Renal System 3

Question 1

What is the osmolarity of extracellular fluid?

Answer 1

Approx 300 mosm/L

Question 2

Sodium and Water Reabsorption

Poximal Convoluted Tubules

Answer 2

Sodium ion reabsorption via: 
1. cotransport with glucose 
2. diffusion through leak channels 
3. counter transport with hydrogen ions 
(luminal membrane) 
4. exchange pump - 3Na+ -2K+ 
(basolateral membrane) 
Water reabsorption via: 
1. osmosis (i.e. follows the sodium)

Question 3

Sodium and Water Reabsorption

Nephron loop

Answer 3

Sodium ion reabsorption via: 
1. co-transport Na+ - K+/2Cl- 
(luminal membrane) 
2. exchange pump Na+ - K+ 
-only in the thick ascending limb 
(basolateral membrane) 
Water reabsorption via: 
1. osmosis (i.e. follows the concentration gradient) 
-only in the thin descending limb

Question 4

The thin descending limb

Answer 4

• is permeable to water and impermeable to solutes
• as tubular fluid flows along thin descending limb:
  
  • osmosis moves water into peritubular fluid, leaving solutes behind
  • osmotic concentration of tubular fluid increases

Question 5

The thick ascending limb

Answer 5

• has highly effective pumping mechanism
  
  • Na+ and Cl- are pumped out of tubular fluid before it reaches DCT
  • solute concentration in tubular fluid declines
• impermeable to water

Question 6

Countercurrent Multiplication **

Answer 6

Countercurrent
Refers to exchange between tubular fluids moving in opposite directions
-fluid in descending limb flows toward renal pelvis
-fluid in ascending limb flows toward cortex

Multiplication
Referes to effect of exchange
-increases as movement of fluid continues

Question 7

Steps in countercurrent multiplication

Answer 7

1. filtrate descend toward the medulla
   - water moves out with osmosis as the medulla peritubular fluid is highly concentrated
   - sodium remains behind making the filtrate more concentrated
2. filtrate ascends towards the cortex
   - sodium and chloride ions are actively moved out
   - water remains in filtrate, making filtrate less concentrated
   - note: the sodium and chloride movement creates the concentration gradient from the medulla to the cortex

Question 8

Benefits of countercurrent multiplication

Answer 8

1. efficiently reabsorbs solutes and water:
   - before tubular fluid reaches DCT and collecting system
2. establishes concentration gradient:
   - that permits passive reabsorption of water from tubular fluid in collecting system:
   - regulated by circulating levels of ADH

Question 9

Sodium and Water Reabsorption

Distal convoluted tubules

Answer 9

Sodium ion reabsorption via: 
1. Na+ - Cl- co-transporter 
(luminal membrane) 
2. exchange pump Na+ - K+ 
-stimulated by aldosterone 
(luminal and basolateral membranes) 
Water reabsorption via: 
1. water channels - aquaporins
-stimulated by ADH

Question 10

Collecting ducts

Water reabsorption

Answer 10

Water reabsorption in medulla region via:

1. water channels - aquaporins
   - stimulated by ADH

Question 11

Regulation of volume and concentration of ECF

Answer 11

Two different hormones controlling:

• Water reabsorption: Antidiuretic hormone (ADH)
  
  • controls fluid concentration
• Sodium reabsorption: Aldosterone
  
  • controls fluid volume

Inter-related process controlling volume and concentration

Question 12

Regulation of ECF concentration

ADH

Answer 12

ADH

• released in response to increased plasma/ECF concentration
• stimulates incorporation of water channels (Aquaporins)
  
  • in DCT and collecting ducts
• allows for water to be reabsorbed along concentration gradient
• fastest way to restore balance to ECF concentration

Question 13

Regulation of ECF volume

Aldosterone

Answer 13

Aldosterone effects of DCT

• stimulates synthesis and incorporation of Na+ - K+ pumps
  
  • in DCT and cortical region of collecting duct
• increases sodium reabsorption
• increased sodium retention results in greater water retention