The human body is provided with an average of 2500 milliliters of water. About 150 milliliters appears in the process of metabolism. For an even distribution of water in the body, its incoming and outgoing amount must correspond to each other.
The main role in the removal of water is played by the kidneys. Diuresis (urination) per day is an average of 1500 milliliters. The rest of the water is excreted through the lungs (about 500 milliliters), skin (about 400 milliliters) and a small amount goes through the feces.
The mechanism of urine formation is a vital process carried out by the kidneys, it consists of three stages: filtration, reabsorption and secretion.
Urine contains water, certain electrolytes, and the metabolic end products of cells. The process of urine formation in the kidneys is carried out by the nephron.
Nefron is a morphofunctional unit of the kidney, providing a mechanism for urination and excretion. Its structure contains the glomerulus, tubule system, Bowman's capsule.
In this article, we will look at the process of urine formation.
Kidney blood supply
Every minute, about 1.2 liters of blood passes through the kidneys, which is equal to 25% of all blood entering the aorta. In humans, the kidneys make up 0.43% of body weight by weight. From this we can conclude that the blood supply to the kidneys is at a high level (as a comparison: in terms of 100 g of tissues, the blood flow for the kidney is 430 milliliters per minute, for the coronary system of the heart - 660, for the brain - 53). What is primary and secondary urine? More on that later.
An important characteristic of the renal blood supply is that the blood flow in them remains unchanged when the arterial pressure changes by more than 2 times. Since the arteries of the kidneys depart from the aorta of the peritoneum, they always have a high level of pressure.
Primary urine and its formation (glomerular filtration)
The first step in the formation of urine in the kidneys originates from the process of filtering blood plasma, which occurs in the renal glomeruli. The liquid part of the blood follows through the wall of the capillaries into the deepening of the capsule of the renal body.
Filtering is made possible by a number of features that are related to anatomy:
- flattened endothelial cells, they are especially thin at the edges and have pores through which protein molecules cannot pass due to their large size;
- The inner wall of the Shumlyansky-Bowman container is formed by flattened epithelial cells, which also prevent large molecules from passing through.
Where is secondary urine formed? More on that below.
What is thiscontributes?
The main force that enables filtration in the kidneys are:
- high pressure in the renal artery;
- not the same diameter of the afferent and efferent arterioles of the renal body.
The pressure in the capillaries is about 60-70 millimeters of mercury, and in the capillaries of other tissues it is 15 millimeters of mercury. The filtered plasma easily fills the nephron capsule, as it has low pressure - about 30 millimeters of mercury. Primary and secondary urine is a unique phenomenon.
Water and substances dissolved in the plasma are filtered from the capillaries into the deepening of the capsule, with the exception of large molecular compounds. S alts related to inorganic, as well as organic compounds (uric acid, urea, amino acids, glucose), enter the capsule cavity without resistance. High-molecular proteins normally do not go into its deepening and are stored in the blood. The fluid that has filtered into the recess of the capsule is called primary urine. Human kidneys form 150-180 liters of primary urine during the day.
Secondary urine and its formation
The second stage of urine formation is called reabsorption (reabsorption), which occurs in the convoluted canals and the loop of Henle. The process takes place in a passive form, according to the principle of push and diffusion, and in an active form, through the cells of the nephron wall themselves. The purpose of this action is to return to the blood all the important and vital substances in the right amount.and remove the final elements of metabolism, foreign and toxic substances.
But where is secondary urine formed?
The third step is secretion. In addition to reabsorption, an active secretion process takes place in the nephron channels, that is, the release of substances from the blood, which is performed by the cells of the nephron walls. During secretion, creatinine, as well as therapeutic substances, goes into the urine from the blood.
During the ongoing process of reabsorption and excretion, secondary urine is formed, which is quite different from the primary urine in its composition. In the secondary urine, a high concentration of uric acid, urea, magnesium, chloride ions, potassium, sodium, sulfates, phosphates, creatinine. About 95 percent of secondary urine is water, the dry residue of other substances is only five percent. About one and a half liters of secondary urine is formed per day. The kidneys and bladder are under great strain.
Regulation of urination
The kidneys are self-regulating, as they are an extremely important organ. The kidneys are supplied with a large number of fibers of the sympathetic nervous system and parasympathetic (endings of the vagus nerve). With irritation of the sympathetic nerves, the amount of blood coming to the kidneys decreases and the pressure in the glomeruli goes down, and the result of this is a slowdown in the process of urine formation. It becomes scarce with painful irritations due to a sharp vascular contraction.
When the vagus nerve is irritated, it leads to increased urination. Also with absolutecrossing all the nerves that come to the kidney, it continues to work normally, which indicates a high ability for self-regulation. This is manifested in the production of active substances - erythropoietin, renin, prostaglandins. These elements control blood flow in the kidneys, as well as the processes associated with filtration and absorption.
What hormones regulate this?
A number of hormones regulate kidney function:
- vasopressin, which is produced by the hypothalamus region of the brain, enhances the return absorption of water in the nephron channels;
- aldosterone, which is a hormone of the adrenal cortex, is responsible for enhancing the absorption of Na+ and K+; ions;
- thyroxine, which is a thyroid hormone, enhances urination;
- adrenaline is produced by the adrenal glands and causes a decrease in urine production.
