The Urinary System
Your “pee” will become a little
more clear!
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We can all learn a little something
from Homer Simpson! 10:20
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The Urinary System
• Consists of the
kidneys and
accessory
structures
• Ureters carry urine
from the kidneys to
the bladder
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Why are those kidneys so
important?
• Structure
– They are bean-shaped and ~10 cm long
– Consists of three parts:
• Medulla (inside)
• Cortex (outside)
• Renal Pelvis (inside)
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Kidneys: Importance
• Function
– to filter and clean blood
– Remove wastes (i.e. nitrogenous wastes)
– Osmoregulation – regulation of water
excretion and reabsorption
– Absorption of vitamins, organic molecules,
nutrients, potassium, and salt
– Maintain pH level by controlled
secretion/absorption of hydrogen (H+) and
bicarbonate ions (HCO3-)
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Kidney Structure
•
3 Main Parts:
– Cortex – the outer
layer of connective
tissue that encircles
the kidney
– Medulla – the inner
layer found beneath
the cortex
– Renal Pelvis – the
hollow chamber that
joins the kidney with
the ureters
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The Kidney: Interesting Facts
• Only 1% of body mass but processes 20% of the
body’s blood with each heart beat
• 99% of water is reabsorbed to keep the body
hydrated
• Nitrogenous wastes (mostly from proteins) are
expelled in the form of urea
• Works 24 hours a day even if you don’t drink  you
will still urinate from extracted water
• Nephrons are the functional unit (approx. 1-1.25
million!) of the kidney  they accomplish the main
functions of the kidney
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Nephron
Structure
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Follow the Flow of Fluid!
• Small branches from
the renal artery,
afferent arterioles,
supply the nephrons
with blood
• These arteries branch
into a capillary bed,
called the glomerulus
• Blood leaves the
glomerulus through
efferent arterioles
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Follow the Flow
• Blood is carried from
the efferent arterioles
to a net of capillaries,
peritubular capillaries,
that wrap around the
kidney tubule
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Follow the Flow
• The glomerulus is
surrounded by a
funnel-like part, called
Bowman’s capsule
• The capsule, the
afferent arteriole, and
the efferent arteriole
are located in the
cortex
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Follow the Flow
• Fluids enter the
Bowman’s capsule
from the blood and it
tapers to a thin
tubule, called the
proximal tubule
• Urine is carried from
the proximal tubule to
the loop of Henle
– Descends into the
medulla
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Follow the Flow
• Urine moves through
the distal tubule, the
last segment of the
nephron, and into the
collecting ducts
• The collecting ducts
collect the urine from
several nephrons
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Urine Formation
Depends on three functions:
1. Filtration
•
•
Process by which blood or body fluids pass
through a selectively permeable membrane
Movement of fluids from the blood into the
Bowman’s capsule
2. Reabsorption
3. Secretion
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Urine Formation
Depends on three functions:
1. Filtration
2. Reabsorption
• Transfer of glomerular filtrate (essential
solutes and water) from the nephron back
into the capillaries
3. Secretion
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Urine Formation
Depends on three functions:
1. Filtration
2. Reabsorption
3. Secretion
 Movement of materials, such as ammonia
and drugs/toxins, from the blood back into
the nephron
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Pathway of Filtrate through the
Nephron
• Filtrate = plasma (no
white/red blood cells,
platelets, hormones, or
plasma proteins)
• Nephron pathway begins in
cortex  Bowman’s Capsule
 proximal tubule  loop of
Henle (medulla)  distal
tubule (cortex)  collecting
tubule (medulla)
• Like a roller coaster!
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Filtration
• Each nephron has an independent
blood supply
• The glomerulus is a high-pressure
filter
– Capillary bed ~ 25 mm Hg
– Glomerulus ~ 65 mm Hg
– The greater the blood pressure,
the more nephric filtrate forms
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Filtration contd.
• Dissolved solutes pass through the walls
of the glomerulus into the Bowman’s
capsule
• So why do you think we feel a greater urge
to urinate when we are cold?
– When we are cold, our blood vessels
constrict and narrow to increase blood
pressure so more urine forms!
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Filtration
• Not all materials enter the Bowman’s capsule:
Solute
Water
NaCl
Glucose
Amino Acids
H+ ions
Plasma Proteins
Erythrocytes
(blood cells)
Platelets
Glomerulus
Bowman’s Capsule
yesWhy can’t plasma
yes
yes proteins, blood
yes
yescells, and platelets
yes
pass through the
yes
yes
glomerulus and
yesenter the nephron?
yes
yes
no
yes
no
yes
no
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Reabsorption
• On average, 600 mL of
fluid flow through the
kidneys every minute
• About 120 mL is filtered
into the nephron
• Only 1 mL of urine is
formed
If we didn’t
reabsorb fluids,
we would have
to drink 1 L of
water every 10
minutes!
– The remaining 119 mL of
fluids and solutes are
reabsorbed
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Selective Reabsorption
• Occurs by both active and passive transport
• Na+ is actively transported out, causing negatively
charged ions to follow
• Glucose and amino acids attach to specific carrier
molecules, which shuttle them out of the nephron
and into the blood
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Proximal Tubule
• Solutes actively transported
out to the interstitial fluid and
so creates an osmotic
gradient
• This draws water from the
nephron (water always flows
toward a hypertonic/hypotonic
environment?)
• The osmotic gradient is
strengthened by the proteins
that are not filtered into the
nephron
– Proteins in the blood draw
water from the interstitial
fluid
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Selective Reabsorption
• What are actively
transported? What are
passively transported?
• NaCl, H+ (to balance pH)
glucose, and nutrients are
actively transported out of
the nephron and to the
interstitial fluid
• Water and K+ are passively
transported to the interstitial
fluid for reabsorption
• Water tends to follow the
direction of NaCl – why?
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Loop of Henle
• As water is reabsorbed, the
remaining solutes become more
concentrated
• Descending loop of Henle is
impermeable to solutes and
ascending the ascending loop is
impermeable to water
• Water passively leaves the
nephron to the interstitial fluid
thereby increasing the solute
concentration
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Loop of Henle
• The ascending limb of the
Loop of Henle is impermeable
to water
• NaCl passively leaves into the
interstitial fluid
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Distal Tubule
• HCO3- is reabsorbed
in the distal tubule (to
regulate pH) by
active transport
• NaCl, K+ and H+ are
actively transported
• Water is passively
reabsorbed
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Collecting Duct
• In the collecting duct, urea
is passively transported and
reabsorbed into the
interstitial fluid
• Water is reabsorbed since
the membrane is
permeable to it
• This results in making the
urine more concentrated
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Other Notes on Reabsorption
The amount of solutes reabsorbed is limited!
• Reabsorption occurs until the threshold level
of a substance is reached
– The maximum amount of material that can be
moved across the nephron
• Once it is reached, excess material stays in
the nephron and is excreted as urine
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Secretion
• This is the movement of wastes from the
blood into the nephron
– i.e. Nitrogen-containing wastes, excess H+ ions,
minerals (e.g. K+)
• Like reabsorption, tubular secretion occurs by
active transport but molecules are shuttled
from the blood into the nephron
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