written by

Paul Hsieh, MD

Dr. Paul Hsieh is a musculoskeletal radiologist in Denver, CO

June 21, 2022,

Paul Hsieh, MD: The Bernoulli Effect On Patients

How The Bernoulli Effect Can Help Patients And Physicians – Paul Hsieh, Contributor May 31, 2022

Like millions of others, I was delighted by this excellent short video by high school science teacher “Wolf_science” explaining how to use the Bernoulli effect to blow up a giant plastic bag with just one breath. If you haven’t watched the video, I highly recommend it. It’s only 2 minutes long. I can wait! (*** Pause ***)

Welcome back! As the teacher explained, the moving air draws in adjacent air into the balloon. This aspect of the Bernoulli effect is known as “entrainment” — where the moving air reduces the local pressure and pulls in nearby air. The entrainment effect also explains why if you open the window while you’re in a rapidly moving car, nearby light objects such as plastic bags might fly out the window if you’re not careful.

Entrainment can also effect personal safety and health. If you stand on a subway platform too close to an incoming train, the rush of air caused by the moving car can pull you in, much to your detriment. That’s one reason subway platforms typically require waiting passengers to stand behind yellow or white safety lines.

The Bernoulli effect can also affect patients with sleep apnea. Some sleep apnea patients have abnormal anatomy of their airway that causes air to flow faster through narrowed passages in their throat. In this case, the reduced air pressure doesn’t pull in more air. Instead, the Bernoulli effect “pulls in” the adjacent soft tissues of the neck, resulting in even greater constriction of the air passage. Proper treatment of sleep apnea thus requires an understanding of the physics of moving air and how it interacts with soft tissues.

The Bernoulli effect can also be used to patients’ benefit, for instance with the Venturi mask. For various medical reasons, some patients may benefit from air with greater than usual 21% oxygen concentration, but should not receive pure oxygen. The Venturi mask thus allows easy administration of, say 40% oxygen concentration. The Venturi mask is designed so that a stream of 100% pure oxygen is directed towards the patient’s face, but with openings in the mask on either side of the oxygen stream. Room air is thus pulled in, mixing with the pure oxygen. Physicians and nurses can control the size of those side openings, and thus control the desired percentage of oxygen delivered to the patient.

Of course, patients don’t need to understand the physics of the Bernoulli effect in order to benefit from it. But it’s cool to know about how it works. Like the teacher says, “I love science!”