Fig 1 represents the lung and upper airway and the pulmonary blood flowing through the lungs.

Oxygen in the air is breathed into the lung Carbon dioxide is breathed out.

Blood is oxygenated in the lungs and is pumped by the heart to the tissues:

• Venous blood from the tissues has delivered some of its oxygen to the tissues. Normally only 75% of the haemoglobin in venous blood is saturated with oxygen
• Blood is oxygenated in the lungs and is pumped by the heart to the tissues. 98% of haemoglobin in arterial blood is saturated with oxygen