Oxygen is picked up in the lungs, carried by the blood, and delivered to tissues where it serves as the terminal electron acceptor during oxidative phosphorylation. During health, oxygen is available in abundance; however, COVID-19 and many other forms of critical illness can damage the lungs and compromise systemic oxygen delivery. Cells that are very active cannot tolerate deficiencies in energy production that result from oxygen deprivation. Hypoxemia that lasts even a few minutes can turn a healthy person into a neurologically devastated patient for life, and when refractory it is often lethal. In this paper, we develop a way to administer oxygen gas to a patient through an intravenous line, replacing or supplementing the function of injured lungs. Here, we show that by coinfusing oxygen gas and a liquid solution through a series of sequential nozzles of decreasing size we are able to create bubbles of oxygen that are smaller than a single red blood cell on demand and in real time. These bubbles are coated with a “membrane” similar to that in every other cell in the body, which 1) prevents them from merging with other bubbles to create larger ones, 2) provides a path for oxygen to diffuse out and into the blood, and 3) minimizes the likelihood of material-related toxicities. Importantly, these devices allow us to control the dosage of oxygen delivered and the volume of fluid administered, both of which are critical parameters in the management of critically ill patients.