The Urinary System
Chapter 26
The organs of the urinary system include:
Functions of the urinary system (all done by
kidneys):
 Regulate the volume, concentration, pH
and content of blood
 Eliminate metabolic wastes as urine
3 processes occur within the kidneys to accomplish
these goals
Filtration (F) – Pressure (blood pressure) forces some fluid (plasma) and
small substances from blood to renal (kidney) tubules. Results in the
formation of “filtrate” in the renal tubules; approx. 180 liters filtrate/day
Reabsorption (R) – Movement (by passive & active means) of most
fluid & many solutes from renal tubules back into the blood; approx.
99% filtate reabsorbed
Secretion (S) – Selective movement of specific substances (e.g. H+, K+)
from blood to tubules
blood
Blood vessels (capillaries)
F
filtrate
R
Renal tubules
Filtered
blood
S
urine
Gross Anatomy of the Kidneys
 Retroperitoneal organs
 Left kidney more superior than right (due to liver)
 Notch at medial border (hilus/hilum) for renal artery & vein, ureter
Each kidney surrounded by 3
layers of CT:
 renal capsule – innermost layer
of dense CT
 adipose capsule – middle layer
protecting & insulating kidneys
 renal fascia – outer layer
holding kidneys in place in
abdominal cavity
Internal Anatomy of the Kidneys
 Cortex
 Medulla
where urine
formation
occurs
 pyramids
 renal papillae
 renal columns
 Pelvis - collecting basin
 minor calyces
 major calyces
 renal pelvis
Microscopic Anatomy of the Kidneys
Functional unit of the kidneys is the “Nephron”
Nephron = renal corpuscle + renal tubules
Most (85%) nephrons
classified as “cortical
nephrons” – corpuscle &
most of tubule located
within cortex
Some (15%) classified as
“juxtamedullary nephrons” –
corpuscle at junction of
cortex & medulla & loop of
nephron extends into
medulla
Renal corpuscle – 1st part of the
nephron; site of filtration;
comprised of
 glomerulus – capillary network
 Bowman’s (glomerular)
capsule – double layered
capsule of epithelial tissue (inner
visceral layer/outer parietal
layer), surrounding glomerulus
Bowman’s capsule
(parietal (capsular)
layer)
Filtration occurs across
“endothelial capsular (filtration)
membrane” – junction
between glomerular
endothelium & podocytes;
results in formation of “filtrate”
in capsular space
capsular space
glomerulus
Visceral layer of
Bowman’s
capsule
(podocytes)
Representative Nephron
proximal convoluted tubule
Filtrate from renal
corpuscle will
move into proximal
convoluted tubule
(PCT)  loop of
Henle distal
convoluted tubule
(DCT); and then
into a collecting
system of tubes
(connecting tubule
 collecting duct
 papillary duct 
minor calyx)
distal convoluted tubule
reabsorption
NEPHRON
Secretion, some
reabsorption
COLLECTING
SYSTEM
renal
corpuscle
filtration
collecting duct
variable
secretion &/or
reabsorption
Loop of Henle
H2O
solutes
filtrate
more
reabsorption
Connecting
tubule
papillary duct
delivery of
urine to minor
calyx
Blood
supply
to
kidneys
(Cortical radiate arteries)
Efferent
arteriole
glomerulus
Afferent arteriole
Efferent arterioles branch into a second capillary network, the peritubular
capillaries, which surround the renal tubules. The peritubular capillaries which
surround the tubules of juxtamedullary nephrons are longer & straighter,
therefore known as vasa recta
Reabsorption & secretion occur between the renal tubules & peritubular
capillaries
Peritubular capillaries  venules  interlobular veins
 arcuate veins  interlobar veins  renal vein
Juxtaglomerular Apparatus (JGA)
Extremely important to regulate the rate of
filtration that occurs at the glomerulus
(glomerular filtration rate “GFR”)
Macula
densa
To regulate glomerular BP, we have a
feedback system – the juxtaglomerular
apparatus (JGA)
JGA =
Juxtaglomerular
cells
 juxtaglomerular cells of the afferent
arteriole – recognize if renal BP is too low &
then can respond
 macula densa cells of the distal convoluted
tubule – recognize if decreased filtrate
produced or too many solutes within filtrate &
then can respond
Histology of the kidneys
Once urine is formed, it will move from the pelvis of the kidneys
into the ureters
 retroperitoneal, muscular tubes running from kidneys to
urinary bladder
 lined with transitional epithelium
 transports urine primarily by peristalsis
 Hollow muscular organ that
temporarily stores urine prior to
“micturition”
 Lined with mucosa of transitional
epithelium with rugae
 Muscularis of 3 layers of smooth
muscle known as “detrussor muscle”
 Internally have “trigone” – triangular
area formed by openings of ureters &
urethra
 Micturition reflex – stretch receptors
in wall of bladder stimulate smooth
muscle of detrussor muscle
(involuntary); stimulation of internal
urethral sphincter (involuntary) &
external urethral sphincter of skeletal
muscle (voluntary)
Urinary bladder
Histology of bladder & ureters
muscularis
Transitional
epithelium
Ureter
Urinary bladder
Urethra
 Transports urine out of body
 Inner urethral sphincter of smooth muscle; external urethral sphincter
of skeletal muscle
 Anatomically different in men & women