VOL: 102, ISSUE: 43, PAGE NO: 26
Marion Richardson, BD, RN, DipN, CertEd, RNT, is senior lecturer, University of Hertfordshire
The average adult passes about 1.5l of urine a day and depends on this urination (or micturition) to rid the body of organic waste products produced as a result of cell metabolism throughout the body. The final composition of urine reflects the filtration, absorption and secretion activities of the nephrons.
The urinary bladder
When it is empty, the bladder collapses into a pyramid shape. Its walls are thick with many folds or rugae. As urine accumulates, the bladder expands and becomes pear-shaped; its muscular wall stretches and thins and the rugae disappear. This allows the bladder to store some urine without an increase in its internal pressure. A reasonably full bladder holds approximately 500ml (one pint) but it can expand to hold more than a litre if necessary. When it is full of urine it can be palpated above the symphisis pubis and if it becomes overdistended, it may burst (Marieb, 2006).
The bladder is lined with mucosa. This is particularly thick in the triangular area formed by the ureter openings and the urethra entrance, known as the trigone (Fig 1), where it acts as a funnel to channel urine into the urethra when the bladder contracts. During micturition, the strong detrusor muscles in the bladder walls contract and compress the bladder, pushing its contents into the urethra.
The internal and external sphincters
The opening between the bladder and the urethra (the neck of the bladder) is closed by two rings of muscle - the internal and external sphincters (Fig 2). The internal sphincter contains smooth muscle fibres, the normal muscle ‘tone’ of which keeps it closed - it is controlled involuntarily. The external sphincter is formed from a circular band of skeletal muscle, supplied by the pudendal nerve, and is under voluntary control. Stimulation from the central nervous system keeps these external sphincter fibres contracted except during micturition.
At its most basic, urination is a simple reflex (Silverthorn, 2003) from the bladder to the spine and back to the bladder and urethral sphincters (Fig 2).
When about 250ml of urine has collected, stretch receptors in the bladder walls are stimulated and excite sensory parasympathetic fibres, which then relay information to the sacral area of the spine. The information is integrated in the spine and relayed to two different sets of neurons:
- Parasympathetic motor neurons are excited and contract the detrusor muscles in the bladder. Bladder pressure increases and the internal sphincter opens;
- When urine enters the urethra, somatic motor neurons supplying the external sphincter via the pudendal nerve are inhibited allowing the sphincter to open and urine to flow out, assisted by gravity.
We have considerable control over when and where we urinate and we can also increase or decrease the rate of flow and even stop and start again, so micturition is clearly more than just a simple reflex. We learn this control in infancy and it involves other sensory fibres in the bladder wall that convey information on fullness of the bladder via the spine to the higher centres in the thalamus and cerebral cortex - we become aware not only that we need to pass water but also of the urgency of the situation. The brain centres can override the micturition reflex by inhibiting the parasympathetic motor fibres to the bladder and reinforcing contraction of the external sphincter (Martini, 2006). When convenient, the brain centres remove the inhibition and permit micturition under our conscious control. These links between the spine and cerebral cortex are not established until about two years of age and toilet training is not physiologically possible until that time (Martini, 2006).
Contraction of the abdominal muscles and the performance of Valsalva’s manoeuvre (forced expiration against a closed glottis) increase the flow of urine (McLaren, 2005). Contraction of the strong pelvic floor muscles can stop urine in mid-flow. The sound of running water encourages micturition (Silverthorn, 2003) but some people are unable to urinate in the presence of others, no matter how great their need.
At the end of micturition, less than 10ml of urine remains in the bladder (Martini, 2006) and the cycle begins again.
The normal composition of urine
- Colour - normally clear and pale to deep yellow due to the presence of urochrome, a pigment from the body’s destruction of haemoglobin. Abnormal colour may result from certain foods (for example beetroot), bile pigments or blood in the urine or some drugs and vitamins. Cloudy urine may indicate infection;
- Smell - slightly aromatic but develops an ammonia smell if allowed to stand. Some drugs, diseases and vegetables (notably asparagus) alter the smell of urine. In diabetes mellitus, urine smells fruity due to the presence of acetone;
- Acidity - usually slightly acidic (pH 6) but changes in body metabolism or diet cause variation from 4.5 to 8.0;
- Specific gravity - urine is heavier than distilled water because it contains solutes. Normal range is 1.001-1.035 depending on the solute concentration (distilled water has a specific gravity of 1.0);
- Constituents - 95% water and 5% solutes, namely urea, sodium, potassium, phosphate and sulphate ions, creatinine and uric acid. The presence of abnormal substances in the urine (glucose, blood cells or bile pigments) is a warning of illness.
For normal micturition to occur we need:
- Intact nerve pathways, including those to the higher brain centres;
- Normal muscle tone in the detrusor muscles, sphincters and pelvic floor;
- Absence of obstruction to urine flow in any part of the urinary tract;
- Normal bladder capacity;
- Absence of environmental or psychological factors that may inhibit normal micturition (McLaren, 2005).
Any of the factors listed below may result in incontinence or urgency to micturate:
- Neurological disorders, including stroke and Alzheimer’s disease, and any condition where nerve pathways to and from the spine and brain are blocked or injured;
- The neurotransmitter acetylcholine (ACh) is involved in the relaying of nerve signals in micturition. ACh can be blocked with the drug atropine so the detrusor muscle will not contract and retention of urine will occur;
- Muscles naturally lose their tone with increasing age and so incontinence is more likely in older people. However, stress incontinence occurs in some individuals when abdominal pressure rises (for example when sneezing or coughing). Stress incontinence is relatively common in men following prostatectomy and in women after childbirth and during the menopause, due to decreased oestrogen secretions (McLaren, 2005);
- Renal stones, inflammation and an enlarged prostate gland are all examples of urine flow problems that may cause urine retention or increased frequency of micturition, as may tumours and pregnancy, which reduce normal bladder capacity.
- This article has been double-blind peer-reviewed.