Tubular Transport of NaCl & Water Flashcards Preview

CVPR: Renal > Tubular Transport of NaCl & Water > Flashcards

Flashcards in Tubular Transport of NaCl & Water Deck (15)
Loading flashcards...
1

Water: daily filtration, excretion and reabsorption

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

2

NaCl: daily filtration, excretion, reabsorption

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

3

Homeostatic control of water/NaCl (general)

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

4

General physiologic locations of reabsorption

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

5

Energy-driven transport of sodium

  • 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 

6

Characteristics of reabsorption of chloride, water, other solute

  • 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

7

Characteristics of reabsorption @ proximal tubule

  • 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

8

Characteristics of reabsorbtion @ loop of Henle

  • 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

9

NaCl transport @ ascending loop of Henle

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

10

Reabsorption of water and NaCl @ distal tubule & collecting duct

  • "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

11

Summary of water/NaCl handling @ nephron

  • 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 

12

Starling equation for transmural flow into capillaries

  • Fic = K’ (Pint + πcap– Pcap - πint)

13

Magnitude of interstitium-to-capillary flow

Pint = 7 mm

πcap =35 mm

Pcap =11 mm

πint = 6 mm

NFP = 25 mm

14

Impact of tubular flow on reabsorption/excretion

  • 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)

15

Compensatory mechanisms to protect against changes in tubular load

  • 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