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The kidney acts as a biological filter, filtering out substances from the blood that
need to be removed from the body. You can see kidney anatomy (figure 24).
Blood enters the kidneys from the aorta through the renal arteries. The purified
blood returns to the inferior vena cava. In the process of urine formation, two phases
are distinguished: the formation of primary and secondary urine.
On the longitudinal section of the kidney, two layers are clearly distinguishable
– this is the outer cortical and the inner cerebral.
The formation of urine occurs in the nephrons – the structural and functional
units of the kidney. The main part of the nephron is located in the cortical layer. There
are about 1 million nephrons in each kidney.

Figure 24. Kidney Anatomy

Through the tubule system, the resulting urine enters the renal pelvis, located at the
concave edge of the kidney. The renal pelvis at the kidney gate passes into the ureter.
THE PROCESS OF URINE FORMATION
The process of urine formation takes place in two stages. The first takes place in
capsules of the outer layer of the kidneys (renal glomerulus). All the liquid part of the
blood that enters the glomeruli of the kidneys is filtered and enters the capsules. This is
how primary urine is formed, which is practically blood plasma.

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