Your body really isn't meant to stay horizontal 23+ hours/day, and that's what many elderly people are reduced to after a bad fall.

Bed sores develop very quickly, and an elderly immune system often simply isn't equipped to fight them. Gangrene and amputations are extremely common in end-of-life care.

For clarity, this sets in as the old body may still be slowly or practically unable to recover from the first event, the fall (e.g. a broken bone).

Your circulatory system is actively pumped by the heart, but also passively pumped by everyday muscle movements. Lack of movement decreases oxygenation and clearance of metabolic byproducts.