Once pulmonary ventilation (inhaled a breathe of air) has taken place and the lungs are filled with air, the second stage of respiration takes place, pulmonary gas exchange.
Pulmonary gas exchange takes place in the lungs between the alveoli and the blood. It is also referred to as ‘external respiration’ as it involves the respiratory processes that have contact with the external environment.
The process of pulmonary gas exchange removes CO2 from the blood and replenishes the bloods O2 supply.
Gas exchange occurs down a pressure gradient, via a process known as ‘diffusion’. When we breathe in we inspire air consisting of a mixture of gases including O2 and CO2.
These gases each have a pressure related to their concentration within the gas mixture. These individual pressures are termed partial pressures.
Differences in the partial pressures between the gases in the alveoli and blood create a pressure gradient across the respiratory membrane (membrane separating the alveoli and blood capillaries).
If the pressure on each side of the membrane were the same there would be no exchange of gas and no movement of O2 and CO2. Where the partial pressures of O2 and CO2 are different gas exchange occurs.
Gases move from an area of high concentration (high pressure) to an area of low concentration (low pressure). As the freshly inspired air in the alveoli is high in O2, the O2 diffuses across the respiratory membrane into the blood where the concentration of O2 is low. This blood is now ‘oxygenated’ and is sent to the tissues of the body for use, as seen in the following image.
The blood that has come from the tissues of the body to the alveoli is high in CO2. The CO2 in this ‘deoxygenated’ blood diffuses across the respiratory membrane into the alveoli to an area of low CO2 concentration, and is subsequently expired (breathed out) from the lungs. This can also be seen in the image above.
In order for O2 to be absorbed into the blood it binds to haemoglobin (Hb) which is a compound that sits on our red blood cells. Oxygenated blood (carried in the arteries) is bright red because of the binding of haemoglobin and oxygen. Deoxygenated blood (carried in the veins) is much darker red because of the lack of available oxygen to bind to haemoglobin.
Deoxygenated blood is often referred to as ‘blue’, as opposed to dark red. This referencing is primarily used to differentiate oxygenated and deoxygenated blood to help people understand the differences and to show differences in diagrams. Deoxygenated blood is not ‘blue’ like the sky on a sunny day (please don’t cut yourself to check this!) rather a very dark red, that can appear bluish when viewed through the superficial veins of the body – check out the colour of the veins in your forearm or hands.