The bladder is an essential organ of the urinary system. It performs two primary functions - temporary storage of urine and assisting with the expulsion of urine. This article looks at the anatomy of the bladder - its shape, vasculature and neurological control - to better understand its functions and role in the body.
The bladder has three external features - the apex, the body and the fundus. The apex, located superiorly, is pointed in the direction of the pubic symphysis. It is attached to the umbilicus by the median umbilical ligament. The body is the main and middle part of the bladder and lies between the apex and the fundus. The fundus is located posteriorly and is triangular in shape, with its tip pointing backwards. The neck is formed at the convergence of the fundus and the two inferolateral surfaces. It is where the bladder and the urethra are connected.
The musculature of the bladder plays a fundamental role in urine storage and expulsion. The bladder wall is made up of specialized smooth muscle fibers, referred to as detrusor muscles. This muscle structure allows the bladder wall to contract during micturition, along with the relaxation of the sphincters. Moreover, in certain conditions, such as benign prostatic hyperplasia, the detrusor muscles hypertrophize, or thicken, in order to compensate for the increased workload of empty the bladder.
The urethra has two muscular sphincters - the internal and external urethral sphincters. The internal urethral sphincter, which is autonomic in nature, consists of circular smooth fibers for males. This is thought to prevent seminal regurgitation during ejaculation. In females, the internal urethral sphincter is formed by the anatomy of the bladder neck and proximal urethra and is thought to act as a functional sphincter. The external urethral sphincter, which is voluntary in nature, is similar in both sexes and is composed of skeletal muscle. However, in males the external sphincteric mechanism is more complex, as it correlates with fibers of the rectourethralis muscle and the levator ani muscle.
The vasculature of the bladder is mainly derived from the internal iliac vessels. Arterial supply is via the superior vesical branch of the internal iliac artery. This is supplemented in males by the inferior vesical artery and in females by the vaginal arteries. The obturator and inferior gluteal arteries may also provide small branches in both sexes. Venous drainage is facilitated by the vesical venous plexus, which empties into the internal iliac veins. In males, this is in continuity at the retropubic space with the prostate venous plexus (plexus of Santorini).
Lymphatic drainage of the bladder typically begins in the superolateral aspect of the bladder, draining into the external iliac lymph nodes. The neck and fundus drain into the internal iliac, sacral and common iliac nodes. The bladder also receives input from both the autonomic (sympathetic and parasympathetic) and somatic arms of the nervous system. The sympathetic branch, which is supplied by the hypogastric nerve from T12 to L2, causes relaxation of the detrusor muscle, promoting urine storage. The parasympathetic branch, sourced from the pelvic nerve from S2-S4, increases signals to cause contraction of the detrousor muscle, stimulating micturition. The somatic branch, supplied by the pudendal nerve from S2-4, innervates the external urethral sphincter to provide voluntary control over evacuation.
In addition to the efferent nerves supplying the bladder, there are also sensory (afferent) nerves in the bladder wall that communicate to the brain when the bladder is full. This is known as the bladder stretch reflex, a primitive spinal reflex in which micturition is stimulated in response to stretch of the bladder wall. The reflex is analogous to a muscle spinal reflex, such as the patella reflex. During toilet training in infants, this reflex is eventually overridden by higher brain centers to achieve voluntary control over micturition.
The bladder is an organ of the urinary system which plays an important role in both the temporary storage and expulsion of urine. It is composed of a hollow organ with distensible walls, and which contains internal folds, known as rugae, that allows to accommodate up to 400-600ml of urine in healthy adults. The bladder externally exhibits an oval shape when full and a flattened shape when empty, being overlaid by the bowel. Its external features include the apex, located superiorly and connected to the umbilicus; the body, the main part located between the apex and the fundus; the fundus or base, which is triangular-shaped with its tip pointing backwards; and the neck, formed by the convergence of the fundus and the two inferolateral surfaces, which is continuous with the urethra. Urine enters the bladder through the left and right ureters, and exits via the urethra, specifically through a triangular area located within the fundus known as the trigone.
The musculature of the bladder is also of key importance in both the storage and expulsion of urine. It contains a specialized smooth muscle, known as detrusor muscle, which contracts during micturition. In order to initiate micturition, signals are transmitted to the spinal cord, by means of interneurons. Then, the signal is relayed to the parasympathetic efferents, specifically to the pelvic nerve, which acts to contract the detrusor muscle and stimulate micturition. Although this reflex is non-functional post-childhood, it needs to be taken into consideration in spinal injuries, where the descending inhibition cannot reach the bladder; and in neurodegenerative diseases, in which the brain is unable to generate inhibition.
The bladder has important clinical considerations when it comes to spinal cord lesions. There are two different clinical syndromes, depending on the location of the damage.
Besides neurogenic dysfunction of the bladder, normal bladder emptying may also be hampered by any form of obstruction at the level of the bladder neck downwards. In males, the most common cause is obstruction due to prostate enlargement (BPH). Other causes include obstruction by a stone or large blood clot. In the case of acute retention, it is a medical emergency due to the accumulation of urine in an acutely obstructed reservoir and the associated excruciating pain. The placement of a urinary catheter immediately alleviates the symptoms. Chronic retention instead results from an incomplete obstruction of the urine outflow, slowly accumulating residual urine in the bladder over months or even years; the bladder is progressively distended in volumes that exceed 1-1.5 lt of urine, accompanied by impairment of renal function, although no pain is usually present as the bladder is gradually stretched.
The bladder is composed of numerous fibres that are orientated in multiple directions in order to provide structural integrity when stretched. This is an important factor, as the detrusor muscle, which is responsible for bladder emptying, often becomes hypertrophic (with the appearance of prominent trabeculae) in order to compensate for increased workload. This is particularly common in conditions that prevent the normal outflow of urine, such as benign prostatic hyperplasia. The bladder also receives innervation from both the sympathetic and parasympathetic nervous systems.
In addition to the detrusor muscle, there are two separate muscular sphincters located in the urethra. The internal urethral sphincter is composed of circular smooth fibres and is under autonomic control. In males, it is thought to prevent seminal regurgitation during ejaculation, while in females, the anatomy of the bladder neck and proximal urethra are thought to form a functional sphincter. The external urethral sphincter is composed of skeletal muscle and is under voluntary control. In males, it is more complex, as it also correlates with fibres of the rectourethralis muscle and levator ani muscle.
The vasculature of the bladder is primarily derived from the internal iliac vessels. The superior vesical branch of the internal iliac artery provides the arterial supply, which in males is supplemented by the inferior vesical artery, and in females, the vaginal arteries. The obturator and inferior gluteal arteries may also contribute small branches. Venous drainage is achieved by the vesical venous plexus, which empties into the internal iliac veins. In males, this plexus is also in continuity with the prostate venous plexus (plexus of Santorini).
The lymphatic drainage of the bladder is fairly straightforward, with the superolateral aspect of the bladder draining into the external iliac lymph nodes. The neck and fundus drain into the internal iliac, sacral and common iliac nodes.
Neurological control of the bladder is complex, with the bladder receiving input from both the autonomic (sympathetic and parasympathetic) and somatic arms of the nervous system. The hypogastric nerve (T12 - L2) of the sympathetic nervous system causes relaxation of the detrusor muscle, promoting urine retention, while the pelvic nerve (S2-S4) of the parasympathetic nervous system causes contraction of the detrusor muscle, stimulating micturition. The somatic arm is linked to the pudendal nerve (S2-4), which innervates the external urethral sphincter, providing voluntary control over micturition.
The bladder stretch reflex is a primitive spinal reflex, in which micturition is stimulated in response to the stretch of the bladder wall. It is analogous to a muscle spinal reflex, such as the patella reflex. During toilet training in infants, the bladder stretch reflex is overridden by the higher centres of the brain, providing voluntary control over micturition.
The reflex arc involves the following steps: bladder filling causes the walls to stretch and sensory nerves detect this stretch. This information is then transmitted to the spinal cord via interneurons, and relayed to the parasympathetic efferents (the pelvic nerve). The pelvic nerve then contracts the detrusor muscle, and facilitates micturition. Although this reflex is non-functional after childhood, it is important to consider in cases of spinal cord injury (due to the lack of descending inhibition) and in neurodegenerative diseases (where the brain may be unable to generate inhibition).
Spinal cord lesions have important clinical implications for the bladder. Two different syndromes are recognised, depending on the level of the injury. A transection of the spinal cord above the T12 level results in a reflex bladder, which is characterised by a complete lack of awareness of bladder filling, and a relaxation of the external urethral sphincter. This is due to the lack of descending control from the brain. The reflex bladder automatically empties as it fills, as the parasympathetic system is still able to initiate detrusor contraction in response to bladder wall stretch.
In contrast, a transection of the spinal cord below the T12 level results in a paralysed detrusor muscle, as the parasympathetic outflow to the bladder is significantly impaired. This is known as a flaccid bladder, as it cannot contract, and thus cannot empty.
The spinal reflex is an essential element of normal bladder function. When it fails to function, then bladder will fill uncontrollably, leading to abnormal distension of the bladder until overflow incontinence occurs. This clinical relevance of spinal cord injuries and urine retention is seen in such cases.
Besides neurogenic dysfunction of the bladder, normal bladder emptying may be hampered by any form of obstruction, from the level of the bladder neck downwards. In males, the most common cause is obstruction due to prostate enlargement (BPH). Other causes include obstruction by a stone or large blood clot.
Acute retention is a medical emergency, as the bladder has a “normal” functional capacity which is pushed to the limit due to accumulation of urine in an acutely obstructed reservoir. The patient feels increasingly excruciating pain and the placement of a urinary catheter immediately alleviates the symptoms.
Chronic retention of urine is a gradual process due to incomplete obstruction of the urine outflow. This leads to accumulation of residual urine in the bladder over a period of months or even years. The bladder is progressively distended in volumes that exceed 1-1.5 liters of urine, yet no pain is usually present. Chronic retention of urine is complicated by urinary tract infections and the formation of bladder stones due to urine stasis and high mineral levels in the urine.
Chronic retention of urine can have a severe and lasting impact on an individual's quality of life. People with bladder retention can experience issues such as frequent trips to the restroom, difficulty in controlling their bladder, pain, increased urinary tract infections, and an overall disruption to their daily activities. Furthermore, chronic retention of urine can lead to a number of long-term health complications such as kidney damage, bladder stones, and an increased risk of overall urinary tract infections.
It is important to identify and treat bladder retention early to avoid these long-term problems. Treatment may include lifestyle changes such as increasing fluid intake or medications to reduce urine production. In more severe cases, surgery or a catheter may be required. In any case, it is important to consult a healthcare provider to discuss the best plan of care.
The clinical relevance of spinal cord injuries and urine retention is significant. Patients who suffer from spinal cord injuries are at an increased risk of experiencing urine retention and should be monitored closely. Early detection and treatment are essential to avoid complications and maintain a high quality of life.
