Through urine, we eliminate residues from cellular work, undesirable substances and the excess of water in the blood. It is a more or less yellowish liquid, whose density and amount depend on each body, its balance, amount of water ingested and activities undertaken.
Generally, a healthy person’s urine is made up of 95% water, which contains dissolved substances the body doesn’t need and discards. Urea stands out (substance fermented in the liver derived from the breaking down of proteins), which reaches 3%, while the remaining 2% are mineral substances like potassium, sodium, chlorine, phosphate and sulfate ions, uric acid and creatinine (a waste of creatine, a muscle element).Daily and under normal conditions, an adult eliminates between 1,200 and 1,500 cm2 of urine.
Formation of urine
The nephron has a vital role in the formation of urinary waste. It is there where the substances are really selected, which, after traveling a sinuous road, will become urine. The filtering of blood begins when the glomerular capillaries found in Bowman’s capsule filter blood for the first time. After this process, blood keeps its cells and some macromolecules (like albumin). The resulting liquid is dubbed primary urine, which has a low concentration.
Later, the remains from the glomerular filtration pass through the proximal convoluted tube, place where nearly 80% of the filtered substances are reabsorbed and continue traveling though our bloodstream. Among them, we point out sodium, water, glucose, amino acids, calcium, phosphate and potassium.
Continuing its cleansing route, we find the nephron loop. It reabsorbs water in its descending portion, while the ascending one is in charge of gathering some ions. The remains of the filtration that reach the distal convoluted tube have already lost all substances vital to the body and only keeps the residues and excess water. Together, these last two components will form urine, which will become more concentrated in the final part of the urinary duct. We must point out that all the elements that are reabsorbed in the different portions of the nephron pass on to the bloodstream, while the liquid that results from filtration will arrive to the inferior pelvis of the kidney and will later be led towards the urinary ways for its evacuation.
Micturition
Micturition is the process through which the urinary bladder empties its content, eliminating all of the useless substances present in urine with it.
As see saw before, the bladder is a storage organ that can only withstand a certain amount of urine before evacuating it. At that moment, a series of mechanisms capable of achieving micturition spring into action. Here, the main parties involved are the muscles of the bladder wall, as well as those housed in the floor of the pelvis.
When the muscle pouch reaches its storage (approximately between 200 and 300 cm3) and stretching threshold, a series of nerve receptors located in its wall send impulses to the spinal cord. This way, in a reflex manner (micturition reflex) return signals are sent back that order the internal sphincter of the bladder (muscle ring located between the bladder and urethra) to relax. At the same time, our brain receives indications that cause the desire to urinate.
Between the sensation and micturition itself, a process we consciously control takes place. Despite the muscles of the bladder wall not being under voluntary control, the ones located under it are, in the floor of the pelvis. Thanks to them we can control the emptying of the urinary bladder, granting a certain lapse of time between the desire we have to urinate and the exact moment of micturition.
Artificial purification
There is an artificial mechanism capable of performing a job similar to the filtering carried out by the kidneys known as dialysis. It is mainly used when there is a chronic or acute renal insufficiency.
It consists on purifying the blood through a machine, making it passs through a semi-permeable membrane housed in a network of tubes and immersed in a special solution called dialyzed (dialysis solution). Once the blood flow begins to transit, said membrane is capable of catching the smaller molecules that are waste substances and urea, among other compounds, which stay in the dialysis solution ready to be eliminated; meanwhile, the larger molecules (like blood cells and plasmatic proteins) are withheld and returned to the blood. Currently, there are two types of dialysis: hemodialysis and peritoneal dialysis, differentiated by the type of membrane used for the collection of waste.
Hemodialysis consists on accessing a blood duct (an intervention is performed to create an areterial-venous fistula, junction between an artery and a vein) around the arm, which will be connected to a machine specially designed for the filtration of blood. It drains it and “washes” it in the dialysis solution, and later, returns it free of waste substances.
Peritoneal dialysis on the other hand, is characterized for using a semi-permeable membrane that belongs to the human body: the peritoneum. This serous layer, that lines the walls of the abdominal and pelvic cavity, is capable of filtering blood; during the procedure, the waste products pass from the capillaries (that line the peritoneal cavity) to the dialysis liquid. A cannula is inserted into the patient in the lower part of the abdomen, through where the liquid that will gather the waste substances is injected.
