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| Anatomy and Physiology | |||||||||||||||
| THE URINARY SYSTEM This information will help you to develop your understanding of the urinary catheters and appliances available to the nurse prescriber. The kidneys The kidneys are responsible for the continuous formation of urine. They are located at the back of the abdomen on either side of the vertebral column. Each kidney is made up of approximately 1 million tiny tubes called nephrons responsible for urine production. The ureters The ureters are about 30cm in length and extend inferiorly from each kidney to the bladder. Urine is forced towards the urinary bladder in spurts by peristaltic contractions of the smooth muscle of the ureters. The bladder The urinary bladder is a hollow muscular organ, which stores urine temporarily. The bladder wall contains layers of longitudinal and circular smooth muscle. These layers form the detrusor muscle. Contraction of this muscle compresses the bladder and causes urine to be expelled into the urethra. The trigone is the triangular area near the mouth of the bladder through which both the ureters and the urethra pass. During contraction of the bladder, the trigone funnels urine into the urethra. The maximum volume of fluid that the bladder can hold varies, but without too much discomfort, it is normally about 0. 5 litres. The neck of the bladder, the area that surrounds the urethral opening, contains an internal urethral sphincter. The smooth muscle fibres of this sphincter involuntarily control the discharge of urine from the bladder. Below the bladder, the urethra passes through the urogenital diaphragm. The urethra The bladder is connected to the exterior surface of the body by the urethra. In the female, the external urethral opening is positioned near the anterior wall of the vagina. In the male, the urethra is divided into three sections:
As the urethra passes through the urogenital diaphragm, a circular band of voluntary skeletal muscle forms the external urethral sphincter. This sphincter is normally contracted, and escape of urine is prevented. The micturition reflex The process of urination is co-ordinated by the micturition reflex, which involves both involuntary and voluntary mechanisms. Involuntary control is by stretch receptors in the wall of the bladder, which become stimulated as the bladder distends (this normally occurs when the bladder contains approximately 500mls of urine). Once these receptors have become stimulated, the smooth muscle of the bladder contracts. These contractions elevate fluid pressure within the bladder. If both external and internal urethral sphincters are relaxed, urine is ejected. The external sphincter is relaxed under voluntary control. If the external sphincter relaxes, then so does the internal sphincter. If the external sphincter does not relax, for example if it is impossible to visit a toilet, the internal sphincter does not open and the bladder is prevented from emptying. The bladder then relaxes. This cycle commences again with further increase in fluid volume in the bladder. This increase in volume leads to an increase in stretch receptor stimulation, making the sensation more acute. If the volume of urine exceeds 500mls, the micturition reflex may generate sufficient pressure, so that the internal urethral sphincter is forced to open. Relaxation of the external sphinter occurs followed by urination, despite possible inconvenience. The pressure exerted by simultaneous contraction of the abdominal muscles also assists bladder emptying. However, the increases in abdominal pressure when sneezing or coughing can also provoke a loss of urine (stress incontinence). This tends to be more common in women, if they have become weakened following childbirth. Multiple Choice Question (MCQs) |
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