28 April 2014
» The urinary system produces urine. The production
of urine has three main functions:
1)
2)
3)
Excretion of waste products of metabolism, especially urea
Osmoregulation – control of the water, salt, and acid-base
balance of the body
Fun fact: Urea is a
Removal of drugs and toxins
nitrogenous compound
» Other functions
produced by the liver when
we digest proteins
˃ Regulation of blood pressure through production of enzyme
renin
˃ Stimulation of red blood cell production of hormone
erythropoeitin
Fun fact: The two major
˃ Conversion of vitamin D to active form places that remove or break
down drugs and toxins are
the liver and the kidney
» Urine is produced
in the cortex by
the nephrons.
The nephrons are
the functional
unit of the kidney.
» Urine is
concentrated in
the medulla
» Urine collects in
the calices, then
flows into the
renal pelvis then
to the ureters
There are ~ 1 million
nephrons in each
kidney
» A nephron consists of 4 main parts:
˃
˃
˃
˃
Bowman’s capsule
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Each nephron empties into a collecting duct
» Capillaries twine
all around the
nephron … why?
» Most nephrons
are entirely within
the cortex; some
have loops of
Henle that extend
into the medulla
The tubules of the nephron
contain filtrate – the urine
that is in process of being
formed.
There are three steps to urine formation
» Filtration
» Reabsorption
» Secretion
» Filtration
• Blood from the glomerular capillaries filters into the Bowman’s capsule
due to extremely high pressure in capillaries
• Filtration is based on size only. Small items (everything in blood except the
cells and most proteins) enter the Bowman’s capsule.
• Passive process
Fun Fact: We produce
~180 L of filtrate a day
… despite the fact that
we have only about 5
L of blood. Nearly
99% of the water that
is filtered out of the
blood is reabsorbed
the capillaries.
» Reabsorption
• Reabsorption of
important
molecules (water,
salts, amino acids,
sugar, minerals)
from tubules to
capillaries
• Some reabsorption
is passive, but most
relies on active
transport
• Different materials
are reabsorbed in
different sections
(see diagram)
More on
Loop of
Henle!
» Secretion
• Active transport of
unwanted materials
from capillaries to
filtrate
• Useful for larger
molecules (some
drugs and toxins)
and for regulation of
blood pH
• Occurs in
convoluted tubules
(esp. distal)
» Responsible for most of
the water and some salt
reabsorption
» Uses a countercurrent
multiplier mechanism 
by having the fluid flow in
opposite direction, the
exchange of materials can
be maximized or
minimized
» The loop dips into
increasingly salty medulla
to extract as much water
as possible from the
filtrate
passive
active
Descending loop of Henle
» Descending loop is
permeable to water, but
impermeable to salt
» So, water passively
diffuses from the loop of
Henle into the interstitial
fluid (and then into the
peritubular capillaries)
passive
active
Ascending loop of Henle
» Ascending loop is
impermeable to water, but
permeable to salt
» So, water CANNOT reenter the tubules, despite
the fact that the filtrate is
now more concentrated
then the interstitial fluid
» NaCl first moves passively
out of the loop, then is
actively transported out.
This both retains our salts
and creates the salty
medulla environment
passive
active
» The reabsorption of water
in the collecting duct is
controlled by hormones.
» If blood solute
concentration is high, the
pituitary releases
antidiuretic hormone
(ADH) which opens
aquaporins in the
collecting duct, making it
permeable to water.
» If blood solute
concentration is low,
aquaporins are closed,
making the duct
impermeable to water
passive
active
» If you are dehydrated,
will you produce ADH?
˃ Yes!
» If you produce ADH, will
you produce a lot of
urine, or a little urine?
˃ Little! Your body will
produce a small
amount of
concentrated urine,
retaining precious
water in your blood.
passive
active
With
ADH
Without
ADH
Think, Pair, Share:
Explain these values
Solute
Plasma
(mg/100 mL)
Filtrate
(mg/100 mL)
Urine
(mg/100 mL)
Glucose
900
900
0
Urea
30
30
1200
proteins
740
0
0
Review: watch me!
1) Draw a nephron, labeling the parts and the major
functions.
2) Draw a nephron, use different colored markers to
show both the relative concentration (use dots!) of
the each of the following chemicals within each
region of the nephron and use arrows to show the
flow of the substances in or out of the nephron.
˃
˃
˃
˃
water
salt
Sugar
Urea
1) What were our objectives, and what did you
learn?
2) What was our learner profile, and how did we
demonstrate it?
3) How does what we did today relate to our unit
question?