Physiological reflexes demanded deeper breaths. The body would become starved of oxygen with greater amount of carbon dioxide in the blood. It would result in tachycardia. Fluctuations in blood pressure and dehydration would lead to heart and lung failure.
Thus crucifixion was the most painful death that could ever be given to anyone. The process of inflicting such extreme pain became synonymous with crucifixion and adverse pain due to it started to be known as excruciating.