Urinary Bladder
Anatomy and Structure
The urinary bladder is a hollow muscular organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. It is located in the pelvis, just above and behind the pubic bone. The bladder has a triangular shape when empty and becomes more spherical as it fills with urine.
Layers of the Bladder Wall
The bladder wall consists of several layers:
- **Mucosa**: The innermost layer, which includes a transitional epithelium that allows the bladder to expand and contract.
- **Submucosa**: A layer of connective tissue that supports the mucosa.
- **Detrusor muscle**: A thick layer of smooth muscle fibers that contract to expel urine.
- **Adventitia**: The outermost layer, which consists of connective tissue.
Blood Supply and Innervation
The bladder receives blood from the superior and inferior vesical arteries, which are branches of the internal iliac artery. Venous drainage is via the vesical venous plexus, which drains into the internal iliac veins. The bladder is innervated by the autonomic nervous system, with parasympathetic fibers from the pelvic splanchnic nerves stimulating contraction of the detrusor muscle, and sympathetic fibers from the hypogastric plexus inhibiting contraction.
Function
The primary function of the urinary bladder is to store urine until it can be excreted. The bladder can hold approximately 400-600 milliliters of urine in adults. When the bladder fills, stretch receptors in the bladder wall send signals to the brain, triggering the urge to urinate.
Micturition Reflex
The process of urination, or micturition, involves both voluntary and involuntary muscle contractions. The micturition reflex is initiated when the bladder fills and stretch receptors are activated. This reflex involves the following steps:
1. Activation of stretch receptors in the bladder wall. 2. Transmission of sensory signals to the sacral spinal cord. 3. Activation of parasympathetic neurons, causing contraction of the detrusor muscle. 4. Relaxation of the internal urethral sphincter. 5. Voluntary relaxation of the external urethral sphincter, allowing urine to flow out of the bladder.
Development
The urinary bladder develops from the urogenital sinus, which is part of the embryonic cloaca. By the fifth week of gestation, the cloaca divides into the urogenital sinus and the anorectal canal. The urogenital sinus further differentiates into the bladder and the urethra. The bladder initially connects to the allantois, a structure involved in early fluid exchange, which later becomes the urachus and eventually forms the median umbilical ligament.
Clinical Significance
Urinary Tract Infections (UTIs)
Urinary tract infections are common and can affect the bladder, leading to cystitis. Symptoms of a bladder infection include frequent urination, urgency, dysuria (painful urination), and hematuria (blood in the urine). UTIs are more common in women due to the shorter length of the female urethra.
Bladder Cancer
Bladder cancer is a malignancy that arises from the epithelial lining of the bladder. The most common type is transitional cell carcinoma. Risk factors include smoking, exposure to certain chemicals, and chronic bladder irritation. Symptoms may include hematuria, pelvic pain, and frequent urination. Diagnosis is typically made through cystoscopy and biopsy.
Neurogenic Bladder
Neurogenic bladder is a condition caused by neurological damage that affects bladder control. It can result from spinal cord injury, multiple sclerosis, stroke, or other neurological disorders. Symptoms may include urinary incontinence, retention, and frequent urinary tract infections.