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BE 203 > Chapter 26: Urine Formation II: Glomerular Filtration and Blood Flow (Discussion 2) > Flashcards

Chapter 26: Urine Formation II: Glomerular Filtration and Blood Flow (Discussion 2) Flashcards

1
Q

Urine Formation Formula

A

Urinary Excretion = Glomerular Filtration – Tubular Reabsorption and Tubular Excretion

Filtration (of substance) = Glomerular Filtration Rate x Plasma Concentration (x filterability)

Filtration & Reabsorption closely coordinated (avoids severe changes in urine output)

  • High volume compared to urine~1.5L/day
    • Filtration - 180L/day
    • Reabsorption - 178.5L/day
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2
Q

Tubular Reabsorption

A

Reabsorption is highly selective

  • Glucose/AA’s completely reabsorbed (Bicarb almost completely reabsorbed)
  • Na+/K+/Cl- mostly reabsorbed but regulated closely
  • Creatine not reabsorbed at all
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3
Q

Luminal Membrane

A
  • Brush-border membrane –> increase surface area for transport and high activity
    • Thin segments of loop of Henle do not have brush border
  • Tight junction –> prevent fluid/solute leakage between cells
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4
Q

Reabsorption Path

A

Transcellular vs Paracellular

  • Transcellular: substance transported cross tubular cell membrane and into cell
  • Paracelullar: substance transported through junctional spaces between cells
  1. Substance must be transported through tubular epithelial membrane –> renal interstitium
  2. Then through peritubular capillary membrane –> blood
    • Achieved through Bulk Flow (non-specific fluid/solute movement caused by hydrostatic and osmotic pressure)
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5
Q

Types of Transport

A
  • Active: uses ATP, against concentration gradient
    • Secondary Active Transport:

uses energy (indirectly) stored in electrochemical gradient of another molecule

  • Diffusion: no ATP, down concentration gradient
    • Facilitated diffusion: binds membrane protein to transport across membrane
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6
Q

Reabsorption of Na+

A
  1. Facilitated diffusion across luminal membrane (primarily into cell/transcellular path)
  2. Active transport by Na-K ATPase across basolateral membrane into interstitium
    • Creates Na+ gradient used to reabsorb Na+ and many other secondary active transport processes
  3. Reabsorption into peritubular capillaries by ultrafiltration
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7
Q

Active Transport in Lumen (Na-K pump)

A

Na-K pump: establishes low [Na+] (major source for secondary transport) and high [K+] in luminal membrane cells

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8
Q

Active Transport in Lumen

A
  • Reabsorption/secondary active co-transport
    • Na/Glucose
    • Na/AA
    • Na/Cl
  • Secretion/secondary active counter-transport
    • Na/H+ (controls acidity of body fluids)
  • Pinocytosis: membrane invaginates and forms vesicle around bulk substance
    • Reabsorbs proteins that end up in the tubule
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9
Q

Tubular Maximum Transport vs Gradient-time Transport

A
  • Active transport/facilitated diffusion has tubular maximum transport rate for a substance depending on proteins involved in transport of the substance
  • Gradient-time transport has no physiological maximum rate b/c not limited by proteins
    • Dependent on: electrochemical gradient, permeability of membrane to substance, time fluid with substance remained in tubule
  • Glomerular filtration has potential to drastically rise
    • Diabetes mellitus (high plasma glucose –> glucose in urine)
  • Na+ exhibits both types of transport
    • Proximal show gradient-time due to back- leakage and distal segments reabsorption can be ↑ by some hormones (ex. aldosterone) but show tubular maximum
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10
Q

Solvent Drag

A

Osmotic flow through tight junctions carrying solutes along as well

Water permeability varies by segment

Water leaving tubule also increase conc gradient of remaining solutes, increasing diffusion

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11
Q

Sections of the Nephron

A
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12
Q

Proximal Tubule

A
  • Reabsorbed
    • Glucose (most by first half)
    • AA (most by first half)
    • Protein
    • Na+ (about 65% of filtered amount)
    • HCO3-
    • Cl-
    • K+
  • Secreted
    • H+
    • Organic acids (PAH)
    • Organic bases (oxalate/urate)
    • Drugs/harmful substances
  • Water
    • Highly permeable
    • Reabsorbed (~65% of filtered amount)
    • Osmolarity relatively constant
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13
Q

Thin Descending Limb

A
  • Reabsorbed
    • Moderately permeable to most solutes (low active transport/reabsorption)
  • Secreted
    • Moderately permeable to most solutes (low active transport/secretion)
  • Water
    • Highly permeable
    • Reabsorbed
    • Increasing osmolarity
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14
Q

Thin Ascending Limb

A
  • Reabsorbed
    • Moderately permeable to most solutes (low active transport/reabsorption)
  • Secreted
    • Moderately permeable to most solutes (low active transport/secretion)
  • Water
    • Impermeable
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15
Q

Thick Ascending Limb

A
  • Reabsorbed
    • Na+
    • Cl-
    • K+
    • HCO3-
    • Ca++
    • Mg++
  • Secreted
    • H+
  • Water
    • Impermeable
    • Decreasing osmolarity
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16
Q

Distal Tubule

A
  • Reabsorbed
    • Na+ (NaCL cotransporter action, inhibited by Thiazide diuretics)
    • Cl-
    • K+
    • Ca++
    • Mg++
  • Secreted
    • No major secretion activity
  • Water
    • Impermeable (also impermeable to urea)
    • Decreasing osmolarity (referred to as the diluting segment)
17
Q

Late Distal Tubule & Cortical Collecting Tubule

A
  • Impermeable to urea
  • Water reabsorption determined by ADH (impermeable normally, become permeable with high ADH)
  • Composed of 2 types of cells
    • Principal cells
    • Intercalated cells
18
Q

Principal Cells

A

Late distal tubule & cortical collecting tubule

  • Reabsorbed
    • Na+
  • Secreted
    • K+
  • Site of action for “potassium sparing diuretics”
    • Aldosterone antagonists: inhibit Na-K pump activity on basolateral membrane
    • Na+ channel blockers: prevent Na+ from diffusing into principal cells from lumen
19
Q

Intercalated Cells

A

(late distal tubule & cortical collecting tubule)

  • Reabsorbed
    • HCO3- (released in cell when H+ is formed to be secreted)
    • K+
  • Secreted
    • H+ (energy from hydrogen-ATPase action, not secondary like other parts)
20
Q

Medullary Collecting Duct

A
  • Reabsorbed
    • Urea
    • Na+
    • Cl-
    • HCO3-
  • Secreted
    • H+
  • Water
    • Variable permeability (controlled by ADH)
21
Q

Na+/2Cl-/K+ Co-transporter

A
  • Located in thick ascending limb
  • Energy supplied by Na+ gradient
  • Loop diuretics (e.g. furosemide) inhibit this action –> very powerful diuretic action
22
Q

Mechanisms of Pressure Natriuresis/Diuresis

A
  • slight ↑ in GFR
  • High peritubular capillary hydrostatic pressure –> ↓ reabsorption of Na/H20
  • High arterial pressure –> ↓ Angiotensin II/aldosterone formation –> ↓ reabsorption of Na/H2O
23
Q

Aldosterone

A

Increase in Na reabsorption and K secretion

Act in collecting tubule/duct

24
Q

Angiotensin II

A

Increase in Na, water reabsorption, and H+ secretion

(also constrict efferent arterioles and increase aldosterone release)

Proximal tubule, thick ascending loop, distal/collecting tubule

25
Q

Atrial Natriuretic Peptide (ANP)

A

Decrease in Na/water reabsorption

Distal/collecting tubule & duct

26
Q

Parathyroid Hormone

A

Increase in Ca++ and Mg++ reabsorption

Decrease phosphate reabsorption

Proximal tubule, thick ascending loop, distal/collecting tubule

27
Q

Antidiuretic Hormone (vasopressing) Mechanism

A

End result: increase water reabsorption

  • late distal tubules, collecting tubules, collecting ducts
  • binds to V2 receptors –> stimulate aquaporin-2 (AQP-2) to move to luminal membrane, cluster & fuse together to form water channels (aquaporins)
  • AQP-3 & AQP-4 on basolateral membrane, permit diffusion out of cells (not regulated by ADH)
  • As ADH ↓, AQPs are shuttled back to cytoplasm removing water channels
28
Q

Calculating GFR

A

(Ucreatinine x Vurine)/ Plasmacreatinine

Can be estimated based on creatinine because creatinine is essentially freely filtered w/o significant secretion/reabsorption

29
Q

Calculating Renal Plasma Flow

A

RPF = Us x V/Ps

  • Substance S must be completely cleared for this to work (requires secretion not just filtration)
  • RPF is then equal to clearance of the substance (PAH is ~90% cleared so can be an estimator)
30
Q

Calculating Extraction Ratio

A

[Ps(renal arterial) – Ps(renal veinous)] / Ps(renal arterial)

Fraction of substance passing through kidney that is excreted

31
Q

Calculating Filtration Fraction (FF)

A

GFR/RPF

Fraction of plasma that passes through glomerular membrane

BE 203 (13 decks)

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