Lack of oxygen.
At high altitude the oxygen concentration in the atmosphere decreases. When oxygen level drops the blood vessels in the lungs constrict:
Hypoxic pulmonary vasoconstriction (HPV), also known as the Euler-Liljestrand mechanism, is a physiological phenomenon in which small pulmonary arteries constrict in the presence of alveolar hypoxia (low oxygen levels).
Wikipedia, Hypoxic Pulmonary Vasoconstriction, 2019
The blood vessels in the lungs constrict when oxygen level drops. This was only known recently however this was portrayed in the Quran 1400 years before it was discovered.
[Quran 6.125] Those whom Allah wants to guide, He opens their chests to Islam; And those whom He wants to leave astray, He makes their chests tight and constricted, as if they are ascending to the sky: Such is the penalty of Allah on those who refuse to believe.
“He makes their chests tight and constricted, as if they are ascending to the sky” today we know why it gets constricted, lack of oxygen. No mistakes in the Quran.