The pressure gradient drives CO 2 out of tissue cells and into the capillaries. At the same time, blood PCO 2=40mmHg and systemic tissue PCO 2=45mmHg. This pressure gradient drives the diffusion of oxygen out of the capillaries and into the tissue cells. In systemic capillaries, PO 2=100mmHg, but in the tissue cells, PO 2=40mmHg. As blood enters the systemic capillaries, the blood will lose oxygen and gain carbon dioxide because of the pressure difference between the tissues and blood. Oxygen and carbon dioxide move independently of each other they diffuse down their pressure gradients as blood leaves the lungs through the pulmonary veins, the venous PO 2=100mmHg, whereas the venous PCO 2=40mmHg. Due to this gradient, CO 2 diffuses down its pressure gradient, moving out of the capillaries and entering the alveoli. At the same time, alveolar PCO 2 is lower than blood PCO 2. Since this pressure gradient exists, oxygen can diffuse down its pressure gradient, moving out of the alveoli and entering the blood of the capillaries where O 2 binds to haemoglobin. More specifically, alveolar PO 2 is higher in the alveoli than blood PO 2 in the capillaries.

These red blood cells carry oxygen to the tissues where oxygen dissociates from the haemoglobin, diffusing into the cells of tissues. Oxygen (about 98 per cent) binds reversibly to the respiratory pigment haemoglobin found in red blood cells. In the lungs, oxygen diffuses out of the alveoli and into the capillaries surrounding the alveoli. The concentration of the gas in a fluid is also dependent on the solubility of the gas in the liquid. The higher the partial pressure of the gas, the higher the number of gas molecules that will dissolve in the liquid. Henry’s law states that the concentration of gas in a liquid is directly proportional to the solubility and partial pressure of that gas. When the RQ is known, the partial pressure of oxygen in the alveoli is calculated using the equation: alveolar PO 2 = inspired PO 2−((alveolar PO 2)/RQ). This results in a lower concentration of oxygen in the lungs than is found in the air outside the body. The lungs never fully deflate with an exhalation therefore, the inspired air mixes with this residual air, lowering the partial pressure of oxygen within the alveoli. The ratio of carbon dioxide production to oxygen consumption is referred to as the respiratory quotient (RQ), which typically varies between 0.7 and 1.0. The RQ is a key factor because it is used to calculate the partial pressure of oxygen in the alveolar spaces within the lung: the alveolar PO 2 (P ALVO 2). In the human body, oxygen is used by cells of the body’s tissues to produce ATP, while carbon dioxide is produced as a waste product. This is driven by the change in partial pressure from the alveoli to the capillaries. Gas exchange occurs in the alveoli where oxygen is exchanged with carbon dioxide between the alveoli and the blood in the capillaries.

Oxygen and carbon dioxide are exchanged