Anatomy and Physiology: When You Gotta Go ?
When You Gotta Go ?
Okay, so you've made the urine. Now what? You can't keep it forever! So how does your bladder let you know that it's full? After all, you don't exactly dribble it out! Similar to the uterine walls, there are stretch receptors in the walls of the urinary bladder. Although the capacity of the urinary bladder is around 750 ml (about the size of a bottle of wine), the stretch receptors start to respond when the volume reaches somewhere between 200 and 400 ml.
Sensory neurons send a message to the brain that makes us consciously perceive a full bladder, and a desire to urinate, or void one's bladder, an act that also carries the highfalutin' name micturition. In addition to the brain's conscious control, there is a reflex action (that evolved first) located in the sacral region of the spinal cord (obviously quite close to the bladder). This reflex is called the micturition reflex.
The reflex starts with parasympathetic nerve impulses (see The Senses) that cause the urinary bladder walls to contract, and the internal urethral sphincter to relax. This, of course, puts a great deal of stress on the walls of the urethra, particularly the external urethral sphincter. Once again, as in the case of the anus, the conscious contraction of our external urethral sphincter is the only thing standing between us and embarrassment!
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.