Respirocytes are hypothetical, microscopic, artificial red blood cells that can emulate the function of its organic counterpart, only with 200 times the efficiency, so as to supplement or replace the function of much of the human body's normal respiratory system. Still entirely theoretical, respirocytes would measure 1 micrometer in diameter. In the original paper by Robert Freitas, titled, "A Mechanical Artificial Red Blood Cell: Exploratory Design in Medical Nanotechnology" (1998), it was proposed that respirocytes would mimic the action of the natural hemoglobin-filled red blood cells. The proposed design of the spherical nanorobot is made up of 18 billion atoms arranged as a tiny pressure tank, which would be filled up with oxygen and carbon dioxide, making one complete transfer point at the lungs, and the reverse transfer at the body's tissues.
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Per proposals, each respirocyte could store and transport 236 times more oxygen than a natural red blood cell, and could release it in a more controlled manner. If an adult human's red blood cells were entirely replaced with these devices, that person could survive a heart attack for hours and get themselves to a hospital or hold his/her breath underwater for hours, making such devices extremely suitable for divers, who would then be able to dive for hours in a single breath, while avoiding both the bends decompression sickness or nitrogen narcosis (Afflictions caused by breathing compressed air under water, thus allowing more nitrogen, than at one atmosphere of pressure, to be dissolved into the bloodstream).
Muscle fatigue results from inadequate supply of oxygen to the muscles during intense exercise, leading to inefficient lactic acid fermentation. If respirocytes could increase the supply of oxygen despite exercise, it should be possible to reduce muscle fatigue, increasing a person's endurance.
Theorist Robert Freitas has proposed respirocytes as a superior alternative to naturally occurring red blood cells, and has similarly proposed "microbivore" robots that would attack pathogens in the manner of white blood cells.
Development and consequences 
By definition, respirocytes qualify as molecular nanotechnology, a field of technology still in the very earliest, purely theoretical phases of development. Current technology could, therefore, not be sufficient to build a respirocyte due to considerations of power, immune reaction or toxicity, computation and communication.
Because respirocytes and related technologies would, if successful, improve the user's abilities beyond normal human limits, their design is associated with the Transhumanism movement which seeks such advances.
- Robert A. Freitas Jr. (1998). "Exploratory Design in Medical Nanotechnology: A Mechanical Artificial Red Cell". Artificial Cells, Blood Substitutes, and Immobil. Biotech. (26): 411–430.
- "RESPIROCYTES – IMPROVING UPON NATURE'S DESIGN: BREATHE EASY WITH RESPIROCYTES".
- The Nano Page. "Respirocyte as an invention of nanotechnology".
- CNN – Scientists: Humans and machines will merge in future
- Kurzweil, Ray. The Singularity Is Near. p. 254.