The urinary bladder is a temporary storage reservoir for urine. It is located in the pelvic cavity, posterior to the symphysis pubis, and below the parietal peritoneum. The size and shape of the urinary bladder varies with the amount of urine it contains and with the pressure it receives from surrounding organs.

The inner lining of the urinary bladder is a mucous membrane of transitional epithelium that is continuous with that in the ureters. When the bladder is empty, the mucosa has numerous folds called rugae. The rugae and transitional epithelium allow the bladder to expand as it fills.

The second layer in the walls is the submucosa, which supports the mucous membrane. It is composed of connective tissue with elastic fibers.

The next layer is the muscularis, which is composed of smooth muscle. The smooth muscle fibers are interwoven in all directions and, collectively, these are called the detrusor muscle. Contraction of this muscle expels urine from the bladder. On the superior surface, the outer layer of the bladder wall is parietal peritoneum. In all other regions, the outer layer is fibrous connective tissue.

There is a triangular area, called the trigone, formed by three openings in the floor of the urinary bladder. Two of the openings are from the ureters and form the base of the trigone. Small flaps of mucosa cover these openings and act as valves that allow urine to enter the bladder but prevent it from backing up from the bladder into the ureters. The third opening, at the apex of the trigone, is the opening into the urethra. A band of the detrusor muscle encircles this opening to form the internal urethral sphincter.

The urinary bladder collects urine from both ureters (Figure). The bladder lies anterior to the uterus in females, posterior to the pubic bone and anterior to the rectum. During late pregnancy, its capacity is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination. In males, the anatomy is similar, minus the uterus, and with the addition of the prostate inferior to the bladder. The bladder is partially retroperitoneal (outside the peritoneal cavity) with its peritoneal-covered "dome" projecting into the abdomen when the bladder is distended with urine.

The bladder is a highly distensible organ comprised of irregular crisscrossing bands of smooth muscle collectively called the detrusor muscle. The interior surface is made of transitional cellular epithelium that is structurally suited for the large volume fluctuations of the bladder. When empty, it resembles columnar epithelia, but when stretched, it "transitions" (hence the name) to a squamous appearance (see Figure). Volumes in adults can range from nearly zero to 500-600 mL.

The detrusor muscle contracts with significant force in the young. The bladder's strength diminishes with age, but voluntary contractions of abdominal skeletal muscles can increase intra-abdominal pressure to promote more forceful bladder emptying. Such voluntary contraction is also used in forceful defecation and childbirth.

Bladder

The bladder , an essential component of the urinary system, acts as a storage reservoir for urine prior to its regulated expulsion from the body (Figure 26.3). Positioned within the pelvic cavity, this hollow and muscular organ possesses a flexible structure, enabling it to expand as it accumulates urine (Shermadou et al., 2023). In females, the bladder lies posterior to the pubic bone and anterior to the uterus and rectum. During late pregnancy, its capacity is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination. In males, the anatomy is similar, minus the uterus and with the addition of the prostate inferior to the bladder.

Figure 26.3 The bladder’s triangular shape is characterized by three openings: two for the ureters and one for the urethra. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

As the bladder gradually fills, stretch receptors in its walls send signals to the central nervous system, indicating the need for voiding. The detrusor muscle , a layer of smooth muscle within the bladder wall, contracts under nervous system control, while the external urethral sphincter relaxes, facilitating the expulsion of urine through the urethra. The bladder’s triangular shape is characterized by three openings: two for the ureters and one for the urethra. The trigone , a triangular area between these openings, acts as a physiological valve, preventing reflux of urine into the ureters. The neck of the bladder connects to the urethra, through which urine exits the body during voiding (Shermadou et al., 2023). The bladder’s intricate anatomy and functional mechanisms enable it to store urine efficiently and facilitate controlled release, contributing to the overall regulation of the body’s fluid and waste balance.

Urethra

The urethra , the final component of the urinary system , serves as the conduit for the expulsion of urine from the body. The urethra is a tubular structure extending from the neck of the bladder to the external urethral orifice. The length of the urethra varies between males and females, with males typically having a longer urethra that also functions as a passageway for semen during ejaculation. The female urethra is shorter and solely dedicated to the elimination of urine. The urethra is lined with mucous membrane and surrounded by muscle, contributing to its role in the voluntary control of urine release. The external urethral sphincter , a ringlike muscle, aids in the regulation of urine flow (Cleveland Clinic, 2022).

The act of micturition involves a complex physiological process governed by the micturition reflex . This reflex is generated by the nervous system, primarily the autonomic nervous system and the spinal cord. As the bladder fills with urine, stretch receptors in the bladder wall are activated, sending signals to the spinal cord. The parasympathetic nervous system is then stimulated, causing the detrusor muscle in the bladder wall to contract while simultaneously relaxing the internal urethral sphincter. The sensation of a full bladder is conveyed to the brain, and when an individual voluntarily decides to empty the bladder, the external urethral sphincter , under conscious control, relaxes. This coordinated relaxation of the sphincters and contraction of the detrusor muscle result in the expulsion of urine through the urethra. The micturition reflex is a finely tuned mechanism that ensures controlled and voluntary release of urine while preventing involuntary leakage. Dysfunction in this reflex can lead to issues such as urinary retention (the inability to empty the bladder fully) or urinary incontinence (involuntary loss of bladder control, leading to the unintentional release of urine) (Flores et al., 2023).

Stages of Micturition

The stages of micturition encompass a series of precisely coordinated events within the urinary system . From the resting and filling stages, in which the bladder gradually accommodates accumulating urine, to the pivotal voiding stage, in which conscious and involuntary mechanisms converge to facilitate controlled urination, each stage plays a crucial role in maintaining continence and ensuring efficient waste elimination. Understanding these stages not only unveils the physiological intricacies of the micturition process but also sheds light on the intricate coordination between nervous system responses and voluntary muscle actions. This exploration into the stages of micturition is essential for comprehending the dynamic interplay of factors that contribute to the regulated release of urine, a fundamental aspect of the body’s homeostatic mechanisms.

Resting and Filling Stages

During the resting stage, the bladder is in a relatively inactive state. As urine accumulates, stretch receptors in the bladder wall are activated, generally sending signals when there is at least 50 to 100 mL, signaling to the nervous system that the bladder is filling and prompting urination. The parasympathetic nervous system is gradually activated, leading to a mild contraction of the detrusor muscle while maintaining closure of the internal and external urethral sphincters. This allows the bladder to gradually expand to accommodate the increasing volume of urine without initiating the urge to void (Flores et al., 2023).

During this filling stage, the bladder continues to receive urine. The stretch receptors continue to send signals to the spinal cord and brain, providing information about the increasing volume of urine. Despite ongoing contractions of the detrusor muscle , the urethral sphincters remain contracted, preventing the involuntary release of urine. The individual typically remains unaware of the filling process during this stage, as the bladder’s main function is to store urine without generating a strong urge to void (Flores et al., 2023).

These early stages are essential for maintaining continence (the ability to control and maintain bladder functions) and preventing the constant need for urination. The transition to the next stage, the voiding or emptying stage, is initiated when the bladder reaches a certain level of distension, triggering the micturition reflex and the conscious sensation of needing to urinate.

Voiding Stage

The voiding stage , also known as the emptying stage, is a crucial phase in the micturition process. It follows the resting and filling stages and involves a coordinated series of events controlled by the micturition reflex and culminating in the release of urine stored in the bladder . The parasympathetic nervous system becomes more active, causing the detrusor muscle in the bladder wall to contract more forcefully. Simultaneously, the internal urethral sphincter relaxes involuntarily, allowing urine to flow from the bladder into the urethra . The conscious component of this stage involves the individual’s decision to voluntarily relax the external urethral sphincter , allowing urine to exit the body. This conscious control is crucial for maintaining continence and preventing involuntary urine release (Flores et al., 2023).

The voiding stage is a dynamic process, and the coordination between involuntary and voluntary muscle actions ensures efficient and controlled urination . Dysfunction in this stage can lead to issues such as urinary retention or incontinence. Overall, the voiding stage represents the culmination of the micturition process, facilitating the expulsion of urine from the body while maintaining voluntary control over the process.

People rarely talk about bladder health, but everyone is affected by it. Located in the lower abdomen, the bladder is a hollow organ, much like a balloon, that stores urine. Urine contains waste and extra fluid left over after the body takes what it needs from what we eat and drink. Each day, adults pass about a quart and a half of urine through the bladder and out of the body.

As people get older, the bladder changes. The elastic bladder tissue may toughen and become less stretchy. A less flexible bladder cannot hold as much urine as before and might make you go to the bathroom more often. The bladder wall and pelvic floor muscles may weaken, making it harder to empty the bladder fully and causing urine to leak.

While you can’t control everything that affects your bladder, here are 15 steps you can take to keep it as healthy as possible:

1. Use the bathroom often and when needed. Try to urinate at least once every 3 to 4 hours. Holding urine in your bladder for too long can weaken your bladder muscles and make a bladder infection more likely.
2. Be in a relaxed position while urinating. Relaxing the muscles around the bladder will make it easier to empty the bladder. For women, hovering over the toilet seat may make it hard to relax, so it is best to sit on the toilet seat.
3. Take enough time to fully empty the bladder when urinating. Rushing when you urinate may not allow you to fully empty the bladder. If urine stays in the bladder too long, it can make a bladder infection more likely.
4. Wipe from front to back after using the toilet. Women should wipe from front to back to keep gut bacteria from getting into the urethra. This step is most important after a bowel movement.
5. Urinate after sex. Sexual activity can move bacteria from the bowel or vaginal cavity to the urethral opening. Both women and men should urinate shortly after sex to lower the risk of infection.
6. Do pelvic floor muscle exercises. Pelvic floor exercises, also known as Kegel exercises, help hold urine in the bladder. Daily exercises can strengthen these muscles, which can help keep urine from leaking when you sneeze, cough, lift, laugh, or have a sudden urge to urinate. These exercises also may help avoid infections by strengthening the muscles that help empty the bladder.
7. Wear cotton underwear and loose-fitting clothes. Wearing loose, cotton clothing will help keep the area around the urethra dry. Tight-fitting pants and nylon underwear can trap moisture and help bacteria grow.
8. Exercise regularly. Physical activity can help prevent bladder problems as well as constipation. It can also help maintain a healthy weight.
9. Keep a healthy weight. People who are overweight may be at higher risk for leaking urine. Making healthy food choices and being physically active can help keep a healthy weight.
10. Watch what you eat. Some people with bladder problems find that some foods and drinks, such as sodas, artificial sweeteners, spicy foods, citrus fruits and juices, and tomato-based foods, make bladder problems worse. Changing your diet may help you feel better.
11. Drink enough fluids, especially water. More than half of the human body is made up of water, so it is important that you are drinking enough. How much water you need can vary based on your size, activity level, and where you live. In general, drink enough fluids so that you need to urinate every few hours. Some people need to drink less water because of certain conditions, such as kidney failure or heart disease. Ask your health care provider how much fluid is healthy for you.
12. Limit alcohol and caffeine. For many people, drinking alcohol can make bladder problems worse. Caffeinated drinks (like coffee, tea, and most sodas) can bother the bladder and increase symptoms such as frequent or urgent need to urinate. Cutting down may help.
13. Avoid constipation. Too much stool built up in the colon, called constipation, can put pressure on the bladder and keep it from expanding the way it should. Eating plenty of high-fiber foods like whole grains, vegetables, and fruits), drinking enough water, and being physically active can help prevent this from happening.
14. Quit smoking. Bladder problems are more common among people who smoke. Smoking can also increase the risk for bladder cancer. If you smoke, take steps to quit.
15. Know your medications. Some medications may make it more likely for your bladder to leak urine. Medications that calm your nerves so you can sleep or relax may dull the nerves in the bladder, and you may not feel the urge to go to the bathroom.