Oxygen cascade

{{Short description|Flow of oxygen from air to mitochondria}} 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.<ref name = "Dominelli2021">{{cite journal |last1=Dominelli |first1=Paolo |last2=Wiggins |first2=Chad |last3=Roy |first3=Tuhin |last4=Secomb|first4=Timothy |last5=Curry |first5=Timothy |last6=Joyner |first6=Michael |title=The Oxygen Cascade During Exercise in Health and Disease |journal=Mayo Clinic Proceedings |date=March 10, 2021 |volume=96 |issue=4 |pages=1017–1032 |doi=10.1016/j.mayocp.2020.06.063 |pmid=33714599 |pmc=8026750 |url=https://www.mayoclinicproceedings.org/article/S0025-6196(20)30924-1/fulltext}}</ref> 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.<ref name="Treacher1998">{{cite journal |last1=Treacher |first1=D F |last2=Leach |first2=R M |title=Oxygen transport—1. Basic principles |journal=BMJ |date=November 1998 |volume=317 |issue=7168 |pages=1302–1306 |doi=10.1136/bmj.317.7168.1302 |pmid=9804723 |pmc=1114207 }}</ref> 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.<ref name="Hirai2018">{{cite journal |last1=Hirai |first1=Daniel |last2=Colburn |first2=Trenton |last3=Craig |first3=Jesse |last4=Hotta |first4=Kazuki |last5=Kano |first5=Yutaka |last6=Musch |first6=Timothy |last7=Poole |first7=David |title=Skeletal muscle interstitial O2 pressures: bridging the gap between the capillary and myocyte |journal=Microcirculation |date=October 2018 |volume=26 |issue=5 |article-number=e12497 |doi=10.1111/micc.12497 |pmid=30120845 |pmc=6379155 }}</ref>

== Table == Table 1 gives the example of a typical oxygen cascade for skeletal muscle of a healthy, adult male at rest who is breathing air at atmospheric pressure at sea level. Actual values in a person may vary widely due to ambient conditions, health status, tissue type, and metabolic demands.

{| class="wikitable" |+ Table 1: An Example Oxygen Cascade |- ! Location !! Partial pressure of oxygen (mmHg) !! Notes |- | Dry air || 159 || Air is ~21% oxygen<ref name="Treacher1998" /> |- | Moist air || 150 || Air is humidified in the respiratory tract<ref name="Treacher1998" /> |- | Alveolar air || 110-100 || Alveolar air includes exhaust gases such as CO₂<ref name="Treacher1998" /><ref name="Hirai2018" /> |- | Arterial blood (PaO₂) || 98-95 || Oxygen must cross the alveoli, leading to a drop in PO₂ called the alveolar-to-arterial gradient (typically a drop of 1-5 mmHg, but sometimes larger).<ref name="Dominelli2021" /><ref name="Hirai2018" /> |- | Venous blood (PvO₂) || 40-35 || Arterial blood offloads oxygen in the capillaries before flowing into the venous system. The partial pressure of oxygen in venous blood (PvO₂) can range widely in different veins that drain different tissues because of differences in oxygen demand of the tissues.<ref name="Treacher1998" /><ref name="Dominelli2021" /> |- | Interstitial space in a resting skeletal muscle || 18-13 || The capillary walls provide physical barriers that partially resist oxygen transfer from the blood to the interstitial space.<ref name="Hirai2018" /> |- | Intracellular space in skeletal muscle cells (PO_2is) || 14-10 || The cell membranes provide physical barriers that partially resist oxygen transfer from the interstitial space to the intracellular space.<ref name="Hirai2018" /> |- | Mitochondria || 10-0 || The partial pressure of oxygen in mitochondria is generally assumed to be lower than the surroundings because the mitochondria consume oxygen.<ref name="Schumacker2014">{{cite journal |last1=Schumacker |first1=Paul T. |last2=Gillespie |first2=Mark N. |last3=Nakahira |first3=Kiichi |last4=Choi |first4=Augustine M. K. |last5=Crouser |first5=Elliott D. |last6=Piantadosi |first6=Claude A. |last7=Bhattacharya |first7=Jahar |title=Mitochondria in lung biology and pathology: more than just a powerhouse |journal=American Journal of Physiology. Lung Cellular and Molecular Physiology |date=1 June 2014 |volume=306 |issue=11 |pages=L962–L974 |doi=10.1152/ajplung.00073.2014 |pmid=24748601 |pmc=4042189 }}</ref> If the oxygen level is too low, mitochondria cannot metabolize nutrients for energy via aerobic metabolism. The shift between aerobic and anaerobic metabolism has profound physiological consequences. |}

== See also ==

* Alveolar–arterial gradient * Alveolar gas equation * Blood gas tension

== References ==

{{reflist}}

Category:Respiratory physiology Category:Cell biology