drwale
New Member
By way of introduction, the urinary system plays a crucial role by maintaining the normal volume and composition of the body fluids by ridding our body systems of harmful waste products and substances. It regulates the concentration of various electrolytes as well as the body pH such that it is neither too acidic nor too alkaline. Anatomically, the urinary system is a continuum which begins from both bean-shaped kidneys where urine is produced and concentrated before it is emptied into two ureters - left and right. Both ureters then open into the bladder through the ureteric orifices since the bladder serves as a temporary reservoir for urine before final excretion through the urethra.
The urinary bladder is a rounded, oval-shaped organ situated anteriorly (in front) within the pelvic cavity. By virtue of its distensibility, it can stretch to hold up to 600ml of urine. In males, it lies between the pubic symphysis in front and the rectum behind. However, in females, the fundus or body of the uterus hangs over the urinary bladder above while the vagina stretches across the base of the bladder.
Furthermore, the urinary bladder consists of two sphincters which control the outflow of urine - the internal urethral sphincter (under reflex control) and the external urethral sphincter which is under voluntary control.
The bladder wall is made up of the detrussor muscle, a specialised smooth muscle which contracts during micturition and expulsion of urine and relaxes during filling of the bladder.
The major blood supply to the urinary bladder is derived from branches of the internal iliac vessels including the superior vesical arteries, inferior vesical arteries, vaginal arteries and the vesical venous plexus that drains blood from the bladder.
Also, the bladder is under complex neurological control from both autonomic (sympathetic and parasympathetic) and the somatic arms of the nervous system. The sympathetic system acts to promote retention of urine whereas the parasympathetic system stimulates contraction of the urinary bladder and relaxes the internal sphincter to facilitate urination.
Primarily, it is instructive to note that the urinary bladder serves two major functions: temporary storage of urine (since it can hold as much as 600ml of urine at a time) and expulsion of stored urine anytime the bladder contracts and both sphincters are relaxed.
The normal adult bladder usually accomodates between 300-600ml of urine. As soon as the volume of stored urine reaches 400ml, an individual perceives a sensation of bladder fullness as well as the need to void. Subsequently, a central nervous system response is triggered to initiate the process of micturition. However, this stretch reflex can be consciously suppressed in an adult by the cortex or by voluntary contraction of the external sphincter when he is in a situation where he lacks the opportunity to urinate. Moreover, it is noteworthy that diseases such as spinal cord damage or diabetes mellitus can disrupt these functions leading to a reflex bladder that automatically empties as it fills or a flaccid bladder (with a paralysed bladder muscle). A flaccid bladder fills uncontrollably to the maximum capacity of the bladder until the person begins to leak urine, a condition referred to as overflow incontinence.
Image Credit:wikipedia.org
