The intake of nutrients in the human body and the removal 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 of excretion of metabolic products formed during evolution, which are filtration, reabsorption and secretion.
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 have a bean-like shape.
This is a paired organ, consisting of cortical and medulla, the pelvis, and it is covered with a fibrous membrane. The renal pelvis consists of a small and a large bowl, and the ureter comes out of it, which delivers urine to the bladder and through the urethra, the final urine is excreted.
The kidneys participate in metabolic processes, and their role in ensuring the body's water balance, maintaining the acid-base balance are fundamental to the full existence of man.
The structure of the kidney is very complex and its structural element is the nephron.
It has a complex structure and consists of a proximal canal, a nephron body, a loop of Henle, a distal canal and a collecting duct giving rise to the ureters. Reabsorption in the kidneys passes through the tubules of the proximal, distal part and the loop of Henle.
Reabsorption mechanism
The molecular mechanisms of the passage of substances during 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 cell 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 the movement is transcellular, which is carried out by passing through the apical membrane and basolateral. Such systems are:
- Primary active transport, which is carried out using energy from the breakdown of ATP. It is used by ions Na +, Ca +, K +, H +.
- Secondary-active transport occurs 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 intercellular fluid with the expenditure of ATP cleavage energy. It is used by amino acids, glucose.
Passive transport follows gradients: electrochemical, osmotic, concentration, and for its implementation does not require energy and carrier formation. The substances that use it are Cl- ions. The movement of substances is carried out paracellularly. This movement is through the cell membrane, which is located between two cells. The characteristic molecular mechanisms are diffusion, transport with a solvent.
The process of protein reabsorption takes place inside the cell fluid, and, after its splitting 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 substances passing through the tubules have different carriers and mechanisms.
150 to 170 liters of primary urine is formed in the kidney per day, which undergoes a reabsorption process 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 bloodstream with other substances.
Proximal reabsorption
In the proximal nephron, which is located in the cortical substance of the kidney, 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 brush border, and it faces the lumen of the renal tubules. The basement membrane forms the folds that form the basal labyrinth, and through them, primary urine enters the peritubular capillaries. Cells are interconnected tightly and form a space that runs along the entire length of the intercellular space of the tubule, and it is called the basolateral labyrinth.
Reabsorption of sodium has a complex three-stage step, and it is a carrier for other substances.
Reabsorption of ions, glucose and amino acids in the proximal tubule
The main stages of sodium reabsorption:
- Passing through the apical membrane. This is the stage of passive sodium transport through Na channels and Na carriers. Sodium ions pass into the cell through membrane hydrophilic proteins that form Na channels.
- The 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, amino acids.
- Passing through the basement membrane. This is the stage of active transport of Na + through Na + / K + pumps using the enzyme ATP, which releases energy when cleaved. Sodium, reabsorbed in the renal tubules, constantly returns to metabolic processes and its concentration in the cells of the proximal tubule is low.
Glucose reabsorption takes place via secondary-active transport and its flow is facilitated by its transfer through the Na-pump, and it completely returns to metabolic processes in the body. An increased glucose concentration does not completely undergo reabsorption in the kidneys and is excreted in the final urine.
Amino acid reabsorption 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 ones.
Reabsorption in the loop of Henle
The Henle loop 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, falling into the descending part of the loop, descending along it, gives off water due to a different pressure gradient and is saturated with sodium and chlorine ions. In this part, water is reabsorbed, and for ions it is impermeable. The ascending part is impermeable to water and when passing through it, primary urine is diluted, while in the descending it is concentrated.
Distal reabsorption
This nephron is located in the cortex of the kidney. Its function is to reabsorb water, which is collected from primary urine and exposes sodium ions to reabsorption. Distal reabsorption is the dilution of primary urine and the formation of final urine from a filtrate.
Entering the distal tubule, primary urine in a volume of 15% after reabsorption in the renal tubules is 1% of the total volume. Gathering after that in a collecting tube, it is diluted, and final urine is formed.
Neuro-humoral regulation of reabsorption
Kidney reabsorption is regulated by the sympathetic nervous system and thyroid hormones, hypothalamic-pituitary, and androgens.
Reabsorption of sodium, water, glucose increases with excitation of the sympathetic and vagus nerves.
The distal tubules and collecting tubules reabsorb water in the kidneys under the influence of antidiuretic hormone or vasopressin, which, with a decrease in water in the body, increases in large quantities, and the permeability of the walls of the tubules increases.
Aldosterone increases the reabsorption of calcium, chlorine and water, as does the atriopeptide, which is produced in the right atrium. Inhibition of sodium reabsorption in the proximal nephron occurs upon receipt of parathyrin.
Activation of sodium reabsorption is due to hormones:
- Vasopressin.
- Glucogan.
- Calcitonin.
- Aldosterone.
Inhibition of sodium reabsorption occurs during the production of hormones:
- 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, regulation of its transport along the tubules, and much more.
Reabsorption Value
The practical application of scientific knowledge about what reabsorption is in medicine has made it possible to obtain informational confirmation about the functioning of the body's excretory system and to look into its internal mechanisms. The formation of urine undergoes very complex mechanisms and the impact on it of the environment, genetic abnormalities. And they do not go unnoticed when problems arise on their background. In short, health is very important. Watch him and all the processes taking place in the body.