Oxygen is transported from the lungs to the body’s tissues via the bloodstream, primarily bound to hemoglobin in red blood cells.

In the lungs, oxygen is inhaled into tiny air sacs known as alveoli, where gas exchange occurs. The oxygen then diffuses across the thin walls of the alveoli into the surrounding capillaries, which are small blood vessels that form part of the circulatory system. This diffusion process is driven by a concentration gradient, as the concentration of oxygen in the alveoli is higher than that in the deoxygenated blood returning from the heart.

Once in the capillaries, oxygen binds to hemoglobin, a protein found in red blood cells. Each hemoglobin molecule can carry up to four oxygen molecules. This binding results in the formation of oxyhemoglobin, a bright red compound that gives oxygenated blood its distinctive color. Notably, the binding of oxygen to hemoglobin is a reversible process, allowing hemoglobin to release oxygen when it is needed by the tissues.

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 via the aorta, the largest artery. As the oxygenated blood reaches the tissues, the oxygen is released from hemoglobin due to the lower concentration of oxygen in the tissues. This release is again facilitated by the concentration gradient, with oxygen moving from an area of high concentration (in the blood) to an area of low concentration (in the tissues).

The oxygen then diffuses into the cells of the tissues, where it plays a crucial role in cellular respiration—a process that generates energy for the cell. Concurrently, carbon dioxide, a waste product of cellular respiration, diffuses out of the cells into the blood, which transports it back to the lungs for exhalation. This entire process of oxygen transport is vital for sustaining life, as oxygen is essential for energy production in all cells.