Chapter 32 - Excretion
The Nephron as a Unit of Kidney Function
Objectives of lesson
1. Outline the structure & associated blood supply & draw
a diagram of the nephron.
2. Explain urine formation, including: Bowman's
capsule/glomerulus / proximal convoluted tubule/ Loop
of Henle/ distal convoluted tubule/ pelvis/ bladder
3. Outline the sites & action of reabsorbing glucose/amino
acids/salts/water
4. State that reabsorbing water occurs in the collecting
duct & is under the influence of ADH.
Each kidney contains more than a million
nephrons.
A nephron is a tube about 3cm long located in
the cortex and medula of the kidney
A section through the kidney shows an outer darker region
(cortex) and a lighter inner zone (medulla).
Pelvis
Cortex
Medulla
Ureter
The Nephron is the functional
unit of the kidney
Blood Supply to the Nephron
Blood enters kidney through renal
artery. This blood vessel divides to
form the renal arterioles which split
further to form afferent (incoming)
arterioles. Each afferent arteriole splits
further to form a cluster of capillaries
called the glomerulus.
The glomerulus fits into the Bowman’s Capsule
which is a cup shaped structure.
Blood leaves the glomerulus through the
efferent (outgoing) arteriole. This then divides to
form capillariers that surround the rest of the
nephron.
The capillaries eventually rejoin to form renal
venules that emerge from the kidney as the renal
vein
NEPHRON
1. Filtration
Filtration means that water and small molecules pass
(under high pressure) from the blood into the nephron.
a) The renal artery divides into afferent arterioles and then
into a capillary network (glomerulus) at the top of each
nephron.
b) A cup-shaped funnel
(Bowman’s capsule)
surrounds each glomerulus
and it is here that smaller
molecules in the blood are
forced, under pressure, out
of the plasma and into the
lumen of Bowman’s capsule,
forming the glomerular
filtrate.
c) The blood pressure is high because the
efferent arteriole is narrower than the
afferent arteriole, so force-filtering the
plasma.
d) Everything (glucose, vitamins, urea,
water etc) except large proteins and blood
cells gets filtered.
About 180L of glomerular filtrate are formed every 24 hours This is
4.5 times the fluid content of the body. Not all this liquid leaves the
body as urine
2. Reabsorption
Reabsorption means that
molecules pass from the
nephron back into the
blood.
a) The body cannot afford
to lose useful chemicals
like food and water, so as
the glomerular filtrate
passes from the
Bowman's capsule into
the proximal convoluted
tubule, glucose, amino
acids, some salts and
water are reabsorbed
back into the blood.
b) The food molecules, including most of
the salt ions, are taken back by active
transport (against the concentration
gradient, so energy is needed for this). Most
of the water is reabsorbed by osmosis—
from the Loop of Henle and convoluted
tubules.
(Active transport means that energy (in the form of ATP)
is used to move molecules often against a concentration
gradient) i.e. from low to high concentrations
3. Secretion
Secretion means that some substances pass
from the blood into the nephron.
Secretion of Potassium ions and hydrogen
ins into the distal convoluted (highly
folded) tubule help maintain the pH of the
blood.
Urea and other wastes, along with some water, are not
reabsorbed. They pass, as urine, into the pelvis of the kidney
and to the bladder for storage. Of the 180L of blood filtered
each day, about 99 % of the filtrate is reabsorbed.
Urine Consists of:
96% water
2.5% nitrogenous waste (mostly urea)
1.5% salts
Urea is produced in the liver when excess
proteins are broken down (de-animation)
Functions of the regions of the nephron
Region
Substance
reabsorbed
Amount of water reabsorbed
Proximal
convoluted
tubule
Most salts, glucose,
amino acids & vitamins
most
Loop of henle
(decending
limb)
Some minerals
A little
Loop of henle
(ascending
limb)
Some salts
none
Distal
convoluted
tubule
Some salts
some
Collecting
duct
None
A little
Quiz
•
•
•
•
•
•
•
Name the parts of the nephron
Where does filtration takes place?
What does not get filtered
Where does reabsorption take place?
What substances are reabsorbed?
What are not reabsorbed?
List some substances present in urine.
The kidney regulates the amount of water in the body by
varying the amount of urine produced.
This is known as osmoregulation, and it is an example of
homeostasis.
ADH [Anti-diuretic hormone]
controls whether the distal
tubule and collecting ducts
reabsorb water or not.
If you drink a great deal of water
 the hypothalamus in the brain detects the diluted
blood and turns off ADH production.
 Less water is reabsorbed, so more water is allowed
to escape to the bladder,
 and a larger volume of dilute urine is produced.
When the body is low on water
 ADH is secreted from the pituitary gland.
 More water is reabsorbed
 and only a small volume of urine is produced.
 When the body has excess water, ADH is NOT secreted from
the pituitary gland. Less water is reabsorbed and so a larger
volume of urine is produced.
If Water Levels Rise in the Body …..
 When the body is low on water, ADH is secreted from the
pituitary gland. More water is reabsorbed and only a small
volume of urine is produced.
If Water Levels Fall in the Body…..
Conditions
Effect on blood
Thirsty/ salty diet/ hot
day/exercise/
sweating
Low water
Released
content and high
salt conc.
More
permeable to
water
Low vol of
water & higher
salt conc. (low
vol of
concentrated
urine
High water
content & low
salt conc
Less
permeable to
water
High vol of
water & lower
salt conc (a
high vol of
dilute urine)
Excessive water
intake/very low salt
diet
High protein diet
ADH
Not
released
Distal tubule Urine
and collecting
duct
Normal
No effect No effect
watercontent and
increased conc
of urine
Same vol of
water,
increased urea
conc. (i.e. the
same vol of
conc urine
Quiz
• What is ADH?
• Where is it made and where does it act?
• Explain what happens in a nephron where
we take in a lot of fluid.
• Explain what happens in each nephron
when we have not taken in fluid for many
hours.