L25 - Renal Function 4: Urine Concentration & Dilution Flashcards
When is the renin-angiotensin-aldosterone system recruited?
when blood pressure drops
What is the role of angiotensin II in the nephron?
↑ sodium reabsorption in the proximal tubule and constricts the efferent arteriole as a means of protecting GFR during ↓ blood pressure
What is the role of aldosterone in the nephron?
↑ sodium reabsorption in the collecting duct
Where does the bulk of reabsorption occur in the nephron?
in the proximal end e.g. proximal convoluted tubule and loop of Henle
Where does fine-tuning occur in the nephron?
in the distal end e.g. distal tubule and collecting duct
How is fine-tuning possible in the distal nephron?
impermeable to Na+ and water unless this permeability is allowed
controlled independently
What stimulates renin release?
renin release is stimulated by volume depletion; fall in pressure at preglomerular arteriole, reduction in NaCl delivery to macula densa and sympathetic nerve activation
Which type of cells is renin released from?
granular cells
What are the effects of aldosterone in the collecting duct?
↑ activity of Na / K pump
↑ expression of ENaCs on luminal membrane
↑ potassium secretion
↑ activity of hydrogen ATPase on luminal membrane
What are the effects of the sympathetic nervous system (noradrenaline) on the nephron?
↑ reabsorption in the proximal convoluted tubule and constriction of the afferent arteriole in an attempt to reroute blood elsewhere
What are the effects of ANP on the nephron?
↓ sodium reabsorption in the collecting duct so that more sodium is excreted in the urine and dilates the afferent arteriole so that more sodium can be excreted by GFR
What is the balance of water in the body?
input: diet (food and drink) = 2200mL metabolism = 300mL intravenous (5% dextrose) = 0mL output: urine = 1500mL lungs = 500mL skin = 400mL feces = 100mL
A negative water balance will be most obvious as: (reduced plasma volume / reduced cardiac venous return / reduced ECF osmolarity / none of the above)
none of the above, not ↓ plasma volume or ↓ cardiac venous return because these also have to do with issues in sodium balance
How does the brain detect changes in osmolarity?
osmoreceptor cells in the anterior hypothalamus respond to changes in cell size
How do osmoreceptor cells respond to a negative water balance?
release antidiuretic hormone (vasopressin) to prevent urination
What things are needed to produce concentrated urine? (aldosterone / vasopressin / angiotensin II / all of the above)
vasopressin manages water balance, aldosterone and angiotensin II manage sodium balance
What is concentration of vasopressin proportional to?
osmolarity
when osmolarity is higher, more vasopressin is released resulting in more concentrated urine
How does the body respond to critically low ECF volume? (10% plasma volume or 500 mL of ECF volume loss)
decreased ECF volume results in increased vasopressin concentration so that water is retained as much as possible
ANGII is involved in vasopressin release. Will ANGII stimulate vasopressin release most of the time?
no, only when ECF volume is particularly low
Which action occurs at the proximal tubule?
bulk absorption
Which action occurs at the ascending limb of the loop of Henle?
dilution of filtrate
Which action occurs at the collecting ducts?
fine-tuning according to needs
What is osmolarity?
solute concentration by volume
What is osmolality?
solute concentration by weight
What is normal urine osmolality?
ranges from 3 to 1200 mOsm/kgH2O
must excrete 600 mOsm of solute per day and at least 500 ml of water per day
Tubular fluid leaving the loop of Henle is normally: (dilute / isosmotic / concentrated)
dilute
How does osmolarity differ throughout the nephron?
isosmotic fluid leaving the PCT becomes more concentrated in the ↓ limb -> removal of solute in the thick ↑ limb creates hyposmotic fluid -> hormones control distal nephron permeability to water and solutes -> urine osmolarity depends on reabsorption in the collecting duct
Why is it called the thick ascending limb?
because it is impermeable to water
What is special about the distal nephron?
impermeable to everything unless permeability is allowed
The medullary concentration gradient is mostly the result of: (sodium / urea / potassium / creatinine)
sodium and urea
What are the two most metabolically active parts of the nephron?
ascending loop of Henle and proximal convoluted tubule
What things are needed to produce concentrated urine?
Na/K/2Cl co-transporter, aquaporin 2 and cAMP
What does activation of vasopressin receptor result in?
receptor activates cAMP second messenger system and cell inserts aquaporin-2 water pores into apical membrane resulting in absorption of water by osmosis into the blood
What happens in the collecting duct when vasopressin is present?
urine concentration increases from 100 mOsM to 1200 mOsM
What happens in the collecting duct when vasopressin is not present?
urine concentration remains at 100 mOsM which is dilute, however it can get more dilute depending on how much solute is reabsorbed
Why is the Na-K-2Cl co-transporter essential?
if solute is not being reabsorbed from the ascending limb and the collecting duct then water is not going to move out as freely, therefore more water is lost
