Filtration

In the Filtration low molecular weight blood components are sorted out in the kidneys. This creates what is known as primary urine, some of which is later excreted. The first stage of filtration takes place in the kidney corpuscles. After the special cross-flow filtration, the smaller parts of the blood plasma remain in an ultrafiltrate. In addition to the substances to be eliminated, this primary urine also contains components that are still important for the body. In the course of the subsequent second filtration stage, valuable substances such as amino acids, electrolytes and sugar are returned to the bloodstream (absorbed) through the kidney tubules.

What is Filtration?

The kidney corpuscles and tubules together form the nephron, the basic functional unit of the kidney. The end products of the metabolism filtered out in the paired organ are called urinary substances. The urine also contains toxins from the body that are supposed to be eliminated via the urinary tract. Most of the higher molecular components of the blood as well as the blood cells are retained in the body thanks to the activity of the kidneys.

In addition, the kidneys regulate the body's water balance, ensure the long-term adjustment of blood pressure and have a control function for the electrolyte balance and the acid-base balance. They carry out the new synthesis of glucose and produce hormones such as erythropoietin, which is important for blood formation.

With filtration, an average of 1,800 liters of blood per day flows through the kidneys of an adult. That is about 300 times the blood volume of the entire body. From this amount, the kidneys filter out around 180 liters of primary urine per day, which in turn is concentrated to around two liters of terminal urine (urine).

The kidneys are brownish-red in color and are bean-shaped. Their weight is 120 to 200 grams each. Often the left of the two kidneys is slightly heavier and larger than the right. The length of a kidney wing is ten to twelve centimeters, the width five to six and a half centimeters. If one kidney is significantly smaller than normal or is completely absent, the other is usually correspondingly larger.

Function & task

The filtration work of the kidneys is driven by the body's systemic blood pressure. This pressure usually fluctuates throughout the day. For example, it is low during sleep, but high during physical activity or high stress.

However, filtration requires adequate blood pressure, which is best not subject to fluctuations. The kidney corpuscles (glomerular capillary network) can largely ensure this constancy of blood pressure without requiring special nervous impulses. Even significant fluctuations in systemic blood pressure do not have a negative effect on the filtration.

This so-called autoregulation of the kidney is achieved by changing the vascular tension and the vascular width in the kidney corpuscles. When systemic blood pressure rises, the renal arteries become narrowed. This can prevent the level of pressure in the afferent vessels of the kidney corpuscles from rising too much.

If the filtration pressure is too low, the sensory system reacts with a higher resistance in the outgoing (efferent) vessels. At the same time, the resistance in the supplying vessels decreases. According to this principle, even strong fluctuations in systolic blood pressure have hardly any effect on the filtration performance.

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Illnesses & ailments

Kidney diseases mostly originate from their vessels and the renal tubules. These are long, very thin tubes that primarily perform important functions in filtration. The vascular diseases are very often related to negative changes in the immune system and are mainly expressed in noticeable changes (mostly increases) in blood pressure. The tubules are usually affected by infections, mostly of bacterial origin, as well as poisoning (intoxication). Genetic diseases also have an effect there very often.

If the vessels and tubules of the kidneys are severely damaged, acute or chronic kidney failure occurs very quickly. This disease, also known as kidney failure, can develop over months or even years and leads to severe kidney failure. It can decrease to such an extent that dialysis treatment or, in an extreme emergency, a kidney transplant is necessary.

If the kidneys no longer adequately perform their detoxifying task, kidney stones may develop. In contrast, kidney cancer is very rare and only accounts for one to two percent of all malignant tumor diseases. However, severe damage to the kidneys and thus restricted filtration always have noticeably negative effects on blood pressure and hormone regulation in the body.