Small engines in our cells called mitochondria stop manufacturing energy for the cells' use. Our cells run out of gas. As oxygen is broken down in our cells, the breakdown products, called free radicals, cause mischief. That is, breathing kills us. Cell boundaries weaken.
Cells lose their ability to regulate the coming and going of calcium and potassium.
They can't keep water out, either, and the water dilutes cell constituents. In effect, says Fred Samson, professor emeritus of molecular and integrative physiology, cells drown. The molecular basis for both arterial narrowing and the heart damage done by blockage of blood flow involves effects from free radicals. Why their reaction with proteins is greater in the elderly is a focus of much current research. It's possible that the reaction is the same but that the normal repair and recycling of damaged proteins becomes defective, so that more and more damaged proteins accumulate.