Transcript Excretion-2

Chapter 23 -2
The Urinary System
Basic processes of urine formation
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Slide # 2
• Filtration
• Blood pressure
• Water and solutes across glomerular capillaries
• Reabsorption
• The removal of water and solutes from the filtrate
(95-99% of fluid filtered is reabsorbed)
• Secretion
• Transport of solutes from the peritubular fluid into the
tubular fluid
Renal function
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• Most regions of the nephron perform a combination of
functions
• General functions can be identified as
• Filtration in the renal corpuscle
• Nutrient reabsorption along the PCT
• Ion and water reabsorption along entire nephron and
collecting ducts
• Active secretion at PCT, DCT, and collecting ducts
• Loop of Henle regulates final volume and solute
concentration
Reabsorption and secretion
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• Accomplished via diffusion, osmosis, and carriermediated transport
• Transcellular transport
• Substances pass through cells of tubule wall.
• Apical membrane: surface that faces filtrate. The
lumenal side.
• Basolateral membrane: faces interstitial fluid. The
sides the part of the cell attached to the basement
membrane
• Paracellular transport – between the cells
Mechanisms of Reabsorption
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Basolateral
membrane
lumen
apical membrane
Paracellular transport
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1
2
3
3
H2O
4
solute
1 solute uptake
2 tranport to basolateral space
3 osmotic gradient pulls water in
4 solvent drag brings in more solute
Carrier Mediated Transport
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• See chapter 3
•
•
•
•
Facilitated diffusion
Active transport
Cotransport
Countertransport
• Carrier proteins have a transport maximum (Tm)
• Determines renal threshold - plasma concentration of
a solute at which it will begin appearing in the urine.
Tubular Maximum (Tm)
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Tm renal threshold
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Tubular maximum
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Figure 27-4;
Guyton and Hall
Overview of Urine Formation
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From Martini
Reabsorption and secretion
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• Glomerular filtration produces fluid similar to plasma
without proteins
• The PCT reabsorbs 60-70% of the filtrate produced
• Reabsorption of most organic nutrients (glucose,
amino acids, lipids, etc)
• Active and passive reabsorption of sodium,
bicarbonate, and other ions
• Reabsorption of water
• Secretion also occurs in the PCT
PCT transport systems
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• Glucose and amino acids
• Sodium
• Bicarbonate
• roles of carbonic anhydrase
CO2 + H2O
H2CO3
• Chloride
• Water channels (aquaporins)
H+ + HCO3-
Transport Activities at the PCT
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Basolateral membrane
Apical membrane
From Seeley, Stephens and Tate
NaHCO3 Reabsorption in the PCT
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From Seeley, Stephens and Tate
The loop of Henle
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• Reabsorbs ~50% remaining water and 66% remaining
solute (about 15% of the filtered water and 25% of the filtered solute)
• Thin segment - concentrates the tubular fluid
• Has water channels to reabsorb water
• Secretes urea into the tubules
• TAL - dilutes the tubular fluid
• Transports Na+/K+/2Cl-
Na+/K+/2 Cl- cotransport in TAL
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Lasix
(furosemide)
From Martini Figure 26.13a
Reabsorption and secretion at the DCT
• DCT performs final adjustment of urine
• Active secretion or absorption
• Absorption
• Tubular cells actively reabsorb Na+ and Cl• In exchange for K+ or H+ (secreted)
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Secretion and Reabsorption at the DCT
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From Martini Figure 26.14
HCO3-
+
and NH4 transport in the DCT
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From Martini Figure 26.14c
Reabsorption and secretion along the
collecting ducts
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• Water and solute loss is regulated by aldosterone and
ADH
• Reabsorption
• Sodium ion, bicarbonate (explained in next chapter), and urea
• Secretion
• hydrogen ions (affected by acid base status as described in next chapter)
Cells of the collecting ducts
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• Principle cells
• Water channels (aquaporins)
• Na + channels
• Sodium reabsorption is regulated by aldosterone and
water reabsorption is regulated by ADH
• Intercalated cells
• Mostly secrete H+ in exchange for K+ via an H+ /K+
ATPase similar to the stomach
• Also have an H+ ATPase
Control of urine volume and osmotic
concentration
• Final urine volume and osmotic concentration are
regulated by controlling water reabsorption in the
collecting ducts
• Precise control allowed via facultative water
reabsorption
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Effects of ADH on DCT & Collecting Ducts
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Figure 26.15a, b
Balance and homeostasis
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Osmotic concentrations in the kidney
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Figure 25.13
Countercurrent multiplication
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• Loop of Henle
• Functional differences between ascending and
descending limbs of loop
• Creates osmotic gradient in medulla
• Facilitates reabsorption of water and solutes before
the DCT
• Permits passive reabsorption of water from tubular
fluid
• Collecting duct reabsorbs urea and sends it back to the
descending loop – keeps recycling and thus
concentrating the urea
Cortical and Juxtamedullary Nephrons
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Urea
• Responsible for large part of
high osmolality in medulla
• Descending limbs of loops of
Henle permeable to urea; urea
diffuses from the interstitial
fluid into the tubule (is
secreted)
• Ascending limbs and distal
tubules are impermeable to
urea
• Collecting ducts permeable to
urea; some diffuses out into
interstitial fluid
• Urea flows in a cycle
maintaining high urea
concentration in medulla
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Countercurrent Multiplication and
Concentration of Urine
Also see Saladin fig 23.20 on slide 32
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Summary of loop of Henle transport
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From Seeley, Stephens and Tate
The Multiplier System (positive feedback)
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Cortical and Juxtamedullary Nephrons
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From Martini Figure 26.7a
Summary table
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Sunmmary table continued
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(Also see Fig 23.22 for a great summary of transport across
the tubule segments and Table 23.1 for hormonal control of
the kidney)
The ureters
• Pair of muscular tubes
• Extend from renal pelvis to the bladder
• Peristaltic contractions force urine toward the
urinary bladder
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The urinary bladder
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• Hollow, muscular organ
• Reservoir for the storage of urine
• Contraction of detrusor muscle voids bladder (read book
for micturation reflex)
• Internal features include
• Trigone
• Neck
• Internal urethral sphincter
• Rugae
Gross Anatomy of the Urinary Bladder
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Figure 26.19c
The urethra
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• Extends from the urinary bladder to the exterior of the
body
• Passes through external urinary sphincter (urogenital
diaphragm)
• Differs in length and function in males and females
Male transverse section
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From Martini Figure 26.19a
Female transverse section
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From Martini Figure 26.19b
Composition of normal urine
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• Varies with the metabolic and hormonal events of the
body
• Reflects filtration, absorption and secretion activity of
the nephrons
• Urinalysis is the chemical and physical analysis of urine
• color
• pH (4.5-8.2)
• specific gravity (measure of concentration of urine water is 1.000)_
• Protein (not normally present)
• glucose (not normally present)
• ions, hormones, drugs, etc