Podcast
Questions and Answers
How does the kidney respond to increased plasma osmolarity to maintain water balance?
How does the kidney respond to increased plasma osmolarity to maintain water balance?
- By stimulating osmoreceptors in the anterior hypothalamus, leading to increased ADH secretion. (correct)
- By decreasing ADH secretion, leading to reduced water reabsorption in the collecting duct.
- By stimulating osmoreceptors in the posterior pituitary, leading to increased thirst.
- By inhibiting osmoreceptors in the hypothalamus, decreasing ADH secretion.
Which of the following accurately describes the state of the collecting duct (CD) when antidiuretic hormone (ADH) is absent?
Which of the following accurately describes the state of the collecting duct (CD) when antidiuretic hormone (ADH) is absent?
- The CD transports large quantities of water, leading to concentrated urine.
- The CD actively transports water, independent of osmotic gradients.
- The CD becomes impermeable to water, resulting in the excretion of dilute urine. (correct)
- The CD is highly permeable to water due to the presence of numerous aquaporins.
What is the primary role of the hypertonic interstitium in the kidney's ability to concentrate urine?
What is the primary role of the hypertonic interstitium in the kidney's ability to concentrate urine?
- To actively secrete water into the collecting duct against the concentration gradient.
- To maintain consistent osmolarity throughout the renal medulla.
- To provide the osmotic gradient necessary for water to be reabsorbed from the collecting duct. (correct)
- To reduce the osmotic gradient, preventing excessive water loss in the collecting duct.
How do juxtamedullary nephrons contribute to the kidney's ability to produce concentrated urine?
How do juxtamedullary nephrons contribute to the kidney's ability to produce concentrated urine?
Which characteristics of the loop of Henle are essential for the countercurrent multiplier system?
Which characteristics of the loop of Henle are essential for the countercurrent multiplier system?
What would be the immediate impact on the osmolarity gradient in the medulla if the salt pumps in the ascending limb of the loop of Henle were suddenly non-functional?
What would be the immediate impact on the osmolarity gradient in the medulla if the salt pumps in the ascending limb of the loop of Henle were suddenly non-functional?
What role does urea play in the kidney's ability to concentrate urine, and how is this process regulated?
What role does urea play in the kidney's ability to concentrate urine, and how is this process regulated?
Why is it crucial that blood flow to the medulla is low (approximately 2% of total kidney flow)?
Why is it crucial that blood flow to the medulla is low (approximately 2% of total kidney flow)?
How does aldosterone affect sodium reabsorption in the distal tubule and collecting duct?
How does aldosterone affect sodium reabsorption in the distal tubule and collecting duct?
What characterizes the countercurrent exchange system in the vasa recta, and why is it important for maintaining the medullary concentration gradient?
What characterizes the countercurrent exchange system in the vasa recta, and why is it important for maintaining the medullary concentration gradient?
How does the kidney respond when a person drinks a large volume of water?
How does the kidney respond when a person drinks a large volume of water?
Which regions of the nephron are primarily responsible for variable reabsorption of salt and water based on the body's needs?
Which regions of the nephron are primarily responsible for variable reabsorption of salt and water based on the body's needs?
In a scenario of severe dehydration, which hormonal and renal responses would be expected to occur to conserve water?
In a scenario of severe dehydration, which hormonal and renal responses would be expected to occur to conserve water?
How does the osmolarity of the fluid change as it passes through the proximal tubule, and what drives the reabsorption in this segment?
How does the osmolarity of the fluid change as it passes through the proximal tubule, and what drives the reabsorption in this segment?
What is the relationship between glomerular filtration rate (GFR) and the percentage of filtrate reabsorbed in the proximal tubule?
What is the relationship between glomerular filtration rate (GFR) and the percentage of filtrate reabsorbed in the proximal tubule?
Which of the following best describes the osmolarity of the filtrate at the end of the loop of Henle as it enters the distal tubule, compared to the initial filtrate in Bowman's capsule?
Which of the following best describes the osmolarity of the filtrate at the end of the loop of Henle as it enters the distal tubule, compared to the initial filtrate in Bowman's capsule?
A patient is administered a drug that inhibits the action of NKCC2 transporters in the thick ascending limb of the loop of Henle. How would this affect the kidney's ability to concentrate urine, and why?
A patient is administered a drug that inhibits the action of NKCC2 transporters in the thick ascending limb of the loop of Henle. How would this affect the kidney's ability to concentrate urine, and why?
In a patient with impaired ADH secretion, which of the following sets of conditions would be expected in the nephron?
In a patient with impaired ADH secretion, which of the following sets of conditions would be expected in the nephron?
A researcher is studying a new diuretic drug that primarily targets the proximal tubule. What mechanism of action would most likely lead to an effective diuretic effect?
A researcher is studying a new diuretic drug that primarily targets the proximal tubule. What mechanism of action would most likely lead to an effective diuretic effect?
A patient presents with a condition causing abnormally high blood flow to the renal medulla. What direct effect would this have on urine concentration, and why?
A patient presents with a condition causing abnormally high blood flow to the renal medulla. What direct effect would this have on urine concentration, and why?
A researcher is comparing urine concentrating abilities in different species of mammals. Which anatomical adaptation would you expect to observe in mammals capable of producing highly concentrated urine?
A researcher is comparing urine concentrating abilities in different species of mammals. Which anatomical adaptation would you expect to observe in mammals capable of producing highly concentrated urine?
In what way does urea recycling contribute to the concentration of urine?
In what way does urea recycling contribute to the concentration of urine?
Which of the following transporters is directly influenced by ADH to enhance water permeability in the late distal tubule and collecting duct cells?
Which of the following transporters is directly influenced by ADH to enhance water permeability in the late distal tubule and collecting duct cells?
What is the normal osmolarity of the renal ultrafiltrate as it enters Bowman's capsule?
What is the normal osmolarity of the renal ultrafiltrate as it enters Bowman's capsule?
What percentage of the filtrate is typically reabsorbed in the proximal tubule under normal physiological conditions?
What percentage of the filtrate is typically reabsorbed in the proximal tubule under normal physiological conditions?
If a substance is freely filtered in the glomerulus, but not reabsorbed or secreted, what is its clearance ratio compared to the glomerular filtration rate (GFR)?
If a substance is freely filtered in the glomerulus, but not reabsorbed or secreted, what is its clearance ratio compared to the glomerular filtration rate (GFR)?
Which of the following best describes the function of the countercurrent multiplier in the loop of Henle?
Which of the following best describes the function of the countercurrent multiplier in the loop of Henle?
How does the medullary osmotic gradient influence water reabsorption in the collecting duct?
How does the medullary osmotic gradient influence water reabsorption in the collecting duct?
How does the administration of a loop diuretic like furosemide affect the countercurrent multiplier system in the kidney?
How does the administration of a loop diuretic like furosemide affect the countercurrent multiplier system in the kidney?
What is the primary function of the countercurrent exchanger mechanism involving the vasa recta in the renal medulla?
What is the primary function of the countercurrent exchanger mechanism involving the vasa recta in the renal medulla?
Which segment of the nephron is impermeable to water regardless of ADH levels?
Which segment of the nephron is impermeable to water regardless of ADH levels?
How does the permeability of the descending limb of the loop affect its function in urine concentration?
How does the permeability of the descending limb of the loop affect its function in urine concentration?
The collecting duct passes through the increasing medullary intersitium, as it proceeds through the medulla what is its function?
The collecting duct passes through the increasing medullary intersitium, as it proceeds through the medulla what is its function?
How does a low salt diet effect urine osmolality?
How does a low salt diet effect urine osmolality?
The kidney creates an environment to reabsorb (or not) water and solutes allowing production of:
The kidney creates an environment to reabsorb (or not) water and solutes allowing production of:
In the face of high plasma osmolality, what is the hormonal response?
In the face of high plasma osmolality, what is the hormonal response?
How would the administration of a drug that inhibits UT-A1 and UT-A3 urea transporters directly impact the kidney's ability to concentrate urine, and what compensatory mechanisms might occur?
How would the administration of a drug that inhibits UT-A1 and UT-A3 urea transporters directly impact the kidney's ability to concentrate urine, and what compensatory mechanisms might occur?
A patient with a genetic mutation has a significantly reduced number of aquaporin channels in the proximal tubule. Although the proximal tubule typically reabsorbs approximately 70% of filtrate, how would you expect this mutation to change the osmolarity and volume of fluid entering the loop of Henle?
A patient with a genetic mutation has a significantly reduced number of aquaporin channels in the proximal tubule. Although the proximal tubule typically reabsorbs approximately 70% of filtrate, how would you expect this mutation to change the osmolarity and volume of fluid entering the loop of Henle?
If the blood flow rate through the vasa recta were to increase significantly, how would this affect the concentration gradient in the medullary interstitium, and what would be the consequences for urine concentration?
If the blood flow rate through the vasa recta were to increase significantly, how would this affect the concentration gradient in the medullary interstitium, and what would be the consequences for urine concentration?
A drug inhibits the action of the Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb. How would this directly disrupt the countercurrent multiplier system and what would be the impact on the osmolarity in the ascending limb?
A drug inhibits the action of the Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb. How would this directly disrupt the countercurrent multiplier system and what would be the impact on the osmolarity in the ascending limb?
How does the differential permeability of the descending and ascending limbs of the loop of Henle, in conjunction with the active transport of solutes in the thick ascending limb, synergistically contribute to the establishment of the medullary osmotic gradient?
How does the differential permeability of the descending and ascending limbs of the loop of Henle, in conjunction with the active transport of solutes in the thick ascending limb, synergistically contribute to the establishment of the medullary osmotic gradient?
What is the osmolarity of the fluid leaving the proximal tubule and entering the loop of Henle under normal physiological conditions?
What is the osmolarity of the fluid leaving the proximal tubule and entering the loop of Henle under normal physiological conditions?
Which of the following best describes the changes in filtrate osmolarity as it flows through the descending limb of the loop of Henle?
Which of the following best describes the changes in filtrate osmolarity as it flows through the descending limb of the loop of Henle?
What is a key feature of the ascending limb of the loop of Henle that allows it to contribute to the establishment of the medullary osmotic gradient?
What is a key feature of the ascending limb of the loop of Henle that allows it to contribute to the establishment of the medullary osmotic gradient?
What is the osmolarity of the medullary interstitium at the tip of the loop of Henle in a healthy human kidney that is producing maximally concentrated urine?
What is the osmolarity of the medullary interstitium at the tip of the loop of Henle in a healthy human kidney that is producing maximally concentrated urine?
How does aldosterone influence the regulation of filtrate osmolarity in the distal tubule and collecting duct?
How does aldosterone influence the regulation of filtrate osmolarity in the distal tubule and collecting duct?
Which of the following nephron segments is primarily responsible for the reabsorption of the remaining approximately 10% of the original filtrate volume?
Which of the following nephron segments is primarily responsible for the reabsorption of the remaining approximately 10% of the original filtrate volume?
What is the expected urinary output of a healthy adult with normal renal function who is experiencing high water intake?
What is the expected urinary output of a healthy adult with normal renal function who is experiencing high water intake?
How does the countercurrent multiplier mechanism contribute to the kidney's ability to produce concentrated urine?
How does the countercurrent multiplier mechanism contribute to the kidney's ability to produce concentrated urine?
What role does urea play in the establishment and maintenance of the medullary osmotic gradient?
What role does urea play in the establishment and maintenance of the medullary osmotic gradient?
When plasma osmolarity increases, where are osmoreceptors stimulated, and what hormone is subsequently released to manage water balance?
When plasma osmolarity increases, where are osmoreceptors stimulated, and what hormone is subsequently released to manage water balance?
Under conditions of low water intake, what adaptive mechanisms occur in the distal tubule and collecting duct to maintain water homeostasis?
Under conditions of low water intake, what adaptive mechanisms occur in the distal tubule and collecting duct to maintain water homeostasis?
The presence of antidiuretic hormone (ADH) is essential for facultative water reabsorption. In the absence of ADH, what is the state of the collecting duct (CD)?
The presence of antidiuretic hormone (ADH) is essential for facultative water reabsorption. In the absence of ADH, what is the state of the collecting duct (CD)?
What is the primary function of the loop of Henle in the process of urine concentration?
What is the primary function of the loop of Henle in the process of urine concentration?
Which characteristic of the loop of Henle is crucial for the countercurrent multiplier system to function effectively?
Which characteristic of the loop of Henle is crucial for the countercurrent multiplier system to function effectively?
If the active transport of sodium chloride (NaCl) in the ascending limb of the loop of Henle were inhibited, what effect would this have on the osmolarity of the medullary interstitium, and consequently, on urine concentration?
If the active transport of sodium chloride (NaCl) in the ascending limb of the loop of Henle were inhibited, what effect would this have on the osmolarity of the medullary interstitium, and consequently, on urine concentration?
What is the role of low blood flow in the vasa recta in maintaining the medullary concentration gradient?
What is the role of low blood flow in the vasa recta in maintaining the medullary concentration gradient?
What would be the effect of administering a drug that blocks aldosterone receptors in the distal tubule and collecting duct?
What would be the effect of administering a drug that blocks aldosterone receptors in the distal tubule and collecting duct?
What characterizes the countercurrent exchange system in the vasa recta, and why is it essential for maintaining the medullary concentration gradient?
What characterizes the countercurrent exchange system in the vasa recta, and why is it essential for maintaining the medullary concentration gradient?
Which of the following best describes the kidney's response when a person ingests a large amount of salt?
Which of the following best describes the kidney's response when a person ingests a large amount of salt?
Which of the following best describes the osmolarity of the fluid entering the distal convoluted tubule (DCT) compared to the fluid in Bowman's capsule?
Which of the following best describes the osmolarity of the fluid entering the distal convoluted tubule (DCT) compared to the fluid in Bowman's capsule?
A patient is diagnosed with central diabetes insipidus, characterized by a lack of ADH production. Which of the following sets of conditions would be most likely to occur in the nephron?
A patient is diagnosed with central diabetes insipidus, characterized by a lack of ADH production. Which of the following sets of conditions would be most likely to occur in the nephron?
A researcher is studying a new diuretic drug that primarily targets the thick ascending limb of the loop of Henle. What drug mechanism of action would most likely lead to an effective diuretic effect?
A researcher is studying a new diuretic drug that primarily targets the thick ascending limb of the loop of Henle. What drug mechanism of action would most likely lead to an effective diuretic effect?
A patient presents with a condition that causes abnormally low blood flow to the renal medulla. What direct effect would this have on urine concentration, and why?
A patient presents with a condition that causes abnormally low blood flow to the renal medulla. What direct effect would this have on urine concentration, and why?
A researcher is comparing urine concentrating abilities in different species of mammals. Which tubular adaptation would you expect to observe in mammals capable of producing highly concentrated urine, like desert rodents?
A researcher is comparing urine concentrating abilities in different species of mammals. Which tubular adaptation would you expect to observe in mammals capable of producing highly concentrated urine, like desert rodents?
How would the administration of a drug that inhibits UT-A2 urea transporters specifically impact the kidney's ability to concentrate urine, and what compensatory mechanisms might occur?
How would the administration of a drug that inhibits UT-A2 urea transporters specifically impact the kidney's ability to concentrate urine, and what compensatory mechanisms might occur?
A patient with a genetic mutation has a significantly reduced number of aquaporin channels in the collecting duct. How would the administration of ADH impact the urine volume and osmolarity?
A patient with a genetic mutation has a significantly reduced number of aquaporin channels in the collecting duct. How would the administration of ADH impact the urine volume and osmolarity?
If fluid moves passively out of the descending limb until the osmolarities are balanced. What active process allows kidneys to maintain the corticopapillary gradient for concentrating urine?
If fluid moves passively out of the descending limb until the osmolarities are balanced. What active process allows kidneys to maintain the corticopapillary gradient for concentrating urine?
What process best defines the countercurrent multiplier mechanism?
What process best defines the countercurrent multiplier mechanism?
How does the urea move in the loop of Henle?
How does the urea move in the loop of Henle?
The active transport of solutes in the thick ascending limb and the differential permeability of the descending and ascending limbs facilitate which primary urine concentrating property?
The active transport of solutes in the thick ascending limb and the differential permeability of the descending and ascending limbs facilitate which primary urine concentrating property?
Aquaporin channels allow for the movement of water. Where does this process primarily help maintain water balance?
Aquaporin channels allow for the movement of water. Where does this process primarily help maintain water balance?
What is the relative NaCl and H2O permeability in the proximal tubule?
What is the relative NaCl and H2O permeability in the proximal tubule?
What is a description of the movement in the thin descending limb?
What is a description of the movement in the thin descending limb?
What determines the permeability of the distal convoluted tubule (DCT) and collecting duct (CD) to water?
What determines the permeability of the distal convoluted tubule (DCT) and collecting duct (CD) to water?
Where does sodium reabsorption occur?
Where does sodium reabsorption occur?
When water intake exceeds the body's needs, what happens to aquaporins?
When water intake exceeds the body's needs, what happens to aquaporins?
What is normal water intake as it related to urinary output?
What is normal water intake as it related to urinary output?
How does the differential permeability of the descending and ascending limbs of the loop of Henle contribute to the establishment of the medullary osmotic gradient?
How does the differential permeability of the descending and ascending limbs of the loop of Henle contribute to the establishment of the medullary osmotic gradient?
If the active transport of NaCl were inhibited in the thick ascending limb (TAL) of the loop of Henle, how would this directly affect the osmolarity of the medullary interstitium?
If the active transport of NaCl were inhibited in the thick ascending limb (TAL) of the loop of Henle, how would this directly affect the osmolarity of the medullary interstitium?
How does the medullary blood flow rate influence the kidney's ability to concentrate urine?
How does the medullary blood flow rate influence the kidney's ability to concentrate urine?
A patient is experiencing severe dehydration. How will this condition most directly affect urea handling in the nephron to help concentrate urine?
A patient is experiencing severe dehydration. How will this condition most directly affect urea handling in the nephron to help concentrate urine?
A patient with a genetic defect has a mutation that impairs the function of aquaporins in the proximal tubule. How would this most likely affect the osmolarity of the tubular fluid entering the loop of Henle, compared to a healthy individual?
A patient with a genetic defect has a mutation that impairs the function of aquaporins in the proximal tubule. How would this most likely affect the osmolarity of the tubular fluid entering the loop of Henle, compared to a healthy individual?
Flashcards
Osmotic Pressure
Osmotic Pressure
Pressure to prevent solvent movement across a semi-permeable membrane.
Nephron Osmolarity Adjustment
Nephron Osmolarity Adjustment
Nephron's ability to modify filtrate concentration based on hydration status.
Proximal Tubule Fluid Composition
Proximal Tubule Fluid Composition
Constituents of blood (minus cells/protein) like salt and water.
Proximal Tubule Regulatory Role
Proximal Tubule Regulatory Role
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Loop of Henle Reabsorption
Loop of Henle Reabsorption
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Distal Tubule and CD function
Distal Tubule and CD function
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Variable Reabsorption
Variable Reabsorption
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Aldosterone
Aldosterone
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Anti-Diuretic Hormone (ADH)
Anti-Diuretic Hormone (ADH)
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Summary in the CD
Summary in the CD
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Sensing Osmolarity
Sensing Osmolarity
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ADH Role
ADH Role
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Hypertonic Interstitium
Hypertonic Interstitium
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Loop of Henle Function
Loop of Henle Function
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Countercurrent
Countercurrent
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Loop Characteristics
Loop Characteristics
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Countercurrent Multiplier
Countercurrent Multiplier
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Countercurrent Exchange
Countercurrent Exchange
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Loop of Henle Regions
Loop of Henle Regions
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Gradient
Gradient
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Ascending Limb Transports
Ascending Limb Transports
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Concentrates
Concentrates
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Collecting Ducts Function
Collecting Ducts Function
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Permeability
Permeability
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Vertical Osmotic Gradient
Vertical Osmotic Gradient
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Vasopressin
Vasopressin
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NKCC2
NKCC2
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Urea Excretion
Urea Excretion
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Ascending LOH and DT increases
Ascending LOH and DT increases
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Vasa Recta function passing
Vasa Recta function passing
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Segments of the nephron
Segments of the nephron
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Water and salt
Water and salt
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Medulla Blood Flow
Medulla Blood Flow
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Vasa Recta's function
Vasa Recta's function
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Osmotic Pressure Defintion
Osmotic Pressure Defintion
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Kidney concentration focus
Kidney concentration focus
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Normal GFR reabsorption
Normal GFR reabsorption
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Countercurrent multiplier system
Countercurrent multiplier system
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High water intake
High water intake
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Aldosterone Role
Aldosterone Role
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ADH Requirements
ADH Requirements
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Hypertonic intersitum means
Hypertonic intersitum means
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Countercurrent definition
Countercurrent definition
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Countercurrent flow
Countercurrent flow
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urea permeability
urea permeability
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Capillaries
Capillaries
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Urea recylcing contribues
Urea recylcing contribues
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Study Notes
- The kidney creates an environment to reabsorb water and solutes, allowing for the production of concentrated, low-volume urine during dehydration, and dilute, large-volume urine when over-hydrated.
Osmotic Pressure
- Osmotic pressure is the pressure to prevent solvent movement into a solution when separated by a semi-permeable membrane.
- Renal ultrafiltrate has an osmotic pressure of 300 mOsm/L as it comprises 140 mmol NaCl = 280 mOsm/L + other solutes.
- 1 mmol of Glucose has an osmotic pressure of 1 mOsm/L.
- 1 mmol of NaCl which dissociates into 1mmol of Na+ and 1mmol of Cl- has an osmotic pressure of 2 mOsm/L
Nephron Osmolarity
- Nephron adjusts osmolality through glomerular filtration.
- Glomerulus filters Na, HCO3, Glucose, Amino acids and H2O.
- Proximal convoluted tubule carries out reabsorption.
- The loop of Henle is hyperosmotic.
- The distal tubule is hyposmotic.
- Medullary adjustments to osmolality happen in the collecting duct
- Filtrate remaining becomes urine
Proximal Convoluted Tubule
- The osmolarity is approximately 300 mOsm/L
- All constitutes of blood minus cells are reabsorbed, mainly salt and water
- Around 70% of filtrate is reabsorbed when the GFR is normal.
- The volume reabsorbed does not usually vary.
- Most of the filtrate is returned back to the blood.
- The proximal convoluted tubule typically has no regulatory role.
Loop of Henle
- Approximately 20% of the filtrate volume is reabsorbed.
- The volume here typically does not vary
- Reabsorption is part of the countercurrent mechanism.
Distal Tubules and Collecting Duct
- Distal tubule and collecting duct enter with approximately 10% of the filtrate.
- This region deals with variable salt and water reabsorption.
- Low water intake leads to high water reabsorption.
- High water intake leads to low or no water reabsorption.
- Sodium reabsorption is controlled by aldosterone.
- Anti-diuretic hormone (ADH/vasopressin) controls water reabsorption.
- Normal water intake is 12 ml entering the collecting duct, with 11 ml almost all reabsorbed, therefore, there is 1 ml left.
- Normal urinary input is equivalent to 1 ml/min
Variation in water
- High water intake (no water reabsorption): urinary output is approximately 12 ml/min.
- Low water intake (maximal water reabsorption): urinary output is approximately 0.5 ml/min.
- Variations in water reabsorption occur in the collecting duct.
- Variations in reabsorption cause urinary output variations from 0.5 ml/min to 12 ml/min.
- Permeability of the collecting duct to water may vary depending on hydration.
Sensing Osmolarity
- Decreased water is involved
- Plasma osmolarity increases
- Osmoreceptors stimulate the anterior hypothalamus
- Thirst increases, as does ADH secretion from the posterior pituitary
- Water drinking increases, as does increased water permeability of principal cells within the late distal tubule and collecting duct.
- Water reabsorption increases
- Urine osmolality increases and urine volume decreases
- Plasma osmolality returns towards normal levels
Adjustable Water Reabsorption in the Collecting Duct Requirements
- Presence of Anti-diuretic hormone (ADH) in collecting duct
- The collecting duct is impermeable to water without ADH.
- ADH inserts aquaporins
- Hypertonic interstitium surrounds the collecting duct
- This provides the osmotic gradient to reabsorb water
Hypertonic explanation
- A hypertonic interstitium means its osmolarity is greater than the interstitium in other tissues (300 mOsm/L).
Loop of Henle Functions
- Loop of Henle generates and maintains the hypertonic interstitium
- Operates as a countercurrent multiplier system.
- Concentrates the kidney interstitium.
- Nephrons with longer loops can best concentrate the interstitium.
- Approx. 15% of human nephrons have long loops called juxtamedullary nephrons.
- It creates a concentration gradient (countercurrent) in the medulla of the kidney.
- Countercurrent comes from the fact that fluid moves in opposite directions in the two limbs of the loop.
Loop Characteristics as a Countercurrent Multiplier
- Countercurrent flow: filtrate flows down the descending limb and ascends up.
- Descending limb permeability is limited to water.
- Ascending limb impermeability is limited to water.
- Lined with pumps which deposit salt in the interstitium.
- The salt pump generates and remains at the ascending limb, which is able to maintain a difference in salt concentration, where approximately 200 mosmol/L is between the filtrate along the limb, and the interstitium that surrounds it
Countercurrent mechanisms
- Countercurrent exchange system: Anatomical arrangement of vessels so that flow direction in one vessel is opposite the direction in an adjacent vessel.
- Countercurrent multiplier: The anatomical arrangement of the loop of Henle concentrates solute in the renal medulla.
Loop Regions
- There are 3 regions of the loop
- Thin-walled descending: permeable to H2O (not solutes) that allows flow from loop to surround medium using osmosis
- Thin-walled lower portion of ascending limb: permeable to Na+ and Cl-, moderately permeable to urea and completely impermeable to H2O
- Thick-walled upper portion of ascending limb: Na+ & Cl- is pumped actively from infiltrate surrounding medium
- The ascending limb transports sodium chloride out of the surrounding interstitium which is 200 mosom/l more concentrated than the tubular fluid
- Water passively moves from the descending limb until the osmolarities become equal
- The entire column of fluid advances around the loop
- Water transports salt out and is actively diffused for descending limb and establishes 200 mosom/l difference between ascending limb and each level on interstitial
- As the process continues, the fluid within the descending limb becomes increasingly hypertonic until it reaches at top of the loop, with maximum of 1,200 mosm/l
- It is crucial as concentration of fluid on ascending limb reduces progressively as salt is pumped out
- Interstitial fluid reaches equilibrium (within medulla)
- Countercurrent flow multiples the gradient
Adjustable Water Reabsorption in the Collecting Duct Requirements
- Permeability of the distal convoluted tubule (DCT) and collecting duct (CD) to water is based on ADH levels.
- A vertical osmotic gradient enables water movement by osmosis down the concentration gradient and the production of highly concentrated low volume urine.
Summary - Low Water Intake
- Filtrate at concentration of 100 mosm/liter enters into distal tubules and collecting tubule
- Distal tubule osmolality is 300
- Cortex osmolality is 300, Medulla is 300
- The loop of henle osmolality is between 600- 1200
- Concentration of Urine may be up to 1200 mosm/liter as it leaves collecting tubule
- Vasopressin present makes distal and collecting permeable to water
- Causes small concentrated volume of urine to be excreted, in which reabsorbed H2O is picked up by peritubular capillaries and conserved
Summary - High Water Intake
- Filtrate at concentration of 100 mosm/liter enters into distal tubules and collecting tubule
- Distal tubule osmolality is 300
- Cortex osmolality is 300
- Medulla osmolality is 300/600
- The loop of henle osmolality is between 400- 1200
- Concentration of urine may be as low to 100 mosm/liter and leaves collecting tubule
- No vasopressin present, therefore, distal and collecting are impermeable to waste
- Volume of diluted urine, leads to no water reabsorption on distal portion
Salt Pumps
- Carrier protein (NKCC2) transports 1Na+, 1K+ + 2Cl-
- Inhibited by loop diuretics (e.g., furosemide, bumetanide)
- Inhibition reduces hypertonic interstitium, limiting the ability to reabsorb osmosis while increasing salt/water excretions
Urea
- Small organic molecule comprising two amide groups joined with carbonyl groups
- Normal plasma concentrations of 2.5 mmol/L - 6.0 mmol/L.
- It it formed in the liver
- It is excreted into urine to remove unwanted aa and nitrogen waste
Urea Handling in Nephron
- Helps maintain the osmotic gradient.
- The ascending Loop of Henle (LOH) and the Distal Tubule (DT) are highly impermeable, so tubular [urea] increases.
- ADH increases urea permeability by promoting urea transporters UT-A1, UT-A3.
Role of Urea
- Segments of the nephron are poorly permeable to urea
- Water is reabsorbed and urea left behind, allowing concentration in tubule to rise
Urea effects on inner medullary collecting duct (IMCD)
- ADH increases urea permeability of the IMCD
- Urea passively diffuses out of the IMCD into the medullary fluids
- Some urea diffuses on thin Ascending Limb via Loop of Henle + recycled
Urea Benefits
- Allows for high [urea] to be built up in the medulla
- It contributes for 50% on osmotic pressure of the medullary fluids via max concentration w/ human kidney
- No ADH urea is recycled and more excreted
Countercurrent
- Why isn't the hypertonic interstitium "washed away"?
- Blood flow on medulla to interstitial fluid is low where 2% goes to inner medulla
- Operations from vasa recta "hairpin" uses countercurrent flow
- Capillaries are permeable to fluid
Loops
- Allow vasa recta to remove salt
- It prevent dispersion within the interstitium
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