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The urinary system - part one

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VOL: 102, ISSUE: 40, PAGE NO: 26

Marion Richardson, BD, RN, DipN, CertEd, RNT, is senior lecturer, University of Hertfordshire

The urinary system consists of the kidneys, ureters, bladder and urethra (Fig 1). While its main function is the pr...

The urinary system consists of the kidneys, ureters, bladder and urethra (Fig 1). While its main function is the production and excretion of urine it is also the major route of excretion for the organic waste products that are manufactured by cells throughout the body. It also plays a vital role in maintaining the composition of the body fluids (McLaren, 2005).

Kidneys

The kidneys are situated in the upper part of the lumbar region on either side of the spine approximately between the level of the 12th thoracic to the third lumbar vertebrae. This means they receive some protection from the rib cage and back muscles. An adult kidney weighs about 150g (five ounces) and it is about the size of a large bar of soap (Marieb, 2006).

The organs are bean-shaped with the convex side pointing laterally. The inner, concave, side contains the renal hilum and it is here that the ureters, renal blood vessels, nerves and lymphatics enter and leave the kidney. The kidneys are encased in a layer of fat, which helps to hold them in place and cushion them from blows.

In addition to their primary function of urine production, the kidneys perform a number of other vital functions:

- They maintain adequate blood volume and blood pressure by regulating the volume of water lost in the urine and by releasing renin;

- They produce the hormone erythropoietin which stimulates the production of red blood cells in bone marrow;

- They regulate the concentration in the plasma of sodium, potassium, chloride and other ions by controlling the amount of these substances excreted in urine;

- They control calcium levels by synthesising calcitriol;

- They help to maintain a stable acid-base balance (pH) in the blood by controlling the amounts of hydrogen ions and bicarbonate ions excreted in the urine;

- They conserve nutrients that are valuable to the body by preventing them from being excreted in the urine;

- They metabolise vitamin D to its active form;

- They help the liver to detoxify poisons infiltrating the body.

Every day the kidneys filter several hundred litres of fluid from the bloodstream, allowing toxins, metabolic wastes and excess ions to be excreted from the body in urine while returning any necessary substances to the blood. At the same time, they regulate the volume and the clinical composition of the blood, maintaining the appropriate balance between water and salts and between acids and bases. If the kidneys fail to function normally, the consequences are serious.

The ureters

The ureters are thin tubes about 30cm long that carry urine from the renal pelvis in the kidneys to the bladder. The ureters enter the bladder wall at an oblique angle at its base and the ureteral openings are slit-like rather than rounded. This shape helps to prevent the back-flow of urine when the bladder contracts. When urine enters the ureters, the muscular walls become distended and this stimulates them to contract and help propel urine into the bladder. A wave of contractile peristalsis sweeps along the ureters approximately every 30 seconds, though the strength and frequency of the contractions are adjusted to the rate of urine production.

Occasionally a ureter may become obstructed by a large renal stone, or calculus, formed when calcium, magnesium or uric acids crystallise, and this prevents drainage of the urine. The resultant back-pressure to the kidneys causes excruciating pain as does peristalsis as the stone is moved along the ureter.

The urinary bladder

The urinary bladder is a smooth, distensible, muscular sac that stores urine temporarily. It lies low in the pelvis, just behind the symphisis pubis and is held in place by strong ligaments. The bladder wall contains a strong, muscular layer - the detrusor ('to thrust out') muscle - which has fibres running both longitudinally and in a circular formation.

In males, the bladder lies in front of the rectum and its bladder neck, where the bladder empties into the urethra, surroundes the prostate gland. In females, the bladder lies in front of the vagina and uterus.

Bladder cancer is three times more common in males than females and is most common in the 60-70 age range (Martini, 2004). Environmental factors appear to play an important role and bladder cancer is more common in smokers and employees of chemical and rubber companies. Prognosis is good if the cancer is caught before it metastasises but poor once it has done so.

The urethra

The urethra is a thin-walled muscular tube that drains urine from the bladder and carries it out of the body. At the bladder-urethra junction a thickening of the detrusor muscle forms the internal urethral sphincter. This sphincter is involuntary and keeps the bladder closed when urine is not being passed. A second sphincter, the external urethral sphincter, surrounds the urethra as it passes through the urogenital diaphragm. This sphincter is formed of skeletal muscle and is voluntarily controlled via a branch of the pudendal nerve. The levator ani muscle of the pelvic floor helps to constrict the urethra.

In females, the urethra is 3-4cm long and runs down the anterior vaginal wall. Its external opening or meatus lies in front of the vaginal orifice. The mucosa of the urethra is continuous with that of the rest of the urinary tract, and an inflammation of the urethra (urethritis) can ascend the tract to cause bladder inflammation (cystitis) or even renal inflammation (pyelitis or pyelonephritis).

Symptoms of UTI include pyrexia, painful urination (dysuria), urinary frequency and urgency, and sometimes cloudy or blood-stained urine which may smell offensive. When the kidneys are involved, back pain and a severe headache can occur as well.

In males, the urethra is about 20cm long and has three sections (Fig 2). The prostatic urethra (2.5cm) runs within the prostate gland. The membranous urethra, which contains the external sphincter, extends for about 2cm from the prostate gland to the beginning of the penis. The spongy, or penile, urethra (about 15cm long) passes through the penis and opens at its tip. The male urethra carries semen as well as urine out of the body.

Ageing

Ageing is often linked with increases in urinary problems such as a decline in kidney function and problems with the micturition reflex. For example, loss of muscle tone after a stroke may lead to inability to control micturition and in males, urinary retention may develop secondary to prostate gland enlargement.

Future articles in this series will examine the structure and function of the kidneys and the physiology of micturition.

- This article has been double-blind peer-reviewed.

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