Filtration

Separation of solid components of a gas or liquid by bodies containing pores of various types, thicknesses and pore sizes (filters).

In the kidney, blood is pressure-filtered through the capillary walls of the clusters of glomeruli vessels and the wall of the inner leaf of Bowman's capsule. The red and white blood cells, as well as most dissolved high-molecular substances such as proteins and peptides, are retained, while inorganic and organic low-molecular compounds with water as primary urine pass into the kidney tubules.

The primary urine is an almost protein-free ultrafiltrate containing almost only substances with a molecular weight below 3-6 x 10⁴.

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