The human body is provided with an average of 2500 milliliters of water. About 150 milliliters appears in the process of metabolism. For a uniform distribution of water in the body, its incoming and outgoing quantity 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 1,500 milliliters. The remaining water is excreted through the lungs (about 500 milliliters), skin (about 400 milliliters) and a small amount leaves with feces.
The mechanism of urine formation is a vital process realized by the kidneys, it consists of three stages: filtration, reabsorption and secretion.
Urine contains water, certain electrolytes, and end products of metabolism in cells. The process of urine formation in the kidneys is carried out by a nephron.
Nephron is a morphofunctional unit of the kidney, providing a mechanism for urination and excretion. In its structure there are a glomerulus, a system of tubules, a Bowman capsule.
In this article, we will consider the process of urine formation.
Renal 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, kidneys by weight account for 0.43% of body 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 tissue, the blood flow for the kidney is 430 milliliters per minute, the coronary system - 660, the brain - 53). What is primary and secondary urine? About it further.
An important characteristic of renal blood supply is that the blood flow in them remains unchanged with changing blood pressure more than 2 times. Since the arteries of the kidneys extend from the peritoneal aorta, 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 kidney 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 recess of the capsule of the renal body.
Filtration becomes possible due to a number of features that are associated with anatomy:
- flattened endothelial cells, at the edges they are especially thin and have pores through which protein molecules cannot pass due to their large size;
- the inner wall of the Shumlyansky-Bowman reservoir is formed by flattened epithelial cells, which also prevent large molecules from passing.
Where is secondary urine formed? About it below.
What contributes to this?
The main force that provides the ability to filter in the kidneys are:
- high pressure in the renal artery;
- not the same diameter of the bringing arterioles of the renal body and the carrying.
The pressure in the capillaries is about 60-70 millimeters of mercury, and in the capillaries of other tissues it is equal to 15 millimeters of mercury. Filtered plasma easily fills the nephron capsule, since it has low pressure - about 30 millimeters of mercury. Primary and secondary urine is a unique occurrence.
Water and substances dissolved in plasma are filtered from capillaries into the cavity of the capsule, with the exception of large molecular compounds. Inorganic salts, as well as organic compounds (uric acid, urea, amino acids, glucose), enter the cavity of the capsule without resistance. High-molecular proteins normally do not go into its deepening and remain in the blood. The liquid that was 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 return absorption (reabsorption), which occurs in the convoluted channels and the loop of Henle. The process takes place in a passive form on the principle of push and diffusion, and in the active, through the cells of the nephron wall itself. 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 reverse absorption, an active secretion process takes place in the channels of the nephron, that is, the release of substances from the blood that is performed by the cells of the walls of the nephron. During secretion, creatinine is released into the urine from the blood, as well as therapeutic substances.
During the process of reverse absorption and excretion, secondary urine is formed, which is quite different from primary urine in its composition. Secondary urine has a high concentration of uric acid, urea, magnesium, chlorine, potassium, sodium, sulfates, phosphates, creatinine. About 95 percent of the secondary urine is water, in the solids of the remaining substances only five percent. About a liter and a half of secondary urine is formed per day. The kidneys and bladder experience a lot of stress.
Urination regulation
The work of the kidneys is self-regulating, as they are an extremely important organ. The kidneys are equipped with a large number of fibers of the sympathetic nervous system and parasympathetic (endings of the vagus nerve). With irritations of the sympathetic nerves, the amount of blood coming to the kidneys drops and the pressure in the glomeruli goes down, and the consequence 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, this leads to increased urination. Also, with the absolute intersection of all the nerves that approach 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 the blood flow in the kidneys, as well as the processes associated with filtration and absorption.
What hormones regulate this?
A number of hormones regulate the functioning of the kidneys:
- vasopressin, which produces the hypothalamus brain region, enhances the return absorption of water in the channels of the nephrons;
- 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 formation.