How is urine produced?

The formation of urine is a process that takes place in the kidneys, which do the main work of the urinary system activity. Their functions are¹:

• regulating the ionic composition of the blood
• regulating blood pH
• controlling blood pressure
• regulating blood osmosis
• producing hormones (calcitriol and erythropoietin)
• regulating blood glucose
• excreting substances through urine

Therefore, we can affirm that the kidneys have a role that goes beyond the excretion of substances and wastes, as is commonly known, since they are responsible for the maintenance of intracellular fluid, electrolyte balance and acid-base balance.

Moreover, renal physiology is directly associated to the excretory apparatus, which is designed to maintain a unidirectional flow. This flow causes urine, which begins its formation in the kidneys, to pass through the ureters to the urinary bladder for storage, so that it can subsequently be eliminated through the urethra.

Urine production starts with filtration, water and most of the plasma solutes pass through the wall of the glomerular capillaries (filtering process), then enter Bowman’s capsule and at last the renal tubule. As the filtered fluid flows along the renal tubules and collecting tubules, tubular cells perform tubular reabsorption, in which about 99% of the filtered water and various useful solutes are reabsorbed. Water and solutes return to the blood as it flows through the capillaries. Inversely, as the filtered fluid flows through the renal tubules and collecting tubules, tubular cells perform tubular secretion of substances such as wastes, drugs, and ionic compounds present in excessive concentrations².

Bibliography

1. Tortora GJ, Bryan D. Principios de anatomía y fisiología, 13va Edición. 13th ed. Editorial Medica Panamericana Sa, editor. México D.F.: Editorial Medica Panamericana Sa; 2013. 1067–1099 p.
2. Carracedo J, Ramírez R. Fisiología Renal [Internet]. 2020. Available from: http://www.nefrologiaaldia.org.es