s

Shape of O₂ dissociation curve of myo Hb: Rectangular hyperbola, i.e.

1. Remains horizontal (at high O₂ tension) → myo Hb does not give up its O₂ easily.

2. Then, suddenly descends vertically (at very low O₂ tension, e.g., severe muscular exercise) → myo Hb gives its O₂ easily (as blood Hb has given most of its O₂).

N.B.: After muscular exercise, myoglobin (myo Hb) absorbs O₂ from blood to replenish O₂ store again.

17.3.2