The Urinary Bladder
The bladder is an organ of the urinary system, situated anteriorly in the pelvic cavity. It is hollow, highly distensible, and rounded in shape.
The main function of the bladder is the collection, temporary storage, and expulsion of urine. It has a unique muscular structure, and a complex neurological control, which shall be considered in detail.
Embryologically, the bladder is derived from the hindgut.
In this article, we shall look at the anatomy and physiological functions of the bladder. We shall also look at the disorders of neurological control that can occur.
Functions of the Bladder
The bladder largely serves two functions:
- Temporary store of urine – The bladder is a hollow organ. The walls are very distensible, with a folded internal lining (known as rugae), this allows it to hold up to 600ml.
- Assists in the expulsion of urine – During voiding, the musculature of the bladder contracts, and the sphincters relax.
Shape of the Bladder
The morphological appearance of the bladder varies with filling. When full, it exhibits an oval shape, and when empty it is flattened by the overlying intestines.
The important external features are the apex, body, fundus and neck:
Apex – This is located superiorly, pointing towards the pubic symphysis. It is connected to the umbilicus by the median umbilical ligament (a remnant of the urachus).
- Body – The main part of the bladder, located between the apex and the fundus
- Fundus (or base) – Located posteriorly. It is triangular-shaped, with the tip of the triangle pointing backwards.
- Neck – Formed by the convergence of the fundus and the two inferolateral surfaces. This structure joins the bladder to the urethra.
Urine enters the bladder by 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.
There are two sphincters controlling the outflow of urine; the internal and external urethral sphincters.
The bladder musculature, and the coordination of its action, plays a key role in the functions of the bladder.
In order to contract during micturition, the bladder wall contains specialised smooth muscle, known as detrusor muscle. Its fibres are orientated in three directions, thus retaining structural integrity when stretched. It receives innervation from both the sympathetic and parasympathetic nervous systems.
There are also two muscular sphincters located in the urethra:
- Internal urethral sphincter:
- Male – consists of circular smooth fibres, which are under autonomic control. It is thought to prevent seminal regurgitation during ejaculation.
- Females – thought to be a functional sphincter (i.e. no sphincteric muscle present). It is formed by the anatomy of the bladder neck and proximal urethra.
- External urethral sphincter – has the same structure in both sexes. It is skeletal muscle, and under voluntary control. During micturition, it relaxes to allow urine flow.
The bladder primarily receives its vasculature from the internal iliac vessels.
Arterial supply is delivered by 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 also contribute small branches.
Venous drainage is achieved by the vesical venous plexus, which empty into the internal iliac vein (also known as the hypogastric vein).
Neurological control is complex, with the bladder receiving input from both the autonomic (sympathetic and parasympathetic) and somatic arms of the nervous system:
- The sympathetic nervous system communicates with the bladder via the hypogastric nerve (T12 – L2). It causes relaxation of the detrusor muscle. These functions promote urine retention.
- The parasympathetic nervous system communicates with the bladder via the pelvic nerve (S2-S4). Increased signals from this nerve causes contraction of the detrusor muscle. This stimulates micturition.
- The somatic nervous supply gives us voluntary control over micturition. It innervates the external urethral sphincter, via the pudendal nerve (S2-S4). It can cause it to constrict (storage phase) or relax (micturition).
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 Reflex
The bladder stretch reflex is a primitive spinal reflex, in which micturition is stimulated in response to stretch. 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 centers of the brain, to give voluntary control over micturition.
The reflex arc:
- Bladder fills with urine, and the bladder walls stretch. Sensory nerves detect stretch and transmit this information to the spinal cord.
- Interneurons within the spinal cord relay the signal to the parasympathetic efferents (the pelvic nerve).
- The pelvic nerve acts to contract the detrusor muscle, and stimulate micturition.
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).
Clinical Relevance: Spinal Cord Injuries and the Bladder
The 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.