Editing Altitude sickness (section)

== Mechanism ==
The [[Breathing#Breathing at altitude|physiology of altitude sickness]] centres around the [[alveolar gas equation]]; the atmospheric pressure is low, but there is still 20.9% oxygen. Water vapour still occupies the same pressure too—this means that there is less oxygen pressure available in the lungs and blood. Compare these two equations comparing the amount of oxygen in blood at altitude:<ref name=":0">{{cite journal |vauthors=Brown JP, Grocott MP |date=2013-02-01 |title=Humans at altitude: physiology and pathophysiology |url=https://academic.oup.com/bjaed/article/13/1/17/281180/Humans-at-altitude-physiology-and-pathophysiology |journal=Continuing Education in Anaesthesia, Critical Care & Pain |volume=13 |issue=1 |pages=17–22 |doi=10.1093/bjaceaccp/mks047 |issn=1743-1816 |doi-access=free}}</ref>

<div class="overflowbugx" style="overflow-x:auto;">
{| class="wikitable"
! Type
!At Sea Level
!At 8400 m (The Balcony of [[Mount Everest|Everest]])
!Formula
|-
|[[Alveolar gas equation|Pressure of oxygen in the alveolus]]
|<math display="inline">21\% \times(101.3\text{ kPa}-6.3\text{ kPa}) - \left (\frac{5.3\text{ kPa}}{0.8} \right ) = 13.3 \text{ kPa O}_2</math>
|<math display="inline">21\% \times(36.3\text{ kPa}-6.3\text{ kPa}) - \left (\frac{1.8\text{ kPa}}{0.74} \right ) = 3.9 \text{ kPa O}_2</math>
|<math display="inline">F_I \text{O}_2 \times(P_\text{B}-P_{\text{H}_2\text{O}}) - \left (\frac{P_{\text{CO}_2}}{\text{RQ}} \right ) </math>
|-
|Oxygen Carriage in the blood
|<math display="inline">\left(0.98 \times 1.34 \times 14\frac{\text{g}}{\text{dL}}\right) + (0.023\times 12\text{ kPa}) = \frac{17.3 \text{ mL O}_2}{100 \text{ mL blood}}</math>
|<math display="inline">\left(0.54 \times 1.34 \times 19.3\frac{\text{g}}{\text{dL}}\right) + (0.023\times 3.3\text{ kPa}) = \frac{14.0 \text{ mL O}_2}{100 \text{ mL blood}}</math>
|<math display="inline">(\text{Sa}_{\text{O}_2}\times 1.34\tfrac{\text{mL}}{\text{g Hb}} \times \text{Hb}) +
(\text{O}_2\text{ carriage in blood} \times \text{Pa}_{\text{O}_2})</math>
|}
</div>

The hypoxia leads to an increase in minute ventilation (hence both low {{chem2|CO2}}, and subsequently bicarbonate), Hb increases through haemoconcentration and erythrogenesis. Alkalosis shifts the haemoglobin dissociation constant to the left, 2,3-BPG increases to counter this. Cardiac output increases through an increase in heart rate.<ref name=":0" />

The body's response to high altitude includes the following:<ref name=":0" />
* ↑ Erythropoietin → ↑ hematocrit and haemoglobin
* ↑ [[2,3-BPG]] (allows ↑ release of {{chem2|O2}} and a right shift on the Hb-{{chem2|O2}} disassociation curve)
* ↑ kidney excretion of bicarbonate (use of acetazolamide can augment for treatment)
* Chronic hypoxic pulmonary vasoconstriction (can cause right ventricular hypertrophy)
People with high-altitude sickness generally have reduced hyperventilator response, impaired gas exchange, fluid retention or increased sympathetic drive. There is thought to be an increase in cerebral venous volume because of an increase in cerebral blood flow and hypocapnic cerebral vasoconstriction causing oedema.<ref name=":0" />