Chapter 26
Urinary System
Urinary System Functions


Filtering of blood
Regulation of
blood volume
 concentration of blood solutes
 pH of extracellular fluid
 blood cell synthesis


Synthesis of Vitamin D
Urinary System Anatomy:
Location and External
Anatomy
of Kidneys

Location



Lie behind peritoneum
on posterior abdominal
wall on either side of
vertebral column
Lumbar vertebrae and
rib cage partially
protect
Right kidney slightly
lower than left

External Anatomy

Renal capsule


Perirenal fat


Engulfs renal capsule and acts as
cushioning
Renal fascia


Surrounds each kidney
Anchors kidneys to abdominal
wall
Hilum

Renal artery and nerves enter
and renal vein and ureter exit
kidneys
Internal Anatomy of Kidneys:

Cortex: Outer area


Medulla: Inner area


Renal pyramids
Calyces



Renal columns
Major: Converge to form
pelvis
Minor: Papillae extend
Nephron: Functional
unit of kidney


Juxtamedullary
Cortical
The Nephron:
Histology of the Nephron
Internal Anatomy of Kidneys

Renal corpuscle

Bowman’s capsule



Parietal layer
Visceral layer

Network of capillaries



Afferent

Blood to glomerulus
Efferent

Drains
Proximal (convoluted)
tubule
Loops of Henle

Arterioles

Tubules

Glomerulus




Descending limb
Ascending limb
Distal (convoluted)
tubules
Collecting ducts
Renal Corpuscle
Kidney Blood Flow:
Ureters and Urinary Bladder

Ureters


Tubes through which
urine flows from kidneys
to urinary bladder
Urinary bladder


Urethra


Stores urine

Transports urine from
bladder to outside of
body
Difference in length
between males and
females
Sphincters


Internal urinary
External urinary
Ureters and Urinary Bladder
Urine Formation
Filtration

Filtration

Renal filtrate



Plasma minus blood cells
and blood proteins
Most (99%) reabsorbed

Filtration pressure


Filtration membrane

Fenestrated
endothelium, basement
membrane and pores
formed by podocytes

Responsible for filtrate
formation
Glomerular capillary
pressure (GCP) minus
capsule pressure (CP)
minus colloid osmotic
pressure (COP)
Changes caused by
glomerular capillary
pressure
Filtration Pressure
Tubular Reabsorption

Reabsorption




Passive transport
Active transport
Cotransport
Specialization of tubule
segments

Substances transported



Active transport moves
Na+ across nephron
wall
Other ions and
molecules moved by
cotransport
Passive transport moves
water, urea, lipidsoluble, nonpolar
compounds
Reabsorption in Proximal Nephron:
Reabsorption in Loop of Henle
Reabsorption in Loop of Henle
Tubular Secretion


Substances enter proximal or distal tubules and
collecting ducts
H+, K+ and some substances not produced in
body are secreted by countertransport
mechanisms
Secretion of Hydrogen and
Potassium
Urine Production


In Proximal tubules




Na+ and other substances
removed
Water follows passively
Filtrate volume reduced
In descending limb of
loop of Henle


Water exits passively,
solute enters
Filtrate volume reduced
15%
In ascending limb of
loop of Henle



Na+, Cl-, K+ transported out
of filtrate
Water remains
In distal 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
Filtrate and Medullary
Concentration Gradient
Medullary Concentration and
Urea Cycling
Urine Concentration
Mechanism

When large volume of
water consumed


Eliminate excess without
losing large amounts of
electrolytes
Response is kidneys
produce large volume of
dilute urine

When drinking water not
available


Kidneys produce small
volume of concentrated
urine
Removes waste and
prevents rapid
dehydration
Urine Concentrating Mechanism I:
Urine Concentrating Mechanism II:
Hormonal Mechanisms

ADH



Secreted by posterior
pituitary
Increases water permeability
in distal tubules and
collecting ducts
Aldosterone


Produced in adrenal cortex
Affects Na+ and Cltransport in nephron and
collecting ducts

Renin


Produced by kidneys,
causes production of
angiotensin II
Atrial natriuretic
hormone

Produced by heart when
blood pressure increases


Inhibits ADH production
Reduces ability of kidney
to concentrate urine
Effect of ADH on Nephron
Aldosterone Effect on Distal
Tubule
Autoregulation and
Sympathetic Stimulation

Autoregulation


Involves changes in
degree of constriction in
afferent arterioles
As systemic BP increased,
afferent arterioles
constrict and prevent
increase in renal blood
flow

Sympathetic stimulation


Constricts small arteries
and afferent arterioles
Decreases renal blood
flow
Clearance and Tubular Load

Plasma clearance




Volume of plasma
cleared of a specific
substance each minute
Used to estimate GFR
Used to calculate renal
plasma flow
Used to determine
which drugs or other
substances excreted by
kidney

Tubular load


Total amount of
substance that passes
through filtration
membrane into
nephrons each minute
Normally glucose is
almost completed
reabsorbed
Tubular Maximum

Tubular maximum


Maximum rate at which
a substance can be
actively absorbed
Each substance has its
own tubular maximum
Urine Flow and
Micturition Reflex

Urine flow


Hydrostatic pressure
forces urine through
nephron
Peristalsis moves urine
through ureters

Micturition reflex


Stretch of urinary bladder
stimulates reflex causing
bladder to contract,
inhibiting urinary
sphincters
Higher brain centers can
stimulate or inhibit reflex
Micturition Reflex
Effects of Aging on Kidneys

Gradual decrease in size of kidney




Decrease in kidney size leads to decrease in renal
blood flow
Decrease in number of functional nephrons
Decrease in renin secretion and vitamin D
synthesis
Decline in ability of nephron to secrete and
absorb
Kidney Dialysis