# Oxygen cascade

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{{Short description|Flow of oxygen from air to mitochondria}}
In [respiratory physiology](/source/respiratory_physiology), the '''oxygen cascade''' describes the flow of oxygen from air to [mitochondria](/source/mitochondria), where it is consumed in [aerobic respiration](/source/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](/source/partial_pressure) of oxygen (PO<sub>2</sub>, also known as [oxygen tension](/source/oxygen_tension)) to areas of lower PO<sub>2</sub>.

Air is typically around 21% oxygen, and at [sea level](/source/sea_level), the PO<sub>2</sub> of air is typically around 159 [mmHg](/source/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](/source/Humidity) dilutes the concentration of oxygen in air. As air is inhaled into the lungs, it mixes with water and exhaust gasses including CO<sub>2</sub>, further diluting the oxygen concentration and lowering the PO<sub>2</sub>. As oxygen continues to flow down the [concentration gradient](/source/concentration_gradient) from areas of higher concentration to areas of lower concentration, it must pass through barriers such as the [alveoli](/source/Lung_alveoli) walls, [capillary wall](/source/capillary_wall)s, capillary [blood plasma](/source/blood_plasma), [red blood cell](/source/red_blood_cell) membrane, [interstitial space](/source/Interstitial_fluid), other [cell membrane](/source/cell_membrane)s, and cell [cytoplasm](/source/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](/source/skeletal_muscle) of a healthy, adult male at rest who is breathing air at [atmospheric pressure](/source/atmospheric_pressure) at [sea level](/source/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](/source/Partial_pressure) of oxygen ([mmHg](/source/mmHg)) !! Notes
|-
| [Dry air](/source/Dry_air) || 159 || Air is ~21% oxygen<ref name="Treacher1998" />
|-
| [Moist air](/source/Moist_air) || 150 || Air is humidified in the [respiratory tract](/source/respiratory_tract)<ref name="Treacher1998" />
|-
| [Alveolar](/source/Alveolar) air || 110-100 || Alveolar air includes exhaust gases such as CO<sub>2</sub><ref name="Treacher1998" /><ref name="Hirai2018" />
|-
| [Arterial blood](/source/Arterial_blood) (PaO<sub>2</sub>) || 98-95 || Oxygen must cross the alveoli, leading to a drop in PO<sub>2</sub> called the [alveolar-to-arterial gradient](/source/alveolar-to-arterial_gradient) (typically a drop of 1-5 mmHg, but sometimes larger).<ref name="Dominelli2021" /><ref name="Hirai2018" /> 
|-
| [Venous blood](/source/Venous_blood) (PvO<sub>2</sub>) || 40-35 || Arterial blood offloads oxygen in the capillaries before flowing into the venous system. The partial pressure of oxygen in venous blood (PvO<sub>2</sub>) can range widely in different veins that drain different tissues because of differences in [oxygen demand](/source/Oxygen_saturation_(medicine)) of the tissues.<ref name="Treacher1998" /><ref name="Dominelli2021" />
|-
| [Interstitial space](/source/Interstitial_fluid) in a resting [skeletal muscle](/source/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](/source/Intracellular_space) in [skeletal muscle cell](/source/skeletal_muscle_cell)s (PO<sub>2is</sub>) || 14-10 || The cell membranes provide physical barriers that partially resist oxygen transfer from the interstitial space to the [intracellular space](/source/intracellular_space).<ref name="Hirai2018" />
|-
| [Mitochondria](/source/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](/source/aerobic_metabolism). The shift between aerobic and [anaerobic metabolism](/source/anaerobic_metabolism) has profound physiological consequences.
|}

== See also ==

* [Alveolar–arterial gradient](/source/Alveolar%E2%80%93arterial_gradient)
* [Alveolar gas equation](/source/Alveolar_gas_equation)
* [Blood gas tension](/source/Blood_gas_tension)

== References ==

{{reflist}}

Category:Respiratory physiology
Category:Cell biology

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Adapted from the Wikipedia article [Oxygen cascade](https://en.wikipedia.org/wiki/Oxygen_cascade) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Oxygen_cascade?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.