38 Questions
What happens to the kidneys' urine production when fluid intake is high?
They produce a large volume of dilute urine
What is the purpose of the osmotic gradient in the medullary interstitial space?
To enable the kidneys to produce urine of various osmolarity
What type of nephrons are responsible for creating the osmotic gradient in the medullary interstitial space?
Juxtamedullary nephrons
What is the range of osmolarity that the kidneys can produce urine in?
65-1200 mOsm/L
What happens to the kidneys' urine production when fluid intake is low or fluid loss is high?
They produce a small volume of concentrated urine
What is the effect of ADH on the late DCT and collecting ducts?
They become permeable to water only in the presence of ADH
What is the osmolarity of the interstitial fluid in the pelvic tip of the medulla?
1200-1400 mosm/L
Which solutes contribute to the osmotic gradient in the renal medulla?
Na+, Cl-, and urea
What is the main function of countercurrent multiplication?
To establish the osmotic gradient in the renal medulla
Why is the proximal tubule not fully permeable to urea?
Because it is impermeable to the proximal tubule
What is the purpose of urea recycling in the kidneys?
To maintain the osmotic gradient in the renal medulla
What type of symporters are present in the thick ascending limb of the LOH?
Na+-K+-2Cl- symporters
What is the direction of fluid flow in countercurrent mechanisms?
In opposite directions
What is the name of the process that establishes the osmotic gradient in the renal medulla?
Countercurrent multiplication
What is the primary function of the kidney in regulating body fluid osmolarity?
To maintain a constant extracellular fluid osmolarity
What is the estimated value of extracellular fluid osmolarity in relation to plasma sodium concentration?
2.1 X plasma Na+ concentration
What is the primary mechanism involved in regulating extracellular fluid osmolarity?
The osmo-receptor –ADH system
What is the result of an increase in body fluid osmolarity on ADH release?
Stimulation of ADH release
What is the role of the kidneys in regulating extracellular fluid osmolarity?
To regulate the amount of water in the body
What is the result of a decrease in body fluid osmolarity on urine production?
The production of dilute urine
Which of the following hormones is involved in the regulation of extracellular fluid osmolarity?
All of the above
What is the main factor that regulates extracellular fluid osmolarity?
The renal excretion of water and sodium
What is the primary function of the high water permeability of the descending limb of LOH?
To create an osmotic gradient in the medullary interstitium
What is the effect of ADH on the collecting duct?
It increases water reabsorption by osmosis
What is the osmolarity of the glomerular filtrate?
300 mOsm/L
What is the role of Na+-K+-2Cl‒ symporters in the ascending limb of LOH?
To decrease the osmolarity of the fluid
What is the primary mechanism of urine concentration?
Urea recycling and countercurrent exchange
What is the effect of the absence of ADH on the collecting duct?
It becomes impermeable to water
What is the osmolarity of the fluid entering the DCT?
Hypotonic
What is the primary function of the countercurrent exchanger?
To conserve the osmotic gradient
What is the minimum volume of urine that must be excreted daily to remove waste products?
0.5 L
What is the osmolarity of urine in a dehydrated state?
1200 mOsm/L
What stimulates the thirst center in the brain?
All of the above
Why are ADH and thirst essential for controlling ECF osmolarity?
Because they absorb water independently of Na+
What is the ideal fluid balance in terms of isotonic urine production?
1 ml/min
What is the osmolarity of waste products that must be excreted daily?
600 mOsm/L
What is the effect of increased ECF osmolarity on ADH secretion?
It stimulates ADH secretion
Why do aldosterone and Angiotensin II have little effect on ECF osmolarity?
Because they reabsorb Na+ and water follows by osmosis
Study Notes
Regulation of Body Fluid Osmolarity
- The kidney plays a major role in regulating body fluid osmolarity.
- Extracellular fluid (ECF) osmolarity is estimated to be 2.1 times the plasma Na+ concentration.
- ECF osmolarity is regulated by the amount of fluid intake and renal excretion of water and Na.
Mechanisms of Regulating ECF Osmolarity
- Two mechanisms are involved: the osmo-receptor-ADH system and the thirst mechanism.
- The osmo-receptor-ADH system regulates ECF osmolarity through the ability of the kidneys to form diluted or concentrated urine.
- The thirst mechanism regulates ECF osmolarity through fluid intake.
Production of Dilute and Concentrated Urine
- Fluid intake is highly variable, but the total volume of body fluid remains stable due to the kidneys' ability to regulate water loss in urine.
- When fluid intake is high, the kidneys produce a large volume of dilute urine.
- When fluid intake is low or fluid loss is high, the kidneys produce a small volume of concentrated urine.
Formation of Dilute Urine
- The glomerular filtrate is isotonic with an osmolarity of about 300 mOsm/L.
- The fluid leaving the proximal convoluted tubule (PCT) is still isotonic, but its osmolarity increases as it moves down the descending limb of the Loop of Henle (LOH).
- The fluid entering the distal convoluted tubule (DCT) is hypotonic.
- In the absence of ADH, collecting ducts are impermeable to water, and urine becomes very diluted (65-70 mOsm/L).
Formation of Concentrated Urine
- Depends on the vertical osmotic gradient established by Na+-K+-2Cl- symporters and urea recycling, and conserving it by the countercurrent exchanger.
- Presence of ADH stimulates aquaporin-2 channels insertion in the apical membrane of the principal cells of the collecting duct, leading to reabsorption of water by osmosis and concentrated urine.
Osmoreceptor-ADH Feedback System
- ADH secretion is more sensitive to increases in ECF osmolarity than reduction in blood volume or blood pressure.
- Stimuli for ADH secretion include:
- Increased osmolarity
- Reduced blood volume (cardiopulmonary receptors)
- Reduced blood pressure (baroreceptors)
Thirst Mechanism
- The thirst center in the brain is stimulated by:
- Increased osmolarity
- Decreased blood volume
- Decreased blood pressure
- Increased Angiotensin II levels
Integrated Responses for Control of ECF Osmolarity
- ADH and thirst mechanism are essential for controlling ECF osmolarity because they absorb water independent of Na+.
- Aldosterone and Angiotensin II have little effect on ECF osmolarity because water follows by osmosis after Na+ reabsorption.
This quiz covers the role of the kidney in regulating body fluid osmolarity, including the production of dilute and concentrated urine. It assesses your understanding of the relationship between extracellular fluid osmolarity and kidney function.
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