Do humans have the same blood that we had as a kid? How come blood doesn’t expire inside of us since we range from different temperatures and are exposed to oxygen etc.

Question

Do we somehow maintain the same blood for the rest of our lives and we just pump more if we lose some. Blood expires in hospitals and not when inside of us?

In: Biology

Answer 1

Red blood cells have a lifespan of about 120 days, no more. When red blood cells mature, they lose their nucleus so that they can’t reproduce at all. That’s why they have to be made in special tissue inside your bone marrow. Not only that, but red blood cells lose pretty much *all* their organelles – they don’t have ribosomes or mitochondria.

Red blood cells are essentially sacks emptied of everything except for hemoglobin to carry oxygen and the bare essentials to keep the cell together to hold it all together. Although they are alive, they are barely alive and barely have a metabolism. They can’t sustain themselves, and don’t really need to.

Other cells in your blood, like some immune cells, are a lot more self-sufficient, but they get replaced often as well. The upshot of this is that your blood *does* expire inside of you, just not all at once. There’s a constant supply of fresh cells, and old cells are cleaned up in your spleen and liver, where they’re destroyed by macrophages (a type of immune cell).

Old blood cells can just fall apart and become food for bacteria. Those bacteria release toxic products. In your body, most blood cells are cleaned up before bacteria eat them, but when it does happen your body quickly cleans up the toxins with your liver before they can reach dangerous levels. In a bag in the hospital, there’s nothing to clean up old cells. They keep dying and falling apart, bacteria start eating them, and there’s nothing to clean up the bacteria or their toxic waste.

For the most part, your body doesn’t have the same *anything* that you did as a kid, with a few exceptions. Neurons in your brain stop reproducing by age 5 or so (probably), so although connections between them change and grow, and other cells in your brain like your glial cells (which support your neurons) continue to divide, you are born with most of the same individual neurons that you die with. Heart muscle cells reproduce slowly, but they do reproduce. And there’s some non-living stuff in your body, like your teeth, that don’t change appreciably. But other than the few exceptions, you body is constantly repairing tissue and organs by replacing cells.