Introduction

The urinary system consists of the kidneys, the ureters, the bladder and the urethra. The urinary system may also be called the ‘renal system’, although strictly speaking ‘renal’ (from the Latin term for kidney) is a term that refers to the kidneys alone. The Greek term for kidney is ‘nephron’, and hence the study of the kidneys is called ‘nephrology’.

The component parts of the urinary system are illustrated in Figure 4.3a-I. It can be seen how the kidneys are situated in the upper part of the back of the abdomen. They rest protected against the ribs between the levels of T11 and L3. The influential acupoints on the back of the Kidney Organ (Shenshu Bl-23) lie directly over the lower parts of the kidneys. The ureters run from the medial aspect of each kidney, down the posterior border of the abdominal cavity, to open into the bladder. The bladder sits protected within the pelvis. Note from the diagram how small the bladder is. Even when uncomfortably full the bladder rarely rises very much higher than the level of the pubic symphysis (the region of acupoint Qugu Rn-2). The urethra (similarly spelled to the more proximal ureter, but a different anatomical structure) is the tube that links the bladder to the external world. It is very short in women, but longer in men because of its passage through the penis.

Figure 4.3a-I • The component parts of the urinary system (excluding the urethra) and related abdominal structures.

The major function of the kidneys is to maintain homeostasis of the composition of the blood by excreting wastes and controlling the concentrations of the various mineral salts within the blood. It does this by the formation and excretion of urine, which is a solution of wastes and salts. Other important functions include the control of the blood pressure, control of the development of the red blood cells by the secretion of the hormone erythropoietin, and production of active vitamin D.

The function of the ureters is to allow the free drainage of the urine into the bladder. The bladder is a reservoir that contains this steady flow of urine from the kidneys until there is an appropriate time for its expulsion. This happens through the process of urination (also known as ‘micturition’), in which the bladder contracts and the urine passes through the urethra and out of the body.

The physiology of the kidneys

The position and structure of the kidneys

The kidneys sit at the back of the upper abdominal cavity embedded in fat. The adrenal glands sit on the top of each kidney, but do not have a direct link with the role of the urinary system. However, both the adrenal glands and the kidneys have functions that would be understood in Chinese medicine as being related to the Kidney Organ. The kidneys sit very close to a number of other deep organs, including the spleen, pancreas, liver, duodenum and the large intestine.

The tissue of the kidney consists of thousands of tiny tubes called ‘nephrons’. Each of these tubes has its origin in the outer parts of the substance of the kidney (the renal cortex). The nephrons converge to open out into wider tubes called ‘collecting tubules’, which drain into a space in the core of the kidney called the ‘renal pelvis’. The renal pelvis drains downwards into the ureter. The collecting tubules are situated in the deeper tissue of the kidney, which is known as the ‘renal medulla’. Figures 4.3a-II and 4.3a-III illustrate the external and internal structure of the kidneys in some detail. Figure 4.3a-II shows the position and size of the adrenal glands, and Figure 4.3a-III indicates the location of the renal cortex, medulla, pelvis and ureter.

Figure 4.3a-II • Anterior view of the kidneys.

Figure 4.3a-III • Longitudinal section of the right kidney.

These diagrams clarify how the kidneys receive their blood supply from a large branch of the aorta called the ‘renal artery’. This artery branches within the substance of the kidney to provide each nephron with its own arteriole. The blood supply to the kidney is very rich. About one quarter of all the blood that has left the heart is directed from the aorta through the kidneys. This is very important because it ensures that the blood undergoes a continual and efficient process of being cleansed and returned to a state of balance.

The formation of urine in the kidneys

Figure 4.3a-IV shows how the blood that reaches the nephron first enters a cluster of capillaries called the ‘glomerulus’. Here, because of a sudden restriction of easy flow, fluid containing salts, wastes and water is forced out of the glomerulus. From here this fluid passes into the cup-like origin of the nephron. However, substances such as plasma proteins and blood cells are too large to be squeezed out, and so these remain within the blood vessel. This process of draining water and small molecules from the blood into the nephron is called ‘simple filtration’.

Figure 4.3a-IV • A nephron and the associated blood vessels.

This fluid then drains down through the tube of the glomerulus towards the collecting tubule. However, it does not yet have the composition of urine. During its passage through the nephron some of the fluid and the salts are reabsorbed back into the bloodstream. The diagram indicates how the arteriole travels on to supply the rest of the nephron with a blood supply for this purpose. Substances can also be secreted from the blood in the arteriole into more distal parts of the nephron.

These two processes of reabsorption and secretion not only make the original fluid that has been drained out of the glomerulus much more concentrated (it is nearly 200 times as concentrated as plasma fluid), but they also allow for a subtle control of its composition according to the body’s requirements. For instance, if the person is dehydrated, more water than normal is reabsorbed, and if the person is depleted in salts, more salts are reabsorbed.

The reabsorption and secretion of the fluid in the nephron are under the control of a number of hormones, including parathyroid hormone (which regulates calcium and phosphate levels), antidiuretic hormone (ADH) (which regulates the concentration of the blood) and aldosterone from the adrenal glands (which regulates the amount of sodium and potassium in the blood).

The fluid that leaves the nephron to enter the collecting tubule, and thence the renal pelvis, has the composition of urine. This yellow-coloured fluid consists mostly (96%) of water, with a further 2% being urea (the waste product of protein metabolism) and the remaining 2% comprising a range of other salts including sodium, potassium and uric acid. In a healthy adult, approximately 1–1.5 litres of urine are passed a day, although this can vary from as little as 500 millilitres to as high as 3 litres, depending on the balance of fluid intake and other fluid losses.

Diuretic drugs that act on the kidney tend to inhibit the amount of water that is reabsorbed, and so increase the volume of urine entering the renal pelvis. Some diuretic drugs may also affect the reabsorption of salts. This is why some diuretics can lead to sodium or potassium depletion, and others may cause a build up of uric acid (predisposing to an attack of gout) (see Q4.3a-1 and Q4.3a-2).

The physiology of the ureters

The ureters receive a steady flow of urine from the renal pelvis at a rate of 20–100 millilitres/hour. These structures run down the back of the abdominal cavity and enter the bladder where it sits within the pelvis. The flow of urine is aided by waves of muscular contractions that are generated by the smooth muscle fibres within the ureter walls.

The urine that enters the bladder passes through a narrowing, akin to a valve, as it enters the bladder. This important structure prevents the return of urine back into the ureter (known as ‘reflux’) from the bladder. The urine enters the bladder in little spurts, which occur every 10 seconds or so.

The physiology of the bladder

The bladder is a hollow, pear-shaped organ, the function of which is to act as a reservoir for urine. Figures 4.3a-V and 4.3a-VI show a vertical section through the pelvis of a woman and a man, respectively. These diagrams illustrate the position of the bladder within the pelvis. Note how in the woman the bladder sits just in front of the cervix and womb. In the man the prostate gland sits at the neck of the bladder, surrounding the first part of the urethra.

Figures 4.3a-V • The pelvic organs associated with the bladder in the female.

Figures 4.3a-VI • The pelvic organs associated with the bladder in the male.

The bladder is lined by a form of epithelium called ‘transitional epithelium’ which has the ability to stretch. Within the wall of the bladder are sheets of smooth muscle, which together are called the ‘detrusor muscle’. When the detrusor muscle contracts, it forces urine out through the urethra.

In a healthy adult the desire to urinate arises when the bladder contains more than 300 millilitres of urine. At this stage, the stretch of the bladder muscle stimulates nerve impulses so that the bladder contracts spontaneously (this is a spinal reflex). Indeed, this is what occurs both in the newborn and in someone who has had a spinal injury. However, in the healthy adult, although the sensation of a desire to urinate occurs at this stage, the brain exerts an inhibitory control on this reflex until an appropriate time arises for it to occur. This voluntary control of bladder function is usually learned during the third year of life.

The physiology of the urethra

The urethra is the canal that leaves the neck of the bladder and links it to the exterior. In women, the urethra is about 4 centimetres long, and it opens to the exterior just in front of the vaginal orifice. In men, the urethra first passes through the tissue of the prostate gland and then travels through the length of the penis to the orifice at the tip of the glans penis. Ducts open out into the first part of the male urethra through which the seminal fluid and semen are passed during ejaculation.

In both women and men the urethra passes through the region of the body called the ‘perineum’ which forms the floor of the pelvis. In doing so, it passes through the pelvic floor muscles, which are slung between the pubic bone and the base of the sacrum and coccyx.

At the origin of the urethra is a smooth muscle ring called the ‘internal urethral sphincter’. This sphincter is generally maintained in a state of contraction, thus holding the urine within the bladder. In addition to the internal sphincter, the muscles of the pelvic floor provide an additional ‘external urethral sphincter’ as a result of their generally high level of tone. Together, these factors exert a closing pressure on the urethra, which is released when the person intends to urinate.

Introduction

Diseases of the urinary system are investigated within three distinct conventional specialties. Medical conditions that affect the kidney are treated by a renal physician within the speciality of renal medicine or nephrology. Conditions that affect the ureters, bladder and urethra, and also those conditions of the kidney that may require surgery, are treated by a surgeon called a ‘urologist’ within the speciality of urology. Conditions that affect the male reproductive organs are also commonly treated by a urologist. However, those conditions that result from sexually transmitted diseases are treated by a genitourinary physician within the speciality of genitourinary medicine or sexual medicine.

The investigation of the urinary system involves:

These are each considered briefly in turn below.

Physical examination

The physical examination of the urinary system involves the stages listed in Table 4.3b-I.

Table 4.3b-I The stages in the physical examination of the urinary system

Examination of the urine

The most common urine tests require a ‘midstream’ sample of urine (MSU). This means that the specimen is taken only after a flow of urine from the urethra has been established. The sample ideally should be free from contamination by the bacteria of the skin, although this is not always possible to ensure, particularly when collecting urine from small children. Any sterile container is suitable for a urine specimen.

Healthy urine is pale yellow and clear. The first urine passed in the morning is usually the most concentrated, and can have a deep amber colour. Visual assessment of urine may reveal a pink discoloration suggestive of blood, or cloudiness suggestive of infection. Both these findings are abnormal, but commonly result from benign causes. For example, a large amount of beetroot in the recent diet as well as some drugs can cause the urine to become pink.

The simplest quantitative test of urine involves ‘dipping the urine’ with a plastic strip that holds tiny indicator cards. These cards change colour according to the concentration of a range of factors within the urine. Use of the plastic dipsticks, commonly termed ‘stix’ or ‘multistix’, can give an instantaneous indication of the amount of sugar, blood, protein and white blood cells in the urine, to name just a few of the factors that can be detected. This test is not as reliable as a laboratory test, but is very commonly used in general practice to check for blood in the urine (haematuria), to give an indication of undiagnosed or poorly controlled diabetes, and to give added weight to a provisional diagnosis of urine infection.

Urine may be sent to a laboratory for microscopic examination. This may reveal the presence of microorganisms and excessive white blood cells in the case of infection. It will also confirm the presence of red blood cells in the urine. In the diseases (classed as glomerulonephritis) that affect the structure of the tubules of the kidney, unusual tube-shaped ‘casts’ of protein may be seen in the urine.

The urine can also be analysed for the substances that are dissolved within it. In addition to the usual body wastes, drugs and hormones can also be found in the urine. Therefore, a urine test can detect the presence of illicit drugs or increased levels of hormones. The protein in the urine should be at a very low level, but in severe kidney disease it may be present in vast quantities. Both the total protein and the albumin (the most common protein in the plasma) levels may be assessed from a urine sample. A 24-hour collection of urine may be used to quantify accurately the protein loss in kidney disease, and can also be used to assess the rate at which blood is filtered by the kidneys (glomerular filtration rate).

Examination of the blood

A serum sample of blood can be analysed to reveal the disturbances in blood chemicals that result from kidney failure. In particular, the by-products of protein metabolism (urea and creatinine) may be increased in quantity. In severe kidney disease there may also be disturbances of the concentration of the basic mineral salts, such as sodium, calcium and potassium, in the blood. The level of creatinine may be inserted into a mathematical formula that incorporates the age, ethnicity and sex of the patient to generate an estimate of the amount of fluid filtered by the kidneys in a fixed amount of time. This is the estimated glomerular filtration rate (eGFR), which is a useful indication of the progression of chronic kidney disease, a condition that becomes increasingly prevalent with ageing.

Other factors that indicate a specific cause of kidney disease may also be assayed (e.g. autoantibodies in systemic lupus erythematosus (SLE)). In disease of the prostate gland, a particular protein called prostate-specific antigen (PSA) is assayed, as this is more likely to be raised if cancer is present.

A full blood count (FBC) may reveal the anaemia of chronic disease that can accompany chronic kidney disease.

Imaging tests to examine the structure of the urinary system

Ultrasound, magnetic resonance imaging (MRI) and computed tomography (CT) scans are used to visualise the shape of the kidneys, ureters and bladder. The images obtained may be used to examine gross defects, such as the presence of cancer or obstruction to the outflow of urine from the bladder, but cannot reveal the more subtle changes that occur in most diseases of the urinary system. An ultrasound scan obtained by means of a probe passed into the anus may also be used to examine the prostate gland.

A plain X-ray image, although it cannot show any detail about the soft tissues, may be used to reveal the presence of kidney stones, also known as ‘calculi’. Figure 4.3b-I shows the X-ray images of the passage over time of a calculus down the left ureter.

Figure 4.3b-I • X-ray images of calculi at different levels of the left ureter.

A catheter (a plastic tube) may be passed into the bladder via the urethra to inject a ‘dye’ which is opaque to X-rays into the bladder. This test, known as ‘cystography’ or ‘urography’, can show up defects in the structure of the bladder.

Imaging tests to examine the function of the urinary system

Until recently, the most common of the imaging tests of function would involve the intravenous injection of a solution that contained iodine. This solution, when concentrated, shows up on X-ray images. After injection, serial X-ray images can be taken of the abdomen. The whole procedure, known as an ‘intravenous urogram’ (IVU) or ‘excretion urogram’, reveals the passage of the ‘dye’ through the urinary system, from when it is first concentrated from the blood by the kidneys and then drained into the bladder. This test can reveal abnormalities in the structure of the kidneys and ureters, as well as in their function of filtering the blood. An excretion urogram is shown in Figure 4.3b-II. An increasingly popular and more accurate form of the IVU is the CT urogram in which the CT scanner is used to locate the obstruction in the urinary system instead of a series of X-ray films.

Figure 4.3b-II • Excretion urogram showing a pale patch of contrast in the calyx of the left kidney
•The pale patch represents a large calculus (kidney stone).

The function of the bladder may be visualised by requesting the patient to urinate after ‘dye’ has been inserted into the bladder for cystography. An X-ray image taken during contraction of the bladder will reveal any abnormalities in the way in which it empties. This test is known as ‘micturating cystourethrography’ (MCU).

A radioactive chemical (containing a radioactive isotope) known as DMSA can be used to assess the filtration function of the kidneys more accurately than the IVU. As in the IVU, DMSA is injected intravenously, and is filtered and concentrated in the kidney. The amount of radioactivity that builds up in the kidneys over the next few hours is then assessed by means of a gamma scanner. The radioactivity dies off very quickly, and so this test is not considered to be harmful to the patient.

Cystoscopy

In cystoscopy, a fibre-optic telescope is inserted into the urethra so that the inside of the bladder can be seen directly. The patient is usually heavily sedated, or has a general anaesthetic. Biopsies of the bladder can also be taken by means of the cystoscope.

Renal biopsy

Under guidance from an ultrasound scan, a needle may be directed through the skin into the kidney to remove a small sample of tissue for examination. This procedure carries a small but significant risk of bleeding into or around the kidney.

Urodynamic studies

Urodynamics is the study of the pressure of the urine in the bladder and the flow of the stream of the urine from the urethra. Specialised tests to measure the pressure within the bladder and the flow rate are used mostly to investigate the underlying problem in cases of incontinence or obstructed flow of urine. They may be used in combination with MCU.

Introduction

The conditions studied in this chapter are (see Q4.3c-1):

All the common diseases of the kidney described in this chapter may be so severe as to lead to either sudden (acute) or gradual (chronic) failure of the kidney to perform its necessary functions.

Part I: Disease processes that affect the kidney

Glomerulonephritis

‘Glomerulonephritis’ is a term used to describe a broad range of conditions that affect the structure of the cup-like origin of the tubules in the kidney and the cluster of blood vessels within this cup known as the ‘glomerulus’. In glomerulonephritis the filtering process is impaired. This has the consequence that red blood cells and protein may leak out of the blood into the urine, and also that excess salts, wastes and water may be retained in the circulation.

In medical textbooks, the condition of glomerulonephritis is categorised according to the precise pathological changes occurring in the glomerulus, and these categories are assigned descriptive terms such as ‘minimal change’, ‘membranous glomerulonephritis’ and ‘mesangiocapillary glomerulonephritis’. This categorisation, which is based on the results of kidney biopsy and is important for predicting prognosis, does not relate directly to the condition that may have caused the disease process. For this reason it is not described further in this chapter.

The causes of glomerulonephritis

Glomerulonephritis can be caused by a variety of underlying disease processes, but also commonly develops without any recognised cause. In many cases the problem results from the deposition of autoantibodies within the glomerulus. Autoantibodies stimulate the immune response, and the inflammation that results leads to glomerular damage. Autoantibodies may develop as a delayed response to infection, as part of an established autoimmune disease such as systemic lupus erythematosus (SLE), or as a reaction to certain drugs. They may also account for many of the cases of glomerulonephritis of unknown cause.

The Streptococcus bacterium, which is a common cause of sore throat and tonsillitis, used to be a very common cause of antibody-related glomerulonephritis. Post-streptococcal glomerulonephritis tends to lead to a characteristic cluster of symptoms known as ‘nephritic syndrome’ or ‘acute nephritis’ (see below). ‘Bright’s disease’ is an old-fashioned name for nephritic syndrome. This term used to be almost synonymous with ‘post-streptococcal kidney disease’ (see Q4.3c-2).

Other infectious agents that may cause the development of damaging autoantibodies include viruses, such as measles and hepatitis, bacteria, such as Salmonella, and parasites, such as malaria.

Glomerulonephritis may also be part of the disease process of a multisystem autoimmune disease. SLE is the most common specific autoimmune disease known to cause glomerulonephritis. In SLE, chronic kidney disease resulting from severe persistent cases of glomerulonephritis is the most common cause of death.

Some drugs may also trigger the development of damaging autoantibodies. The drug which most commonly has this effect is penicillamine, which is prescribed as a disease-modifying treatment in rheumatoid arthritis.

The consequences of glomerular damage

The effects of glomerulonephritis can be broadly divided into four categories, although in practice, for any individual patient, it may be difficult to assign them to a category. The categories are:

• haematuria (blood in the urine)

• proteinuria (protein in the urine)

• nephritic syndrome (blood and protein in the urine, with high blood pressure and oedema)

• nephrotic syndrome (excessive protein in the urine leading to severe oedema).

Haematuria and proteinuria in glomerulonephritis

Some forms of glomerulonephritis cause sufficient damage to the glomerulus to bring about a leakage of red blood cells, protein or both. This leakage may not be sufficiently severe to be visible to the patient, and instead may be picked up on routine urine testing. Greater numbers of red blood cells might cause the urine to appear pink or smoky coloured, and excess protein might cause the urine to appear frothy. There may be no other symptoms.

Nevertheless, despite the mildness of the symptoms, the finding of even small amounts of blood or protein in the urine in the absence of urinary infection can be a warning sign of a progressive disease of the kidney. Chronic kidney disease can be the long-term consequence of glomerulonephritis which may be signalled by haematuria or proteinuria. For this reason, all patients who are found to have unexplained haematuria or proteinuria above a certain level will be referred to a renal physician for further investigations.

Some forms of glomerulonephritis that cause haematuria or proteinuria may respond to treatment with immunosuppressant drugs, including corticosteroids. Other immunosuppressant medications that may be prescribed at the same time include azathioprine, cyclophosphamide and chlorambucil. All these drugs can have a wide range of toxic effects.

Nephritic syndrome (acute nephritis or Bright’s disease)

‘Nephritic syndrome’ is a term used to describe a glomerulonephritis that causes severe symptoms to appear rapidly. In nephritic syndrome the patient, commonly a child, becomes unwell with fever, vomiting and pain in the back. There may be a history of a streptococcal infection within the past 2–4 weeks, although streptococcal infection is not always the cause. The face becomes puffy with oedema and the blood pressure is raised. The urine may be pink or smoky coloured with red blood cells. In severe cases the production of urine becomes severely reduced as the filtration function of the kidney is extremely impaired. The patient is then at risk of death from acute kidney failure or malignant hypertension.

Information Box 4.3c-I Glomerulonephritis: comments from a Chinese medicine perspective

In Chinese medicine, the underlying pathological process of damage to the structure of the kidney in glomerulonephritis suggests that Internal Heat (which may have had External origins if following an infection) is causing Kidney Yin depletion. Blood in the urine can be a result of transmission of Heart Fire via the Small Intestine or Damp Heat in the Bladder. Protein in the urine may be interpreted as the draining of Essence from the Kidneys.

The predominant energetic effect of immunosuppressant drugs such as azathioprine and cyclophosphamide is that of Heating and may cause severe damage to Blood and Yin.

A patient with nephritic syndrome requires close medical observation. In most childhood cases there is full recovery from the condition after 4–7 days, although a minority of cases take a more chronic course. In contrast to the forms of glomerulonephritis that cause haematuria and proteinuria alone, most of these chronic cases of nephritic syndrome do not respond to immunosuppressant drugs. In post-streptococcal nephritis a course of penicillin is given to ensure eradication of the Streptococcus.

In the acute cases, as the tissue of the kidneys recovers, the toxins and excess salts that have built up in the body are cleared by means of profuse urination. However, in rare cases, the patient may die during the acute illness from acute renal failure and from damage caused to the brain by a rapidly rising blood pressure. Convulsions may develop in these severe cases. This consequence can be prevented with the help of emergency dialysis, which takes over the function of the failing kidney (see later in this chapter).

In the more chronic cases, and more commonly in adults, there may be persisting damage to the kidneys. This damage can progress to chronic kidney failure over the course of months to years.

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