As a respiratory physiologist, I can explain the process of how oxygen enters the bloodstream. "Oxygen enters the bloodstream" from the alveoli, which are tiny air sacs located in the lungs. These alveoli are surrounded by a network of capillaries, the smallest blood vessels in the body. The walls of both the alveoli and capillaries are extremely thin, which facilitates the exchange of gases. The process begins when we inhale air rich in oxygen. This oxygen travels down the respiratory tract and fills the alveoli. Due to the concentration gradient, oxygen molecules diffuse across the thin walls of the alveoli and into the capillaries, where they bind to hemoglobin molecules within red blood cells. Hemoglobin is a protein that has a high affinity for oxygen and can carry up to four oxygen molecules at a time. Once oxygen is bound to hemoglobin, the oxygen-rich blood is carried away from the lungs and pumped by the heart through the systemic circulation to the rest of the body. This oxygen is then used by cells for various metabolic processes, including the production of energy. In summary, the process of oxygen entering the bloodstream involves inhalation, diffusion across the alveolar-capillary membrane, binding to hemoglobin, and systemic circulation. read more >>

The oxygen enters the bloodstream from the alveoli, tiny sacs in the lungs where gas exchange takes place (Figure below). The transfer of oxygen into the blood is through simple diffusion. The oxygen-rich blood returns to the heart.read more >>