Respiration is composed of inspiration which is an active process, and expiration which is a passive process. The inspiration is controlled by several centres in the medulla oblongata and pons that initiate contractions of the diaphragm and the intercostal muscles. The result is that the diaphragm is pressed against the abdomen and the chest lifted forward and somewhat extended. This creates a negative pressure within the thorax which thus draws air into the lungs. Every time you inhale, you inhale about 500 ml. Of this approximately 350 ml is participating in gas exchange. The air in the lungs ”freshen up” a little every time you breathe, and the gas mixture in the alveoli is not equal to the air outside of you. Oxygen diffuses from the alveoli into the blood, which becomes 100% saturated, and is transported dissolved in plasma or bound to haemoglobin in the blood. In normal resting conditions about 25% of the oxygen is delivered in the tissues. In the tissues carbon dioxide diffuses into the blood and is transported in plasma, bound to haemoglobin or as bicarbonate. When the blood arrives in the lungs, carbon dioxide enters the alveoli by diffusion. Blood arriving at the lungs under normal, resting conditions is about 75% saturated with oxygen.