Tubular Transport of NaCl & Water

Question 1

Water: daily filtration, excretion and reabsorption

Answer 1

• filtration = 190 L
• excretion = 0.5 - 25 L
• reabsorption = 165 - 189.5 L

Question 2

NaCl: daily filtration, excretion, reabsorption

Answer 2

• filtration = 1500 g
• excretion = 0.05 - 30 g
• reabsorption = 1470 - 1500 g

Question 3

Homeostatic control of water/NaCl (general)

Answer 3

• most of filtered load is obligatorily reabsorbed ==> only small fraction is controlled by homeostasis

Question 4

General physiologic locations of reabsorption

Answer 4

• most obligatory reabsorption ==> @ proximal segments (proximal tubule + loop of Henle)
• homeostatic control/reabsorption ==> @ “fine tuning” segments (distal tubule + collecting duct)

Question 5

Energy-driven transport of sodium

Answer 5

• primary energetic event = Na⁺ from interior of tubular epithlelial cells ==> serosal side via Na⁺/K⁺ ATPase pump
• K⁺ leaks out to establish @ negative potential
• Na⁺ transport ==> gradient for Na⁺ to flow into cells via sodium channels

Question 6

Characteristics of reabsorption of chloride, water, other solute

Answer 6

• coupled to active transport of sodium
• ==> creates positive charge that is generally balanced via reabsorption of chloride ==> Na⁺ and Cl^- on serosal side ==> osmotic gradient
• water will flow down osmotic gradient via peracellular or transcellular routes
• glucose reabsorbed via secondary active transport = lumen ==> cell via Na⁺/glucose co-transporter

Question 7

Characteristics of reabsorption @ proximal tubule

Answer 7

• obligatory reabsorption of the majority of filtered water and NaCl + small filtered metabolites
• water via transcellular transport @ aquaporins
• capacity for reabsorption of metabolities is finite ==> “transport maximum”
• filtrate remains ~isotonic through proximal tubule

Question 8

Characteristics of reabsorbtion @ loop of Henle

Answer 8

• ascending limb = transports 25% of NaCl from lumen ==> interstitium
  
  • NOT permeable to water
  • apical transport of NaCl via Na/K/2Cl co-transporter
• descending limb = IMPERMEABLE to NaCl, but permeable to water
  
  • water flows ==> interstitium due to gradient created by ascending limb
  • 15% of water is reabsorbed ==> relatively less reabsorbption vs. NaCl
• ==> hypertonic interstitium + hypotonic tubular fluid

Question 9

NaCl transport @ ascending loop of Henle

Answer 9

• via Na/K/2Cl transporter @ apical membrane ==> Na/K ATPase or diffusion ==> serosa

Question 10

Reabsorption of water and NaCl @ distal tubule & collecting duct

Answer 10

• “fine tuning” segments ==> rate of transport can be varied by hormones
• tight jxns ==> reabsorption must occur transcellularly via selective transporters
• NaCl
  
  • aldosterone ==> upregulation of apical Na⁺ channels + basolateral Na⁺ pumps ==> increased sodium tranport
• water
  
  • hypertonic interstitium + NaCl transport @ these segments ==> osmotic gradient
  • however, @ base rate, segments ~impermeable to water flow
  • ADH ==> aquaporin deposition @ apical membrane

Question 11

Summary of water/NaCl handling @ nephron

Answer 11

• proximal tubule = obligatory reabsorption of filtered load in isotonic fashion
• LOH = spatial separation of NaCL and water reabsorption + creation of hypertonic interstition
• fine tuning segments =
  
  • obligatory + homeostatic (aldosterone) NaCl reabsorption
  • ADH-controlled/osmotic gradient-driven water reabsorption

Question 12

Starling equation for transmural flow into capillaries

Answer 12

• F_ic = K’ (P_int + π_cap– P_cap - π_int)

Question 13

Magnitude of interstitium-to-capillary flow

Answer 13

P_int =** 7 mm**

π_cap =35 mm

P_cap =11 mm

π_int =** 6 mm**

**NFP = **25 mm

Question 14

Impact of tubular flow on reabsorption/excretion

Answer 14

• faster flow ==> less time for reabsorption = greater proportion of tubular substances escape ==> excretion increases
• slower flow ==> greater proportion of substances reabsorbed ==> excretion decreases
• diuretics ==> increased flow ==> increased excretion rate of Na, K, and Cl (indirectly)

Question 15

Compensatory mechanisms to protect against changes in tubular load

Answer 15

• small changes in GFR ==> large impact on tubular flow
• glomerulotubular balance
  
  • obligatory reabsorption mechanisms @ proximala tubule compensate for changes in filtered load ==> applies to reduced filtration
• tubuloglomerular feedback
  
  • regulates GFR of nephron in response to changes in NaCl concentration at macula densa