Human Physiology: Water Balance and Reabsorption

Podcast Beta

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What mechanism primarily controls water intake in the body?

Antidiuretic hormone

Proximal tubule activity

Thirst mechanism (correct)

Sodium reabsorption

What percentage of filtered water is reabsorbed in the proximal tubules?

65% (correct)

50%

25%

10%

During dehydration, which statement is true regarding the secretion of antidiuretic hormone (ADH)?

ADH secretion is increased. (correct)

ADH secretion is decreased.

ADH is secreted primarily in the proximal tubules.

ADH has no effect on water balance.

What is true about the ascending limb of the loop of Henle?

It is the diluting segment of the nephron. Signup and view all the answers

How much filtered water is reabsorbed in the late distal tubules and collecting ducts under the influence of ADH?

10% Signup and view all the answers

What type of urine does the kidney produce when there is decreased ADH secretion?

Diluted urine Signup and view all the answers

What is the osmolarity range that the kidney can achieve when concentrating urine?

1200-1400 m.osmol/L Signup and view all the answers

What happens to the fluid entering the loop of Henle from the proximal tubule?

It remains isotonic. Signup and view all the answers

What is the primary function of the thin ascending limb of the loop of Henle?

Reabsorption of NaCl passively Signup and view all the answers

How does increased flow rate in the loop of Henle affect the osmotic gradient?

It decreases the gradient Signup and view all the answers

What leads to an increase in the osmolarity of the medullary interstitium?

Increased diffusion of urea Signup and view all the answers

Which factor significantly increases the osmotic gradient in the loop of Henle?

Higher percentage of long loops of Henle Signup and view all the answers

What role does ADH play in the osmotic gradient of the renal system?

Increases water reabsorption in the distal tubule Signup and view all the answers

Which factor can lead to the washout of the medullary gradient?

High medullary blood flow Signup and view all the answers

How does the length of the loop of Henle affect urine concentration?

Longer loops lead to more concentrated urine Signup and view all the answers

What impact does a high protein diet have on urine concentration?

It increases the amount of available urea Signup and view all the answers

What is the primary mechanism that contributes to the formation of the medullary gradient in the kidneys?

Reabsorption of sodium chloride at the thick ascending limb Signup and view all the answers

How does the counter-current multiplier system contribute to urine concentration?

By maintaining a flow of fluids in opposite directions Signup and view all the answers

What effect does ADH have on the kidney's ability to concentrate urine?

It promotes water reabsorption in the collecting ducts Signup and view all the answers

What is the osmolarity at the tip of the renal papillae compared to the cortico-medullary junction?

It increases to approximately 1200-1400 mosmol/L Signup and view all the answers

Which statement describes the role of urea in the kidney's medullary gradient?

Urea recycling helps to shift the horizontal gradient to a vertical one Signup and view all the answers

What primarily drives the reabsorption of NaCl at the thick ascending limb?

Na-K-2Cl co-transport mechanism Signup and view all the answers

In the renal system, what role does the loop of Henle play in osmolarity regulation?

It provides selective permeability to water and solutes Signup and view all the answers

Which of the following factors is essential for the maintenance of the medullary gradient?

Counter-current flow of fluid in the nephron Signup and view all the answers

What is the primary mechanism of urea handling in the pars recta of the renal tubules?

Passive reabsorption of urea. Signup and view all the answers

What role does ADH play in urea reabsorption in the renal tubules?

Increases the permeability of the collecting duct to urea. Signup and view all the answers

How does urea concentration change in the descending limb of Henle (DLH)?

Increases because water is reabsorbed. Signup and view all the answers

What physiological characteristic of the vasa recta allows it to function as a counter-current exchanger?

Permeability to solutes and water. Signup and view all the answers

Which statement describes the significance of urea recycling in the renal system?

It helps concentrate the urea in the inner medulla. Signup and view all the answers

Which feature of the medullary collecting duct is important for urea reabsorption?

Response to antidiuretic hormone (ADH). Signup and view all the answers

In what location does significant urea secretion occur due to a high concentration of urea in the inner medullary interstitium?

Inner medullary collecting duct. Signup and view all the answers

What is a characteristic of the vasa recta during dehydration?

Increased viscosity of the blood. Signup and view all the answers

What is the main effect of vasopressin (ADH) on the principal cells of the collecting duct?

Inserts aquaporin channels into the apical membrane Signup and view all the answers

Which of the following statements accurately describes the mechanism of action of ADH?

C-AMP activates protein kinase which then phosphorylates aquaporin Signup and view all the answers

Which of the following factors can stimulate the feeling of thirst?

Dryness of the mouth Signup and view all the answers

In the renal system, urea permeability is primarily increased by which mechanism?

Insertion of specific aquaporin channels Signup and view all the answers

What is the role of urea in maintaining the medullary osmolarity?

It builds up to share 50% of the medullary osmolarity Signup and view all the answers

At which location in the nephron is sodium, potassium, and chloride co-transport stimulated?

Thick ascending limb of the loop of Henle Signup and view all the answers

How much change in plasma osmolarity is required to strongly evoke thirst?

2-3% Signup and view all the answers

Which of the following statements about aquaporins is correct?

Aquaporins help facilitate the transport of both urea and water Signup and view all the answers

Which segment of the nephron is primarily responsible for reabsorbing the highest percentage of filtered water?

Proximal tubules Signup and view all the answers

When the kidney concentrates urine, which mechanism is primarily involved?

Osmotic work by the thick ascending limb of the loop of Henle Signup and view all the answers

Which of the following correctly describes the osmolarity of fluid at the exit of the loop of Henle compared to that entering the distal tubules?

Hypotonic compared to the fluid entering the distal tubules Signup and view all the answers

During maximal diuresis, the secretion of which hormone is significantly reduced?

Antidiuretic hormone (ADH) Signup and view all the answers

The medullary collecting duct allows for reabsorption of filtered water under what circumstance?

In the presence of ADH Signup and view all the answers

Which physiological condition leads to an increased secretion of ADH?

High plasma osmolarity Signup and view all the answers

What percentage of filtered water is typically reabsorbed in the late distal tubules and collecting ducts when ADH is present?

10% Signup and view all the answers

Under which condition does the kidney produce concentrated urine?

Increased osmolarity in the medullary interstitium Signup and view all the answers

What percentage of urea is passively reabsorbed from the pars recta of the renal tubules?

50% Signup and view all the answers

What role does the thick ascending limb of the loop of Henle play in urea handling?

Does not reabsorb urea Signup and view all the answers

How does dehydration affect the vasa recta's function?

Maintains sluggish blood flow leading to better concentration gradients Signup and view all the answers

At what point does the urea concentration in the descending limb of Henle (DLH) increase significantly?

When water is reabsorbed Signup and view all the answers

Which statement correctly reflects the effect of ADH on urea handling in the renal system?

ADH fosters urea recycling in the inner medullary collecting duct Signup and view all the answers

What is one consequence of urea recycling in the renal medulla?

Increased medullary gradient Signup and view all the answers

Which statement about the vasa recta’s capillary walls is correct?

They permit solute and water movement in both descending and ascending vessels Signup and view all the answers

What defines the significance of urea's concentration in the inner medulla?

It increases the medullary osmotic gradient Signup and view all the answers

What is the role of the counter-current multiplier system in the formation of the medullary gradient?

It creates a horizontal gradient that shifts to a vertical one. Signup and view all the answers

What is the significance of the single effect in the Na-K-2Cl co-transport process?

It contributes to the development of the medullary gradient through NaCl reabsorption. Signup and view all the answers

What is the primary role of the distal tubule and collecting duct (CCD) in relation to water reabsorption?

They reabsorb about 2/3 of the water delivered to them. Signup and view all the answers

How does urea recycling play a role in maintaining the medullary osmolarity?

It increases the concentration of urea in the inner medullary collecting duct. Signup and view all the answers

Which feature of the loop of Henle contributes to its differing permeability to water and solutes?

The varying epithelial cell types along the length of the limb. Signup and view all the answers

How does the length of the loop of Henle influence the urine concentration capacity?

A longer loop of Henle increases the osmotic gradient. Signup and view all the answers

What effect does the presence of ADH have on the collecting duct and its interaction with urea?

ADH enhances the passive reabsorption of water without affecting urea levels. Signup and view all the answers

Which statement outlines a factor that can lead to a decrease in the renal osmotic gradient?

Increased flow rate through the loop of Henle. Signup and view all the answers

What is a significant consequence of high medullary blood flow in the vasa recta?

Washout of the medullary osmotic gradient. Signup and view all the answers

What is the primary mechanism behind the active transport of NaCl in the thick ascending limb of the loop of Henle?

Na-K-2Cl co-transport mechanism. Signup and view all the answers

Which factor is crucial for the formation of concentrated urine in species with longer loops of Henle, such as camels and rodents?

Increased length of the loop of Henle. Signup and view all the answers

How does the osmolarity change from the cortico-medullary junction to the tip of the renal papillae?

It shows a gradual increase up to 1200-1400 mosmol/L. Signup and view all the answers

In what way does urea contribute to the osmotic gradient in the renal medulla?

Urea diffusion increases medullary osmolarity. Signup and view all the answers

What does the term 'medullary gradient' refer to in renal physiology?

The gradient of osmolarity within the renal medulla due to solute concentration. Signup and view all the answers

What impact does the presence of ADH have on the kidney's ability to concentrate urine?

Enhances water reabsorption in the connecting tubules and CCD. Signup and view all the answers

What happens to the osmotic gradient if loop diuretics are utilized?

The osmotic gradient is decreased. Signup and view all the answers

What is the effect of ADH on aquaporin channels in renal principal cells?

Increases water permeability by phosphorylating aquaporins Signup and view all the answers

How does the outer medullary collecting duct primarily contribute to water handling?

By increasing urea permeability Signup and view all the answers

What triggers the sensation of thirst when blood volume decreases?

Decrease of 10-15% in blood volume Signup and view all the answers

Which statement correctly describes urea's role in the medullary gradient?

Urea assists in maintaining the osmolarity of the medulla Signup and view all the answers

What is the primary physiological function of aquaporin located in the brain?

Regulating brain water homeostasis Signup and view all the answers

Which tubular segment is primarily stimulated to reabsorb Na+, K+, and Cl- ions?

Thick ascending limb of the loop of Henle Signup and view all the answers

What triggers the activation of protein kinase by C-AMP following ADH binding?

Formation of cyclic AMP from ATP Signup and view all the answers

What is the primary effect of the vasa recta on medullary osmolarity during dehydration?

It maintains the medullary gradient by counter-current exchange Signup and view all the answers

What effect does increased flow rate in the loop of Henle have on the osmotic gradient?

Increased flow rate decreases the osmotic gradient because there is less time for reabsorption. Signup and view all the answers

How does the presence of long loops of Henle in certain animals affect their urine concentration?

Long loops of Henle increase urine concentration by enhancing the osmotic gradient, allowing for the formation of more concentrated urine. Signup and view all the answers

What role does urea play in the medullary osmotic gradient?

Urea contributes to increasing the medullary osmolarity by diffusing from the collecting duct to the interstitium. Signup and view all the answers

How does the action of diuretics influence the magnitude of the renal gradient?

Diuretics lower the magnitude of the single effect, resulting in a decreased osmotic gradient. Signup and view all the answers

What is the effect of high medullary blood flow in vasa recta on the renal osmotic gradient?

High medullary blood flow can wash out the medullary gradient, reducing its effectiveness. Signup and view all the answers

In the context of urea reabsorption, what occurs when ADH is present?

ADH enhances the reabsorption of urea in the collecting duct, increasing medullary osmolarity. Signup and view all the answers

What physiological mechanism leads to increased urine concentration due to a high protein diet?

A high protein diet leads to an increased availability of urea, enhancing urine concentration. Signup and view all the answers

Explain how the counter-current flow in the loop of Henle affects the reabsorption process.

Counter-current flow allows water reabsorption in the descending limb and solute reabsorption in the ascending limb, optimizing concentration gradients. Signup and view all the answers

What role does the loop of Henle play in urine concentration?

The loop of Henle, particularly the thick ascending limb, exerts osmotic work to create a medullary osmotic gradient necessary for urine concentration. Signup and view all the answers

Describe how dehydration affects ADH secretion and urine concentration.

Dehydration increases ADH secretion, which enhances water reabsorption in the collecting ducts, leading to the production of concentrated urine. Signup and view all the answers

What happens to the osmolarity of urine when there is an increase in fluid intake?

Increased fluid intake leads to diluted urine with osmolarity as low as 25-50 mOsmol/L due to decreased ADH secretion. Signup and view all the answers

Explain the relationship between the medullary interstitium and urine concentration.

The medullary interstitium's osmolarity is essential for water reabsorption in the collecting ducts, enabling the kidney to concentrate urine. Signup and view all the answers

How does the structure of the renal tubules facilitate water reabsorption?

The renal tubules have selectively permeable segments, with the proximal tubule reabsorbing 65% of filtered water passively, and the late distal tubules reabsorbing water depending on ADH presence. Signup and view all the answers

What is the significance of osmotic work in the kidney?

Osmotic work, primarily done by the loop of Henle, is crucial for forming concentrated urine and maintaining water balance. Signup and view all the answers

Describe the process that occurs when fluid enters the loop of Henle.

Fluid entering the loop of Henle from the proximal tubule is isotonic, and as it passes through, it leaves hypotonic to the distal tubules due to solute reabsorption. Signup and view all the answers

What effect does the impermeability of the ascending limb of Henle have on urine composition?

The ascending limb's impermeability to water prevents water reabsorption, leading to the dilution of tubular fluid as it ascends. Signup and view all the answers

How does the passive reabsorption of urea in the pars recta affect its concentration in the renal tubules?

It increases urea concentration due to the reabsorption of a large amount of water. Signup and view all the answers

What is the significance of urea recycling in maintaining the medullary osmotic gradient?

Urea recycling augments its concentration in the inner medulla, enhancing the medullary gradient. Signup and view all the answers

What role does ADH play in the reabsorption of urea in the inner medullary collecting duct?

ADH promotes the passive reabsorption of urea in the inner medullary collecting duct. Signup and view all the answers

How does the flow rate in the loop of Henle influence osmotic gradients?

An increased flow rate reduces time for solute reabsorption, potentially diminishing the osmotic gradient. Signup and view all the answers

Describe the counter-current exchange mechanism of the vasa recta and its physiological importance.

The vasa recta facilitates solute reabsorption and water loss, maintaining the osmotic gradient in the medulla. Signup and view all the answers

What effect does dehydration have on the functioning of the vasa recta?

Dehydration leads to increased solute reabsorption in the descending vasa recta and water retention in the ascending vasa recta. Signup and view all the answers

How does ADH influence water reabsorption in the thick ascending limb of the loop of Henle?

ADH does not influence water reabsorption directly in the thick ascending limb as it is impermeable to water. Signup and view all the answers

What characterizes the permeability of the capillary wall in the vasa recta and how does this affect solute and water movement?

The capillary wall in the vasa recta is permeable to both solutes and water, allowing for counter-current exchange. Signup and view all the answers

What process is stimulated by ADH to increase water reabsorption in principal cells?

The insertion of phosphorylated aquaporin channels into the apical membrane. Signup and view all the answers

How does the reabsorption function of the vasa recta impact the maintenance of the medullary osmolarity?

It decreases medullary blood flow, helping to maintain the medullary osmotic gradient. Signup and view all the answers

What is the significance of urea recycling in maintaining the osmolarity of the renal medulla?

Urea helps to maintain 50% of the medullary osmolarity, contributing to the concentration gradient. Signup and view all the answers

Which stimuli are primarily responsible for triggering thirst in relation to osmolarity?

A 2-3% increase in plasma osmolarity and a 10-15% decrease in blood volume. Signup and view all the answers

How does the presence of aquaporin affect water transport in the kidney?

Aquaporins facilitate the transmembrane transport of water, particularly in the collecting ducts. Signup and view all the answers

What role does Na+, K+, and Cl- co-transport play in the thick ascending limb of the loop of Henle?

It contributes to the reabsorption of solutes and helps establish the osmotic gradient. Signup and view all the answers

Describe the effect of hyperosmolarity on thirst and its regulatory mechanism.

Hyperosmolarity leads to a strong desire to drink, which is mediated by increased plasma osmotic pressure. Signup and view all the answers

How does the connecting tubule and cortical collecting duct influence renal function?

They increase water permeability, allowing for more efficient water reabsorption. Signup and view all the answers

What is the significance of the medullary gradient in kidney function?

The medullary gradient allows the kidney to concentrate urine by facilitating the reabsorption of water and solutes through varying osmolarity levels, ultimately leading to efficient water conservation. Signup and view all the answers

Describe how the counter-current multiplier system enhances urinary concentration.

The counter-current multiplier system enhances urinary concentration by ensuring that fluid flows in opposite directions through parallel structures, allowing NaCl and other solutes to be efficiently reabsorbed from the thick ascending limb into the medullary interstitium. Signup and view all the answers

How does ADH influence urea handling in the kidney?

ADH increases urea reabsorption in the inner medullary collecting duct by enhancing urea permeability, which aids in increasing the osmolarity of the medullary interstitium. Signup and view all the answers

Explain the role of Na-K-2Cl cotransport in developing the medullary gradient.

Na-K-2Cl cotransport in the thick ascending limb actively transports NaCl into the interstitium, establishing the medullary gradient necessary for osmotic balance and urine concentration. Signup and view all the answers

What effect does the increasing osmolarity from the cortico-medullary junction to the renal papillae have on urine concentration?

The increasing osmolarity enhances the kidney's ability to concentrate urine by facilitating water reabsorption and driving osmotic gradients. Signup and view all the answers

How does urea recycling contribute to osmolarity in the renal system?

Urea recycling contributes to osmolarity by allowing urea to re-enter the medullary interstitium from the collecting duct, which increases the osmolarity and helps retain water. Signup and view all the answers

Describe the impact of the loop of Henle's structure on urine concentration.

The loop of Henle's unique structure, with its thin descending limb and thick ascending limb, creates a counter-current flow that maximizes solute reabsorption and water retention, which is vital for concentrating urine. Signup and view all the answers

What results from the interaction of differing water and solute permeabilities along the loop of Henle?

The interaction results in a vertical osmotic gradient that enhances the kidney's ability to concentrate urine by allowing differential absorption of water and solutes. Signup and view all the answers

Study Notes

Water Balance

Daily water input and output are approximately 2.5 liters.
Thirst mechanism primarily controls water intake.
Kidney, under ADH control, mainly regulates water output.

Water Reabsorption in Nephrons

Proximal tubules reabsorb 65% of filtered water passively via osmosis, secondary to solute reabsorption.
Loop of Henle's descending limb (DLH) passively reabsorbs 15% of filtered water due to medullary hyperosmolarity.
Loop of Henle's ascending limb (ALH) is impermeable to water.
Early distal tubules are impermeable to water.
Late distal tubules and cortical collecting ducts reabsorb 10% of filtered water, becoming permeable to water in the presence of ADH.
Medullary collecting ducts reabsorb 4% of filtered water.
Approximately 1% of filtered water (0.5-1 ml/min) forms urine.

Urine Concentration

Kidneys produce diluted urine (25-50 mOsmol/L) in overhydration with decreased ADH.
Concentrated urine (1200-1400 mOsmol/L) is produced in dehydration with increased ADH.
Kidney's osmotic work, especially the thick ALH, is crucial for urine dilution/concentration.
Fluid entering the loop of Henle is isotonic; it exits hypotonic.
Medullary gradient is formed by entrapped solutes (NaCl and urea).

Medullary Gradient Formation

Medullary osmolarity gradually increases from 300 mOsmol/L at the cortico-medullary junction to 1200-1400 mOsmol/L at the renal papillae tip.
Mechanisms include the counter-current multiplier system and urea recycling between DLH and papillary collecting duct (PCD).
Counter-current systems require active transport (thick ALH's Na-K-2Cl cotransport), differential permeability of the loop of Henle, counter-current flow, water reabsorption from distal parts of nephron, and the medullary collecting duct's osmotic equilibration.

Factors Affecting Medullary Gradient

Magnitude of the single effect (active NaCl transport in thick ALH) influences gradient, affected by loop diuretics and ADH levels.
Higher flow rates in the loop of Henle reduce the gradient due to insufficient reabsorption time.
Longer loops of Henle create a steeper gradient. Animals like camels and rodents have longer loops.
Higher medullary blood flow washes out the gradient.
High protein diets increase urine concentration due to increased available urea.
ADH presence greatly affects water permeability and concentration.

Urea Handling in Renal Tubules

Urea concentration increases as water is reabsorbed in the pars recta and DLH.
Approximately 50% of urea passively reabsorbs from the pars recta.
Urea concentration continues to increase in the thin ALH.
No urea reabsorption occurs in the thick ALH.
ADH enhances urea concentration in the inner medullary collecting duct (IMCD) leading to passive reabsorption in the PCD.
Urea recycling occurs between the IMCD, inner medullary interstitium, and various parts of the nephron, increasing medullary osmolarity.

Vasa Recta Function

Vasa recta acts as a counter-current exchanger.
Its capillary walls are permeable to water and solutes.
Solute influx and water efflux occur in the descending vasa recta (DVR), and the reverse in the ascending vasa recta (AVR).
Long capillaries with sluggish blood flow and high blood viscosity are characteristics of vasa recta.
In dehydration, more water is absorbed than solutes; in overhydration, more solutes are absorbed than water.

ADH Mechanism of Action

ADH binds to basolateral membrane receptors activating cAMP.
cAMP activates protein kinase which phosphorylates aquaporins (water channels).
Phosphorylated aquaporins are inserted into the principal cells' apical membrane.

Aquaporin Types

Aquaporins are present in the apical border of proximal tubules and DLH, unaffected by ADH.
Some aquaporins are present in the apical border of collecting ducts.
Others are in the basolateral border of collecting ducts, facilitating water and urea transport.
Aquaporins are also found in the brain, salivary/lacrimal glands, and respiratory system.

Thirst Stimuli

Hyperosmolarity (2-3% plasma osmolarity change) strongly induces thirst.
Substantial blood volume decrease (10-15%) triggers thirst similar to hyperosmolarity's effect.
Angiotensin II directly stimulates the thirst center.
Dry mouth and gastric water metering may also contribute to thirst.

Water Balance

Daily water input and output are approximately 2.5 liters.
Thirst mechanism primarily controls water input.
Kidney, under ADH control, primarily regulates water output.

Water Reabsorption in the Nephron

Proximal tubules reabsorb 65% of filtered water passively via osmosis secondary to solute reabsorption.
Loop of Henle's descending limb (DLH) passively reabsorbs 15% of filtered water due to medullary hyperosmolarity.
Loop of Henle's ascending limb (ALH) is impermeable to water.
Early distal tubules are impermeable to water.
Late distal tubules and cortical collecting ducts reabsorb 10% of filtered water.
Medullary collecting ducts, under ADH influence, reabsorb 4% of filtered water.
Approximately 1% of filtered water (0.5-1 ml/min) forms urine.

Urine Concentration

Kidneys produce diluted urine (25-50 mOsmol/L) in overhydration with decreased ADH.
Kidneys produce concentrated urine (1200-1400 mOsmol/L) in dehydration with increased ADH.
Kidney performs osmotic work to create both diluted and concentrated urine, primarily via the loop of Henle (thick ALH).
Fluid entering the loop of Henle is isotonic, leaving hypotonic.
Medullary osmotic gradient is established by excess solute entrapment (NaCl and urea).

Medullary Osmotic Gradient

Gradually increases from 300 mOsmol/L at the cortico-medullary junction to 1200-1400 mOsmol/L at the renal papillae tip.
Mechanisms include counter-current multiplier system and urea recycling between DLH and papillary collecting duct (PCD).
Counter-current multiplier system involves active transport in the thick ALH, differential water and solute permeability in the loop of Henle, counter-current flow, water reabsorption from late distal tubule and collecting ducts, and medullary collecting duct's osmotic equilibration.

Factors Affecting Medullary Gradient

Magnitude of the single effect (active NaCl transport in thick ALH) – reduced by loop diuretics and ADH deficiency.
Flow rate in the loop of Henle – higher flow reduces the gradient.
Length of the loop of Henle – longer loops create steeper gradients.
Percentage of long loops of Henle – higher percentage (e.g., camels and rodents) produces more concentrated urine.
Rate of medullary blood flow in vasa recta – high flow washes out the gradient.
Amount of urea available – higher protein diets increase urea availability and urine concentration.
ADH presence or absence.

Urea Handling in Renal Tubules

Urea concentration increases due to water reabsorption.
Approximately 50% urea passively reabsorbed from pars recta.
Urea concentration further increases in the DLH.
Urea is secreted in the inner medulla due to high interstitial urea concentration.
No urea reabsorption in thick ALH.
ADH increases urea concentration in the inner medullary collecting duct, leading to passive reabsorption in PCD.
Urea recycling occurs between the inner medullary collecting duct (PCD), inner medullary interstitium, thin ALH, thick ALH, distal convoluted tubule (DCT), connecting tubules, cortical collecting duct (CCD), medullary collecting duct (MCD), PCD, and back to the interstitium. This enhances the medullary gradient.

Vasa Recta

Counter-current exchanger system with permeable capillary walls.
Solute entry and water exit in descending vasa recta (DVR).
Solute exit and water entry in ascending vasa recta (AVR).
Long capillaries cause slow blood flow.
Blood viscosity is high.
Vasa recta reabsorb equal amounts of water and solutes in steady state (normal body water). In dehydration, more water is reabsorbed; in overhydration, more solutes are reabsorbed.

ADH Mechanism of Action

ADH binds to basolateral cAMP receptors.
cAMP activates protein kinase A, phosphorylating aquaporins.
Phosphorylated aquaporins are inserted into the apical membrane of principal cells; increase water permeability.

Aquaporin Types

Aquaporins are present in proximal tubule (PT) and DLH apical borders; unaffected by ADH.
Aquaporins are present in collecting duct (CD), especially principal cells.
Basolateral aquaporins facilitate water and urea transport.
Aquaporins are located in the brain, salivary and lacrimal glands, and respiratory system.

Thirst Stimuli

Hyperosmolarity (2-3% plasma osmolarity change).
Blood volume decrease (10-15%).
Angiotensin II (AII) direct action.
Dry mouth.
Gastric water metering.

Water Balance

Daily water input and output are approximately 2.5 liters.
Thirst mechanism controls water input; the kidneys control water output under ADH influence.
Proximal tubules reabsorb 65% of filtered water passively via osmosis.
Loop of Henle's descending limb (DLH) passively reabsorbs water due to medullary osmolarity; its ascending limb (ALH) is impermeable to water.
Early distal tubules are impermeable to water.
Late distal tubules and cortical collecting ducts reabsorb 10% of filtered water; this is ADH-dependent.
Medullary collecting ducts reabsorb 4% of filtered water.
About 1% of filtered water forms urine (0.5-1 ml/min).

Urine Concentration

The kidney produces diluted urine (25-50 mOsmol/L) in overhydration with decreased ADH; concentrated urine (1200-1400 mOsmol/L) forms in dehydration with increased ADH.
This process requires osmotic work, mainly by the loop of Henle's thick ascending limb (ALH).
The medullary gradient is formed by trapping excess solutes (NaCl and urea) in the medulla.

Medullary Gradient Formation

Medullary osmolarity gradually increases from 300 mOsmol/L at the cortico-medullary junction to 1200-1400 mOsmol/L at the renal papillae.
Mechanisms include the counter-current multiplier system and urea recycling.
The counter-current multiplier involves active NaCl transport in the thick ALH, differing water and solute permeability of the loop, counter-current flow, water reabsorption in the late distal tubule and collecting duct, and the medullary collecting duct's osmotic equilibrating device.

Counter-Current Multiplier

Active NaCl transport in the thick ALH via Na-K-2Cl co-transport is key.
This creates a horizontal osmolarity gradient between the ALH and interstitium (about 200 mOsmol/L at each level).
ADH presence leads to water absorption without urea, increasing urea concentration in the inner medullary collecting duct (MCD).
Urea reabsorption into the medullary interstitium further increases osmolarity.
The horizontal gradient is shifted into a vertical one.

Loop of Henle Permeability & Counter-Current Flow

The DLH is permeable to water but not solutes facilitating water reabsorption.
The thin ALH is permeable to solutes, allowing passive NaCl reabsorption into the medullary interstitium.
These factors, combined with counter-current flow, shift the horizontal gradient vertically.

Distal Tubule, Collecting Duct & ADH

About 2/3 of fluid delivered to connecting tubules and collecting ducts is reabsorbed.
This concentrates urea and promotes its diffusion into the medullary interstitium, increasing osmolarity.
The medullary collecting duct helps reabsorb urea and solutes from the interstitium, further increasing osmolarity.

Factors Affecting Medullary Gradient

The magnitude of the single effect (e.g., reduced by loop diuretics or ADH deficiency) impacts the gradient.
Higher flow rates in the loop of Henle decrease the gradient (insufficient reabsorption time).
Longer loops of Henle create a steeper gradient.
A higher percentage of long loops (e.g., in camels and rodents) allows for more concentrated urine.
High medullary blood flow washes out the gradient.
More urea (e.g., from a high-protein diet) increases urine concentration.
ADH presence or absence significantly influences the gradient.

Urea Handling

Urea concentration increases in the pars recta, DLH, and thin ALH due to water reabsorption.
About 50% of urea is passively reabsorbed in the pars recta.
The thick ALH doesn't reabsorb urea; ADH-mediated water reabsorption in the collecting duct increases urea concentration.
Urea recycling between the inner medullary collecting duct and the interstitium augments the medullary gradient.

Vasa Recta

The vasa recta functions as a counter-current exchanger.
Its capillary walls are permeable to water and solutes.
Long capillaries cause sluggish blood flow.
Blood viscosity is high.
In steady-state, water and solute reabsorption are equal.
In dehydration, more water is reabsorbed; in overhydration, more solutes are reabsorbed, washing out the gradient.

ADH Mechanism of Action

ADH binds to basolateral receptors, activating cAMP.
cAMP activates protein kinase, phosphorylating aquaporins.
Phosphorylated aquaporins are inserted into the apical membrane, increasing water permeability.

Aquaporin Types

Aquaporins are present in various locations (proximal tubules, DLH, collecting ducts, etc.) and have different roles in water and urea transport.

Thirst Stimuli

Hyperosmolarity (2-3% plasma osmolarity change).
Decreased blood volume (10-15% decrease).
Angiotensin II (AII).
Mouth dryness.
Stomach water metering.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz covers the essential aspects of water balance and reabsorption within the human kidneys. It focuses on the mechanisms regulating water intake and output, the roles of nephrons, and the process of urine concentration. Test your knowledge on how the body's hydration status is maintained.