Gas Transport in Blood

Human Physiology

Lesson 35 : Gas Transport & Respiratory Adaptation

Gas Transport in Blood

Blood gases are oxygen and carbon dioxide that are required and evolved during cell metabolism and required to be exchanged with environment through respiratory system. Blood is the only medium that distributes oxygen and collects carbon dioxide between lungs and tissue cells. Respiratory membrane consists of anatomical structures that these respiratory gases required to cross between blood and alveolar air.

Transport of O2 The concentration of oxygen in arterial blood leaving heart in 20ml per 100 ml of blood and has a partial pressure of 100 mm Hg. The venous blood returning to heart has oxygen concentration of 40ml per 100 ml of blood at a partial pressure of 40mm Hg. Oxygen is partially soluble in plasma and only 0.7% of total blood oxygen is carried in physical solution form. Rest 99.3% is transported in combination with hemoglobin present in erythrocytes. Hemoglobin combines with oxygen in physical but reversible manner. Oxygen molecule is attached to Fe++ (Ferrous) part of heme molecule in hemoglobin. One hemoglobin molecule can binds to four molecules of oxygen and one gram of hemoglobin can bind with 1.34 ml of oxygen. Therefore, oxygen carrying capacity of hemoglobin is 1.34 ml per gram of hemoglobin, Hemoglobin concentration of blood is 15g/100ml and thus arterial blood can carry about 20 ml (15x1.34) of oxygen in full saturation state. Combination of hemoglobin with oxygen is termed as oxyhemoglobin (or oxygenated hemoglobin). Oxidized hemoglobin is one in which chemical transformation of ferrous occurs to ferric form and this form is called met hemoglobin (met Hb). Mt Hb is abnormal form of hemoglobin and it doesn't have capacity to bind to oxygen. The formation of metHb occurs when blood is exposed to some oxidizing agents such as in cyanide, nitrate poisoning. Formation of met Hb is irreversible and such Hb is required to be destroyed and eliminated from the body.

Transport of Carbon dioxide (CO2) Carbon dioxide is 20 times more soluble in plasma than O2. The partial pressure of CO2 in arterial blood is 40 mm Hg and venous blood is 46 mm Hg. Venous blood contains more CO2 than arterial blood. It is transported in following forms in blood:

1. Dissolved CO2


2 As Carbamino Hb


3 As plasma bicarbonate


CO2 being readily soluble in water combine with water molecule to form carbonic acid as shown in reaction.
CO2 + H2O ? H2CO3
In this form only 8% of CO2 can be transported.

Carbamino Hb is formed when CO2 combines with hemoglobin. The attachment of CO2 to Hb is a chemical reaction and it binds to globin (protein) part of hemoglobin. This reaction occurs when blood is passing through tissues.

Most carbon dioxide in blood is transported as plasma bicarbonate. CO2 first enters the erythrocytes and with the help of carbonic anhydrase enzyme, it is converted to bicarbonate ions which then diffuses out of erythrocytes and comes in plasma for transportation to lungs.

In the lungs all above reactions are reversed and CO2 diffuses out into alveolar air for further elimination through respiratory tract.

Last modified: Tuesday, 10 April 2012, 12:45 PM