Urinary Physiology

1
Urinary Physiology

• Central Texas College
• BIOL 2402
• Chelsea Loafman, MS

2
Urinary System Anatomy

• See figure 19-1

3
Nephrons

• Functional unit of the kidney
• 80 cortical nephrons
• 20 juxtamedullary nephrons
• Nephron is a collection of tubules
• Bowmans capsule
• Proximal tubule
• Loop of Henle
• Descending limb
• Ascending limb
• Distal tubule
• Collecting duct
• Surround by a dense network of blood vessels
• Afferent arteriole
• Glomerulus in Bowmans capsule (together makes
  the renal corpuscle)
• Efferent arteriole
• Peritubular capillaries
• Vasa recta projects into medulla
• Juxtaglomerular apparatus overlap between
  ascending limb and arterioles allowing
  autoregulation

• See figure 19-1 in text

4
Processes of Fluid/Solute Movement

• In order to produce urine and filter the blood in
  the proper way, three processes occur
• Filtration
• Moves fluid from blood into lumen of the nephron
  creating filtrate
• Reabsorption
• Moves substances in the filtrate back into the
  blood
• Secretion
• Removes molecules from the blood and adds them to
  filtrate

• See figure 19-2 in text

5
Modification of Fluid

• 180 liters filtered into Bowmans capsule per day
• Fluid is normal body osmolarity (300mOsm)
• At then end of the proximal tubule
• Volume54 L/day
• Osmolarity300mOsm
• After leaving the loop of Henle
• Volume18 L/day
• Osmolarity100mOsm
• At the end of the collecting duct (urine)
• Volume1.5 L/day
• Osmolarity50-1200mOsm
• Final volume and osmolarity depend on bodys
  physiology, water balance, and electrolyte balance

• See figure 19-3 in text

6
Filtration Fraction

• Filtrate filtered off the blood should only
  contain water and dissolved solutes
• 1/5 of plasma entering kidneys is filtered off
• This leaves 4/5 to flow through the peritubular
  capillaries for secretion and reabsorption
• Percentage of total plasma that is filtered into
  the tubule is the filtration fraction
• See figure 19-4 in text

7
Renal Corpuscle

• 3 filtration barriers that must be crossed
• Glomerular capillary endothelium
• Fenestrated capillaries contain large pores
• Mesangial cells have actin-like filaments
  allowing for contraction to alter blood flow
• Basement membrane (basal lamina)
• Excludes most proteins from crossing
• Bowmans capsule epithelium
• Made up of podocytes that create filtration slits

• See figure 19-5 in text

8
Filtration at the Glomerulus

• Hydrostatic pressure in glomerular capillaries
  (PH) forces fluid out of leaky endothelium
• Average 55mmHg
• Colloid osmotic pressure inside glomerular
  capillaries (p) draws fluid back into capillaries
• Averages 30mmHg
• Hydrostatic pressure inside Bowmans capsule
  (Pfluid) moves fluid into the capillaries
• Averages 15mmHg
• Creates an overall net filtration

• See figure 19-6

9
Glomerular Filtration Rate (GFR)

• Volume of fluid filtering into Bowmans capsule
  per unit time
• Average is 125ml/min or 180L/day
• Plasma volume filtered about 60 times per day
• 2 factors influence rate
• Surface area of glomerular capillaries
• Permiablity of capillary and capsule interface
• Rate fairly constant

• See figure 19-7 in text

10
Glomerular Filtration Rate

• See figure 19.8 in text

11
GFR Autoregulation

• Myogenic Response
• Smooth muscles in arteriole walls stretch with
  increased BP leading to contraction of the
  muscles
• GFR drops when BP dips below 80mmHg
• Tubuloglomerular feedback
• Amount of fluid moving through loop of Henle
  alters flow through glomerulus due to paracrine
  secretions
• Ascending loop of Henle passes between afferent
  and efferent arteriole
• Creates the Juxtaglomerular apparatus
• Macula densa modified tubule epithelial cells
• Granular cells modified smooth muscle cells of
  the afferent arteriole

12
Juxtaglomerular Apparatus

• See figure 19-10 in the text

13
Influences of GFR

• Hormones and autonomic nervous system alter GFR
  through changes in arteriole resistance and
  filtration coefficient
• Autonomic control
• Sympathetic innervation to afferent and efferent
  arterioles causes vasoconstriction to decrease
  GFR and retain fluid volume
• Hormones
• Angiotension II vasoconstrict
• Prostaglandins vasodilate
• Both affect filtration coefficient by acting on
  podocytes and mesangial cells

14
Reabsorption

• 99 of filtered fluid must be reabsorbed to
  maintain fluid balance
• Peritubular capillaries enhance reabsorption
• Hydrostatic pressure10mmHg
• Colloid osmotic pressure 20mmHg
• Active transport moves water and solutes from
  tubules to interstitial fluid
• Solute moved actively and water follows
  osmolarity
• Accomplished through 2 mechanisms
• Epithelial transport cross apical and basal
  surface
• Paracellular pathway movement through junctions
  between cells

15
Active Sodium Transport

• See figure 19-12 in text

16
Sodium Linked Glucose Reabsorption

• See figure 19-13 in text

17
Reabsorption of Other Molecules

• Urea moves across membrane through diffusion due
  to urea concentration gradient (passive)
• Urea becomes more concentrated inside tubules
  once water leaves due to osmotic gradient
• Proteins taken into cells of the proximal tubule
  through receptor mediated endocytosis
• Digested by enzymes
• Delivered to extracellular fluid through
  exocytosis

18
Saturation

• See figure 19-14 in text

19
Glucose Handling

• Glucose excretedglucose filtered-glucose
  reabsorbed
• See figure 19-15 in text

20
Secretion

• Transfers molecules from ECF to nephron lumen
• Allows enhanced secretion of substances
• K and H secretion important to maintain
  homeostasis
• Active transport processes because particles
  moved against concentration gradient

21
Excretion

• Excretion is the result of all three processes to
  create urine output
• Excretionfiltration-reabsorptionsecretion
• Excretion rate of a substance does not tell us
  how the substance was handled in the nephron
• Excretion rate of a substance depends on
• Filtration rate
• If the substance is reabsorbed, secreted, or both

22
Renal Clearance

• Rate that a solute disappears from the body by
  excretion or metabolism
• Expressed as volume of plasma moving through
  kidneys that has been cleared of a substance in a
  given time, not how much has been excreted

23
Inulin Clearance

• Rate that a solute disappears from the body by
  excretion or metabolism
• Expressed as volume of plasma moving through
  kidneys that has been cleared of a substance in a
  given time, not how much has been excreted
• Indicates glomerular filtration rate
• Example Inulin clearance
• GFRexcretion rate of inulin/inulin concentration
  in plasma
• Samples of blood and urine needed

• See figure 19-16 in text

24
Renal Handling

• Assuming the solute is freely filtered
• Filtered loadconcentration of solute x GFR
• Glucose
• Filtered load(100mg glucose/100ml plasma) x 125
  ml plasma/min
• 125 mg glucose/min
• Clinically important to determine the net
  handling of a solute for diagnostic purposes

25
Renal Handling

• See table 19-2 in text

26
Clearance and Excretion

• See figure 19-17 in text

27
Micturition Reflex

• See figure 19-18 in text