LECTURE 7:
REGULATION OF WATER AND
ELECTROLYTE BALANCE (I)
RENAL FUNCTION
Eamonn O’Connor
Trinity College Dublin
Lecture Outline
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Functions of the Urinary System
  Anatomy of the Urinary System
  Basic Renal Exchange Processes
 
  Filtration
  Reabsorption
  Secretion
  Excretion
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I. Functions of the Urinary System
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 
Main functions of the urinary system:
 
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Other functions of the kidney:
 
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Anatomy of the Urinary System: Structures
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 
Kidneys – form urine
 
Ureters – transport urine
from kidneys to bladder
 
Bladder – store urine
 
Urethra – excrete urine
from bladder to outside
of body
Stanfield 4th Ed: Figure 18.1
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Anatomy of the Kidney
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 
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Paired, bean shaped
Approximate size of fist, 115–170
grams
Retroperitoneal
Less than 1% of body weight
Receive 20% of cardiac output at rest
Utilize 16% of ATP usage by body
Function is to filter blood
Renal arteries enter kidney at hilus
Renal veins exit at hilus
Stanfield 4th Ed: Figure 18.2
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Anatomy of the Nephron
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Nephron = functional unit
  Renal corpuscle
 
  Glomerulus
= capillary
network for filtration
  Bowman’s capsule
  Receives
the filtrate
  Inflow to renal tubules
Stanfield 4th Ed: Figure 18.3
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Renal Tubules
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 
Proximal tubule:
  Proximal
convoluted tubule
  Proximal straight tubule
 
Loop of Henle:
  Descending
limb
  Thin ascending limb
  Thick ascending limb
 
Distal convoluted tubule:
  Connecting
 
tubule
Collecting duct
Stanfield 4th Ed: Figure 18.3
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Cortical/Juxtamedullary Nephrons
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 
Juxtamedullary nephron
Long loop of Henle
extends into medulla
  Responsible for the
medullary osmotic
gradient
 
 
Cortical
 
 
 
Short loop of Henle
Most numerous, 80–85%
Both types produce
urine
Stanfield 4th Ed: Figure 18.4
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Juxtaglomerular Apparatus
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Stanfield 4th Ed: Figure 18.5
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Kidney and Nephron Blood Supply
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Kidney
Nephron
Stanfield 4th Ed: Figure 18.6
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Basic Renal Exchange Processes
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 
Glomerular filtration – from glomerulus to Bowman’s
capsule
  Glomerular
filtration rate = 125 mL/min or 180 liters/
day
Reabsorption – from tubules to peritubular
capillaries
  Secretion – from peritubular capillaries to tubules
  Excretion – from tubules out of body
 
Stanfield 4th Ed: Figure 18.7
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Starling Forces & Filtration Across Glomerulus
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 
Forces in favour of filtration:
  Glomerular
  High
due to resistance of efferent arteriole
  Bowman’s
  Low
 
capillary hydrostatic pressure: 60 mm Hg
capsule oncotic pressure: 0 mm Hg
due to lack of protein in filtrate
Forces opposing filtration:
  Bowmans’
capsule hydrostatic pressure: 15 mmHg
  Relatively
high (compared to systemic capillaries) due to large
volume of filtrate in closed space
  Glomerular
oncotic pressure: 29 mmHg
  Higher
than in systemic capillaries due to plasma proteins in
smaller volume of plasma
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Glomerular Filtration Pressure/
Glomerular Filtration rate (GFR)
Stanfield 4th Ed: Figure 18.9 a & b
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Filtered Load
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Filtered Load = GFR x Px
  Small molecules that are filtered without impedance
are freely filterable
 
  Quantity
filtered = filtered load
  Depends
on plasma concentration of solute
and GFR
 
Example: Filtered Load of Glucose
 
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Regulation of GFR
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 
180 liters fluid filtered/day
  Only
1.5 liters urine excreted/day (<1%)
  >99% of filtered fluid is reabsorbed
 
 
 
Small increase in GFR  large increase volume fluid
filtered and excreted
GFR highly regulated
Intrinsic and extrinsic mechanisms
  Intrinsic
mechanisms:
  Myogenic
regulation
  Tubuloglomerular feedback
  Extrinsic
mechanisms:
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Effect of MAP on GFR
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Stanfield 4th Ed: Figure 18.10
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Intrinsic control of GFR: Myogenic Regulation &
Tubuloglomerular Feedback
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Stanfield 4th Ed: Figure 18.11
Extrinsic Control of GFR
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 
Decreases in MAP (<80mmHg)
can decrease GFR
  Directly
(through decrease in
filtration pressure)
  Indirectly (through extrinsic controls)
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Stanfield 4th Ed: Figure 18.12
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Reabsorption
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 
Movement from tubules into peritubular capillaries
(returned to blood)
  Most
occurs in proximal tubule
  Some
  Most
 
in distal convoluted tubule
is not regulated
Barriers to reabsorption
  Epithelial
cells of renal tubules
  Endothelial cells of capillary (minimal)
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Reabsorption Barrier
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Stanfield 4th Ed: Figure 18.13
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Transport Maximum
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Rate of transport when carriers are saturated
  When solute transported across epithelium by
carrier protein, saturation of carriers can occur
  Renal Threshold
  For a solute which is normally 100% reabsorbed
 
  If
solute in filtrate saturates carriers, then some solute
excreted in urine
  Solute in plasma that causes solute in filtrate to saturate
carriers and spillover into urine = renal threshold
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Example: Glucose Reabsorption
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 
 
 
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Freely filtered at glomerulus
Normally 100% actively reabsorbed in proximal tubule
Normally, no glucose appears in urine
Carrier proteins for glucose reabsorption
 
 
Apical membrane: secondary active transport
Basolateral membrane: facilitated diffusion
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Stanfield 4th Ed: Figure 18.15
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Glucose Renal Curve
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Stanfield 4th Ed: Figure 18.16
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Secretion
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Solute moves from peritubular capillaries
into tubules
  Barriers same as for reabsorption
  Transport mechanisms same but opposite direction
  Secreted substances: Potassium, Hydrogen ions,
Choline, Creatinine, Penicillin
 
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Excretion
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Amount of substance excreted = amount filtered
+ amount secreted – amount reabsorbed
  Amount excreted depends on
 
  Filtered
load
  Secretion rate
  Reabsorption rate
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