During filtration, low-molecular-weight blood components are sorted out in the kidney. This produces the so-called primary urine, part of which is later excreted. In this process, the first stage of filtration takes place in the renal corpuscles. There, after special cross-flow filtration, the smaller parts of the blood plasma remain in an ultrafiltrate. In addition to the substances to be excreted, this primary urine also contains components that are still important for the body. In the course of the subsequent second filtration stage, substances of value such as amino acids, electrolytes, and sugars are returned to the bloodstream (reabsorbed) through the renal tubules.
What is filtration?
Renal corpuscles and renal tubules together form the nephron, the basic functional unit of the kidney. The end products of metabolism filtered off in the paired organ are called urinary substances. The urine also contains toxins from the body that are to be excreted via the urinary tract. Most of the higher molecular components of blood as well as blood cells are retained in the body thanks to kidney activity. In addition, the kidneys regulate the water balance of the body, ensure the long-term adjustment of blood pressure and have a control function for the electrolyte balance as well as the acid-base balance. They carry out the re-synthesis of glucose and produce hormones such as erythropoietin, which is important for blood formation. During filtration, the kidneys of an adult human are flown through by an average of 1800 liters of blood per day. That is about 300 times the blood volume of the entire body. From this volume, the kidneys filter out about 180 liters of primary urine each day, which in turn is concentrated into about two liters of final urine. The kidneys have a brown-red 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, and the width is five to six and a half centimeters. If one kidney is significantly smaller than normal or missing entirely, the other is usually correspondingly larger.
Function and purpose
The kidneys’ filtration work 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. Filtration, however, requires sufficient blood pressure, which ideally is not subject to fluctuations. The renal corpuscles (glomerular capillary network) can largely ensure this constancy of blood pressure without requiring special nervous impulses. Even considerable fluctuations in systemic blood pressure thus have no negative effect on filtration. This so-called autoregulation of the kidney is achieved by changes in vascular tension and vascular width in the renal corpuscles. If systemic blood pressure rises, the renal arteries are constricted. This prevents the pressure in the afferent vessels of the renal corpuscles from increasing 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 afferent vessels decreases. According to this principle, even severe fluctuations in systolic blood pressure have little effect on filtration efficiency.
Diseases and complaints
Diseases of the kidneys mostly originate from their vessels and 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 manifested by noticeable changes (mostly increases) in blood pressure. The tubules are usually affected by infections, mostly of bacterial origin, as well as poisonings (intoxications). Likewise, genetic diseases very often have an effect there. If the vessels and tubules of the kidneys are severely damaged, acute or chronic kidney failure occurs quite quickly. This disease, also known as renal insufficiency, can develop over months or even years and leads to serious failures of kidney function. It can decline to the point where dialysis treatment or, in an extreme emergency, a kidney transplant becomes necessary.If the kidneys no longer perform their detoxifying task adequately, kidney stones may develop. In contrast, kidney cancer is very rare and accounts for only one to two percent of all malignant tumor diseases. However, severe damage to the kidneys and thus impaired filtration always have noticeable negative effects on blood pressure and hormone regulation in the body.
