Dominic Stone
The human urinary system is a complex network of organs and muscles that work together to filter waste from the body and excrete urine. The kidneys, ureters, bladder, and urethra are the key components of this system, with the detrusor and sphincter muscles playing a crucial role in controlling urine flow. When urine fills the bladder, the detrusor muscle contracts and the internal urethral sphincter relaxes, allowing urine to exit the bladder and enter the urethra. The external urethral sphincter is under voluntary control, requiring conscious relaxation for urine to pass through the urethra and exit the body. Pelvic floor muscles also contribute to urine control, and their weakening can lead to urinary incontinence. Understanding the interplay between these muscles and their impact on urine flow is essential for maintaining urinary health and addressing related issues.
| Characteristics | Values |
|---|---|
| Muscle that operates urine flow | The detrusor muscle, urethral sphincters, pelvic floor muscles |
| Location | The detrusor muscle is located within the walls of the bladder; the urethral sphincters are located at the bladder's inferior end and the urethra's proximal end; the pelvic floor muscles are located at the floor of the bladder |
| Composition | The detrusor muscle is composed of smooth muscle fibers that are longitudinal and circular; the urethral sphincters are composed of smooth muscle; the pelvic floor muscles are skeletal muscles |
| Control | The detrusor muscle is under autonomic control; the internal urethral sphincter is involuntary and under involuntary or autonomic control; the external urethral sphincter is voluntary and under somatic nervous system control; the pelvic floor muscles are under voluntary control |
| Function | The detrusor muscle contracts during urination to push urine out of the bladder and relaxes to store urine; the internal urethral sphincter relaxes to allow urine to pass; the external urethral sphincter must be relaxed for urination to occur; the pelvic floor muscles control the release of urine and provide support to the bladder |
| Related Conditions | Detrusor muscle pathology can lead to urinary retention, incontinence, or both; abnormalities of the detrusor muscle can lead to deterioration of the upper urinary tracts; damage to the urethral sphincters can lead to urinary incontinence; weak pelvic floor muscles can lead to urinary incontinence and prolapse of organs |
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What You'll Learn
Pelvic floor muscles
The pelvic floor muscles are a group of muscles that form the base of the core muscles. They stretch from the pubic bone at the front of the body to the tailbone (coccyx) at the back, and from one sitting bone (ischial tuberosity) to the other. These muscles support the pelvic organs, including the bladder, bowel, and uterus (womb) in women. They also assist with essential bodily functions like urination, defecation, and sexual intercourse.
The pelvic floor is composed of layers of muscles and connective tissues. The two main muscles that intertwine to form the pelvic floor are the levator ani, which includes the pubococcygeus, puborectalis, and iliococcygeus, and the coccygeus. The levator ani wraps around the entirety of the pelvis. The deep pelvic floor muscles include the pubococcygeus, iliococcygeus, coccygeus, and puborectalis.
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Detrusor muscle
The detrusor muscle is a smooth muscle located within the walls of the bladder. It is composed of smooth muscle fibres that are longitudinal and circular, with three layers of muscles arranged longitudinal-circular-longitudinal from the innermost to the outermost layer. The detrusor muscle is responsible for the contraction of the bladder during urination to push urine out of the bladder and into the urethra.
The detrusor muscle is under autonomic control, with the parasympathetic nervous system stimulating the muscarinic stretch receptors in the bladder through the pelvic nerve fibres. When the bladder fills with urine, the M3 receptors in the bladder become stretched and stimulated, leading to the contraction of the detrusor muscle for urination. The parasympathetic fibres also inhibit the internal urethral sphincter, causing relaxation and allowing for bladder emptying.
During bladder emptying, the stretch fibres become inactivated, and the sympathetic nervous system is stimulated to activate the beta-3 receptors through the adenylyl cyclase-cAMP pathway in the bladder. This process causes the relaxation of the detrusor muscle. The detrusor muscle remains relaxed when the bladder is storing urine, allowing the bladder to expand as it fills.
The detrusor muscle is derived from the upper segment of the urogenital sinus in the fetus and is connected with the allantois, which eventually becomes the urachus, leaving just the bladder. The upper portion of the bladder is primarily supplied by the superior vesical artery, a branch of the hypogastric artery. The lower portion receives supply from the vaginal artery in females and the inferior vesicular artery in males, with minor blood supply from the inferior gluteal and obturator arteries.
The detrusor muscle is essential for normal urinary function, and abnormalities or pathology of this muscle can lead to urinary retention, incontinence, or deterioration of the upper urinary tracts if left untreated. In older adults, the detrusor muscle may cause issues with bladder voiding, resulting in uncomfortable urinary retention.
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Urethral sphincters
The urethral sphincter is an integral part of maintaining urinary continence. The urethra is encircled by a complex of muscles known as the urethral sphincters, which control the flow of urine. There are two urethral sphincters: an internal urethral sphincter and an external urethral sphincter.
The internal urethral sphincter is located at the junction of the urethra and the urinary bladder. It is made of smooth muscle and is under involuntary control via the autonomic nervous system and hypogastric nerves. The internal sphincter controls involuntary urine flow from the bladder to the urethra. When the bladder is full, the parasympathetic nervous system stimulates the bladder, causing it to contract. Simultaneously, the parasympathetic fibres inhibit the internal urethral sphincter, leading to its relaxation and allowing urine to pass from the bladder to the urethra.
The external urethral sphincter, also known as the rhabdosphincter, is located distal to the internal urethral sphincter in the deep perineal pouch. It is formed from skeletal muscle fibres, which arise from the ischial rami, inferior pubic rami, and adjacent fascia. This muscle surrounds the membranous or intermediate part of the urethra. The external sphincter is under voluntary control via the somatic nervous system and nicotinic receptors. The external urethral sphincter controls voluntary urine flow from the bladder to the urethra.
The urethral sphincters work together to maintain urinary continence. In their resting state, both sphincters compress or close the urethra. When urinating, the sphincters relax to allow urine to pass through the urethra and out of the body. Weak pelvic floor muscles, intrinsic sphincter damage, or damage to the surrounding nerves and tissue can cause urinary incontinence by preventing the urethral sphincter from closing fully.
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Bladder muscles
The human urinary system is a complex network of organs and muscles that work together to filter toxins and wastes from the body. The bladder is a vital component of this system, acting as a reservoir for urine before it is eliminated from the body.
The bladder is a hollow, muscular organ that sits in the pelvis between the hip bones. Its walls are primarily composed of a smooth muscle called the detrusor muscle. This muscle is responsible for the contraction of the bladder during urination and relaxation to hold urine. The detrusor muscle surrounds the bladder and allows it to expand as it fills with urine. It contracts to push urine out of the bladder and into the urethra, the tube located at the bottom of the bladder that allows urine to exit the body.
The detrusor muscle is under autonomic control, regulated by the parasympathetic nervous system. When the bladder fills with urine, stretch receptors in the bladder wall are activated, triggering the micturition reflex. This leads to the contraction of the detrusor muscle and relaxation of the internal urethral sphincter, allowing urine to pass out of the bladder. The internal urethral sphincter is a ring-like muscle that seals the urethra shut to prevent urine leakage. It is composed of smooth muscle and is under involuntary or autonomic control.
The pelvic floor muscles also play a crucial role in bladder control. They support the bladder and provide additional strength to the urethral sphincters in preventing urine leakage. Weak pelvic floor muscles can lead to urinary incontinence and affect both men and women. Pelvic floor exercises, also known as Kegel exercises, can help strengthen these muscles and maintain healthy bladder function.
In summary, the detrusor muscle and internal urethral sphincter work together to control urine flow from the bladder, while the pelvic floor muscles provide additional support and control. Maintaining the health and strength of these muscles is essential for proper bladder function and urinary control.
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Kidney function
The kidneys are two bean-shaped organs, each about the size of a fist, located just below the rib cage, one on each side of the spine. They are part of the urinary system, which works as the body's filtration system.
The kidneys perform several important functions that help keep the body in balance. Firstly, they filter blood, removing wastes and extra water to make urine. Each kidney contains up to a million tiny filtering units called nephrons, which are responsible for this process. Blood flows into the kidney through the renal artery and passes through the nephrons, where it is filtered by tiny blood vessels called glomeruli. The wastes and extra water removed by the nephrons become urine. This process also helps regulate blood pressure.
Once the blood is filtered, it exits the kidney through the renal vein, and the waste materials, now in the form of urine, are moved into the bladder. The urine flows from the kidneys to the bladder through two thin tubes of muscle called ureters, one on each side of the bladder. The bladder acts as a reservoir, storing the urine until it is ready to be emptied through urination.
The kidneys also help maintain a healthy balance of water, salts, and minerals, such as sodium, calcium, phosphorus, and potassium, in the blood. This balance is crucial for the normal functioning of nerves, muscles, and other tissues in the body.
It is important to regularly check kidney function through simple lab tests, such as the estimated glomerular filtration rate (eGFR) and the urine albumin-creatinine ratio (uACR). Early detection and treatment of any issues can help slow down or stop the progression of kidney disease.
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Frequently asked questions
The detrusor muscle contracts to push urine out of the bladder and into the urethra. The internal and external urethral sphincters also control urine flow.
The detrusor muscle is a smooth muscle that surrounds the bladder. It is located within the bladder walls and is composed of longitudinal and circular smooth muscle fibres.
The internal urethral sphincter is a ring-like muscle that controls involuntary urine flow from the bladder to the urethra. The external urethral sphincter controls voluntary urine flow from the bladder to the urethra.
If the detrusor muscle is damaged, it can lead to urinary retention, incontinence, or both. Weak pelvic floor muscles can also lead to stress urinary incontinence, as they are responsible for controlling the release of urine.
