TRANSPORT OF OXYGEN

Blood serves to transport the respiratory gases. Oxygen, which is essential for the cells is transported from alveoli of lungs to the cells. Carbon dioxide, which is the waste product in cells is transported from cells to lungs.

TRANSPORT OF OXYGEN

Oxygen is transported from alveoli to the tissue by blood in two forms:

1. As simple physical solution

2. In combination with hemoglobin.

AS SIMPLE SOLUTION

Oxygen dissolves in water of plasma and is transported in this physical form. Amount of oxygen transported in this way is very negligible. It is only 0.3 mL/100 mL of plasma. It forms only about 3% of total oxygen in blood. It is because of poor solubility of oxygen in water content of plasma. Still, transport of oxygen in this form becomes important during the conditions like muscular exercise to meet the excess demand of oxygen by the tissues.

IN COMBINATION WITH HEMOGLOBIN

Oxygen combines with hemoglobin in blood and is transported as oxyhemoglobin. Transport of oxygen

in this form is important because, maximum amount (97%) of oxygen is transported by this method.

Oxygenation of Hemoglobin

Oxygen combines with hemoglobin only as a physical combination. It is only oxygenation and not

oxidation. This type of combination of oxygen with hemoglobin has got some advantages. Oxygen can be

readily released from hemoglobin when it is needed. Hemoglobin accepts oxygen readily whenever the partial pressure of oxygen in the blood is more. Hemoglobin gives out oxygen whenever the partial pressure of oxygen in the blood is less. Oxygen combines with the iron in heme part of hemoglobin. Each molecule of hemoglobin contains 4 atoms of iron. Iron of the hemoglobin is present in ferrous form. Each iron atom combines with one molecule of oxygen. After combination, iron remains in ferrous form only. That is why the combination of oxygen with hemoglobin is called oxygenation and not oxidation.

Oxygen Carrying Capacity of Hemoglobin

Oxygen carrying capacity of hemoglobin is the amount of oxygen transported by 1 gram of hemoglobin. It is 1.34 mL/g.

Oxygen Carrying Capacity of Blood

Oxygen carrying capacity of blood refers to the amount of oxygen transported by blood. Normal hemoglobin content in blood is 15 g%. Since oxygen carrying capacity of hemoglobin is

1.34 mL/g, blood with 15 g% of hemoglobin should carry 20.1 mL% of oxygen, i.e. 20.1 mL of oxygen in 100 mL of blood.

But, blood with 15 g% of hemoglobin carries only 19 mL% of oxygen, i.e. 19 mL of oxygen is carried by 100 mL of blood. Oxygen carrying capacity of blood is only 19 mL% because the hemoglobin is not

fully saturated with oxygen. It is saturated only for about 95%.

Saturation of Hemoglobin with Oxygen

Saturation is the state or condition when hemoglobin is unable to hold or carry any more oxygen. Saturation of hemoglobin with oxygen depends upon partial pressure of oxygen. And it is explained by oxygenhemoglobin dissociation curve.

OXYGEN-HEMOGLOBIN DISSOCIATION CURVE

Oxygen-hemoglobin dissociation curve is the curve that demonstrates the relationship between partial

pressure of oxygen and the percentage saturation of hemoglobin with oxygen. It explains hemoglobin’s

affinity for oxygen. Normally in the blood, hemoglobin is saturatedwith oxygen only up to 95%. Saturation of hemoglobin with oxygen depends upon the partial pressure of

oxygen. When the partial pressure of oxygen is more hemoglobin accepts oxygen and when the partial pressure of oxygen is less, hemoglobin releases oxygen.

Method to Plot Oxygen-hemoglobin

Dissociation Curve

Ten flasks or tonometers are taken. Each one is filled with a known quantity of blood with known

concentration of hemoglobin. Blood in each tonometer is exposed to oxygen at different partial pressures.

Tonometer is rotated at a constant temperature till the blood takes as much of oxygen as it can. Then, blood is analyzed to measure the percentage saturation of hemoglobin with oxygen. Partial pressure of oxygen and saturation of hemoglobin are plotted to obtain the oxygen-hemoglobin dissociation curve.

Normal Oxygen-hemoglobin Dissociation Curve

Under normal conditions, oxygen-hemoglobin dissociation curve is ‘S’ shaped or sigmoid shaped.

Lower part of the curve indicates dissociation of oxygen from hemoglobin. Upper part of the curve indicates the uptake of oxygen by hemoglobin depending upon partial pressure of oxygen.

P50

P50 is the partial pressure of oxygen at which hemoglobin saturation with oxygen is 50%. When the partial pressure of oxygen is 25 to 27 mm Hg, the hemoglobin is saturated to about 50%. That is, the blood contains 50% of oxygen. At 40 mm Hg of partial pressure of oxygen, the saturation is 75%. It becomes 95% when the partial pressure of oxygen is 100 mm Hg.

Factors Affecting Oxygen-hemoglobin

Dissociation Curve

Oxygen-hemoglobin dissociation curve is shifted to left or right by various factors:

1. Shift to left indicates acceptance (association) of oxygen by hemoglobin

2. Shift to right indicates dissociation of oxygen from hemoglobin.