Oxygen is relatively insoluble in liquid which means only a small amount is dissolved in blood. Instead, the oxygen molecules bind to Haemoglobin (Hb + O₂ → HbO₂), which is found in red blood cells. Once the oxygen molecules arrive at an area of the body that has a low concentration of oxygen, the Haemoglobin releases oxygen (HbO₂ → Hb + O₂).

Haemoglobin is a protein, which carries 98% of the oxygen in the blood. Haemoglobin can hold four molecules of oxygen, but it’s affinity for oxygen changes depending on how many oxygen molecules are bound to it. This relationship is represented by the oxyhaemoglobin dissociation curve, which we will explore in more detail later in the unit.

In contrast, carbon dioxide has a higher solubility in liquid than oxygen. One of the main methods of transport of CO₂ in blood is by dissolving it in the blood. Similarly, carbon dioxide can react with water in the blood to form carbonic acid, which is another important method of transporting carbon dioxide from the tissues to the lungs. It is also an important factor when it comes to controlling the acid levels of the body, which we will discuss later. In addition, there are some blood proteins which carbon dioxide can bind to and be transported. Finally, the carbon dioxide can bind to haemoglobin to form carbaminohaemoglobin.