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The Urinary System
By the end of this class you should
understand:
• The major functions of the urinary system
• The major parts of the nephron and the function
of each
• The purpose of reabsorption and what materials
are always or sometimes reabsorbed
• The major components of urine and how this
reflects the state of the body
• How major defects of the urinary system put the
body at risk
The Urinary System
• Also known as the renal
system and excretory
system
• Consists primarily of the
kidneys which remove
waste from the blood
• The waste becomes urine
which collects in the
bladder and is excreted by
urination (peeing)
Three Components of Urine
• Water
– If the body is dehydrated, a
minimal amount of water is still
excreted
• Excess salts and any nutrients
that are way above normal levels
– Diabetes was originally diagnosed
by testing urine for sweetness
• Anything in the blood that the
body does not recognize as useful
(including drug metabolites)
Urine Drug Test
• Why must we pee in a cup for a
job?
• Drugs are not recognized by the
body as being nutrients and so
are not reabsorbed back into
the blood when they are filtered
• The urine essentially reflects the
state of the blood, including
drugs and metabolites (brokendown drugs after processing by
the liver)
Kidneys
• The kidneys are wrapped
in layers of connective
tissue
– Up to a quarter of all blood
flow can pass through
kidneys, so bruising and
laceration can be lethal
• The outer layer is the
cortex
• The inner core is the
medulla
– The medulla is very salty
Kidneys
• The kidneys produce filtrate
when they filter blood plasma
• The filtrate is then processed
– The end result is urine
• A huge amount of blood flows
through the kidneys every day
(up to ¼ of all blood flow)
– The kidneys can excrete excess
water at an impressive rate if
hyperhydrated
Nephron
• The kidney is composed
mostly of a set of
tubules, called a nephron
– The kidney may have up
to a million nephrons
• The nephron is mostly a
tube of epithelial tissue
called a tubule
– The tubule contains
filtrate, which becomes
urine
Nephron Diagram
Filtration
• Blood enters a special leaky
artery called the glomerulus
– Plasma leaks out of the
glomerulus into the glomerular
capsule (AKA Bowman’s capsule)
• RBCs and proteins are too large
to enter the capsule
• Plasma, including salts,
nutrients, and wastes, pass
through the glomerulus
– Referred to as glomerular
filtration
Renal Circulation
• The blood flow through the
kidneys is called renal
circulation
• After blood enters the renal
artery, it becomes tiny
afferent arterioles
• These become the glomeruli
(leaky arteries), which then
become efferent arterioles
• The efferent arterioles
branch to become the
capillaries
Tubule Function
• The tubule’s primary function is
to reabsorb nutrients
– Primarily glucose, amino acids
and vitamins
• This is performed by active
transport and can only be
performed at a certain rate
– The concentration of nutrients
also pulls water out via osmosis
– Excess nutrients will end up in
the urine
• The nutrients are put back into
the peritubular capillaries that
follow the nephron
Additional Reabsorption
• In addition to reabsorbing nutrients and
salts, certain nutrients are reabsorbed
only if they are in demand
• Primary example is calcium
– Reabsorbed from the kidney only when
PTH is in the blood, which is the signal for
low calcium
– What gland releases PTH?
• Additional example is sodium
– Reabsorbed more strongly when
aldosterone is in the blood, which is the
signal for low sodium
– What gland releases aldosterone?
Tubular Secretion
• Some waste products and
drugs are also actively
transported into filtrate by
tubular secretion
– Also requires active transport
• Creatinine (waste product
from anaerobic cell activity) is
reliably filtered and secreted
by the kidney
– Blood tests in the hospital for
creatinine levels test kidney
function
Loop of Henle
• The loop of Henle is the middle
part of the tubule
– The loop of Henle takes advantage
of the medulla’s saltiness
• The descending limb allows
water to leave the tubule
through osmosis
• The ascending limb is
impermeable to water but has
ion channels so that salts diffuse
out of the tubule
Loop of Henle function
• The Loop of Henle is designed
to reclaim almost all the water
and salt from the filtrate
• The capillaries of the medulla
are called vasa recta, which
reclaim water and salt for the
blood
– If the Loop of Henle did not
reclaim the water and salt, we
would pee as fast as we filtered
– Instead, we can concentrate our
urine, allowing us to not pee all
day
Collecting Duct
• The tubules begin fusing to
become collecting ducts
– The collecting duct still contains
dilute urine (with a lot of water)
• There are pores that allow water
to leave the collecting ducts and
enter the medulla
• These pores are only opened by
the action of a hormone called
ADH, to conserve water
– Where is ADH secreted from?
Release of Urine
• When filtrate leaves the collecting
duct, it goes through the ureter to
the bladder, where it is now
officially urine
• The release of urine (urination, or
micturition) is under
parasympathetic control
– Which is why for some people it’s
hard to go when you’re being
watched
– There are sympathetic nerve fibers
in the smooth muscle of the
bladder, which is why panic can
cause some to wet themselves
Urination
• The urethra is the tube that carries
urine from the bladder to the
outside of the body
– In men the urethra meets with the
vas deferens, so urine exits the same
opening of the penis that semen
exits
– In women the urethra has a separate
opening from the vagina
• The urethra has sphincter muscles
that can hold it closed, but the
brain must be trained to control
them (hence diapers for babies)
Kidney Homeostasis
• Kidney function is vital to life
– Many nitrogenous wastes that are eliminated by
the kidney are toxic to our cells
– The body can survive with one kidney, but that
kidney can become overworked and fail
• Kidney injuries are very hard to survive
– So much blood flow through the kidney that being
stabbed in the kidney can result in death from loss
of blood in seconds, similar to the heart
Dialysis
• If both kidneys fail through
nonviolent means, the patient
may require dialysis, which is
essentially artificial kidney
function
– There are several options, all of
which are inferior to having actual
working kidneys but all of which
are superior to death
– The most effective is hemodialysis,
where blood is pumped through a
dialysis machine that cleanses the
blood
Other Kidney Problems
• Kidney Stones
– Crystals form in the kidney or
bladder, caused by a combination
of factors including high calcium,
alkaline blood, and not drinking
enough water
• Urinary Tract Infection
– Bacteria can invade the urethra
and potentially damage the body
– Infected bladder is worse,
infected kidney can be fatal
That’s the end of class!
• For those of you disappointed in the lack of
naughty bits, Wednesday’s lesson is all about
naughty bits!
• For those of you who thought this was too
much naughty bits already, you may want to
reconsider coming on Wednesday…