The intake of nutrients into the human body and the excretion of metabolic products is carried out by the human excretory system. The work of the organs of the human excretory system has its own mechanisms for the excretion of metabolic products, which are filtration, reabsorption and secretion, formed in the process of evolution.
Human excretory system
Excretion of metabolic products from the body is carried out by the organs of the excretory system, which consist of the kidneys, ureters, bladder and urethra.
The kidneys are located in the retroperitoneal space in the lumbar region and are bean-shaped.
This is a paired organ, consisting of the cortex and medulla, the pelvis, and it is covered with a fibrous membrane. The pelvis of the kidney consists of a small and a large bowl, and the ureter exits from it, which delivers urine to the bladder and through the urethra the final urine is excreted from the body.
The kidneys are involved in metabolic processes, and their role in ensuring the body's water balance, maintaining acid-base balance are fundamental forfull human existence.
The structure of the kidney is very complex and its structural element is the nephron.
It has a complex structure and consists of the proximal canal, the nephron body, the loop of Henle, the distal canal and the collecting duct, which gives rise to the ureters. Reabsorption in the kidneys passes through the tubules of the proximal, distal and loop of Henle.
Reabsorption mechanism
Molecular mechanisms of the passage of substances in the process of reabsorption are:
- diffusion;
- endocytosis;
- pinocytosis;
- passive transport;
- active transport.
Of particular importance for reabsorption is active and passive transport and the direction of reabsorbed substances along the electrochemical gradient and the presence of a carrier for substances, the operation of cellular pumps and other characteristics.
Active transport of substances goes against the electrochemical gradient with the expenditure of energy for its implementation and through special transport systems. The nature of movement is transcellular, which is carried out by the transition through the apical membrane and basolateral. These systems are:
- Primary active transport, which is carried out with the help of energy from the breakdown of ATP. It is used by Na+, Ca+, K+, H+ ions.
- Secondary active transport takes place due to the difference in the concentration of sodium ions in the cytoplasm and in the lumen of the tubules, and this difference is explained by the release of sodium ions into the interstitial fluid withenergy consumption of ATP splitting. It uses amino acids, glucose.
Passive transport runs along gradients: electrochemical, osmotic, concentration, and its implementation does not require energy and the formation of a carrier. Substances that use it are Cl- ions. The movement of substances is paracellular. This is movement across the cell membrane, which is located between two cells. Characteristic molecular mechanisms are diffusion, transport with a solvent.
The process of protein reabsorption takes place inside the cellular fluid, and after splitting it into amino acids, they enter the intercellular fluid, which occurs as a result of pinocytosis.
Types of reabsorption
Reabsorption is a process that takes place in the tubules. And the substances passing through the tubules have different carriers and mechanisms.
During the day, the kidneys form from 150 to 170 liters of primary urine, which goes through the process of reabsorption and returns to the body. Substances with highly dispersed components cannot pass through the membrane of the tubules and, in the process of reabsorption, enter the blood with other substances.
Proximal Reabsorption
In the proximal nephron, which is located in the renal cortex, reabsorption takes place for glucose, sodium, water, amino acids, vitamins and protein.
The proximal tubule is formed by epithelial cells that have an apical membrane and a brush border, andit is directed towards the lumen of the renal tubules. The basement membrane forms folds that form the basal labyrinth, and through them the primary urine enters the peritubular capillaries. The cells are interconnected tightly and form a space that runs throughout the intercellular space of the tubule, and it is called the basolateral labyrinth.
Sodium is reabsorbed in a complex three-step process and is a carrier for other substances.
Reabsorption of ions, glucose and amino acids in the proximal tubule
Key steps in sodium reabsorption:
- Passing through the apical membrane. This is the stage of passive transport of sodium, through Na-channels and Na-carriers. Sodium ions enter the cell through membrane hydrophilic proteins that form Na-channels.
- Entry or passage through the membrane is associated with the exchange of Na + for hydrogen, for example, or with its entry as a carrier of glucose, an amino acid.
- Passing through the basement membrane. This is the stage of active transport of Na+, through the Na+/K+ pumps with the help of the enzyme ATP, which, when broken down, releases energy. Sodium, being reabsorbed in the renal tubules, is constantly returned to metabolic processes and its concentration in the cells of the proximal tubule is low.
Reabsorption of glucose passes through secondary active transport and its intake is facilitated by transferring it through the Na-pump, and it is completely returned to the metabolic processes in the body. The increased glucose concentration is not completely reabsorbed in the kidneys and is excreted withfinal urine.
Reabsorption of amino acids proceeds similarly to glucose, but the complex organization of amino acids requires the participation of special transporters for each amino acid for less than 5-7 additional.
Reabsorption in the loop of Henle
The loop of Henle passes through the medulla of the kidney, and the process of reabsorption in the ascending and descending parts of it is different for water and ions.
The filtrate, getting into the descending part of the loop, descending along it, releases water due to a different pressure gradient and is saturated with sodium and chlorine ions. In this part, water is reabsorbed, and it is impermeable to ions. The ascending part is impermeable to water and when passing through it, the primary urine is diluted, while in the descending one it is concentrated.
Distal Reabsorption
This part of the nephron is located in the cortex of the kidney. Its function is to reabsorb water that is collected in the primary urine and reabsorbs sodium ions. Distal reabsorption is the dilution of the primary urine and the formation of the final urine from the filtrate.
Entering the distal tubule, primary urine at 15% after reabsorption in the renal tubules is 1% of the total volume. Collecting after that in the collecting duct, it dilutes, and the final urine is formed.
Neurohumoral regulation of reabsorption
Reabsorption in the kidneys is regulated by the sympathetic nervous system and thyroid, hypothalamic-pituitary and androgens.
Reabsorption of sodium, water, glucoseincreases with excitation of the sympathetic and vagus nerves.
Distal tubules and collecting ducts reabsorb water in the kidneys under the influence of antidiuretic hormone or vasopressin, which increases in large quantities with a decrease in water in the body, and also increases the permeability of the walls of the tubules.
Aldosterone increases the reabsorption of calcium, chloride and water, as does the atriopeptide, which is produced in the right atrium. Inhibition of sodium reabsorption in the proximal nephron occurs when parathyrin enters.
Activation of sodium reabsorption comes from hormones:
- Vasopressin.
- Glucogan.
- Calcitonin.
- Aldosterone.
Inhibition of sodium reabsorption occurs during hormone production:
- Prostaglandin and prostaglandin E.
- Atriopeptide.
The cerebral cortex regulates the excretion or inhibition of urine.
Tubular reabsorption of water is carried out by many hormones responsible for the permeability of the membranes of the distal nephron, the regulation of its transport through the tubules, and much more.
Reabsorption value
The practical application of scientific knowledge about what reabsorption is - this in medicine made it possible to obtain informational confirmation of the work of the excretory system of the body and look into its internal mechanisms. The formation of urine goes through very complex mechanisms and the influence of the environment, genetic abnormalities on it. And they don't go unnoticed when problems arise.against their backdrop. In a word, he alth is very important. Follow him and all the processes occurring in the body.
