The fluid dynamics of urination

Los Angeles Times : From elephants to cats to rats, it has been observed that most animals take about the same amount of time—21 seconds—to urinate regardless of the vastly different quantities of liquid involved. But until now, no one had studied the fluid dynamics required. David Hu of Georgia Tech and coworkers visited Zoo Atlanta to study the flow rate and duration for different animals. Once they figured out how to catch their subjects in the act and found the proper vessel to collect the samples, they were able to see that the process differed depending on the size of the animal. Animals smaller than 3 kg, such as rats, don’t generate enough liquid to flow; instead, rats urinate droplets of liquid one by one. Animals larger than 3 kg generate and store sufficient amounts of urine that they are able to dispense it in a jet or stream. However, as both elephants and cats fall into that category, and an elephant’s bladder is some 3600 times as large as a cat’s, in order for both animals to void in the same amount of time, the design of the urethra is key. The longer and wider the urethra, the more pressure builds up at the bottom and the faster the flow of liquid. Better understanding of the process of urination is important for human health. And even though rats have been extensively studied in the laboratory because of their similarities to humans in terms of behavior and physiology, they may not be the best examples for understanding the urinary tract.