BIOCHEMISTRY - Key Regulators and Biological Fluids of the Human Body - 2016

4. CHEMICAL COMPOSITION OF URINE

4.2. Mechanism of Urine Formation

The nephron is the Structural and functional unit of the Kidneys responsible for urine production. Each Kidney contains approximately 1 million nephrons.

Urine Formation in the nephrons occurs in three stages.

Stage 1 - Ultrafiltration of Blood Plasma in the renal glomeruli (Formation of primary urine). Approximately 10% of the blood flowing through the kidneys undergoes ultrafiltration. All blood components are filtered except for Proteins; thus, primary urine is essentially a protein-free blood plasma. A human produces 150 - 180 L of primary urine per day.

Stage 2 - Reabsorption. As primary urine moves along the renal tubules (which have a total length of about 120 km), it returns the majority of its components (almost all glucose, 99% of Water, sodium, chlorine, bicarbonates, Amino Acids, 93% of potassium, 45% of urea, etc.) back into the blood flowing through the capillary network surrounding the renal tubules.

Reabsorption requires significant Energy Expenditure, with ATP serving as the energy source.

Stage 3 - Secretion. During secretion, certain blood substances — potassium, ammonium, and hydrogen ions, as well as foreign substances (drugs, toxins) — are transported from the nephron's capillary network into the lumen of the renal tubules. This process also actively consumes ATP.

Reabsorption and secretion transform primary urine into secondary, or definitive, urine, which is then excreted from the body.