How is oxygen transported from the lungs to the tissues?

Oxygen is transported from the lungs to the tissues via the bloodstream, specifically bound to haemoglobin in red blood cells.

In the lungs, oxygen is inhaled into the alveoli, tiny air sacs where gas exchange occurs. The oxygen diffuses across the thin walls of the alveoli and into the surrounding capillaries, tiny blood vessels that are part of the body's circulatory system. This process is driven by the concentration gradient, as the concentration of oxygen is higher in the alveoli than in the deoxygenated blood arriving from the heart.

Once in the capillaries, the oxygen binds to a molecule called haemoglobin, which is found in red blood cells. Each haemoglobin molecule can carry up to four oxygen molecules. This binding forms oxyhaemoglobin, a bright red substance that gives oxygenated blood its colour. The binding of oxygen to haemoglobin is a reversible process, meaning it can easily release the oxygen when needed.

The oxygen-rich blood then travels from the lungs, through the pulmonary veins, and into the left side of the heart. From there, it is pumped out to the rest of the body through the aorta, the largest artery in the body. As the oxygenated blood reaches the tissues, the oxygen is released from the haemoglobin due to the lower oxygen concentration in the tissues. This is again due to the concentration gradient, as oxygen moves from an area of high concentration (the blood) to an area of low concentration (the tissues).

The oxygen then diffuses into the cells of the tissues, where it is used in cellular respiration, a process that produces energy for the cell. Meanwhile, carbon dioxide, a waste product of cellular respiration, diffuses out of the cells and into the blood, where it is carried back to the lungs to be exhaled. This entire process of oxygen transport is crucial for maintaining life, as oxygen is needed for the production of energy in all cells.