What we know as “urine”, the liquid that is excreted to the exterior of the body through the urethra, is not produced only by the kidneys but, as we will see, it is produced by the coordinated action of the urinary system, or urinary tract, as a whole: kidneys, ureters, bladder and urethra.
Improperly, when we speak of “urine” we usually refer without distinction, not only to the urine that comes out through the urethra, but we also call “urine” to the liquid that, once produced by the kidneys, flows into the ureters.
However, it is necessary to take into account that the “urine” that comes out of the kidneys undergoes a further and profound transformation along its journey through the rest of the urinary tract: ureters, urinary bladder and urethra.
We can divide the urine formation process into 3 steps:
First step in the formation of “primary urine”. In this first step, the filtration of the blood plasma by the kidneys takes place.
What is blood plasma? Blood plasma is the liquid fraction of the blood, that is, the blood devoid of cells, cells such as white blood cells and red blood cells.
The plasma filtered in the kidneys by the “renal glomeruli”, is called “primary urine”. The composition of primary urine is very similar to that of blood plasma, except for the larger plasma molecules, which do not pass this first renal filtrate.
The kidneys filter about 1,500 liters of blood per day, which generates about 170 liters of primary urine. The primary urine empties into the “collecting ducts”. The collecting ducts are connected to the blood by surrounding blood capillaries.
A double exchange of substances between urine and blood occurs along the collecting ducts: from the collecting ducts to the bloodstream and from the bloodstream to the collecting ducts.
In the filtering process, re-absorption and secretion of urine on its way through the collecting ducts takes place, primary urine returns most of the filtered water and components to the blood.
The 170 liters of primary urine, produced by the renal glomeruli, undergo a profound transformation in composition and volume of water as they travel through the collecting ducts. This urine, highly concentrated and turned into what we will name “SECONDARY URINE” leaves the kidneys through the ureters.
Second step, formation of secondary urine. Indeed, and as we have just said, primary urine, after its journey through the ducts, leaves the kidneys as SECONDARY URINE.
Approximately one and a half liters of “secondary urine” are generated per day, which represents 1% of the total volume of “primary urine” (170 litres).
From these pages, it is proposed, I believe for the first time in the scientific literature, to name “secondary urine” to the urinary fluid which exits the kidneys.
Third step: the formation of “urine” proper, as is commonly referred to, or “final urine”. Secondary urine undergoes a further transformation on its way through the rest of the urinary tract, ureters, urinary bladder and urethra, to finally come out to the exterior as urine proper.
We reserve the common term “urine”, or “final urine” to the fluid that is finally excreted to the exterior of the body through the urethra, after having journeyed the entire urinary tract: kidneys, ureters, bladder and urethra.
Summing up, we are faced with a fundamental terminological and conceptual distinction: primary urine (elaborated and filtered in the kidneys), secondary urine (the one that comes out of the kidneys), and urine proper, or “final urine” (the one that is excreted to the exterior through the urethra).
I consider important the terminological distinction we have just made in reference to urine. The confusion, or lack of distinction between these terms and concepts can give rise (and possibly will have given rise to it) to various types of diagnostic and treatment errors.
Thus, for example, in a urinalysis, we can generically attribute to the kidney a disease or dysfunction attributable to another section of the urinary tract: the urinary bladder or the urethra.
Epithelia, peptidome and stem cells
What are peptides and how are they different from proteins? Peptides are made up of chains of amino acids. In the scientific terms “peptides” and “proteins” are often used interchangeably.
Molecules made up of chains of up to 50 amino acids are usually called “peptides”. Peptides formed by amino acid chains of over 50 units are called proteins. Simply put, proteins are larger molecules and with higher molecular weight than peptides.
Primary urine, the result of blood plasma filtration by the renal glomeruli, has a very similar composition to that of the blood plasma itself, except for large protein molecules.
Primary urine, in its journey through the collecting ducts, varies its composition, concentrating and exchanging components with the blood. Regarding its peptidome, primary urine is “enriched” with the contributions of exfoliated cells of the epithelia that cover the surfaces of the kidneys, organs with a very complex structure.