Secondary urine is a fluid formed in the kidneys after removal from the primary urine of excess water, mineral salts and organic matter valuable to the body. It is the secondary urine that is collected into the ureters , then into the bladder and is released into the environment.
The volume of secondary urine in the human body is 1-1.5 liters per day. Secondary urine consists of water, salts, urea, uric acid, ammonia. Phosphates and sulphates are also present there. The reabsorption of substances from primary urine depends on their concentration at a given time. For example, if the blood glucose is in excess (more than 0.15-0.18%), then part of the glucose from the primary urine is not absorbed back, but is excreted with the urine. On the contrary, for example, with a lack of salt in food, its elimination with urine almost stops. For each substance, there is a certain limit of their concentration in the blood, above which they cannot be completely reabsorbed in the tubules. This limit is called the elimination threshold. However, some substances are non-threshold, as they are not reabsorbed even with very low blood levels. These include urea, sulfates, creatinine.
Urine formation
Primary urine formation
Due to the fact that the blood pressure in the capillaries of the glomeruli of the renal corpuscle is high (approximately 70 mmHg), the constituents of the blood are filtered through the single-layer cells of these capillaries. They penetrate the slit-like cavity located between the two layers of the capsule. This forms the primary urine. Studies have shown that its composition is very close to the composition of the blood plasma. Primary urine contains approximately 0.1% glucose, 0.3% sodium ions, 0.37% chlorine ions, 0.02% potassium ions, 0.03% urea. However, not all substances that make up the blood plasma are able to penetrate through the walls of capillaries into the glomerulus capsules. Thus, proteins, fats and glycogen in the blood plasma 7-9%, and in the primary urine they are not at all. This is due to the fact that the molecules of these substances are large and can not penetrate through the wall of capillaries and capsules.
Secondary urine formation
Primary urine flows from the capsules to the renal tubules. Here the most important process of the reverse suction of a number of constituent parts of the primary urine in the blood flowing through the capillary network intertwining these tubules takes place. This occurs contrary to the laws of diffusion. So, despite the equality of glucose concentration in the primary urine and blood plasma, this monosaccharide completely passes from the renal tubules into the blood. A similar pattern is observed during the reverse absorption into the blood of other components of the primary urine. Reabsorption occurs due to the vigorous activity of the epithelial cells that form the renal tubules.
As a result, from 150-170 l. during the course of the day, primary urine produces only about 1.5 liters. secondary urine.
The composition of urine is subject to fluctuations depending on which substances are in excess blood plasma. It occurs under the influence of nervous and humoral mechanisms. So, when the concentration of salts in the blood plasma increases, its osmotic pressure increases. It irritates special receptors called osmoreceptors. Under the influence of osmoreceptor irritation, the secretion of one of the pituitary hormones - antidiuretic hormone - is enhanced . Bringing blood to the kidney, this hormone enhances the reabsorption of water from the primary urine. And the secondary urine becomes more concentrated, so that a lot of salt is removed from the body with a small loss of water. And when the water content in the blood increases, for example, due to excessive drinking, the amount of antidiuretic hormone decreases, which leads to a decrease in the reabsorption of water from the primary urine. Then the secondary urine becomes less concentrated and the body gets rid of excess water.
Removal of urine from the kidneys
The urine formed in the kidneys from the renal cups enters the ureters. In the ureters, urine is dripped into the bladder, where it accumulates before filling the bladder. Emptying the bladder is a reflex. When urine accumulates in the urinary bladder in an amount up to 250-300 ml. accumulated urine begins to put pressure on the walls of the bladder and urge to urinate. The nerve impulses that have arisen in the receptors of the walls of the bladder are sent to the center of urination, located in the sacral spinal cord. Higher centers are also located in the frontal lobes of the cerebral hemispheres, and they also regulate urination.
In the newborn, the ureters have a tortuous course . The length of the ureter reaches 5-7 cm. By 4 years, its length increases to 15 cm. The muscular membrane in early childhood is poorly developed.
The bladder in newborns is spindle-shaped, in children of the first years of life - pear-shaped. In the period of the second childhood (8-12 years), the bladder is ovoid. The bladder capacity in newborns is 50-80 ml, by 5 years 180 ml, and after 12 years - 250 ml. In the newborn, the circular muscular layer in the walls of the bladder is weakly expressed, the mucous membrane is well developed. The top of the bladder in newborns reaches half the distance between the navel and the pubic symphysis. At the age of 1-3 years, the bottom of the bladder is located at the level of the upper pubic symphysis. In adolescents, the bottom of the bladder is at the level of the middle, and in adolescence - at the level of the lower edge of the pubic symphysis. Subsequently, the bottom of the bladder is lowered, depending on the state of the muscles of the urogenital diaphragm.
Kidney buds differ from adult kidneys in size and weight. They are relatively more - in newborns their weight is about 1/100 - 1/125 of body weight, and in adults - 1/200 - 1/225.
The kidneys grow and develop with age, this process is most intensive in the first year of life.
During the first year of life, urination is involuntary. In the future, as the central nervous regulatory mechanisms mature and nurture, urination becomes arbitrary. However, some children still have bedwetting. This phenomenon may be due to the irrational mode of life of the child: food before bedtime, plenty of fluid, abnormal sleep. With appropriate hygiene measures, you can stop bed-wetting. But these phenomena can be the result of a violation of the neuro-psychological sphere of the child. In this case, the child must undergo a course of treatment as directed by a specialist.