L25 - Renal Function 4: Urine Concentration & Dilution

Question 1

When is the renin-angiotensin-aldosterone system recruited?

Answer 1

when blood pressure drops

Question 2

What is the role of angiotensin II in the nephron?

Answer 2

↑ sodium reabsorption in the proximal tubule and constricts the efferent arteriole as a means of protecting GFR during ↓ blood pressure

Question 3

What is the role of aldosterone in the nephron?

Answer 3

↑ sodium reabsorption in the collecting duct

Question 4

Where does the bulk of reabsorption occur in the nephron?

Answer 4

in the proximal end e.g. proximal convoluted tubule and loop of Henle

Question 5

Where does fine-tuning occur in the nephron?

Answer 5

in the distal end e.g. distal tubule and collecting duct

Question 6

How is fine-tuning possible in the distal nephron?

Answer 6

impermeable to Na+ and water unless this permeability is allowed
controlled independently

Question 7

What stimulates renin release?

Answer 7

renin release is stimulated by volume depletion; fall in pressure at preglomerular arteriole, reduction in NaCl delivery to macula densa and sympathetic nerve activation

Question 8

Which type of cells is renin released from?

Answer 8

granular cells

Question 9

What are the effects of aldosterone in the collecting duct?

Answer 9

↑ activity of Na / K pump
↑ expression of ENaCs on luminal membrane
↑ potassium secretion
↑ activity of hydrogen ATPase on luminal membrane

Question 10

What are the effects of the sympathetic nervous system (noradrenaline) on the nephron?

Answer 10

↑ reabsorption in the proximal convoluted tubule and constriction of the afferent arteriole in an attempt to reroute blood elsewhere

Question 11

What are the effects of ANP on the nephron?

Answer 11

↓ 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

Question 12

What is the balance of water in the body?

Answer 12

input:
diet (food and drink) = 2200mL
metabolism = 300mL
intravenous (5% dextrose) = 0mL
output:
urine = 1500mL
lungs = 500mL
skin = 400mL
feces = 100mL

Question 13

A negative water balance will be most obvious as: (reduced plasma volume / reduced cardiac venous return / reduced ECF osmolarity / none of the above)

Answer 13

none of the above, not ↓ plasma volume or ↓ cardiac venous return because these also have to do with issues in sodium balance

Question 14

How does the brain detect changes in osmolarity?

Answer 14

osmoreceptor cells in the anterior hypothalamus respond to changes in cell size

Question 15

How do osmoreceptor cells respond to a negative water balance?

Answer 15

release antidiuretic hormone (vasopressin) to prevent urination

Question 16

What things are needed to produce concentrated urine? (aldosterone / vasopressin / angiotensin II / all of the above)

Answer 16

vasopressin manages water balance, aldosterone and angiotensin II manage sodium balance

Question 17

What is concentration of vasopressin proportional to?

Answer 17

osmolarity

when osmolarity is higher, more vasopressin is released resulting in more concentrated urine

Question 18

How does the body respond to critically low ECF volume? (10% plasma volume or 500 mL of ECF volume loss)

Answer 18

decreased ECF volume results in increased vasopressin concentration so that water is retained as much as possible

Question 19

ANGII is involved in vasopressin release. Will ANGII stimulate vasopressin release most of the time?

Answer 19

no, only when ECF volume is particularly low

Question 20

Which action occurs at the proximal tubule?

Answer 20

bulk absorption

Question 21

Which action occurs at the ascending limb of the loop of Henle?

Answer 21

dilution of filtrate

Question 22

Which action occurs at the collecting ducts?

Answer 22

fine-tuning according to needs

Question 23

What is osmolarity?

Answer 23

solute concentration by volume

Question 24

What is osmolality?

Answer 24

solute concentration by weight

Question 25

What is normal urine osmolality?

Answer 25

ranges from 3 to 1200 mOsm/kgH2O

must excrete 600 mOsm of solute per day and at least 500 ml of water per day

Question 26

Tubular fluid leaving the loop of Henle is normally: (dilute / isosmotic / concentrated)

Answer 26

dilute

Question 27

How does osmolarity differ throughout the nephron?

Answer 27

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

Question 28

Why is it called the thick ascending limb?

Answer 28

because it is impermeable to water

Question 29

What is special about the distal nephron?

Answer 29

impermeable to everything unless permeability is allowed

Question 30

The medullary concentration gradient is mostly the result of: (sodium / urea / potassium / creatinine)

Answer 30

sodium and urea

Question 31

What are the two most metabolically active parts of the nephron?

Answer 31

ascending loop of Henle and proximal convoluted tubule

Question 32

What things are needed to produce concentrated urine?

Answer 32

Na/K/2Cl co-transporter, aquaporin 2 and cAMP

Question 33

What does activation of vasopressin receptor result in?

Answer 33

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

Question 34

What happens in the collecting duct when vasopressin is present?

Answer 34

urine concentration increases from 100 mOsM to 1200 mOsM

Question 35

What happens in the collecting duct when vasopressin is not present?

Answer 35

urine concentration remains at 100 mOsM which is dilute, however it can get more dilute depending on how much solute is reabsorbed

Question 36

Why is the Na-K-2Cl co-transporter essential?

Answer 36

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