Science Bytes: Red is the Color of Oxygen

Red is the color of oxygen, which is distributed in just the right amount by an army of twenty trillion red blood cells in our body. It is a fine balance; too little or too much oxygen is potentially fatal. This is another example of nature’s miracles that often goes unnoticed.

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Red blood cells under the electron microscope. Photo: BBC.

We think of oxygen as an unqualified good thing, a precious life-giving substance. While it is true that our body tissues indeed need oxygen to perform its myriad functions, what many people don’t know is that oxygen also a highly reactive gas (think of fire). Because of that, it helps to have a mechanism that keeps oxygen from reacting with any passing body part until it gets to where it is needed. Our red blood cells are that mechanism.

At just 7.5 millionth of a meter across, red blood cells are the among our smaller cells. Their small size is actually an asset because it allows them to fit through the narrowest of blood vessels. We harbor an army of red blood cells. At any one time, some 20 trillion of them course inside us. Each cell is powered by the energy molecule, ATP, which causes them to vibrate like stretched drums at millions of times per second. Those vibrations help the cells maintain their characteristic flattened oval or disc shape, which is critical to their ability to perform as they traverse blood vessels in the body to deliver oxygen to tissues. In particular, their pizza-like shape helps to maximize their surface area for oxygen absorption.

For all their importance, red blood cells have very short lives. Each red blood cell live for 120 days before they are replaced by new ones. With trillions of them in the body, the cycle of creation and dissolution number in the hundreds of billions per day, a spectacle that is part of the rhythm of life and death.

To visualize 20 trillion restless red blood cells is almost beyond the powers of human imagination. So, it helps to zoom in to look at a tiny section. When the 19th century anatomist and Nobel laureate Santiago Ramon y Cajal (1852 – 1934) did that, he was completely floored by the beauty of what he saw. This was how he described it:

During the sublime spectacle, I felt as though I were witnessing a revelation. Enraptured on seeing the red and white blood cells move about like pebbles caught in the force of a torrent; on seeing how the elastic properties of red corpuscles allow them to suddenly regain their shape like a spring after laboriously passing through the finest capillaries … it seemed to me as though a veil had been lifted from my soul.”

Artist rendering of human red blood cell. Park et al. (2009) show that dynamic remodeling in the coupled membranes powered by adenosine 5’-triphosphate (ATP), results in non-equilibrium membrane fluctuations and also maintains the biconcave shape of red blood cells. Credit: Photo: Alex Jerez, YongKeun Park, Gabriel Popescu, and Subra Suresh [1]

The key to the oxygen-regulation role of red blood cells is haemoglobin. This is the protein that binds oxygen with iron, and which gives blood its red color. Every red blood cell is made up of a vast number of haemoglobin, which performs a life-critical function: it locks away inhaled oxygen from our lungs and releases the right amount for aerobic respiration to provide energy that drives cellular functions. Too much oxygen, like uncontrolled fire, can damage the body’s tissues. Too little of it (or too much carbon dioxide) can be fatal too [2]. When this happens, haemoglobin releases more oxygen to counter the increased acidic environment caused by a high concentration of carbon dioxide.


[1] YongKeun Park, Catherine Best, Thorsten Auth, Nir Gov, Samuel Safran, Gabriel Popescu, Subra Suresh and Michael Feld, “Metabolic remodeling of the human red blood cell membrane”, Proceedings of the National Academy of Sciences, Week of Dec. 21, 2009. Read more at:

[2] Hypercapnia is the condition where there is an elevated levels of carbon dioxide. This condition may result from inadequate ventilation, rebreathing of exhaled carbon dioxide, exposure to environment with abnormally high concentrations of carbon dioxide (such as from volcanic or geothermal activity), lung disease or diminished consciousness. Hypercapnia may lead to elevated arterial blood pressure, increased risk of irregular heart beat (arrhythmias), disorientation, panic, hyperventilation, convulsions, unconsciousness and even death.

Further Study

The material for this blog is based on Caspar Hendersen, A Map of New Wonders, Granta Books, London, 2017, chapter 3.

Here’s a good bite-sized writeup on the wonders of red blood cells. https://www.

And here’s a video on the life cycle of red blood cells (erythrocytes).

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