PSIO202 Exam 4 - Urinary Concentration, Water Balance, and Micturition

Question 1

What are the two main things happening in a countercurrent mechanism?

Answer 1

countercurrent flow and osmotic gradient

Question 2

What are the two types of countercurrent mechanisms, and describe them?

Answer 2

countercurrent multiplication: progressively increasing osmotic gradient in the interstitial fluid of the renal medulla

countercurrent exchange: enables oxygen delivery to cells of the renal medulla without loss of the osmotic gradient in the interstitial fluid of the renal medulla

Question 3

What is the basic difference between concurrent flow and countercurrent flow?

Answer 3

concurrent - flowing same direction, start 0/100 and end at 50/50
countercurrent - slowing opposite directions, start 0/100 and end 100/0

Question 4

What two factors create the osmotic gradient in the ISF of the renal medulla?

Answer 4

permeability differences in different sections of the nephron (descending = water, ascending = solutes, DCT and collecting duct = water only with ADH)

countercurrent flow of fluid through the ascending and descending limbs of the loop of henle

Question 5

Because of the permeability differences and countercurrent flow in the loop of henle, there is a constant difference in ———.
These effects build on each other, making the loop of henle a ——— ———–.

Answer 5

osmolarity

countercurrent multiplier

Question 6

What are the three major solutes involved in the osmotic gradient?

Answer 6

Na+ Cl- and urea

Question 7

How does ADH effect the osmotic gradient?

Answer 7

• stimulates symporters in the thick ascending limb, which builds the osmotic gradient
• stimulates facultative reabsorption of water in the upper collecting duct
• stimulates water reabsorption and urea recycling in the lower collecting duct

Question 8

What is the sequence of events in countercurrent multiplication?

Answer 8

1. symporters in thick ascending limb build up the Na+ and Cl- in the renal medulla
2. countercurrent flow through the descending and ascending limbs create an osmotic gradient
3. cells in the collecting duct reabsorb water and urea (water via ADH, and urea because of permeability)
4. urea is recycled and builds up in the renal medulla

Question 9

What is the difference between exchange and countercurrent multiplication happening at the vasa recta?

Answer 9

countercurrent multiplication establishes the osmotic gradient which allows for motion between the vasa recta and the nephron

countercurrent exchange is the delivery of nutrients to the nephron cells, which is made possible by the previously established gradient

Question 10

During countercurrent multiplication, the osmolarity outside the descending limb is (higher/lower) than the fluid inside, so water moves (in/out) of the tube into the ISF via osmosis.

Answer 10

higher outside
water moves out

Question 11

When the tubule releases water into the ISF, urea in the tube becomes (more/less) concentrated in the tube. The duct cells (are/are not) permeable to urea, so it will…

Answer 11

becomes more concentrated in the tube
duct cells are permeable to urea
so it will diffuse out into the interstitial fluid of the medulla

Question 12

When tubule fluid flows through the ascending limb, DCT, and cortical portion of the collecting ducts, what happens to urea, and why?

Answer 12

it remains in the tubule fluid because these cells are not permeable to urea

Question 13

In the collecting ducts, what happens to urea?

Answer 13

urea concentration increases in the tubular fluid, so it diffuses out into the interstitial fluid of the medulla

Question 14

In which structure does countercurrent exchange occur?

Answer 14

vasa recta

Question 15

The vasa recta provides —– and —— to the renal medulla without washing out the ————.

Answer 15

oxygen and nutrients

osmotic gradient

Question 16

By the end of the DCT, what percent of solutes and water have been reabsorbed?

Answer 16

90-95%

Question 17

What types of cells in the DCT and CD make the final adjustments to how much water and solutes are reabsorbed?

Answer 17

principal cells - reabsorb Na+ and H2O, secrete K+
intercalated cells - reabsorb K+ and HCO3- and secrete H+

Question 18

How do the principal cells affect Na+ and K+?

Answer 18

Na+ enters the cells through leak channels
Na+ pumps keep the concentration in the tube low
K+ can be secreted to adjust for dietary changes in K+ intake —> down gradient because of the Na+ pump

Question 19

How does aldosterone affect the principal cells?

Answer 19

increases Na+ and water reabsorption and K+ secretion by stimulating the synthesis of new pumps and channels in the principal cells

Question 20

How does ADH affect the principal cells?

Answer 20

trigger the insertion of aquaporin channels into the apical membrane,
increase the water permeability of the principal cells,
water will move more rapidly from tubular fluid into the interstitial fluid and blood

Question 21

What are the two main actions of intercalated cells?

Answer 21

secrete H+, absorb HCO3-

Question 22

How do intercalated cells secrete H+?

Answer 22

proton pumps (ATPases)
can pump against a gradient 1000 times higher than blood
urine is buffered by HPO4 2- and NH3, both which can combine with H+ to carry away excess H+ ions

Question 23

How do intercalated cells absorb HCO3-?

Answer 23

via Cl-/HCO3- antiporters
new HCO3- is generated when carbonic acid disassociates (generated rapidly by carbonic anhydrase)
Cl-/HCO3- antiporters exchange chloride in ISF for bicarbonate in the cells
HCO3- enters the blood and pH rises

Question 24

What do intercalated cells do when blood pH is low?

Answer 24

• secrete more H+ into the tubular fluid
• form carbonic acid from CO2 and H2O
• exchange more Cl- for HCO3-
• more HCO3- enters the blood and raises blood pH

Question 25

What do intercalated cells do when blood pH is high?

Answer 25

• form more carbonic acid from CO2 and H2O
• pump H+ into the ISF
• exchange HCO3- for Cl- in the tubular fluid
• more H+ enters blood, causing blood pH to drop

Question 26

What are diuretics, and some examples?

Answer 26

they slow renal reabsorption of water and cause diuresis (increase urine flow rate)

caffeine, alcohol, and some prescription medications

Question 27

How does caffeine act as a diuretic?

Answer 27

inhibit Na+ reabsorption and therefore, obligatory water reabsorption

Question 28

How does alcohol act as a diuretic?

Answer 28

inhibit secretion of ADH which then inhibits facultative water reabsorption

Question 29

What muscle causes the bladder to contract?

Answer 29

the detrusor muscle

Question 30

What allows the bladder to expand/distend?

Answer 30

extensive rugae and transitional epithelial lining

Question 31

What is the capacity of the bladder?

Answer 31

700-800 mL

Question 32

What is the trigone?

Answer 32

a smooth, flat area bordered by the 2 ureteral openings and the urethral opening

Question 33

What is micturition?

Answer 33

urination, or voiding your bladder

Question 34

How does the micturition reflex function?

Answer 34

stretch receptors signal spinal cord and brain stem when volume exceeds 200-400 mL

impulses are sent to the micturition center of the sacral spinal cord

parasympathetic fibers cause the detrusor muscle to contract and the eternal and internal sphincters to relax

Question 35

How does the micturition reflex differ from the sensations of filling/desire to urinate/conscious control?

Answer 35

filling causes a sensation of fullness that initiates a desire to urinate (before the actual reflex)
In this case…
- there is conscious control over the external sphincter
- the cerebral cortex can initiate micturition or delay its occurrence for a limited period of time