Ch 25: Urinary Part 3 Flashcards
What is osmolality?
Number of solute particles in 1 kg of H2O that reflects ability to cause osmosis
How is osmolality regulated by the kidneys?
Countercurrent mechanism by regulating urine concentration and volume
What is the mOsm of kidneys?
300
What is the countercurrent mechanism?
Occurs when fluid flows in opposite directions in 2 adjacent segments of same tube with hair pin turn
What is countercurrent multiplier?
The interaction of filtrate in ascending/descending limbs of nephron loops of juxtamedullary nephrons
What is the countercurrent exchanger?
Blood flow in ascending/descending limbs of vasa recta
What is the role of countercurrent mechanisms?
- Establish and maintain osmotic gradient from renal cortex through medulla
- Allow kidneys to vary urine concentration
Describe the countercurrent multiplier of the descending limb of the nephron loop
- Freely permeable to H2O
- H2O passes out of filtrate into hyperosmotic medullary interstitial fluid
- Filtrate osmolality increases to 1200mOsm
Describe the countercurrent multiplier of the ascending limb of the nephron loop
- Impermeable to H2O
- Na+ and Cl– actively reabsorbed in thick segment; some passively reabsorbed in thin segment
- Filtrate osmolality decreases to 100mOsm
Describe the route of the nephron loop
- Filtrate entering the loop is isosmotic to both plasma and cortical interstitial fluid
- Water moves out of the filtrate in the descending limb down its osmotic gradient concentrating the filtrate
- Filtrate reaches its highest concentration at the bend of the loop
- Na+ and Cl- are pumped out of the filtrate increasing the interstitial fluid osmolality
- Filtrate is most dilute as it leaves nephron
Describe the countercurrent exchanger in vasa recta
- Preserves medullary gradient
- Prevent rapid removal of salt from interstitial space
- Remove reabsorbed water
What happens to urine during overhydration?
- Decrease osmolality
- ADH decreases
- Aldosterone removes additional ions
- Decrease in aquaporins
- Less H2O reabsorption
- Large volume of dilute urine
What happens to urine during dehydration?
- Increase osmolality
- Maximal ADH released
- Increased aquaporins
- Increase H2O reabsorption
- Small volume concentrated urine
How is urea recycled?
- Enters filtrate in ascending thin limb of nephron loop
- Cortical collecting duct reabsorbs water; leaves urea
- In deep medullary region now highly concentrated urea → interstitial fluid of medulla → back to ascending thin limb → high osmolality in medulla
What is a diuretic?
Chemicals that enhance urinary output
What are examples of diuretics?
- ADH inhibitors: alcohols
- Na+ reabsorption inhibitors: caffeine, hypertension drugs, edema
- Loop diuretics inhibit medullary gradient formation
- Osmotic diuretics: substance not reabsorbed so water remains in urine
What is required for assessing renal function?
Blood and urine examination
What is renal clearance?
Volume of plasma kidneys clear of particular substance in given time
What is the purpose of renal clearance tests?
- Determine GFR
- Detects glomerular damage
- Follows progress of renal disease
What factors are proportional to renal clearance?
- Concentration of substance in urine
- Flow rate of urine formation
What factors are inverse to renal clearance?
Concentration of same substance in plasma
What is the standard use of inulin in regards to renal clearance?
Freely filtered; neither reabsorbed nor secreted by kidneys
Describe what occurs during the change of renal clearance
- If C < 125 ml/min, substance reabsorbed
- If C = 0, substance completely reabsorbed, or not filtered
- If C = 125 ml/min, no net reabsorption or secretion
- If C > 125 ml/min, substance secreted (most drug metabolites)