Question

How come blood doesn't get mixed up in capillaries?

Answer 1

It is a conceptual question, and you will need basic physics for flow and knowledge in diffusion to understand this problem.

Blood is present in capillaries. Capillaries are near cells. The cells are bathed in blood too.
The blood in capillaried has oxy-haemoglobin, which is oxygen + haemoglobin complex. Lets call this A.
The blood near cells, outside capillaries have carbamino harmoglobin, which is CO2 + haemoglobin, lets call this B.

So,. Inside capillary, we have more A. Outside capillary, we have more B. So, there is concentration difference . Capillaries are permeable membrane. So, diffusion occurs. A from capillaries, move to blood outside the capillaries, reach cells, and lose oxygen, and becomes B.
As B is less inside capillaries, B moves from cells, to, inside the capillaries, thus oxygen is delivered and CO2 is taken away.

The concept that blood actually moves from capillary outside, and deoxygenated blood moves inside is a simple way of explaining things, not the actual process.

The actual process is diffusion of haemoglobin according to concentration difference. So, blood doesn't move in and out, so blood won't mix up. Only haemoglobin , vitamins, WBC etc moves in and out through capillaries.

That is why we don't call capillaries as arteries or veins. We cannot actually say of capillary has oxygenated or deoxygenated blood.

Note, the oxygenated blood has 90% oxygen. Deoxygenated blood has 80-85% oxygen. But blood near cells have as low as 40-60% oxygen only. Capillaries use this concentration difference, and it is not the actual blood that is pumped outside and then inside.

These are much higher concepts, you will only study in MBBS or degree levels.

Try to understand as much as you can. It is not possible to explain this in any simpler way.