Tissues can utilise only 25% of O 2 carried by the blood.
Every 100 ml of oxygenated blood can deliver around 5 ml of O2 to the tissues under normal physiological conditions.
The venous blood will have approx. 15 ml oxygen/100 ml of blood as it comes our the tissues in normal conditions.
Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO2 to the alveoli.