Urine formation process: its 4 stages and characteristics.

Overview of the process of urine formation and how it enables the elimination of substances.

Urine is an essential fluid for the maintenance of homeostasis in the human body. Thanks to the action of the kidneys and the mechanisms of urination in the human being, this fluid allows us to eliminate toxic substances produced by the body itself during metabolism (urea), the ejection of harmful and toxic compounds coming from the outside (drugs and medicines), the maintenance of the electrolyte balance of salts in the bloodstream and an infinite number of other things.

For these reasons and many others, we affirm without fear that the quantity, nature and properties of 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 tract, hematuria (bloody urine) is usually an indication of kidney cancer or a serious infection, and proteinuria (excessive protein in the urine) is an indication of poor kidney function in the patient.

The act of urinating provides medical professionals with a great deal of information, since the waste we produce is a reflection of what is going on inside us. Based on this premise, we ask you the following question: do you know what the process of urine formation is like? If not, don't worry, because here we dissect it for you.

The starting point: the kidneys

Before talking about the formation of urine itself, we must lay a number of basics about the kidneys, because without understanding their structure it is impossible to correctly understand the processes of urination. Let's be quick.

The kidneys are the main organs of the urinary system.The kidneys are the main organs of the urinary system, since with a relatively small bean shape (about 10 cm long) and weighing about 170 grams, an average of 1,500 liters of Blood pass through these tireless organs every day. In order to eliminate 2 liters of waste products and excess water, a kidney needs to purify about 190 liters of blood. This is an astronomical figure, considering that an adult human being contains at most 5.5 liters of blood fluid.

Because of their functionality and physiological demands, the kidneys account for 22% of the individual's cardiac output, i.e., a little more than one fifth of the entire volume of blood expelled by the cardiac ventricle every minute goes to these purifying micro-factories. Thus, the renal blood supply is said to be largely linked to the patient's blood pressure.

The functional complex 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 takes place. This network of capillaries allows the filtration of blood plasma, and 75% of them are located in the renal cortex (external part of the kidney).

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 distinct blocks, which are as follows:

• 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 wider diameter than the efferents and, therefore, the blood leaving the glomerulus creates a characteristic hydrostatic pressure. This glomerular hydrostatic pressure "forces" fluids and small solutes to leave the blood capillaries into the glomerular capsule, while cell bodies and other large molecules remain in the bloodstream. Being a passive process, it requires no energy expenditure.

The result is a freshly filtered fluid containing large amounts of water, electrolytes and organic substances such as glucose, vitamins and amino acids.. This whole process is represented by a value known as the "glomerular filtration rate" (GFR), which is generally around 125 ml/min.

Tubular reabsorption

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

We are not going to focus on the particularities of each section in particular, but we will give a few relevant figures and hints. For example, in the proximal tubule (PCT) all glucose, amino acids and 65% of sodium (Na) and water are reabsorbed into the blood. In the loop of Henle, much 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 entails the expenditure of energy or, alternatively, the use of some type of specific electrochemical gradient.

Tubular secretion

This is the opposite process to reabsorption, since throughout the urine's journey through the tubules and loops, it is also used to excrete harmful substances from the peritubular blood capillaries to the renal tubular lumen.

This diffusion occurs thanks to active transport and passive diffusion, physical processes that we will not dwell on here. Basically, passive diffusion takes place on the basis of a concentration gradient: products pass from an area with a high concentration (blood) to an area with a low concentration (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, it also promotes a secretion of hydrogen ion. As you can see, tubular secretion is a situational mechanism, which depends entirely on the individual physiological state.

4. Storage of urine

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

The bladder is basically a muscular tissue in the form of a sack with 3 layerswhich expand depending on the amount of urine to be stored. A functional bladder can hold up to 1,000 milliliters of urine, although normally the urge to urinate is activated at 400-500 milliliters. Sometimes this muscular sac does not empty completely with urination, a pathological condition known as "urinary retention".

Summary

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

Therefore, when a human being has protein or glucose in the urine, it is usually a sign that something is wrong. The body does not waste useful compounds lightly, so these atypical values usually indicate poor kidney function or that, failing that, some pathological condition causes excess circulating elements (as in the case of diabetes and excess blood sugar). For this reason, health professionals see these parameters as warning signs.

Bibliographic references:

• Moore, L. C., & Marsh, D. J. (1980). How descending limb of Henle's loop permeability affects hypertonic urine formation. American Journal of Physiology-Renal Physiology, 239(1), F57-F71.
• Ogobuiro, I., & Tuma, F. (2019). Physiology, renal. StatPearls [Internet].
• Pickering, G. W., & Prinzmetal, M. (1940). The effect of renin on urine formation. The Journal of physiology, 98(3), 314.
• Richards, A. N. (1938). The Croonian lecture: processes of urine formation. Proceedings of the Royal Society of London. Series B, Biological Sciences, 126(844): pp. 398 - 432.
• Sangre en la orina, Hematuria, Mayo Clinic. Recogido a 17 de marzo en https://www.mayoclinic.org/es-es/diseases-conditions/blood-in-urine/symptoms-causes/syc-20353432#:~:text=La%20sangre%20que%20puedes%20ver,determinar%20la%20causa%20del%20sangrado.
• Tolls, R. E., & Dille, J. M. (1955). The relation between bladder pressure and urine formation. The Journal of urology, 74(2): pp. 197 - 201.