© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

8.5.1 Cytometrics

How many cells are there in the human body? Serious estimates vary from a low of 10 trillion^312,931,2185 up to 50 trillion,⁸⁷⁰ 75 trillion,^71,863 and even 100 trillion.^{836,932,938,2022} The numbers show considerable spread because some authors refer only to tissue cells while others may refer to all cells physically present, both native and foreign.

The single most common native cell in the human body is the red blood cell (RBC), or erythrocyte. The average adult male carries ~5.2 x 10⁹ RBCs/cm³ (Appendix B) in his ~5400 cm³ blood volume (Table 8.1), giving ~28.1 trillion RBCs. The spleen has an additional ~70 cm³ of noncirculating RBC storage volume, giving another 0.4 trillion RBCs for a total of ~28.5 trillion RBCs in the body. Also confined to the bloodstream are a median ~2.5 x 10⁸ platelets/cm³, ~7.0 x 10⁶/cm³ leukocytes including neutrophils, eosinophils, basophils, lymphocytes and monocytes (Appendix B), plus another ~0.7 trillion platelets sequestered in a circulation-accessible pool in the spleen,²¹²² giving ~2.1 trillion platelet cells in the human body and a white cell count of 0.4 trillion in the blood. There are also ~0.7 trillion lymphocytes in the lymphatic system (Table 8.5) and ~0.2 trillion macrophages and other reticuloendothelial (mononuclear phagocyte) cells throughout the human tissues.⁷⁴³ Thus there are ~31.5 trillion native non-tissue cells in the human body. There are also ~40 trillion foreign single-celled bacteria inhabiting the human colon, assuming ~35% of the ~340 cm³ colorectal contents are bacteria each averaging ~1.8 microns in diameter.

As for tissue cells, a 70 kg adult male reference body includes ~28.8 kg of body cell mass (BCM), defined as the amount of body tissue calculated to contain potassium at a concentration of 120 mEq (millimole)/kg.⁸¹⁷ This figure is favored over the LBM (lean body mass) because BCM defines the component of body composition that contains the oxygen-containing, potassium-rich, glucose-oxidizing, work-performing tissue and contains all cellular elements concerned with respiration, physical and chemical work, and mitotic activity.

Most human cells fall within a size range of 2-120 microns.⁸⁶⁶ Platelets are ~2 microns, red cells ~3 microns x ~8 microns, neutrophils ~8-10 microns, lymphocytes ~6-12 microns in size, exocrine cells ~10 microns, fibroblasts 10-15 microns, osteocytes ~10-20 microns including processes, chondrocytes and liver cells ~20 microns, goblet and ciliated cells ~50 microns long and 5-10 microns wide, macrophages at ~20-80 microns, hematopoietic stem cells ~30-40 microns, and adipocytes filled with stored lipid are typically 70-120 microns in diameter (but may be up to five times larger in very obese people).^935,936 Neurons vary enormously in size and shape, their bodies ranging from 4-120 microns in diameter with axonal processes varying between ~0.1-20 microns in diameter and ranging in length from a few microns up to ~1 meter;⁷⁹⁹ muscle cells (~30% of all tissue cells) also vary from 10-100 microns in diameter and may run up to ~50 cm in length. However, the "average" tissue cell, if such a thing may be said to exist, is probably ~20 microns in diameter,³¹³ weighing ~10 nanograms.

Since the reference human has a mean density of 1064 kg/m³ excluding bone minerals,⁸¹⁷ a BCM of 28.8 kg makes ~0.027 m³ of cells; taking 8000 micron³ per cell gives a total of ~3.4 trillion tissue cells. Thus the human body contains ~35 trillion native cells, of which only ~10% are tissue cells, plus ~40 trillion foreign (mostly bacterial) cells in the colon for a grand total of 75 trillion cells. The number of tissue cells comprising each organ listed in Table 8.9 may be crudely estimated by multiplying tissue volume by the mean tissue cellular number density, or ~125 million cells/cm³.

Last updated on 20 February 2003