The marrow replaces the blood without pause
A typical adult produces on the order of two million red blood cells every second to replace those that are retired.
Established2 min read
A red cell circulates for about 120 days and is then taken apart, mostly in the spleen. An adult body holds on the order of 25 trillion of them. Divide one number by the other and the required production is a few million cells a second. The marrow does this in the ordinary course of a quiet afternoon.
The rate is adjustable. Bleed, or go to altitude, or stop making the hormone erythropoietin, and the number moves. The 'two million' is the resting order of magnitude that keeps the count steady.
Caveats
- The figure is derived from blood volume, cell count, and lifespan. It is not a direct tally of one person's marrow.
- White cells and platelets are replaced on their own schedules, some of them much faster.
Sources
- 1Kaushansky K. et al. (eds.). Williams Hematology. McGraw-Hill, 9th edition, 2016.