Kidneys in the human body perform a number of functions: it is the regulation of the volume of blood and intercellular fluid, and the removal of decomposition products, and stabilization of acid-base balance, and regulation of water-salt balance, and so on. All these tasks are solved due to urination. Tubular reabsorption is one of the stages of this process.
Tubular reabsorption
For a day, the kidneys are passed up to 180 L of primary urine. This liquid is not removed from the body: the so-called filtrate passes through the tubules, where practically all the liquid is absorbed, and the substances necessary for life-amino acids, trace elements, vitamins, return to the blood. The products of disintegration and metabolism are removed with secondary urine. Its volume is much less - about 1.5 liters per day.
The structure of the
nephron The "working" kidney cell consists of the following parts.
- Renal body - glomerular capsule, inside are capillaries.
- Proximal convoluted tubule.
- The loop of Henle consists of a descending and ascending part. The subtle descending is located in the brain substance, bends under 180 degrees in order to rise into the cortical substance to the level of the glomerulus. This part forms an ascending thin and thick part.
- Distal, convoluted tubule.
- The end department is a short fragment connected to the collecting tube.
- Collecting tube - located in the brain substance, diverts secondary urine into the renal pelvis.
The general principle of placement is as follows: in the cortex are located the glomerulus, proximal and distal tubules, in the cerebral - descending and thick ascending parts and collecting tubes. Thin sections, collecting tubes remain in the inner brain substance.
On the video of the nephron:
The mechanism of reabsorption
For the realization of tubular reabsorption, molecular mechanisms are used, similar to the movement of molecules through plasma membranes: diffusion, endocytosis, passive and active transport, and so on. The most significant is active and passive transport.
Active - conducted against the electrochemical gradient. It requires energy and special transportation systems.
Two types of active transport are considered:
- Primary-active - the energy released during the cleavage of adenosine triphosphate is used. Thus, for example, ions of sodium, calcium, potassium, hydrogen move.
- Secondary - the energy is not spent on transport. The driving force is a difference in the concentration of sodium in the cytoplasm and the lumen of the tubule. The carrier necessarily includes a sodium ion. In this way, glucose and amino acids pass through the membrane. The difference in the amount of sodium - less in the cytoplasm than outside, is explained by the removal of sodium into the intercellular fluid with the participation of ATP.
After overcoming the membrane, the complex is split into a carrier - a special protein, sodium ion and glucose. The carrier returns to the cage where the next metal ion is ready to attach. Glucose from the intercellular fluid should flow into the capillaries and return to the bloodstream. The glucose is reabsorbed only in the proximal part, since only here the required carrier is formed.
Passive transport - suction is performed on an electrochemical gradient and does not need support: for example, the absorption of chloride ions in the distal tubule. It is possible to move along the concentration, electrochemical, osmotic gradients.
In practice, reabsorption is carried out according to schemes that include a variety of transportation methods. And depending on the area of the nephron absorbed substances can be differently or not absorbed at all.
For example, water is absorbed in any part of the nephron, but by different methods:
- , about 40-45% of water is absorbed in the proximal tubules by the osmotic mechanism - following ions;
- 25-28% of water is absorbed in the loop of Henle by the rotary-proto-flow mechanism;
- in the distal convoluted tubules is absorbed up to 25% of water. And if in the two previous departments water absorption is made regardless of the water load, then in the distal process is regulated: water can be excreted with secondary urine or retained.
The volume of secondary urine reaches only 1% of the primary volume.
On the video reabsorption process:
Movement of the reabsorbed substance
There are 2 ways of moving the reabsorbed substance into the intercellular fluid:
- paracellular - the transition is made through one membrane between two tightly connected cells. This, for example, diffusion, or transport with a solvent, that is, passive transport;
- transcellular - "through the cage".The substance overcomes 2 membranes: luminal or apical, which separates the filtrate in the tubular lumen from the cell cytoplasm, and basolateral, which acts as a barrier between the interstitial fluid and the cytoplasm. At least one transition is implemented by the mechanism of active transport.
Types of
Various methods of reabsorption are implemented in different departments of the nephron. Therefore, in practice, separation is often used according to the characteristics of work:
- proximal part - convoluted part of the proximal tubule;
- thin - loop parts Henle: thin ascending and descending;
- distal - distal convoluted tubule, connecting and thick ascending part of the loop of Henle.
Proximal
It absorbs up to 2/3 of water, as well as glucose, amino acids, proteins, vitamins, a large number of calcium, potassium, sodium, magnesium, chlorine ions. The proximal tubule is the main supplier of glucose, amino acids and proteins to the blood, so this stage is mandatory and independent of the load.
Reabsorption schemes are used different, which is determined by the type of absorbed substance.
Glucose in the proximal tubule is absorbed almost completely. From the lumen of the tubule to the cytoplasm, it follows the luminal membrane through countertransport. This is a secondary active transport, for which energy is needed. It uses the one that is released when the sodium ion moves along the electrochemical gradient. Then, glucose passes through the basolateral membrane by diffusion: the glucose accumulates in the cell, which provides a difference in concentration.
Energy is needed when passing through the luminal membrane, transport through the second membrane does not require energy costs. Accordingly, the primary factor of glucose uptake is the primary-active sodium transport.
By the same scheme, amino acids, sulfate, inorganic calcium phosphate, nutrients are reabsorbed.
Weak organic acids and weak bases due to a low degree of dissociation are reabsorbed by the method of nonionic diffusion. The substances dissolve in the lipid matrix and are absorbed by the concentration gradient. Absorption depends on the pH level: when it decreases, dissociation of the acid decreases, and the dissociation of the bases rises. At a high pH level, dissociation of acids increases.
This feature has found application in the production of poisonous substances: when poisoned into the blood injected drugs, alkalizing it, which increases the degree of dissociation of acids and helps to remove them with urine.
Loop Henle
If ions of metals and water are reabsorbed in the same proportion in the proximal tubule, then mainly sodium and chlorine are absorbed in the Henle loop. Water is absorbed from 10 to 25%.
In the Henle loop, a rotary-prototype mechanism is realized, based on the singularity of the location of the descending and ascending part. The descending part does not absorb sodium and chlorine, but remains permeable to water. The ascending sucks ions, but for water it is impenetrable. As a result, the absorption of sodium chloride by the ascending part determines the degree of water absorption by the descending part.
Primary filtrate enters the initial part of the downward loop, where the osmotic pressure is lower compared to the pressure of the intercellular fluid. Urine goes down the loop, giving up water, but retaining the ions of sodium and chlorine.
As water is removed, the osmotic pressure in the filtrate rises and reaches a maximum value at the turning point. Then, urine follows the ascending area, keeping water, but losing sodium and chlorine ions. In the distal canaliculus urine falls hypoosmotic - up to 100-200 mosm / l.
As a matter of fact, in the descending section of the loop, Henle's urine concentrates, and in the ascending section it is divorced.
On the video the structure of the loop Gentle:
Distal
The distal tubule weakly passes water, and organic substances here are not absorbed at all. In this department, further dilution is carried out. In the distal tubule gets about 15% of the primary urine, and about 1% is excreted.
As it moves along the distal tubule, it becomes more and more hyperosmotic, as here mainly ions are absorbed and in part water - no more than 10%.The dilution continues in the collecting tubes, where the final urine is formed.
The special feature of this segment is the possibility of adjusting the absorption of water and sodium ions. For water, the regulator is an antidiuretic hormone, and for sodium it is aldosterone.
Norm
To assess the functionality of the kidney, various parameters are used: the biochemical composition of blood and urine, the value of the concentration ability, as well as partial indices. The latter include the glomerular filtration rate and tubular reabsorption.
Glomerular filtration rate - indicates the excretory power of the organ, this is the filtration rate of primary urine that does not contain protein, through the glomerular filter.
The norm of GFR is 90-140 ml / min. The highest indicator is in the daytime, decreases by the evening, and in the morning is at the lowest level. With physical activity, shocks, kidney or heart failure and other ailments, the GFR falls. May increase in the initial stages of diabetes and hypertension.
Tubular reabsorption is not measured directly, but is calculated as the difference between GFR and minute diuresis according to the formula:
P =( GFD-D) x 100 / GFR, where,
- GFR - glomerular filtration rate;
- D - a minute diuresis;
- P - tubular reabsorption.
With a decrease in blood volume - surgery, loss of blood, there is an increase in tubular reabsorption in the direction of growth. Against the background of diuretics, with some renal ailments - decreases.
The norm for tubular reabsorption is 95-99%.Hence, and so much difference between the volume of primary urine - up to 180 liters, and the volume of secondary - 1-1.5 liters.
To obtain these values, a Reberg sample is used. With its help, the clearance is calculated - the coefficient of purification of endogenous creatinine. For this index, GFR and tubular reabsorption are calculated.
The patient is held in a lying position for 1 hour. During this time, the urine is collected. The analysis is carried out on an empty stomach.
Half an hour later, blood is taken from the vein.
Then, the amount of creatinine is found in the urine and blood and the GFR is calculated by the formula:
GFR = M x D / P, where
- M is the urinary creatinine level;
- P - level of substance in plasma
- D - minute volume of urine. It is calculated by dividing the volume by the time of separation.
According to the Reberg sample, the degree of kidney damage can be classified:
- Reducing the filtration rate to 40 ml / min is a sign of kidney failure.
- Reduction of GFR to 5-15 ml / min indicates the terminal stage of the disease.
- Reducing the CD usually follows a water load.
- The growth of CD is associated with a decrease in blood volume. The cause may be loss of blood, as well as nephritis - with this ailment the glomerular apparatus is damaged.
Regulation of tubular reabsorption
Blood circulation in the kidneys is a relatively autonomous process. With changes in blood pressure from 90 to 190 mm.gt;Art. The pressure in the renal capillaries is maintained at the usual level. This stability is explained by the difference in diameter between the bringing and taking out blood vessels.
There are two most significant methods: myogenic autoregulation and humoral.
Myogenic - with the growth of blood pressure arteriol arteriolar walls contract, that is, the body receives a smaller volume of blood and the pressure drops. Constriction most often causes angiotensin II, in the same way act thromboxanes and leukotrienes. Vasodilators are acetylcholine, dopamine and so on. As a result of their action, the pressure in the glomerular capillaries is normalized in order to maintain a normal level of GFR.
Humoral - that is, with the help of hormones. In fact, the main indicator of tubular reabsorption is the level of water absorption. This process can be divided into 2 stages: mandatory - the one that is performed in the proximal tubules and is independent of the water load, and the dependent one - is realized in the distal tubules and collecting tubes. This stage is regulated by hormones.
The main among them - vasopressin, antidiuretic hormone. It retains water, that is, promotes fluid retention. A hormone is synthesized in the nuclei of the hypothalamus, moves to the neurohypophysis, and from there it enters the bloodstream. In the distal areas, there are receptors for ADH.The interaction of vasopressin with receptors leads to an improvement in the permeability of membranes to water, so that it is absorbed better. In this case, ADH not only increases permeability, but also determines the level of permeability.
Due to the difference in pressure in the parenchyma and the distal tubule, the water from the filtrate remains in the body. But against a background of low absorption of sodium ions diuresis can remain high.
Sodium ion absorption regulates aldosterone - the hormone of the adrenal cortex, as well as natriuretic hormone.
Its effect on the section of collecting tubes is especially strong. The hormone "works" both in the kidneys, and in the glands, and in the digestive tract, improving the absorption of sodium. Also, aldosterone regulates the sensitivity of receptors to ADH.
Aldosterone appears for another reason. With a decrease in blood pressure, renin, a substance that controls the tone of the vessels, is synthesized. Under the influence of renin, agglobulin from blood is transformed into angiotensin I, and then into angiotensin II.The latter acts as the strongest vasoconstrictor. In addition, it triggers the production of aldosterone, which causes the reabsorption of sodium ions, which causes a water retention. This mechanism - water retention and vasoconstriction, creates optimal blood pressure and normalizes blood flow.
Natriyurethic hormone is formed in the atrium when it is stretched. Once in the kidneys, the substance reduces the reabsorption of sodium and water ions. In this case, the amount of water that enters the secondary urine increases, which reduces the total volume of blood, that is, the stretching of the atria disappears.
In addition, the level of tubular reabsorption is affected by other hormones:
- parathyroid hormone - improves absorption of calcium;
- thyreocalcioninine - reduces the level of reabsorption of ions of this metal;
- epinephrine - its effect depends on the dose: with a small amount of adrenaline reduces GFR filtration, in a large dose - here tubular reabsorption is increased;
- thyroxine and somatropic hormone - increase diuresis;
- insulin - improves the absorption of potassium ions.
The mechanism of influence is different. So, prolactin increases the permeability of the cell membrane for water, and parathyrene changes the osmotic gradient of interstitium, thereby affecting the osmotic transport of water.
Canal reabsorption is a mechanism that causes the return of water, trace elements and nutrients to the blood. There is a return - reabsorption, in all areas of the nephron, but according to different schemes.