Concentration Of Urine Introduction
The osmolarity of glomerular filtrate is the same as that of plasma and it is 300 mOsm/L. But, normally urine is concentrated and its osmolarity is four times more than that of plasma, i.e. 1200 mOsm/L.
Table of Contents
The osmolarity of urine depends upon two factors:
- Water content in the body
- Antidiuretic hormone.
Read And Learn More: Medical Physiology Notes
Formation Of Dilute Urine
- The mechanism of urine formation is the same for dilute urine and concentrated urine till the fluid reaches the distal convoluted tubule.
- Whether it has to be excreted as dilute urine or concentrated urine depends upon the water content of the body.
- When the water content in the body increases, the kidney excretes dilute urine.
- It is achieved by the inhibition of ADH secretion.
- ADH is secreted by posterior pituitary. The stimulus for its secretion is the decreased body fluid volume and/or increased sodium concentration (hyper-osmolarity).
- ADH increases the water reabsorption from the distal convoluted tubule and collecting duct resulting in a concentration of urine.
- But when the volume of body fluid increases or the osmolarity of body fluid decreases, ADH secretion stops.
- So water reabsorption from renal tubules does not take place
- This leads to the excretion of large amounts of water in urine making the urine dilute.
- It brings back the normalcy of water content and osmolarity of body fluids.
Formation Of Concentrated Urine
- When the water content in the body decreases, the kidney excretes concentrated urine.
- The formation of concentrated urine is not as simple as that of dilute urine.
It involves two important processes:
Medullary gradient which is developed and maintained by the countercurrent system
- Secretion of ADH.
- Medullary Gradient
Medullary Hyperosmolarity
- The osmolarity of the interstitial fluid in the renal cortex is similar to that of plasma and it is 300 mOsm/L.
- The osmolarity of the interstitial fluid in the renal medulla near the cortex also is 300 mOsm/L.
- However, while proceeding from the outer part towards the inner part of the medulla, it increases gradually and, reaches the maximum at the innermost part of the medulla near the renal sinus.
- Here, it is 1200 mOsm/L.
- This type of gradual increase in the osmolarity of the medullary interstitial fluid is called the medullary gradient.
- It plays an important role in the concentration of urine.
Development And Maintenance Of Medullary Gradient
The kidney has some unique anatomical arrangements called countercurrent systems, which are responsible for the development and maintenance of medullary gradient and hyperosmolarity of interstitial fluid in the inner medulla.
Countercurrent Mechanism
Countercurrent Flow
- A countercurrent system is a system of ‘U’ shaped tubules (tubes) in which, the flow of fluid is in opposite directions in two limbs of the ‘IT shaped tubules.
- In the kidney, the structures, which form the counter-current system, are the loop of Henle and the vasa recta.
- In both, the direction of flow of fluid in the descending limb is just opposite to that in the ascending limb.
- The loop of Henle forms the countercurrent multiplier and, the vasa recta forms the countercurrent exchanger.
Countercurrent Multiplier
Loop of Henle; Loop of Henle functions as a countercurrent multiplier. It is responsible for the development of hyperosmolarity of medullary interstitial fluid and medullary gradient.
Role of Loop of Henle in the Development of Medullary Gradient
- The loop of Henle of juxtamedullary nephrons plays a major role as a countercurrent multiplier.
- It is because the loop of the juxtamedullary nephrons is long and extends up to the deeper parts of the medulla.
- The major cause for the hyperosmolarity of medullary interstitial fluid is the active reabsorption of sodium, chloride, and other solutes from ascending limb of Henle’s loop into the medullary interstitium.
- These solutes accumulate in the medullary interstitium and increase the osmolarity.
- Now, due to the concentration gradient, the sodium and chloride ions diffuse from the medullary interstitium into the descending limb of Henle’s loop and reach the ascending limb again via a hairpin bend.
- Thus, the sodium and chloride ions are repeatedly recirculated between the descending limb and ascending limb of Henle’s loop through medullary interstitial fluid leaving a small portion to be excreted in the urine.
- Apart from this, there is the regular addition of more and more new sodium and chloride ions into descending limbs by constant filtration.
- Thus, the reabsorption of sodium chloride from ascending limb and the addition of new sodium chloride ions into the filtrate increase or multiply the osmolarity of medullary interstitial fluid and medullary gradient. Hence, it is called countercurrent multiplier.
- Other Factors Responsible for Hyperosmolarity of Medullary Interstitial Fluid.
- In addition to the countercurrent multiplier action provided by the loop of Henle, two more factors are involved in the hyperosmolarity of medullary interstitial fluid.
- Reabsorption of sodium from the medullary part of the collecting duct into the medullary interstitium, which adds to the osmolarity.
- Urea recirculation: Urea is completely filtered in the glomeruli. As it is a waste product, it is not reabsorbed from the renal tubule.
- So, all the molecules of urea reach the collecting duct, and the concentration of urea increases in the collecting duct.
- Now, due to the concentration gradient, urea diffuses from the collecting duct into the inner medullary interstitium.
- So, the osmolarity increases in the inner medulla.
- Due to the continuous diffusion, the concentration of urea increases in the medullary interstitium.
- Again, by the concentration gradient, urea enters the ascending limb. From here, it passes through the distal convoluted tubule and reaches the collecting duct.
- From here, urea enters the medullary interstitium and the cycle repeats.
In this way, urea recirculates repeatedly and helps to maintain hyperosmolarity in the inner medullary interstitium.
- Only a small amount of urea is excreted in the urine.
- Countercurrent Exchanger
- Vasa Recta
Vasa recta functions as countercurrent exchanger, it is responsible for the maintenance of the hyperosmolarity of medullary interstitial fluid and the medullary gradient developed by the countercurrent multiplier.
- Role of Vasa Recta in the Maintenance of Medullary Gradient Vasa recta acts like a countercurrent exchanger because of its position.
- It is also a ‘U’ shaped tubule with a descending limb, hairpin bend, and an ascending limb.
- Vasa recta runs parallel to the loop of Henle. Its descending limb runs along the ascending limb of Henle’s loop and its ascending limb runs along with descending limb of Henle’s loop.
- The sodium chloride reabsorbed from ascending limb of Henle’s loop enters the medullary interstitium.
- From here it enters the descending limb of the vasa recta.
- Simultaneously water diffuses from descending limb of the vasa recta into the medullary interstitium.
- The blood flows very slowly through the vasa recta.
- So, a large quantity of sodium chloride accumulates in descending limb of the vasa recta and flows slowly towards ascending limb.
- By the time the blood reaches the ascending limb of the vasa recta, the concentration of sodium chloride increases very much.
- This causes the diffusion of sodium chloride into the medullary interstitium.
- Water from the medullary interstitium enters the ascending limb of the vasa recta And the cycle is repeated.
- If the vasa recta would be a straight vessel without a hairpin arrangement, blood would leave the kidney quickly at the renal papillary level.
- In that case, the blood would remove all the sodium chloride from the medullary interstitium and thereby the hyperosmolarity will be decreased.
- However, this does not happen, since the vasa recta have a hairpin-like structural configuration.
- Therefore, when blood passes through the ascending limb of the vasa recta, sodium chloride diffuses out of the blood and enters the interstitial fluid of the medulla and, water diffuses into the blood.
- Thus, the vasa recta retain sodium chloride in the medullary interstitium and remove water from it.
- So, the hyperosmolarity of the medullary interstitium is maintained.
- The blood passing through the ascending limb of the vasa recta may carry a very little amount of sodium chloride from the medulla.
- Recycling of urea also occurs by vasa recta. From the medullary interstitium, along with sodium chloride, urea also enters the descending limb of the vasa recta.
- When blood passes through ascending limb of the vasa recta, urea diffuses back into the medullary interstitium along with sodium chloride.
- Thus, sodium chloride and urea are exchanged for water between the ascending and descending limbs of the vasa recta, hence this system is called a countercurrent exchanger.
Role Of Adh
The final concentration of urine is achieved by ADH. Normally, the distal convoluted tubule and the collecting duct are not permeable to water.
- In the presence of ADH, distal convoluted tubule and collecting duct become permeable to water resulting in water reabsorption.
- The water reabsorption induced by ADH is called the facultative reabsorption of water.
- A large quantity of water is removed from the fluid while passing through the distal convoluted tubule and collecting duct.
- So, the urine becomes hypertonic with an osmolarity of 1200 mOsm/L.
Summary Of Urine Concentration
When the glomerular filtrate passes through renal tubule, its osmolarity is altered in different segments as described below.
1. Bowman’S Capsule
- The glomerular filtrate collected at the Bowman’s capsule is isotonic to plasma.
- This is because it contains all the substances of plasma except proteins.
- The osmolarity of the filtrate at Bowman’s capsule is 300 mOsm/L.
2. Proximal Convoluted Tubule
- When the filtrate flows through a proximal convoluted tubule, there is active reabsorption of sodium and chloride followed by obligatory reabsorption of water.
- So, the osmolarity of fluid remains the same as in the case of Bowman’s capsule, i.e. 300 mOsm/L.
- Thus, in proximal convoluted tubules, the fluid is isotonic to plasma.
3. Thick Descending Segment
- When the fluid passes from the proximal convoluted tubule into the thick descending segment, water is reabsorbed from the tubule into the outer medullary interstitium by means of osmosis.
- It is due to the increased osmolarity in the medullary interstitium, i.e. outside the thick descending tubule.
- The osmolarity of the fluid inside this segment is between 450 and 600 mOsm/L. That means the fluid is slightly hypertonic to plasma.
4. Thin Descending Segment Of Henle’S Loop
- As the thin descending segment of Henle’s loop passes through the inner medullary interstitium (which is increasingly hypertonic) more water is reabsorbed.
- This segment is highly permeable to water, and so the osmolarity of tubular fluid becomes equal to that of the surrounding medullary interstitium.
- In the short loops of cortical nephrons, the osmolarity of fluid at the hairpin bend of the loop becomes 600 mOsm/L.
- And, in the long loops of juxtamedullary nephrons, at the hairpin bend, the osmolarity is 1200 mOsrn; L.
- Thus in this segment, the fluid is hypertonic to plasma
5. Thin Ascending Segment Of Henle’S Loop
- When the thin ascending segment of the loop ascends upwards through the medullary region, osmolarity decreases gradually.
- Due to the concentration gradient, sodium chloride diffuses out of tubular fluid and osmolarity decreases to 400 mOsm/L.
- The fluid in this segment is slightly hypertonic to plasma.
6. Thick Ascending Segment
- This segment is impermeable to water. But there is active reabsorption of sodium and chloride from this.
- Reabsorption of sodium decreases the osmolarity of tubular fluid to a greater extent.
- The osmolarity is between 150 and 200 mOsm/L. The fluid inside becomes hypotonic to plasma.
7. Distal Convoluted Tubule And Collecting Duct
- In the presence of ADH, distal convoluted tubule and collecting duct become permeable to water resulting in water reabsorption and the final concentration of urine.
- Recently, it is suggested that in the collecting duct, P cells are responsible for ADH-induced water reabsorption also.
- Reabsorption of a large quantity of water increases the osmolarity to 1200 mOsm/L. The urine becomes hypertonic to plasma.
Applied Physiology
Kidneys fail to concentrate or dilute the urine in some pathological conditions.
1. Osmotic Diuresis
- The loss of a large quantity of water through urine is called diuresis.
- The excretion of large amounts of water through urine due to the osmotic effects of solutes like glucose is called osmotic diuresis.
- It is common in diabetes mellitus.
2. Polyuria
- Increased urinary output with increased frequency of voiding is called polyuria.
- It is common in diabetes insipidus. In this disorder, the renal tubules fail to reabsorb ‘water because of ADH deficiency.
3. Syndrome of Inappropriate Hypersecretion of ADH (SIADH)
- It is a pituitary disorder characterized by hypersecretion or ADH.
- Excess ADH causes water retention which decreases the osmolarity of ECF.
4. Nephrogenic Diabetes Insipidus
- Sometimes, ADH secretion is normal but the renal tubules fail to give a response to ADH resulting in polyuria.
- This condition is called nephrogenic diabetes insipidus.
5. Bartter’s Syndrome
- It is a genetic disorder characterized by a defect in the thick ascending segment.
- This causes decreased sodium and water reabsorption resulting in loss of sodium and water through urine.
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