This a medical description of what happens:

DEATH BY CRUCIFIXION: SLOW SUFFOCATION

1 - Shallowness of breathing causes small areas of lung collapse.

2 - Decreased oxygen and increased carbon dioxide cause acidic conditions in the tissues.

3 - Fluid builds up in the lungs. Makes the situation in step 2 worse.

4 - The heart is stressed and eventually fails.

https://www.blueletterbible.org/Comm/terasaka_david/misc/crucify.cfm