People have two kidneys which lie on the right and left sides of the abdomen, well protected by the back musculature. In adults, each kidney is about 12 cm long and 5–7 cm wide and weighs between 150 and 200 grams. The kidneys have many tasks and functions to fulfill.

Amongst other things, they clear the blood of waste products that arise from the metabolic processes of various cell types. They also excrete toxins that are taken in with food, as well as breaking down and excreting drugs.

In addition to this function as a ‘clearing house’, the kidneys regulate the water content of the body and the composition of the blood salts, namely the concentrations of sodium, potassium, calcium and phosphorus.

Finally, the kidneys make hormones, which are essential for certain bodily functions, e.g. renin, which regulates the blood pressure, and erythropoietin (‘Epo’), which is needed to make new blood cells.

For all these tasks to be fulfilled, a good blood supply is essential. The renal arteries, which carry the blood to the kidneys, arise as thick branches directly from the body’s largest artery.

A person’s entire blood content passes through the kidneys every 20 minutes. After passage through the kidneys, the blood is returned to the circulation through a large collecting vessel, the renal vein.

The essential tasks of blood cleansing and water excretion are done by the glomeruli, of which each kidney has about one million.

Each glomerulus consists of a ball of tiny blood vessels (capillaries). The unpurified blood flows through an incoming arteriole into the ball, where it is filtered.

The filtered blood is then led back into the circulation through another blood vessel. The incoming and outgoing vessels have special properties: they can regulate the flow of blood very precisely by narrowing or widening. This is particularly important for two reasons:

• First, it is essential to have exactly the right blood pressure in the glomeruli for the blood cleansing to occur. This is known as the filtration pressure and in a healthy kidney it is regulated automatically.

• Second, it gives the glomeruli the ability to protect themselves when the circulatory blood pressure is too high. The incoming vessel is narrowed, reducing the blood flow, and the renal cells are thereby guarded against excessive pressure. If the circulatory blood pressure falls, the kidney can expand the incoming vessel and narrow the outgoing one to maintain the required filtration pressure.

The actual blood purification occurs via the basal membrane in the capillaries. This contains small pores and acts as a filter: cells and large protein fragments that will be needed again are held back in the blood; water, salts and metabolic waste products pass through the membrane.

The filtered liquid accumulates in a capsule that surrounds the glomeruli and is then led through the kidney tubules. 180 liters of this so-called ‘primary urine’ pass through the filter every day.

So that not too much fluid is lost from the body, the filtered liquid is built up again: water and salts are taken back by the kidney and returned to the body. Only a small percentage of the original filtered urine ends up in the kidney collecting duct, from where it is passed through the ureter and excreted as urine.

When the kidneys are no longer working efficiently, tell-tale signs appear in the blood and the urine – but not until a certain stage. In the blood, for example, creatine and urea accumulate. Creatine is a waste product of muscle metabolism that is normally excreted through the kidneys.

Healthy people have a creatine serum concentration of between 0.6 and 1.2 mg/100 ml. The normal value in an individual depends on the muscle mass. In someone with less muscle, it is lower; in ‘muscle men’, it is higher. If kidney function fails, the creatine concentration in the blood gradually rises.

Generally, though, mild kidney impairment does not lead to a noticeable change in serum creatine concentration. It rises only when kidney function has decreased by more than half. In people with little musculature, the low starting point for the creatine concentration can mean that it stays within the normal range for a long time, even when kidney function is seriously compromised.

A more exact picture of kidney function is given by the so-called creatine clearance rate. To measure this, the urine must be collected for 24 hours or overnight, then the creatine concentration in the blood and in the urine is determined. A formula can then be used to calculate the filtration rate and thereby the activity of the kidney.

However, a precise value is obtained only when all the urine is collected. In practice, this is where mistakes can easily be made. A simpler and faster method is to calculate the creatine clearance using a formula based on serum creatine concentration, body weight and age.

This gives reasonable values for a wide range of kidney function. Normally, the rate of creatine clearance lies between 80 and 140 ml/min. Urea is the end product of protein metabolism and is excreted through the kidneys. In healthy people, the concentration of urea in the blood is less than 45 mg/100 ml.

As kidney function fails, the amount of urea in the blood rises. This occurs particularly when a person eats a lot of protein. The concentrations of the blood salts, potassium, calcium and phosphorus, as well as that of uric acid, can also give information about the state of the kidneys.

In the urine, the most important marker for impaired renal function is protein excretion. Normally, only a very small amount of albumin, a special blood protein, is found in the urine – less than 20 mg/l, which is too little for the standard tests to detect.

If the albumin concentration rises above a certain threshold, this can be one of the first signs of kidney failure. Protein may occur in the urine as a result of other circumstances, such as after major bodily damage, infection, pressure on the kidneys or low temperatures.

Examination of the urine composition can also give clues regarding the state of the kidneys. Usually, no red or white blood cells should be visible in the urine under a microscope. If they are present, it is an indication that something is wrong with the kidney–bladder system. Such a finding should be the starting point for further investigations.