The bladder is an organ of the urinary system. It plays two main roles: Show
In this article, we shall look at the anatomy of the bladder - its shape, vasculature and neurological control. [caption id="attachment_9168" align="aligncenter" width="425"] Shape of the BladderThe appearance of the bladder varies depending on the amount of urine stored. When full, it exhibits an oval shape, and when empty it is flattened by the overlying bowel. The external features of the bladder are:
Urine enters the bladder through the left and right ureters, and exits via the urethra. Internally, these orifices are marked by the trigone - a triangular area located within the fundus. In contrast to the rest of the internal bladder, the trigone has smooth walls (this is explained by the different embryological origin: the trigone is developed by the integration of two mesonephric ducts at the base of the bladder). [caption id="attachment_43413" align="aligncenter" width="768"] MusculatureThe musculature of the bladder plays a key role in the storage and emptying of urine. In order to contract during micturition, the bladder wall contains specialised smooth muscle - known as detrusor muscle. Its fibres are orientated in multiple directions, thus retaining structural integrity when stretched. It receives innervation from both the sympathetic and parasympathetic nervous systems. The fibers of the detrusor muscle often become hypertrophic (presenting as prominent trabeculae) in order to compensate for increased workload of the bladder emptying. This is very common in conditions that obstruct the urine outflow such as benign prostatic hyperplasia. There are also two muscular sphincters located in the urethra:
[caption id="attachment_40009" align="aligncenter" width="1024"] VasculatureThe vasculature of the bladder is primarily derived from the internal iliac vessels. Arterial supply is via the superior vesical branch of the internal iliac artery. In males, this is supplemented by the inferior vesical artery, and in females by the vaginal arteries. In both sexes, 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. The vesical plexus in males is in continuity at the retropubic space with the prostate venous plexus (plexus of Santorini), which also receives blood from the dorsal vein of the penis [caption id="attachment_41855" align="aligncenter" width="635"] LymphaticsThe superolateral aspect of the bladder drains into the external iliac lymph nodes. The neck and fundus drain into the internal iliac, sacral and common iliac nodes. Nervous SupplyNeurological control is complex, with the bladder receiving input from both the autonomic (sympathetic and parasympathetic) and somatic arms of the nervous system:
In addition to the efferent nerves supplying the bladder, there are sensory (afferent) nerves that report to the brain. They are found in the bladder wall and signal the need to urinate when the bladder becomes full. The Bladder Stretch ReflexThe bladder stretch reflex is a primitive spinal reflex, in which micturition is stimulated in response to stretch of the bladder wall. It is analogous to a muscle spinal reflex, such as the patella reflex. During toilet training in infants, this spinal reflex is overridden by the higher centres of the brain, to give voluntary control over micturition. The reflex arc:
Although it is non-functional post childhood, the bladder stretch reflex needs to be considered in spinal injuries (where the descending inhibition cannot reach the bladder), and in neurodegenerative diseases (where the brain is unable to generate inhibition). [start-clinical] Clinical Relevance: Spinal Cord Injuries and the BladderThe bladder has important clinical considerations when it comes to spinal cord lesions. There are two different clinical syndromes, depending on where the damage has occurred. Reflex Bladder - Spinal Cord Transection Above T12 In this case, the afferent signals from the bladder wall are unable to reach the brain, and the patient will have no awareness of bladder filling. There is also no descending control over the external urethral sphincter, and it is constantly relaxed. There is a functioning spinal reflex, where the parasympathetic system initiates detrusor contraction in response to bladder wall stretch. Thus, the bladder automatically empties as it fills - known as the reflex bladder. Flaccid Bladder - Spinal Cord Transection Below T12 A spinal cord transection at this level will have damaged the parasympathetic outflow to the bladder. The detrusor muscle will be paralysed, unable to contract. The spinal reflex does not function. In this scenario, the bladder will fill uncontrollably, becoming abnormally distended until overflow incontinence occurs. [end-clinical] [start-clinical] Clinical Relevance: Urine RetentionBesides 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 with 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 alleviates the symptoms immediately. Chronic retention is a gradual procedure due to incomplete obstruction of the urine outflow. This leads to accumulation of residual urine in the bladder through months or even years; the bladder is therefore progressively distended in volumes that exceed 1-1.5 lt of urine. Chronic retention is often accompanied by impairment of renal function. However no pain is usually present as the bladder is gradually stretched. Chronic retention of urine is often complicated by infections and formation of bladder stones due to urine stasis and accumulation of minerals in the urine [caption id="attachment_40010" align="aligncenter" width="434"] [end-clinical] |