H 3.4.8 The Nephron as a Unit of Kidney Function
Objectives – What you will need to
know from this section
 Outline the structure & associated blood supply & draw
a diagram.
 Explain urine formation, including: Bowman's
capsule/glomerulus/ proximal convoluted tubule/Loop of
Henle/distal convoluted tubule/pelvis/bladder
 Outline the sites & action of reabsorbing glucose/amino
acids/salts/water
 State that reabsorbing water occurs in the collecting
duct & is under the influence of ADH.
OUR EXCRETORY SYSTEM
 The urinary system consists of the kidneys, the bladder and
some ducts (tubes).
 A section through the kidney shows an outer darker region
(cortex) and a lighter inner zone (medulla).
Pelvis
Cortex
Medulla
Ureter
LEARNING CHECK
– Identify Cortex, Medulla, Pelvis, Ureter
D
A
B
C
A = Cortex
B = Medulla
C = Ureter
D = Pelvis
KIDNEY LS
H 3.4.8 The Nephron as a Unit of Kidney Function
 The structural
and functional
unit of the
kidney is the
nephron.
 The renal artery
divides into afferent
arterioles and then into
a capillary network
(glomerulus) at the top
of each nephron.
NEPHRON
1. Filtration
 The renal artery divides into afferent arterioles and then into
a capillary network (glomerulus) at the top of each
nephron.
 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.
 The blood pressure is high
because the efferent arteriole
is narrower than the afferent
arteriole, so force-filtering the
plasma.
 Everything except large proteins and blood cells gets filtered.
2. Reabsorption
 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.
 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.
 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.
LEARNING CHECK
•
•
•
•
•
•
•
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…..
LEARNING CHECK
• 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.