Pathophysiological background of the disease
The oxygen sensors in the kidney control the hematocrit, produce erythropoietin in the cortex, and are able to translate tissue oxygen pressure into a measure of plasma volume [94]. However, in severe hypoxia below pO2 40 mmHg, glomerular filtration rate declines, leading to sodium retention and water retention [95, 96]. Hence, renal function in chronic respiratory failure, like OSA or COPD, is often impaired and these patients show fluid and sodium retention [97]. This excess retention has also been described at high altitude, especially in altitude maladapted individuals [98]. The effect of hypoxia on patients with impaired renal function can be studied in hypoxia models and this would have implications for determining their risks of long air travel or dwelling at high altitude.