Chapter 26
Urinary System
26-1
I. Functions of the Urinary System
• A. Filtering of blood: involves three processesfiltration, reabsorption, secretion.
• B. Regulation of
– 1. Blood volume
– 2. Concentration of blood solutes: Na+, Cl-, K+, Ca2+,
HPO4-2
– 3. pH of extracellular fluid: secrete H+
– 4. Blood cell synthesis
• C. Synthesis of vitamin D
26-2
II. Kidney Anatomy and
Histology
• A. Retroperitoneal
• B. Right kidney
overshadowed by
liver
• C. Renal artery and
renal vein enter and
exit hilum
• D. Protected by
layer of fat
26-3
E. Internal Anatomy of Kidneys
•
•
•
•
•
•
•
•
•
•
•
1.
2.
3.
4.
5.
6.
cortex
pyramids
renal papilla
medulla
renal columns
calyx
a. Minor
b. Major
7. renal pelvis
8. ureter
9. hilum
26-4
F. The Nephron
1.
Functional and histological unit
of the kidney
2. Renal corpuscle
a. Bowman’s capsule
b. Glomerulus
3. Proximal convoluted tubule
4. Loop of Henle
5. Distal convoluted tubule
6. Collecting Ducts
7. Cortex
8. Renal pyramid
9. Renal papillae
10. Juxtamedullary nephrons
(15%)
11. Cortical nephrons
26-5
G. Renal Corpuscle-General view
Bowman’s capsule
a. Parietal layer
b. Visceral layer made
up of podocytes
2. Glomerulus
3. Afferent arteriole
4. Efferent arteriole
5. Juxtaglomerular cells
1.
26-6
H. Bowman’s Capsule more closely
• 1. parietal layer
• 2. visceral layer with
podocytes with filtration
slits
• 3. glomerular capillary
covered with visceral
layer of Bowman’s
capsule
• 4. juxtaglomerular cells
with macula densa
making up the
juxtaglomerular
apparatus-site of renin
production
• 5. notice diameters of
afferent and efferent
arterioles
26-7
I. Filtration Membrane
1. Fenestrae:
2. Filtrations slits:
26-8
J. Cells of nephron reflect function
1. Proximal tubule: simple
cuboidal epithelium with many
microvilli-site of reabsorption
2. Loops of Henle-production of
osmotic gradient
– Descending limb: simple
squamous epithelium
– Ascending limb: distal
part thicker and simple
cuboidal
3. Distal tubule: shorter than
proximal tubule. Simple cuboidal,
but smaller cells and very few
microvilli-site of secretion
4. Collecting ducts: Larger in
diameter, simple cuboidal
epithelium. Site of urine
concentration
26-9
K. Arteries and Veins of the Kidneys
1.
2.
3.
4.
Afferent arterioles
Glomerulus
Efferent arterioles
Peritubular
capillaries form a
plexus around the
proximal and distal
tubules
5. Vasa recta: specialized
parts of peritubular
capillaries that course into
medulla along with loops
of Henle, then back toward
cortex
26-10
III. Urine Production A. Three basic steps
1.
Filtration
2.
Reabsorption
3.
Secretion
26-11
B. Amount of Filtration
• Movement of fluid, derived from blood flowing through the glomerulus,
across filtration membrane
• Filtrate: water, small molecules, ions that can pass through membrane
• Pressure difference forces filtrate across filtration membrane
• Renal fraction: part of total cardiac output that passes through the kidneys.
averages 21%
• Renal blood flow rate: 5600 ml/min X .21 = 1176 ml/min
• Renal plasma flow rate: renal blood flow rate X fraction of blood that is
plasma: 1176 ml/min X .55 = 650 mL/min
• Filtration fraction: part of plasma that is filtered into lumen of Bowman’s
capsules; average 19%
• Glomerular filtration rate (GFR): amount of filtrate produced each minute.
therefore 650ml/min X .19 = 124ml/min
• Amount of filtrate formed per day = 180 liters/day
• Average urine production/day: 1-2 L. Most of filtrate must be reabsorbed
26-12
C. Filtration Pressure = GCP-CHP-BCOP
Changes in afferent and efferent arteriole diameter alter filtration pressure
Dilation of afferent arterioles/constriction efferent arterioles increases glomerular
capillary pressure, increasing filtration pressure and thus glomerular filtration
26-13
D. Regulation of GFR
1. Autoregulation
– Involves changes in degree of
constriction in afferent
arterioles
– Myogenic mechanism: As
systemic BP increases,
afferent arterioles constrict
and prevent increase in renal
blood flow
– Tubuloglomerular feedback:
Increased rate of blood flow
of filtrate past cells of macula
densa: signal sent to
juxtaglomerular apparatus,
afferent arteriole constricts
26-14
2. Sympathetic regulation:norepinephrine
a. What do you think will happen?
b. Constricts small arteries and afferent arterioles
c. Decreases renal blood flow and thus filtrate formation
d. During shock or intense exercise: intense sympathetic
stimulation, rate of filtrate formation drops to a few mm
e. Not a problem over short duration-normal response as
blood is shunted to skeletal muscle
f. Prolonged shock can damage kidneys
26-15
E. Tubular Reabsorption in general:
1. occurs as filtrate flows through the lumens of proximal tubule
(majority), loop of Henle, distal tubule, and collecting ducts
2. Processes involve diffusion, facilitated diffusion, active
transpot, and osmosis
3. Substances transported to interstitial fluid and reabsorbed into
peritubular capillaries
4. 99% of filtrate volume is reabsorbed.
5. These substances return to general circulation through venous
system
26-16
F. Reabsorption in PCT
1. Substances pass
through
– Apical surface
– Basal surface:
– Lateral surfaces
2. Active transport of Na+
across the basal
membrane from
cytoplasm to interstitial
fluid linked to
reabsorption of most
solutes
3. Number of carrier
molecules limits rate of
transport ie. diabetes
mellitus
4. Filtrate volume
reduced by 65% due to
osmosis of water
5. Osmotic pressure of
filtrate remains 300
mOsm due to
permeability char of
membrane
26-17
G. Reabsorption in Loop of Henle
1. Descending loop
of Henle passes into
an ever increasing
concentrated
environment. Simple
squamous epithelium.
2. Descending limb
is highly permeable
to water and
moderately
permeable to urea,
sodium, most other
ions
3. In descending
limb, water moves
out of nephron,
solutes in. Volume of
filtrate reduced by
another 15%.
26-18
Reabsorption in Loop of Henle
4. The wall of the ascending
limb of the loop of Henle is
not permeable to water.
5. Ascending limb moves Na+
across the wall of the basal
membrane by active
transport.
6. At the end of the loop of
Henle, inside of nephron is
100 mOsm/kg.
7. Interstitial fluid in the
cortex is 300mOsm/kg.
8. Filtrate within DCT is much
more dilute than the
interstitial fluid which
surrounds it.
9. Loop of Henle like the trap
below the sink in the
kitchen
10. Establishes concentration
gradient as you pass into the
medulla
26-19
H. Reabsorption in DCT and Collecting Duct
1.
2.
3.
4.
5.
6.
Filtrate which reaches DCT is
dilute with respect to interstitial
fluid
DCT and collecting duct are the
sites where hormonal control of
urine volume occurs-finishing
touches
Two ducts are affected by ADH
In presence of ADH these two
become very permeable to water
In absence of ADH, relatively
impermeable
DCT can also reaborb more
sodium ion, but this is also under
hormonal control of aldosterone
26-20
I. Tubular Secretion
1. Moves metabolic by-products, drugs, molecules not normally produced by the body into
tubule of nephron
2. Active or passive
3. Ammonia: produced by epithelial cells of nephron from deamination of amino acids.
Diffuses into lumen
4. H+, K+, penicillin, and substances such as para-aminohippuric acid (PAH): actively
secreted into nephron
5. Hydrogen ions are moved into tubule as Na ions are reabsorbed-major mechanism of body
in regulating tissue fluid pH
26-21
Urine Production Summary
• In Proximal convoluted
tubules
– Na+ and other substances
removed
– Water follows passively
– Filtrate volume reduced by
65%
• In descending limb of loop
of Henle
– Water exits passively, solute
enters
– Filtrate volume reduced an
additional 15%-only 20% of
original filtrate remains
• In ascending limb of loop of
Henle
– Na+, Cl-, K+ transported out of
filtrate
– Water remains
• In distal convoluted tubules
and collecting ducts
– Water movement out regulated
by ADH
• If absent, water not
reabsorbed and dilute urine
produced
• If ADH present, water moves
out, concentrated urine
produced
26-22
Loop of Henle
26-23
26-24
Urine Concentrating Mechanisms
26-25
IV. Regulation of Urine Concentration and
Volume: A. Renin/Angiotensin/Aldosterone
Mechanism
1.
2.
3.
4.
5.
6.
7.
8.
9.
Mainly a mechanism to regulate blood pressure
Cascade of events occurs to result in Na ion reabsorption and therefore water
Sensory mechanism for the system is located in the juxtaglomerular complex
Two stimuli result in the release of renin from this area
a. Decreased stretch of the afferent arteriole of the glomerulus
b. Decreased concentration of Na ions in the distal convoluted tubule
Released renin converts a liver protein “angiotensinogen” into angiotensin I
AngiotensinI -> angiotensinII by a proteolytic enzyme found in capillaries of
lung
Enzyme is called angiotensin-converting enzyme (ACE)
Angiotensin II is a very powerful vasoconstrictor
Angiotensin II also increases aldosterone secretion-increases reabsorption from
distal convoluted tubule
26-26
The juxtaglomerular complex is the site of
renin release
26-27
26-28
B. ADH Mechanism
1.
2.
3.
4.
5.
6.
7.
8.
9.
Mainly regulating osmolarity of the blood and tissue fluids
Secondarily affecting blood pressure
Osmoreceptors are found in hypothalamus of the brain
Reacts secondarily to pressure receptors in atria, carotid sinuses, and
aortic arch
ADH effects the permeability of the distal convoluted tubule and
collecting ducts
Water reabsorption before this portion of the nephron is obligatory
Filtrate entering the distal convoluted tubule possesses an osmolarity
of 100 mOsm
Depending upon the amount of ADH released, urine’s final
osmolarity can vary from 100 to 1200 mOsm
From as little as less than 1 liter to 20 liters in the total absense of
ADH (diabetes insipidus)
26-29
ADH Mechanism
26-30
C. ANH-atrial natriuretic hormone
1.
2.
3.
4.
5.
Produced by right atrium of heart when blood volume increases
stretching cells
Inhibits Na+ reabsorption
Inhibits ADH production
Increases volume of urine produced
Venous return is lowered, volume in right atrium decreases
26-31
Some Youtube sites
• http://www.youtube.com/watch?v=uo-NOrP49I
26-32
V. Urine Movement
A. Hydrostatic pressure forces urine through
nephron
B. Peristalsis moves urine through ureters from
region of renal pelvis to urinary bladder. C. Occur
from once every few seconds to once every 2-3
minutes
1. Parasympathetic stimulation: increase
frequency
2. Sympathetic stimulation: decrease frequency
D. Ureters enter bladder obliquely through trigone.
Pressure in bladder compresses ureter and prevents
backflow
26-33
VI. Anatomy and Histology of Ureters and
Bladder
A. Ureters: bring urine from
renal pelvis to urinary bladder.
Lined by transitional epithelium
B. Urinary bladder: hollow
muscular container. In pelvic
cavity posterior to symphysis
pubis. Lined with transitional
epithelium; muscle part of wall is
detrusor
C. Trigone: interior of urinary
bladder. Triangular area between
the entry of the two ureters and the
exit of the urethra. Area expands
less than rest of bladder during
filling
26-34
Anatomy and Histology of Urethra
• Male: extends from the inferior part of the urinary bladder
through the penis
• Female: shorter; opens into vestibule anterior to vaginal
opening
• Internal urinary sphincter: in males, elastic connective tissue
and smooth muscle keep semen from entering urinary bladder
during ejaculation
• External urinary sphincter: skeletal muscle surrounds urethra
as it extends through pelvic floor. Acts as a valve
26-35