Our bodies are constantly making cells to replace aging parts. The latest scientific calculations reveal the high performance of this mechanism. Biologists Ron Sende and Ron Milo of the Weizmann Institute of Science in Israel say that the human body replaces about 330 billion cells every day. At that rate, the body produces more than 3.8 million new cells per second.
Most of those are blood cells, followed by intestinal cells. Having this data can help scientists better perceive how the human body functions and the role of cell renewal in health and disease.
The famous modern myth says that the human body is made up entirely of brand new cells every seven years. The actual situation is much more complex. Some cells survive for only a few days, while others (such as neurons in the cerebellum and lipids in the lens of the eye) are limited only by the lifespan of their host (you). So there is still a gap between humans and the Ship of Theseus.
But although scientists had previously estimated the total number of cells in the human body, there was no detailed census of the number of each cell type and how long they lived.
So Sende and Milo got to work.
“Here, we fill in the gaps in knowledge about the overall cell renewal kinetics of the human body by investigating changes in the lifespan of different cell types and quantifying the renewal rate of cells in terms of both quality and number.”
They based their calculations on a standard human: a healthy male between 20 and 30 years old, weighing 70 kg and 170 cm tall.
They drew on the literature to obtain the survival cycles of various cells and then derived the total mass of each cell type based on the average cell mass.
Based on this information, they both calculated that the daily cell renewal rate of a standard reference human is about 80 grams, or 330 billion cells. Eighty-six percent are blood cells, mainly erythrocytes (red blood cells, the most abundant cells in the body) and neutrophils (white blood cells). Another 12% were epithelial cells of the gastrointestinal tract with a small number of skin cells (1.1%), endothelial cells lining the blood vessels and lung cells (0.1% each).
Although blood cells make up the majority in terms of the number of individual types of cells, qualitatively it is a different matter. All blood cells accounted for only 48.6% of the total mass. Gastrointestinal cells accounted for another 41%. Skin cells make up 4%, and adipocytes or fat cells (which are minuscule by number) make up 4% of the mass.
It is important to note that these numbers may actually vary from person to person, depending on factors such as age, health status, height and sex. But this work provides a baseline.
There are many topics that can be developed around the topic of biological renewal in health and disease,” the researchers write in the paper. For example, what is the difference in the turnover rate of tumor cells compared to the total cell turnover rate in the patient’s body? What is the cost of energy and biosynthesis for tumor growth, and does it directly affect resource allocation? And so on.”
The study has been published in Nature Medicine.