It begins with a bit of discomfort and soon becomes a pressing sensation that's impossible to ignore. Finally, it's all you can think about. And out of sheer desperation, you go on a hunt for a bathroom until...ahhh.
Humans should urinate at least four to six times a day, but occasionally, the pressures of modern life forces us to clench and hold it in. How bad is this habit? And how long can our bodies withstand it? The answers lie in the workings of the bladder, an oval pouch that sits inside the pelvis.
Surrounding this structure are several other organs that together make up the whole urinary system: two kidneys, two ureters, two urethral sphincters, and a urethra. Constantly trickling down from the kidneys is the yellowish liquid known as urine. The kidneys make urine from a mix of water and the body's waste products, funneling the unwanted fluid into two muscular tubes called ureters. These carry it downward into the hollow organ known as the bladder. This organ's muscular wall is made of tissue called detrusor muscle, which relaxes as the bladder fills, allowing it to inflate like a balloon. As the bladder gets full, the detrusor contracts. The internal urethral sphincter automatically and involuntarily opens, and the urine is released.
Whooshing downwards, the fluid enters the urethra and stops short at the external urethral sphincter. This works like a tap. When you want to delay urinating, you keep the sphincter closed. When you want to release it, you can voluntarily open the flood gates. But how do you sense your bladder's fullness so you know when to pee?
Inside the layers of detrusor muscles are millions of stretch receptors that get triggered as the bladder fills. They send signals along your nerves to the sacral region in your spinal cord. A reflex signal travels back to your bladder, making the detrusor muscle contract slightly and increasing the bladder's pressure so you're aware that it's filling up. Simultaneously, the internal urethral sphincter opens. This is called the micturition reflex. The brain can counter it if it's not a good time to urinate by sending another signal to contract the external urethral sphincter.
With about 150 to 200 milliliters of urine inside of it, the bladder's muscular wall is stretched enough for you to sense that there is urine within. At about 400 to 500 milliliters, the pressure becomes uncomfortable. The bladder can go on stretching, but only to a point. Above 1000 milliliters, it may burst. Most people would lose bladder control before this happens, but in very rare cases, such as when a person can't sense the need to urinate,
the pouch can rupture painfully, requiring surgery to fix. But under normal circumstances, your decision to urinate stops the brain's signal to the external urethral sphincter, causing it to relax and the bladder to empty.
在裡面有大約 150 到 200 毫升的尿液時，膀胱肌壁的延展程度足以讓你感覺到裡面有尿液。在大約 400 到 500 毫升時，那種壓迫感變得不舒服。膀胱可以繼續延展，但只能到一定程度。超過 1000 毫升，膀胱可能會撐爆。大多數人在這發生前膀胱就會失控，但在非常少數的案例中，例如當一個人無法察覺排尿需求時，尿袋可能就會很疼痛地破裂，需要靠手術修復。但在正常情況下，你排尿的決定會阻斷大腦信號傳到尿道外括約肌，使括約肌放鬆，然後膀胱排空。
The external urethral sphincter is one of the muscles of the pelvic floor, and it provides support to the urethra and bladder neck. It's lucky we have these pelvic floor muscles because placing pressure on the system by coughing, sneezing, laughing, or jumping
could cause bladder leakage. Instead, the pelvic floor muscles keep the region sealed until you're ready to go.
But holding it in for too long, forcing out your urine too fast, or urinating without proper physical support may over time weaken or overwork that muscular sling. That can lead to an overactive pelvic floor, bladder pain, urgency, or urinary incontinence.
So, in the interest of long-term health, it's not a great habit to hold your pee. But in the short term, at least, your body and brain have got you covered, so you can conveniently choose your moment of sweet release.