Secondary urine is formed as a result of the filtration process. Primary and secondary urine: what it is, composition and stages of formation. Additional factors affecting urine status
The mechanism of final urine formation. Primary urine from the capsule enters the renal tubules. As it passes through the tubules, reabsorption occurs, i.e. reabsorption into the blood of glucose, amino acids, vitamins, most salts and water. In this case, from 150 liters of primary urine, 1.5 liters of final urine are formed. The absorption process is a complex physiological process. It occurs due to the expenditure of chemical energy by tubular epithelial cells and is called active transport. At the same time, a large amount of oxygen is consumed in the kidneys, which indicates a high metabolism. Oxygen consumption by the kidneys averages 1/11 of the body's total oxygen consumption, although the kidneys account for only 1/12 of the body's weight: Consequently, a very large amount of energy is expended to form urine.
Primary urine, flowing through the system of tubules and collecting ducts, is concentrated. A large amount of water and all the substances necessary for the body are reabsorbed. The epithelium of the renal tubules is characterized by the ability to selectively reabsorb. Thus, substances necessary for the body: glucose, amino acids, table salt and other salts can be absorbed entirely if the body needs them. If they are in excess in the blood, such as glucose in diabetes or after taking a large amount of sugar, then some of the glucose is excreted in the urine. If there is a lack of table salt in food, its excretion in the urine almost stops. Thus, the kidneys finely regulate the content of substances necessary for the body in the blood, protecting valuable substances that the body lacks and removing unnecessary ones.
Some end products of metabolism: creatine, sulfates are not absorbed at all by the tubular epithelium, urea is poorly absorbed. They are contained in the final urine in high concentrations and are removed from the body. Thus, there is 67 times more urea in the final urine than in the blood, 75 times more creatinine, and 90 times more sulfates.
The epithelium of the tubules has not only an absorptive function, but also a secretory one. Thanks to the secretory function of the tubules, substances that do not pass through the renal filter in the glomeruli are removed from the blood. These include some paints, diodrast, and many medications, such as penicillin.
Urine and its properties
Urine and its properties. Urine is a clear, light yellow liquid. It contains 95% water and 5% solids. Its main components are urea (2%), uric acid (0.05%) and creatinine (0.075%). Urine contains various sodium and potassium salts. During the day, 25-30 g of urea and 15-25 g of inorganic salts are excreted in the urine. Urine has a relative density of 1.010-1.020.
Its reaction can be slightly acidic, neutral or alkaline and depends on the type of food taken. When eating meat food, it is slightly acidic or neutral, while vegetable food is slightly alkaline.
Regulation of water and salt balance occurs through the neurohumoral pathway. With severe painful stimulation, diuresis decreases and even stops.
Urine excretion
Excretion of urine. Urine formed in the kidneys is directed from the renal pelvis to the ureters, which, through peristaltic movements, move it drop by drop to the bladder. The bladder, free of urine, is in a contracted state. As it fills, it stretches. No urine comes out V urethra, since there are sphincters of the urethra (internal And outer). The bladder is greatly stretched, but the pressure in it increases slightly. In humans, with the accumulation of 250-300 ml of urine and a pressure of approximately 12-15 cm of water. Art., a feeling arises, characterized as the urge to urinate. The involuntary center of urination is located in the lumbosacral part of the spinal cord. Emptying the bladder occurs reflexively. Impulses from the bladder receptors are sent to the micturition center in the spinal cord, and from it along the parasympathetic pelvic nerve to the bladder muscles, causing them to contract and at the same time relax the sphincter.
Voluntary urinary retention is absent in newborns. It appears only at the end of the 1st year of life and becomes stronger by the age of 2, when the conditioned reflex of urinary retention is developed. The higher cortical centers that regulate urination are located in the frontal lobes of the cerebral hemispheres. As a result of upbringing, the child develops a conditioned reflex delayed urge and a conditioned situational reflex: urination when certain conditions for its implementation appear.
It is needed to excrete urine, and in men, seminal fluid is excreted through it.
Woman's urethra is a short tube 3-6 cm long, which is located behind the pubic symphysis. Wall myocytes form two layers: internal longitudinal and more pronounced outer - circular. The external opening is located in the vestibule of the vagina, in front and above the entrance to the vagina and is surrounded by a striated external urethral sphincter.
Male urethra - a curved tube 18-23 cm long. Urine and seminal fluid come out through it. The canal passes through the prostate gland, the urogenital diaphragm and the corpus spongiosum of the penis.
Based on their location, in the male urethra there are three parts– prostatic, membranous, spongy.
Prostatic part– length 2.5 cm. This is the widest part of the male urethra. On its back wall there is an elevation - seed mound there is a hole on it prostatic uterus and holes ejaculatory ducts. On the sides of the mound are the openings of the excretory ducts of the prostate gland.
Membranous part- length 1 cm - this is the narrowest part of the male urethra, surrounded by the sphincter.
Spongy part– length 15-20cm, has two extensions:
a) in the bulb of the corpus spongiosum;
b) in the scaphoid fossa of the glans penis.
The ducts of the bulbourethral glands open into the spongy part.
The male urethra has two curvatures– front and back
The front straightens when the penis is raised, the back remains fixed
Curvatures, narrowings and expansions of the male urethra are taken into account when inserting a catheter.
Mechanisms of formation and excretion of urine.
During the day, a person consumes approximately 2.5 liters of water, including 1.5 liters. in liquid form and about 650 ml with solid food. In addition, about 400 ml of water is formed in the body during the breakdown of proteins, fats and carbohydrates. Water is excreted from the body mainly through the kidneys - 1.5 liters per day.
Urine formation in the kidneys
The formation of urine in the nephrons of the kidneys is isolated two phases :
First phase - glomerular ultrafiltration - this is education primary urine in the glomeruli of nephrons. In the renal glomeruli, water and substances dissolved in it are filtered from the renal capillaries of the nephrons in the first phase. Ultrafiltration occurs due to the difference in pressure in the capillaries of the glomeruli and in the nephron capsule. In the capillaries of the glomeruli there is very high blood pressure - 60-70 mm Hg. Art. The creation of high pressure in the capillaries of the renal glomeruli is facilitated by a noticeable difference in the diameter of the vessels that bring blood to the glomeruli and carry blood away from them. The afferent arterioles of the glomeruli have a diameter 2 times larger than the efferent arterioles. Thus, the capillary network of the glomerulus is located between two arterial vessels.
More than 1 liter of blood flows through the kidneys within 1 minute. During the day, up to 1700-1800 ml of blood passes through the kidneys. Thus, in 24 hours all the blood flows through the capillaries of the glomeruli more than 200 times. This blood comes into contact with the inner surface of the capillaries, the area of which in the glomeruli of the kidneys is 1.5-2 m 2. The amount of primary urine produced reaches 150-180 liters per day. Thus, from 10 liters of blood flowing through the kidneys, 1 liter of primary urine is filtered. Primary urine contains all components of blood plasma, except high molecular weight proteins. Primary urine contains amino acids, glucose, vitamins and salts, blood cells, as well as small amounts of urea, uric acid and other substances.
Second phase - tubular reabsorption (reabsorption) , occurs in the nephron tubules. As a result, a concentrated, so-called, secondary (final) urine. In the second phase, primary urine, similar in structure to blood plasma, enters the nephron tubules from the glomerular capsules. In the tubules, amino acids, glucose, vitamins, and most of the water and salts are absorbed into the blood from primary urine. Ultimately, during the day from 150-180 liters. Primary urine produces up to 1.5 liters. secondary urine. Secondary urine enters the bladder through the urinary tract and is excreted from the body. 99% of the water contained in primary urine is absorbed in the tubules. Secondary urine lacks glucose, amino acids, many salts, proteins, and blood cells. At the same time, in secondary urine the concentration of sulfates, phosphates, urea and uric acid is sharply increased.
Selection - the final stage of metabolism, ensuring the constancy of blood. With. O. by removing metabolic products, excess water and salts from the body. The excretion function is carried out by the kidneys, gastrointestinal tract, skin, lungs, liver, salivary glands. There are relationships between them. But the main role in the processes of excretion belongs to the urinary system, which is represented by the kidneys, ureters, bladder and urethra.
Kidneys – paired bean-shaped organ; they are located on both sides of the spine in the upper part of the abdominal cavity. During the process of urine formation, the kidneys
− free the body from unnecessary substances; − maintain consistency c. With. O.;
− perform a humoral function (renin);
− participate in the regulation of blood volume, lymph, tissue fluid; in the metabolism of proteins and carbohydrates.
After the function of both kidneys is turned off, a condition called uremia. The concentration of nitrogen metabolic products in the blood increases, weakness develops, respiratory distress and death occur.
On a section in the kidney, two layers are distinguished: the outer - cortical and the inner - medulla. In the medulla, pyramids are clearly visible, the tops of which are directed towards the center of the kidney, where the renal pelvis is located. At the tops of the pyramids, the lumens of the renal ducts open, through which urine flows into the renal pelvis.
The structural and functional unit of the kidney is nephron; there are about 1 million of them in each kidney. The nephron is formed by a vascular glomerulus, surrounded by a capsule that passes into a system of convoluted tubules. The afferent arterial vessel enters the cavity of the capsule; it branches into capillaries that form a glomerulus. Its capillaries merge into the efferent artery, the lumen of which is smaller than that of the afferent artery, so the blood pressure in the glomerulus is increased. This facilitates the filtration of blood plasma from the capillaries into the capsule cavity. The efferent artery weaves around the renal tubules. Thus, in the kidney, the blood passes through a double system of tubules and then enters the renal vein.
Blood plasma filtered into a capsule contains a large amount of water, salts, and some organic substances and is primary urine (150-180 l), which, passing through the renal tubular system, is gradually converted into secondary . In this case, by reabsorption, amino acids, glucose, K + ions, Na + and most of the water are absorbed back into the blood. Urine becomes more concentrated, saturated with urea and other harmful metabolic products. Approximately 1.5 liters of secondary urine are produced per day. From the tubules it is collected into the renal pelvis, then through the ureters into the bladder.
Compared to adults, children have a limited ability to concentrate urine. The amount of urine per 1m2 in children is greater than in adults due to intensive metabolism and more water in food. The amount of urine is affected by temperature, air humidity, clothing, and the child’s mobility.
In children, the bladder is located higher than in adults; only by the age of 2 does it descend into the pelvic cavity. During the first year of life, children experience involuntary urination. But as the nervous regulatory mechanisms and education mature, it becomes arbitrary.
Kidney activity is regulated by neurohumoral factors.
Some children may experience bedwetting - enuresis. Causes: insufficient maturation of the urination center in the brain, increased inhibition of its neurons, inflammatory processes of the kidneys and bladder, neuroses, spicy food, large amounts of liquid drunk before bed, mental trauma, physical fatigue, hypothermia, sleep disturbance. More often these disorders appear in boys, but by the age of 8 they disappear. Children with enuresis should be examined by doctors - a urologist and a neurologist.
Urine is produced continuously. This process is in two stages: first, primary urine is formed, and then secondary, or concentrated, urine is formed. Formation of primary urine. Unlike ordinary capillaries, glomerular capillaries allow fluid to flow in only one direction—outward.
The diameter of the drive arteriole is larger than the outlet arteriole, so high pressure is created in the capillaries of the glomerulus. Under pressure, a liquid similar to blood plasma is filtered through the walls of the capillaries and the renal capsule in its cavity. It lacks only the formed elements of blood and large protein molecules that do not pass through the pores of the walls of the glomerular capillaries. This liquid is called primary urine, and the process of its formation is called filtration. During the day, 170-180 liters of primary urine are formed in the kidneys.
From the renal capsule, primary urine enters the first convoluted tubule. This is where reabsorption begins - substances vital to the body are returned from primary urine to the capillaries. Unlike filtration, the reabsorption of some substances occurs with significant energy expenditure: the movement of certain molecules and ions from the nephron to the capillaries requires the participation of carrier molecules (see § 31).
From the first tubule, part of the water, glucose, amino acids returns to the capillaries, from the nephron loop - water, sodium, chlorine ions, etc.. To the second tubule, substances from the blood are actively transported: potassium ions, hydrogen ions, drug components, etc. .d.. This process is called secretion. Secondary urine enters the cleaning tube of the nephron. The result of what is happening in the non-Fron itself is the formation of secondary urine and the restoration of blood homeostasis. Normally, there are no red blood cells, proteins, amino acids, or glucose in the urine. In general, uric acid, drugs, heavy metal ions, and partially urea do not turn from primary urine into the blood.
Analysis of urine
Analysis of urine, done in clinics, gives the doctor important information about the functioning of the kidneys, and about the metabolic processes occurring in other organs.
The role of the kidneys in maintaining water-salt balance in the body. An important indicator of homeostasis is the concentration of ions in the blood, in particular sodium and chlorine. How is it maintained constant, since we sometimes eat very salty foods?
This process is regulated by the nervous system and humoral. An increase in salt concentration in the blood is recorded by neurons, most of which are located in the brain (in the hypothalamus). In response to their irritation, the hypothalamus produces an antidiuretic hormone (vasopressin), which acts on the nephron, increasing water reabsorption. The concentration of sodium and chlorine ions in the blood decreases, and homeostasis is restored. Accordingly, the concentration of salt in the urine increases, the body gets rid of it
The human body is provided with an average of 2500 milliliters of water. About 150 milliliters appears during metabolism. For uniform distribution of water in the body, its incoming and outgoing amounts must correspond to each other.
The kidneys play the main role in removing water. Diuresis (urination) per day is on average 1500 milliliters. The rest of the water is excreted through the lungs (about 500 milliliters), the 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.
The nephron is the morphofunctional unit of the kidney, providing the mechanism of urine formation and excretion. Its structure contains a glomerulus, a system of tubules, and Bowman's capsule.
In this article we will look at the process of urine formation.
Blood supply to the kidneys
About 1.2 liters of blood passes through the kidneys every minute, which is equal to 25% of all blood entering the aorta. In humans, the kidneys 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 of the heart - 660, the brain - 53). What is primary and secondary urine? More on this later.
An important characteristic of the renal blood supply is that the blood flow in them remains unchanged when the blood pressure changes by more than 2 times. Since the arteries of the kidneys arise from the aorta of the peritoneum, there is always a high level of pressure in them.
Primary urine and its formation (glomerular filtration)
The first stage of urine formation in the kidneys begins with the process of filtration of 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 is made possible due to a number of features that are associated with 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 do not allow large molecules to pass through.
Where is secondary urine formed? More on this below.
What contributes to this?
The main forces that provide the ability to filter in the kidneys are:
- high pressure in the renal artery;
- the diameter of the afferent arteriole of the renal body and the efferent arteriole are not the same.
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 are a unique phenomenon.
Water and substances dissolved in the plasma, with the exception of large molecular compounds, are filtered from the capillaries into the recess of the capsule. Salts classified as 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 recess and remain in the blood. The liquid that has filtered into the recess of the capsule is called primary urine. Human kidneys produce 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 ducts and 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 all important and vital substances to the blood in the required quantity and remove the final elements of metabolism, foreign and toxic substances.
The third stage is secretion. In addition to reverse absorption, an active secretion process takes place in the nephron channels, that is, the release of substances from the blood, which is carried out by the cells of the nephron walls. During secretion, creatinine and therapeutic substances are released from the blood into the urine.
During the ongoing process of reabsorption and excretion, secondary urine is formed, which is quite different from primary urine in its composition. Secondary urine contains a high concentration of uric acid, urea, magnesium, chlorine ions, potassium, sodium, sulfates, phosphates, and creatinine. About 95 percent of secondary urine is water, the remaining substances are only five percent. About one and a half liters of secondary urine are produced per day. The kidneys and bladder experience greater stress.
Regulation of urine formation
The work of the kidneys is 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 (vagus nerve endings). When the sympathetic nerves are irritated, the amount of blood flowing to the kidneys decreases 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 during painful stimulation due to sharp vascular contraction.
When the vagus nerve is irritated, it leads to increased urine production. Also, with the absolute intersection of all the nerves that approach the kidney, it continues to function 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 processes associated with filtration and absorption.
What hormones regulate this?
A number of hormones regulate kidney function:
- vasopressin, which is produced by the hypothalamus, enhances the reabsorption of water in the nephron canals;
- 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, increases urine formation;
- adrenaline is produced by the adrenal glands and causes a decrease in urine production.
