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Review
Name the 4 regions of the renal tubules
2. The structural and functional regions that form
urine are the __________.
3. Micturition is controlled by the ____________
nervous system.
4. How is filtrate different that urine? (2 sentences)
1.
Part 2
Urine Formation
 Filtrate
 Blood plasma minus most proteins
 Urine
 <1% of total filtrate
 Contains metabolic wastes and unneeded substances
Urine Formation
Glomerular filtration
Tubular reabsorption
1.
2.
Returns components to blood


3.
Glucose amino acids, water and salt
Tubular secretion
 Reverse of reabsorption
 Selective addition to urine
4.
Water conservation
Afferent arteriole
Glomerular capillaries
Efferent arteriole
Cortical
radiate
artery
Glomerular capsule
Rest of renal tubule
containing filtrate
Peritubular
capillary
Three major
renal processes:
Glomerular filtration
Tubular reabsorption
Tubular secretion
To cortical radiate vein
Urine
Figure 25.10
Glomerular Filtration
 Occurs at renal corpuscle
 Passive process driven by hydrostatic pressure
 Glomerulus is a very efficient filter
 Permeable membrane

Water and small solutes pushed through filter
 Large surface area
 Higher blood pressure
Efferent
arteriole
Glomerular capsule
Glomerulus
Afferent
arteriole
Parietal layer
of glomerular
capsule
Capsular
space
Foot processes
of podocytes
Podocyte cell
body (visceral
layer)
Red blood cell
Proximal
tubule cell
Efferent
arteriole
Juxtaglomerular
apparatus
• Macula densa cells
of the ascending limb
of loop of Henle
• Extraglomerular
mesangial cells
• Granular cells
Afferent arteriole
Juxtaglomerular
apparatus
Lumens of
glomerular
capillaries
Endothelial cell
of glomerular
capillary
Mesangial cells
between capillaries
Renal corpuscle
Figure 25.8
Capillary
Plasma
Fenestration
(pore)
Filtration membrane
• Capillary endothelium
• Basement membrane
• Foot processes of podocyte
of glomerular capsule
Filtration
slit
Slit
Filtrate in diaphragm
capsular
space
Foot processes
of podocyte
(c) Three parts of the filtration membrane
Filtration slits
Podocyte
cell body
Foot
processes
(b) Filtration slits between the podocyte foot processes
Efferent
arteriole
Glomerular capsular space
Proximal
convoluted
tubule
Afferent
arteriole
Glomerular capillary
covered by podocytecontaining visceral
layer of glomerular
capsule
Cytoplasmic extensions
of podocytes
Filtration slits
Parietal layer
of glomerular
capsule
(a) Glomerular capillaries
and the visceral layer of
the glomerular capsule
Podocyte
cell body
Fenestrations
(pores)
Glomerular capillary
endothelium (podocyte
covering and basement
membrane removed)
Foot processes
of podocyte
Figure 25.9a
Glomerular Filtration
 Filtration membrane
 Allows passage of water and small solutes


Fenestrations prevent filtration of blood cells
Negatively charged basement membrane repels large anions
Glomerular Filtration
 Net Filtration Pressure (NFP)
 Glomerular hydrostatic pressure (HPg)
 Capsular hydrostatic pressure (HPc)
 Blood osmotic pressure (OPg)
Glomerular Filtration
 Net Filtration Pressure (NFP)
 The pressure responsible for filtrate formation
NFP = HPg – (OPg + HPc)
Afferent
arteriole
Glomerular
capsule
10
mm
Hg
Net
filtration
pressure
Glomerular (blood) hydrostatic pressure
(HPg = 55 mm Hg)
Blood colloid osmotic pressure
(Opg = 30 mm Hg)
Capsular hydrostatic pressure
(HPc = 15 mm Hg)
Figure 25.11
Net Filtration Pressure
NFP = HPG – (OPG + HPC)
= 55 mm Hg – (30 mmHg + 15 mmHg)
= 10 mmHg
Glomerular Filtration
 Glomerular Filtration Rate (GFR)
 125 ml/min
1800 liters of blood through kidneys/day
= 1200 ml/min = about 180 liters filtrate/day
Glomerular Filtration
 Factors affecting GFR
 Kidney disease

↓ blood osmotic pressure, ↑ capsular osmotic pressure
 Hemorrhage

↓ glomerular blood hydrostatic pressure
 Hypotension

Glomerular blood hydrostatic pressure = capsule hydrostatic
and blood osmotic pressure = filtration stops!
 Renal suppression
Glomerular Filtration
 GFR is tightly controlled by two types of mechanisms
 Intrinsic controls (renal autoregulation)

Act locally within the kidney
 Extrinsic controls

Nervous and endocrine mechanisms that maintain blood
pressure and affect kidneys
Glomerular Filtration
 Intrinsic controls
 Maintain a nearly constant GFR when MAP is in the
range of 80–180 mm Hg
 Renal autoregulation

Mechanisms that cause vasoconstriction of afferent arterioles
in response to increased BP
Extrinsic Controls
 Sympathetic nervous system
 At rest



Renal blood vessels are dilated
Renal autoregulation mechanisms prevail
GFR maintained
 Extreme stress


Norepinephrine and epinephrine released
Both cause constriction of afferent arterioles
 Inhibits filtration
 Shunts blood to other vital organs
Tubular Reabsorption





125 ml/min of filtrate produced
Most of this fluid is reabsorbed
A selective transepithelial process
Includes active and passive process
Most occurs in PCT
Afferent arteriole
Glomerular capillaries
Efferent arteriole
Cortical
radiate
artery
Glomerular capsule
Rest of renal tubule
containing filtrate
Peritubular
capillary
Three major
renal processes:
Glomerular filtration
Tubular reabsorption
Tubular secretion
To cortical radiate vein
Urine
Figure 25.10
Tubular Reabsorption
 PCT
 Site of most reabsorption




65% of Na+ and water
All nutrients
Ions
Small proteins
Tubular Reabsorption
 Transcellular route
Luminal membranes of tubule cells → cytosol of tubule
cells → basolateral membranes of tubule cells →
endothelium of peritubular capillaries
Movement via the
transcellular route
involves:
1
Transport across the
luminal membrane.
2 Diffusion through the
cytosol.
3
Transport across the basolateral
membrane. (Often involves the lateral
intercellular spaces because
membrane transporters transport ions
into these spaces.)
4 Movement through the interstitial
fluid and into the capillary.
Lateral intercellular space
Tight junction
Filtrate
in tubule
lumen
The paracellular route
involves:
• Movement through
leaky tight junctions,
particularly in the PCT.
Interstitial
fluid
Capillary
endothelial
cell
Tubule cell
Peritubular
capillary
Paracellular
H2O
1
2
3
4
Transcellular
Luminal
membrane
1
Transcellular
3
4
2
Solutes
3
Paracellular
4
Basolateral
membranes
Active
transport
Passive
transport
Figure 25.13
Tubular Reabsorption
 Paracellular route
 Between cells
 Limited to water movement and reabsorption of Ca2+,
Mg2+, K+, and some Na+ in the PCT where tight junctions
are leaky
Movement via the
transcellular route
involves:
1
Transport across the
luminal membrane.
2 Diffusion through the
cytosol.
3
Transport across the basolateral
membrane. (Often involves the lateral
intercellular spaces because
membrane transporters transport ions
into these spaces.)
4 Movement through the interstitial
fluid and into the capillary.
Lateral intercellular space
Tight junction
Filtrate
in tubule
lumen
The paracellular route
involves:
• Movement through
leaky tight junctions,
particularly in the PCT.
Interstitial
fluid
Capillary
endothelial
cell
Tubule cell
Peritubular
capillary
Paracellular
H2O
1
2
3
4
Transcellular
Luminal
membrane
1
Transcellular
3
4
2
Solutes
3
Paracellular
4
Basolateral
membranes
Active
transport
Passive
transport
Figure 25.13
Tubular Reabsorption
 Sodium
 Most abundant cation in filtrate
 Primary active transport out of the tubule cell by
Na+-K+ ATPase in the basolateral membrane
 Na+ passes in through the luminal membrane by
secondary active transport or facilitated diffusion
mechanisms
1 At the basolateral membrane,
Nucleus
Filtrate
in tubule
lumen
Na+
Tubule cell
3Na+
1
2K+
2K+
3
K+
4
Lipid-soluble 5
substances
Cl–, Ca2+, K+
and other
ions, urea
Peritubular
capillary
3 Reabsorption of organic
2
3Na+
Glucose
Amino
acids
Some
ions
Vitamins
H2O
Interstitial
fluid
Na+ is pumped into the
interstitial space by the Na+-K+
ATPase. Active Na+ transport
creates concentration gradients
that drive:
2 “Downhill” Na+ entry at the
luminal membrane.
6
Tight junction
Primary active transport
Secondary active transport
Passive transport (diffusion)
Cl–
Paracellular
route
Transport protein
Ion channel or aquaporin
nutrients and certain ions by
cotransport at the luminal
membrane.
4 Reabsorption of water by
osmosis. Water reabsorption
increases the concentration of
the solutes that are left
behind. These solutes can
then be reabsorbed as
they move down their
concentration gradients:
5 Lipid-soluble
substances diffuse by the
transcellular route.
6 Cl– (and other anions),
K+, and urea diffuse by the
paracellular route.
Figure 25.14
Tubular Reabsorption
 Sodium
 Low hydrostatic pressure and high osmotic pressure in
the peritubular capillaries

Promotes bulk flow of water and solutes (including Na+)
Tubular Maximum
 Transport maximum (Tm) reflects the number of
carriers in the renal tubules available
 When the carriers are saturated, excess of that substance
is excreted
 Example: too much glucose in the blood entering
glomerulus will cause glucosuria
Tubular Reabsorption
 Reabsorption of nutrients, water, and ions
 Blood becomes hypertonic to filtrate
 Water is reabsorbed by osmosis
 Cations and fat-soluble substances follow by diffusion
1 At the basolateral membrane,
Nucleus
Filtrate
in tubule
lumen
Na+
Tubule cell
3Na+
1
2K+
2K+
3
K+
4
Lipid-soluble 5
substances
Cl–, Ca2+, K+
and other
ions, urea
Peritubular
capillary
3 Reabsorption of organic
2
3Na+
Glucose
Amino
acids
Some
ions
Vitamins
H2O
Interstitial
fluid
Na+ is pumped into the
interstitial space by the Na+-K+
ATPase. Active Na+ transport
creates concentration gradients
that drive:
2 “Downhill” Na+ entry at the
luminal membrane.
6
Tight junction
Primary active transport
Secondary active transport
Passive transport (diffusion)
Cl–
Paracellular
route
Transport protein
Ion channel or aquaporin
nutrients and certain ions by
cotransport at the luminal
membrane.
4 Reabsorption of water by
osmosis. Water reabsorption
increases the concentration of
the solutes that are left
behind. These solutes can
then be reabsorbed as
they move down their
concentration gradients:
5 Lipid-soluble
substances diffuse by the
transcellular route.
6 Cl– (and other anions),
K+, and urea diffuse by the
paracellular route.
Figure 25.14
Afferent arteriole
Glomerular capillaries
Efferent arteriole
Cortical
radiate
artery
Glomerular capsule
Rest of renal tubule
containing filtrate
Peritubular
capillary
Three major
renal processes:
Glomerular filtration
Tubular reabsorption
To cortical radiate vein
Tubular secretion
Urine
Figure 25.10
Tubular Secretion
 Reabsorption in reverse
 K+, H+, NH4+, creatinine, and organic acids move from
peritubular capillaries or tubule cells into filtrate
 Involves active transport since no concentration
gradients in this case
Na+ (65%)
Glucose
Amino acids
H2O (65%) and
many ions (e.g.
Cl– and K+)
Milliosmols
Cortex
(d)
(a)
300
(e)
Outer
medulla
(b)
(c)
600
Some
drugs
–
H+, HCO3
NH4+
Inner
medulla
Blood pH regulation
1200
(a) Proximal convoluted tubule:
• 65% of filtrate volume reabsorbed
• Na+, glucose, amino acids, and other nutrients actively
transported; H2O and many ions follow passively
• H+ and NH4+ secretion and HCO3– reabsorption to
maintain blood pH (see Chapter 26)
• Some drugs are secreted
Active transport
(primary or
secondary)
Passive transport
Figure 25.18a
Tubular Secretion
 Principle effects
 Rids body of



Foreign substances (penicillin and other drugs)
Nitrogenous wastes
Excess K+
 Controls blood pH:

Altering amounts of H+ or HCO3– in urine
Review continued…
5. Name the 4 steps to urine formation
6.
7.
Dilute urine = _________ urine
Concentrated urine = ___________ urine
Questions?