Most of us picture blood as plumbing — red liquid in a pipe, going round. It is closer to a transport network that never closes. About five litres of it moves through an adult, and it completes a circuit in roughly a minute at rest.
Four jobs at once
Red cells carry oxygen out from the lungs and bring carbon dioxide back. Plasma — the straw-coloured fluid the cells travel in — carries nutrients, hormones and waste. White cells handle defence. Platelets handle repair, sealing damage in vessel walls before it becomes a leak.
None of these happen in turn. They happen continuously and simultaneously, in the same fluid, which is part of why blood is a useful thing to look after as a system rather than one number at a time.
A red cell lives about 120 days. Your body replaces roughly two million of them every second.
The shape is the point
A red blood cell is a biconcave disc — a dimple on each side. That shape gives it more surface area for its volume than a sphere would, and enough flex to fold through capillaries narrower than the cell itself. It carries no nucleus, which leaves more room for haemoglobin, the protein that actually binds the oxygen.
Flow is a property of the whole system
How easily blood moves depends on the vessels it moves through, its own thickness, and how readily platelets stick together. These are ordinary, continuous physiological processes, and they shift with hydration, activity, sleep and age.