What is the process by which oxygen moves from the alveoli into the blood?

Alveolar diffusion is the fundamental process responsible for the movement of oxygen from the lungs into the blood and for the removal of carbon dioxide from the blood to the lungs.

Consider the alveoli as minute balloons located at the very end of the respiratory tree. Their walls are incredibly thin, about one cell thick, and they are closely enveloped by a network of capillaries. This proximity facilitates the process of alveolar diffusion as gases can easily move across the alveolar and capillary walls. Oxygen present in the alveoli after inhalation naturally diffuses into the blood because the oxygen pressure is greater in the alveoli than in the blood. Conversely, carbon dioxide released from the blood into the alveoli is exhaled from the body. This movement occurs due to the differences in partial pressures, or concentration gradients, making diffusion an efficient and vital process in respiratory physiology.

The respiratory system's main function is to supply the body with oxygen and remove carbon dioxide. It begins at the nose or mouth, continues through the trachea, into the bronchi, down to the smaller bronchioles, and finally reaching the alveoli.

During inhalation, the diaphragm contracts and the rib cage expands, drawing air into the lungs. Once in the alveoli, oxygen transfers into the bloodstream, while carbon dioxide moves from the blood to the alveoli to be expelled during exhalation. This process of gas exchange is a continuous cycle, meticulously regulated to adjust to the body's oxygen demands and metabolic waste removal.

Once inside the blood, oxygen transport is mainly managed by hemoglobin, a protein found within red blood cells. Each hemoglobin molecule can bind up to four oxygen atoms, forming oxyhemoglobin.

The efficiency of oxygen transport in the blood is influenced by the oxygen-hemoglobin dissociation curve, which describes how readily hemoglobin acquires and releases oxygen molecules into the tissues. Factors like pH, temperature, and the concentration of carbon dioxide can affect this curve, in turn influencing oxygen delivery to where it's needed most. When the body is at rest, the hemoglobin releases oxygen slowly; however, when active, changes in the blood environment promote oxygen release, ensuring adequate supply to the muscles.

Capillary gas exchange is associated not only with the lungs and respiratory physiology but also with the systemic circulation throughout the body tissues. This exchange uses the same principle of diffusion as alveolar transfer.

At the site of tissues, capillaries release oxygen into the cells and pick up carbon dioxide—the product of cellular respiration. The 'offloading' of oxygen and 'loading' of carbon dioxide occurs due to the differing pressures in the capillaries and tissue fluid. Oxygen is driven into the tissues where its concentration is lower, whereas carbon dioxide moves from the tissues (where its concentration is higher) into the capillaries, to be transported back to the lungs for release.