Urine formation process: its 4 stages, and characteristics

Urine is an essential fluid for the maintenance of homeostasis in the human body. Thanks to the action of the kidneys and the urination mechanisms in humans, this fluid allows us to eliminate toxic substances produced by the body itself during metabolism (urea), the ejection of harmful and toxic compounds from abroad (drugs and medicines), the maintenance of the electrolytic balance of salts in the bloodstream and an infinity of things further.

For these reasons and many others, we fearlessly affirm that the quantity, nature and properties of the urine can tell a lot about the patient's state of health. For example, anuria (total lack of urination) may be due to severe obstructions of the urinary system, hematuria (bloody urine) is usually an indication of a kidney cancer or a serious infection and, for example, proteinuria (excessive presence of protein in the urine) will be indicative of poor kidney function in the patient.

The act of urination provides a lot of information to medical professionals, since the waste we produce is a reflection of what happens inside us. Based on this premise, we ask you the following question:

Do you know how the urine formation process is? If not, don't worry, because here we dissect it for you.

• Related article: "Excretory system: characteristics, parts and operation"

The starting point: the kidneys

Before talking about the formation of urine itself, we must establish a series of bases about the kidneys, for without understanding their structure it is impossible to correctly understand the processes of urination. We will be fast.

The kidneys are the main organs of the urinary system., since with a relatively small bean shape (about 10 cm long) and about 170 grams in weight, an average of 1,500 liters of blood pass through these tireless organs per day. Without going any further, to eliminate 2 liters of waste products and excess water, it is necessary for a kidney to purify about 190 liters of blood. We move in astronomical figures, taking into account that an adult human being contains within it, at most, 5.5 liters of blood fluid.

Due to its functionality and physiological demands, the kidneys account for 22% of the individual's cardiac output, that is, a little more. one fifth of all the volume of blood ejected by the cardiac ventricle every minute ends up in these microfactories purifying. Therefore, it is said that the renal blood supply is largely linked to the patient's blood pressure.

The complex functional unit of the kidney is the nephron.. In each of these organs there are approximately one million of them, which in turn contain the glomeruli, the exact places where blood purification occurs. This network of capillaries allows the filtration of blood plasma, and 75% of them are found in the renal cortex (outer part of the kidney).

• You may be interested in: "The 4 most important parts of the kidney, and their functions"

The process of urine formation

Once we have put into perspective the figures related to urination and the generalities of the kidneys, we are ready to explain the process of urine formation. We will divide the explanation into 4 different blocks, which are the following:

• glomerular filtration.
• tubular reabsorption.
• tubular secretion.
• Urine storage.

1. glomerular filtration

Glomerular filtration is the first step in the formation of urine, and it should be noted that it is a passive process in which hydrostatic pressure pushes fluids and solutes across the relevant membrane. This exchange takes place in the semi-permeable walls of the glomeruli, which in turn are surrounded by an external envelope called "Bowman's capsule".

The arterioles (very small arterial branches) that reach the glomeruli (afferents) have a diameter larger than wider than the efferents and therefore the blood leaving the glomerulus creates a characteristic hydrostatic pressure. This glomerular hydrostatic pressure “forces” fluids and small solutes out of the blood capillaries. into the glomerular capsule, while cell bodies and other large molecules remain in the torrent sanguine. Being a passive process, it does not require energy expenditure.

The result is a freshly filtered liquid that contains large amounts of water, electrolytes, and organic substances, such as glucose, vitamins, and amino acids.. This entire process is represented by a value known as “glomerular filtration rate” (GFR), which generally ranges from 125 ml/min.

2. tubular reabsorption

The problem with this process, as you can imagine, is that a not inconsiderable amount of useful substances “sneaks” into the liquids that will later be excreted. For this reason, the nephron has 4 different tubes, through which the "proto-urine" passes, which has been collected by Bowman's capsule (where the glomerulus is located) in the previous section. These are the proximal tubule, the loop of Henle, the distal tubule, and the collecting duct.

We are not going to focus on the particularities of each specific section, but we will give a few figures and relevant brushstrokes. For example, in the proximal tubule (PCT) all glucose, amino acids, and 65% sodium (Na) and water are reabsorbed into the blood. In the loop of Henle, a lot of water, sodium, and chlorides are also reabsorbed, to the point that only 20% of what was originally filtered reaches the distal tubule..

It should be noted that many of the substances reabsorbed at this point must be actively transported, which that entails the expenditure of energy or, failing that, the use of some type of electrochemical gradient specific.

3. tubular secretion

It is the opposite process to reabsorption, because during the entire journey of urine through the tubules and loops, also used to excrete noxious substances from the peritubular blood capillaries into the tubular lumen renal.

This diffusion happens thanks to active transport and passive diffusion, physical processes in which we are not going to dwell too much. Basically, passive diffusion is done based on a concentration gradient: the products pass from an area with a high concentration (blood) to another with little (urine).

For example, tubular secretion is responsible for the disposal of excess potassium in the blood when necessary (hyperkalemia), an action that is mediated by the hormone aldosterone. When the blood pH falls below the normal range, a hydrogen ion secretion is also encouraged. As you can see, tubular secretion is a situational mechanism, which depends entirely on the individual physiological state.

4. urine storage

Once the urine has formed, a series of collecting ducts, papillary ducts and calyces collect the liquid and gather it to a common outlet point, as if it were the branches and trunk of a tree. Finally, the urine that we all know reaches the ureters, where it is transported to the bladder.

The bladder is basically a sac-shaped muscle tissue with 3 layers., which distend depending on the amount of urine that must be stored. A functional bladder can hold up to 1,000 milliliters of urine, although normally the desire to urinate is activated at 400-500 milliliters. Sometimes this muscular sac is not completely emptied with urination, a condition known as "urinary retention."

Summary

At the end of this dizzying process, Humans excrete a fluid consisting of 95% water, 2% mineral salts, and 3% urea and uric acid.. It's not a perfect mechanism, but it certainly allows us to systemically reabsorb a large amount of organic and inorganic compounds useful for the body that should not be lost in the urination process.

Therefore, when a human being presents protein or glucose in the urine, it is usually an indication that something is wrong. Helpful compounds are not wasted lightly by the body, so these outliers often indicate poor kidney function or, failing that, some pathological picture causes excess circulating elements (as is the case of diabetes and excess sugar in blood). For this reason, healthcare professionals view these parameters as red flags.

Bibliographic references:

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• Blood in the urine, Hematuria, Mayo Clinic. Collected on March 17 in https://www.mayoclinic.org/es-es/diseases-conditions/blood-in-urine/symptoms-causes/syc-20353432#:~:text=La%20sangre%20que%20puedes%20ver, determine%20the%20cause%20of%20bleeding.
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