In respiratory physiology, the oxygen cascade describes the flow of oxygen from air to mitochondria, where it is consumed in aerobic respiration to release energy.[1] Oxygen flows from areas with high partial pressure of oxygen (PO2, also known as oxygen tension) to areas of lower PO2.
Air is typically around 21% oxygen, and at sea level, the PO2 of air is typically around 159 mmHg.[2] Humidity dilutes the concentration of oxygen in air. As air is inhaled into the lungs, it mixes with water and exhaust gasses including CO2, further diluting the oxygen concentration and lowering the PO2. As oxygen continues to flow down the concentration gradient from areas of higher concentration to areas of lower concentration, it must pass through barriers such as the alveoli walls, capillary walls, capillary blood plasma, red blood cell membrane, interstitial space, other cell membranes, and cell cytoplasm. The partial pressure of oxygen drops across each barrier.[3]
^Dominelli, Paolo; Wiggins, Chad; Roy, Tuhin; Secomb, Timothy; Curry, Timothy; Joyner, Michael (March 10, 2021). "The Oxygen Cascade During Exercise in Health and Disease". Mayo Clinic Proceedings. 96 (4): 1017–1032. doi:10.1016/j.mayocp.2020.06.063. PMC 8026750. PMID 33714599.
^Treacher, D F; Leach, R M (November 1998). "Oxygen transport—1. Basic principles". BMJ. 317 (7168): 1302–1306. doi:10.1136/bmj.317.7168.1302. PMC 1114207. PMID 9804723.
^Hirai, Daniel; Colburn, Trenton; Craig, Jesse; Hotta, Kazuki; Kano, Yutaka; Musch, Timothy; Poole, David (October 2018). "Skeletal muscle interstitial O2 pressures: bridging the gap between the capillary and myocyte". Microcirculation. 26 (5): e12497. doi:10.1111/micc.12497. PMC 6379155. PMID 30120845.
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