How Many Red Blood Cells Do We Lose A Day?

Quote of the Day

Well, did we pass?

— Paul Samuelson's PhD adviser to the rest of Paul's dissertation committee after his thesis defense. Samuelson, arguably the greatest economist of his generation, was intimidating to his professors even as a student.

Introduction

Figure 1: Image of Human Red Blood Cells (Source).

I regularly visit the RefDesk website to pick up general information. Refdesk has a section that contains a Fact of the Day from the Random History website. Unfortunately, these "facts" are occasionally just plain wrong (example). Today, another one of these random facts did not seem correct and I thought I would perform a quick Fermi analysis here to show that it cannot be correct. I assume that they confused hours and minutes in their analysis. I will present my argument below.

The questionable random "Fact of the Day" is a simple one.

Every hour, about 180 million newly formed red blood cells enter the bloodstream. Red blood cells are basically shells. Before being released from the bone marrow, most of a red blood cell's internal structure is ejected, creating a disc-shaped balloon that is ideal for carrying oxygen and a small amount of the body's carbon dioxide.

A rough mental calculation told me that this number was way too small. I will perform a more detailed calculation below that supports my argument that this number is low by a factor of ~60. Did they really mean to say one minute instead of one hour? That is my theory.

Analysis

Figure 2 shows my detailed calculations with links that support each result.

Figure 2: My Calculations for the Average Number of Blood Cells Lost Per Minute.

Conclusion

My calculations show that ~180 million red blood cells are replaced every minute – not ~180 million every hour as stated in the "Fact of the Day."

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