Urinary System
Topics & Objectives
Kidney
1.
•
•
Anatomy
Function
2.
Glomerular filtration
3.
Tubular reabsorption & secretion
4.
Urine excretion & plasma clearance
Kidney Functions:

Ultimately regulate ECF volume (receive ~
20% of cardiac output!)
1. Maintain H2O balance in the body
2. Maintain osmolarity
3. Regulation of ECF ions
•
Na+, Cl-, K+, H+, etc.
4. Maintain plasma volume & acid-base balance
5. Excretion of end products and foreign compounds
6. Producing EPO & renin
Renal
cortex
Renal
pyramid
Renal
medulla
Renal
pelvis
Renal
vein
Inferior
vena cava
Urinary
bladder
Urethra
Renal
artery
Ureter
Kidney
Aorta
Ureter
Figure 14.1
Page 513
Nephrons
•
~ 1 million within kidney
Medulla
Cortex
 Functional unit for urine
formation
•
Arrangement comprises renal
cortex & renal medulla
•
Each nephron composed of:
1. Vascular component
2. Tubular component
Proximal tubule
Juxtaglomerular
apparatus
Distal
tubule
Collecting
duct
Efferent
arteriole
Afferent
arteriole
Bowman’s
capsule
Glomerulus
Cortex
Medulla
Figure 14.3
Page 514
Peritubular
capillaries
Loop of
Henle
To renal
pelvis
Vascular Component:

Glomerulus
• Location for H2O and
solute filtration from
blood
Efferent
arteriole
Afferent
arteriole
Glomerulus

Arterioles
• Afferent: to glomerular
capillaries
• Efferent: drains
capillaries
No O2 extraction!

Peritubular capillaries
• Supply renal tissue
Peritubular
capillaries
Tubular Component:
Bowman’s capsule

•

Proximal tubule
Distal
tubule
Juxtaglomerular
apparatus
Collects fluid from
glomerulus
Fluid travels to:
Bowman’s
capsule
1. Proximal tubule
2. Loop of Henle
3. Passes through
juxtaglomerular
apparatus
 Vascular/tubular component
4. Distal tubule
Loop of
Henle
Cortical nephron
Juxtamedullary
nephron
Urine Formation
Glomerular filtration (protein-free)
1.
•
•
~ 20% of the plasma (1st step of urine formation)
~ 50 gallons each day (PV ~ 65x/day)
Tubular reabsorption
2.
•
Of the 50 gallons filtered, about 98% reabsorbed
Tubular secretion
3.
•
~ 80% of the plasma into the peritubular
capillaries
Efferent
arteriole
Afferent
arteriole
Glomerulus
80% of the plasma
that enters the
glomerulus is
not filtered
and leaves through
the efferent arteriole.
GF
Bowman’s
capsule
20% of the
plasma
that
enters the
glomerulus
is filtered.
TR
TS
Kidney
tubule
(entire
length,
uncoiled)
Urine excretion
(eliminated
from the body)
Peritubular
capillary
To venous system
(conserved
for the body)
Figure 14.6
Page 516
Figure 14.7
Page 517
Blood
pathway
Filtrate
pathway
Glomerular
capillaries
Glomerular
filtration
Bowman’s
capsule
Efferent
arteriole
Venous
blood
Peritubular
capillaries
Tubular
reabsorption
Tubular
secretion
Tubule (from proximal
tubule to collecting duct)
Urine
Glomerular Filtration
Afferent arteriole
Efferent arteriole
Glomerulus
Glomerular
capillary
Bowman’s
capsule
Basement
membrane
Proximal convoluted tubule
Figure 14.8 (1)
Page 518
Glomerular Filtration
Endothelial
cell
Lumen of glomerular
capillary
pores
Basement
membrane
podocytes
Lumen of
Bowman’s capsule
Figure
14.8 (3)
Page 518
Glomerular Filtration (cont.):

Occurs through pressure gradients…
1. Capillary blood pressure (~55mmHg)
 Favors filtration
2. Plasma osmotic pressure (~30mmHg)
 Caused by distribution of plasma proteins across
glomerular membrane
 Cannot cross into Bowman’s capsule
3. Bowman’s capsule hydrostatic pressure
(~15mmHG)
 Pressure by the fluid

All three pressures determine filtration rate!
1) Changes in BP
2) Osmotic
pressure
Look at Table 14.1!
3) Hydrostatic pressure
GFR Regulation
1.
Autoregulation
• Prevents spontaneous changes in GFR
 Vasoconstriction & vasodilation
a. Myogenic mechanism – response to stretch
b. Tubuloglomerular feedback mechanism
2.
Extrinsic sympathetic control
• Long-term regulation of arterial BP
 Sympathetic nervous system (no
parasympathetic activity)
• Baroreceptor reflex
GFR autoregulation
1.
Alterations in arteriolar afferent & efferent
blood pressures
Glomerulus
Afferent arteriole
Arterial blood
pressure
(increases blood
flow into the
glomerulus)
Figure 14.10
Page 520
Glomerular
capillary
blood pressure
Net filtration
pressure
GFR
Efferent arteriole
GFR autoregulation (cont.)
Glomerulus
Afferent arteriole
Vasoconstriction
(decreases blood flow
into the glomerulus)
Glomerular
capillary
blood pressure
Net filtration
pressure
GFR
Efferent arteriole
GFR autoregulation
Glomerulus
Afferent arteriole
Glomerular
capillary
blood pressure
Net filtration
pressure
Vasodilation
(increases blood flow
into the glomerulus)
GFR
Efferent arteriole
GFR Autoregulation – Tubuloglomerular feedback
1. Smooth muscle cells
within afferent arteriole
 Granular cells – secretory
capabilities
Distal
tubule
Bowman’s
capsule
2. Tubular cells (macula
densa)
 Detect changes in the rate
of fluid passing through
tubule
Bring about
vasoconstriction or
vasodilation
Efferent
arteriole
Afferent
arteriole
Efferent
arteriole
Endothelial
cell
Lumen of
Bowman’s
capsule
Smooth
muscle
cell
Glomerular
capillaries
Macula
densa
Podocyte
Granular cells
Distal
tubule
Afferent
arteriole
Arterial blood pressure
Driving pressure into glomerulus
Glomerular capillary pressure
GFR
Rate of fluid flow
through tubules
Stimulation of macula densa cells
to release vasoactive chemicals
Chemicals released that induce
afferent arteriolar vasoconstriction
Blood flow into glomerulus
Glomerular capillary pressure
to normal
GFR to normal
Extrinsic Control – Baroreceptors

Response to decreased BP
• Sympathetically induced vasoconstriction
• Afferent arterioles (sympathetically innervated)

Response to increased BP
• Sympathetic stimulation decreases
Short-term
adjustment
for
Arterial
blood
pressure
Cardiac
output
Total
peripheral
resistance
Arterial blood
pressure
Detection by aortic
arch and carotid sinus
baroreceptors
Sympathetic activity
Generalized
arteriolar vasoconstriction
Afferent arteriolar
vasoconstriction
Glomerular capillary
blood pressure
GFR
Urine volume
Conservation of fluid and salt
Arterial blood pressure
Long-term
adjustment for
Tubular Reabsorption
Efferent
arteriole
Afferent
arteriole
Glomerulus
GF
Bowman’s
capsule
TR
Peritubular
capillary
Figure 14.6
Page 516
Tubular reabsorption:

Ultimately attempting to maintain body’s
internal environment
• Proper composition & volume
Table 14.2
Filtered substance
reabsorbed
Filtered substance
excreted
99
1
Sodium
99.5
0.5
Glucose
100
0
Urea (waste product)
50
50
Phenol (waste product)
0
100
Water

Material must pass through the cells (5 steps)
Tubular
lumen
Peritubular
capillary
Tubular
epithelial cell
Plasma
Tight
junction
1) Luminal
membrane
4) Interstitial
fluid
2) Cytosol
3) Basolateral
membrane
5) Capillary
wall
Figure 14.17
Page 526
Transepithelial transport
Passive & Active Reabsorption
1.
Passive: all steps follow electrochemical or
osmotic gradients
2.
Active: any one of the steps requiring energy
•
Sodium (80% of kidney’s total energy
requirement)
 ~ 67% in proximal tubule
Constant percentage of
Na+ reabsorption
 ~ 25% in loop of Henle
 ~ 8% in distal and collecting tubules
•
•
Glucose
Phosphate

Na+ pumped out against concentration gradient
• Creates higher concentration in interstitial fluid &
allows for passive diffusion back into lumen
Lumen
Tubular cell
Interstitial fluid
Diffusion
Na+
channel
Active transport
Basolateral
Na+– K+ ATPase
carrier
Diffusion
Figure 14.18
Page 527
Peritubular
capillary

Distal tubule (~ 8% of
total reabsorption) is
hormonally regulated
• Related to total Na+ load in
body
Changes in ECF affect
osmotic pressures
ex: Increased Na+ in ECF
causes increased H2O in ECF
• Ultimately regulates blood
pressure
Renin-angiotensinaldosterone system ↑
Atrial natriuretic peptide ↓
Distal
tubule
Within juxtaglomerular apparatus…

Granular cells release renin
• In response to fall of NaCl/ ECF volume/ BP
• Recognized by intrarenal baroreceptors
Sympathetic response to secrete more renin
• Ultimately trying to increase plasma volume
NaCl /
ECF volume /
Arterial blood pressure
Liver
Kidney
Adrenal
cortex
Lungs
Kidney
H2O
conserved
Na+ (and CI–)
osmotically hold
more H2O in ECF
Renin
Angiotensinogen
Na+ (and CI–)
conserved
Angiotensinconverting
enzyme
Angiotensin I
Angiotensin II
Vasopressin
Thirst
H2O reabsorption
by kidney tubules
Fluid intake
Aldosterone
Na+ reabsorption
by kidney tubules
( CI–
reabsorption
follows passively)
Arteriolar
vasoconstriction
Figure
14.19
Page 529
Helps correct
NaCl / ECF volume /
Arterial blood pressure
Helps correct
Cardiac
atria
Atrial natriuretic
peptide
Na+ reabsorption
by kidney tubules
Salt-conserving
renin-angiotensinaldosterone system
Inhibits aldosterone
& renin secretion
Na+ excretion
in urine
Smooth muscle
of afferent arterioles
Afferent
arteriolar
vasodilation
GFR
Na+ and H2O
filtered
H2O excretion
in urine
Sympathetic
nervous system
Cardiac
output
Total
peripheral
resistance
Arterial blood
pressure
Figure
14.20
Page 530

Glucose and amino acid reabsorption
• Na+ dependent secondary active transport
Co-transporters that do not require energy
• Maximal reabsorption rate depends on substance
No energy
required
Cotransport carrier
Luminal border
Energy
required
Na+–K+ pump
No energy
required
Blood vessel
Glucose
carrier
Basolateral
border

Phosphate & Calcium reabsorption
• Dependent upon total body content
• Regulated by kidneys
Hormonally (parathyroid hormone)

Na+ reabsorption responsible for passive
reabsorption of Cl-, H2O, and urea

H2O (passive) reabsorption
• 80% in proximal tubules & loops of Henle
Lumen
Proximal tubular cell
Interstitial fluid
Peritubular
capillary
Osmosis
Figure 14.22
Page 533
Water
channel
Osmosis
Hydrostatic
pressure

Urea (passive)
reabsorption
• Waste product of
protein
• Becomes
increasingly
concentrated
Glomerulus
Bowman’s
capsule
Peritubular
capillary
125 ml
of
filtrate
Beginning of
proximal
tubule
Na+ (active)
H2O (osmosis)
Na+ (active)
H2O (osmosis)
Figure
14.23
Page 534
End of
proximal
tubule
= Urea molecules
44 ml
of
filtrate
Passive diffusion
of urea down its
concentration gradient
Tubular Secretion
Efferent
arteriole
Afferent
arteriole
Glomerulus
GF
Bowman’s
capsule
TS
Kidney
tubule
(entire
length,
uncoiled)
Peritubular
capillary
Figure 14.6
Page 516
Tubular secretion:

Hydrogen ions (H+)
• Acid-base regulation throughout the body

Potassium (K+)
• Early reabsorption into tubules not regulated
• Secretion in distal tubules regulated
Na+-K+ pump
Aldosterone

Organic anions & cations
• Foreign compounds, chemical messengers
Na+/ ECF volume/
arterial pressure
Aldosterone dual regulation
Renin
1) Na+ & K+
Angiotensin I
Plasma K+
Angiotensin II
Figure 14.25
Page 536
Aldosterone
Tubular K+ secretion
Urinary K+ excretion
Tubular Na+ reabsorption
Urinary Na+ excretion
Plasma Clearance
Substance:
1) Filtered
Peritubular
capillary
2) NOT reabsorbed
3) NOT secreted
Glomerulus
All filtered plasma is
cleared of substance
Tubule
In
urine
Fig. 14.26a Page 539
Substance:
1) Filtered
2) NOT secreted
3) Completely
reabsorbed
None of filtered plasma is
cleared of substance
Substance:
1) Filtered
2) NOT secreted
3) Partially
reabsorbed
Portion of filtered plasma is
cleared of substance
Substance:
1) Filtered
2) Secreted
3) NOT reabsorbed
ALL of filtered plasma is
cleared of substance
Regulation of plasma H2O

Fluids & solutes
• Normal balance in body fluids called isotonic
300 mosm/liter
• Hypotonic
Too much H2O compared to solute (osmolality < 300)
• Hypertonic
Too little H2O compared to solute (osmolality > 300)

Osmotic gradient maintained in interstitial fluid of
medulla
• Dependent upon hydration levels
From
proximal
tubule
To
distal
tubule
Medullary
interstitial fluid
Medullary
interstitial fluid
Descending limb
of loop of Henle
of juxtamedullary
nephron
Ascending limb
of loop of Henle
of juxtamedullary
nephron
Initial scene
Fig. 14.28a (2)
Page 542
Step 1
From
proximal
tubule
Step 2
To
distal
tubule
Step 3
From
proximal
tubule
Step 4
To
distal
tubule
Step 5
From
proximal
tubule
To
distal
tubule
Step 6 and on
Further regulation

Vasopressin
• Anti-diuretic hormone from hypothalamus
Tubular
lumen
filtrate
Water
channel
Distal tubular cell
Increases permeability of
luminal membrane to H2O
by inserting new
water channels
Peritubular
capillary
plasma
Figure
14.29
Page 544