The process of excretion in humans, for IGCSE Biology.

1. EXCRETION IN HUMANS.
IGCSE BIOLOGY.
BY
MR. WILLY WAMBUA
BIOLOGY/CHEMISTRY TEACHER
AGA KHAN HIGH SCHOOL, NAIROBI
KENYA.

2. Excretion in Humans
Excretion is the removal of toxic
materials, the waste products of
metabolism and substance in excess
of requirements from organisms.
Metabolism is chemical reactions
taking place inside cells, including
respiration.

3. Main waste products
The body excretes three main waste
materials. These are Carbon Dioxide,
Urea and Water. Excretion is a very
important feature to us because
without if toxic substances will build
up in our bodies they might kill us. It
also helps in maintaining the
composition of body fluids.

4. Components of excretory system
The Excretory System of humans is
made up of 4 structures: Two
kidneys, two ureters, a bladder, and
the urethra. The kidneys act as a
filter to filter the waste products
from the blood, the ureters are tubes
that transport the main waste
products (urine) from the kidneys to
the bladder, where it is stored until it
is excreted out of the body through
the urethra.

5. Urinary system

6. Kidneys Structure:

7. A kidney consists of two main
structures:
• Cortex (outer layer)
• Medulla
Between the cortex and the Medulla, there is a
structure called the nephrone.
The nephrone is the where filtration of toxic
materials from the blood takes place. We have
many of them in each kidney.
In the centre of the kidney there is a cavity
called the pelvis which leads to the ureter.

8. Structure of Nephrone:

9. The nephrone starts with a cup shaped structure
called Bowman’s capsule. Inside the Bowman’s
capsule there is a very dense network of blood
capillaries entering as capillaries from the renal
artery and exiting as capillaries from the renal
vein. This dense network of capillaries is called
Glomerulus.

10. The rest of the nephrone is a long coiled tube
where materials filtered from the blood flow in.
At some point the coiled tube becomes straight
and is bent in a U shape tube, this part is called
loop of Henle and it is surrounded by a network
of capillaries from the renal vein, it is where
reabsorption takes place. All nephrones end at a
large tube called the Collecting duct where
content of the nephrones are transported to the
pelvis, to be secreted in the ureter

11. Mechanism of the Kidneys:
Ultrafiltration:
The blood in the renal artery contains large amounts
of urea, glucose, water, mineral ions and some amino
acids. When it reaches the glomerulus, the high
pressure of the blood and the concentration gradient
of these materials between the blood and the
nephrone cause most of these substances to diffuse
from the blood to the bowman’s capsule and become
content of the nephrone, which is called glomerular
filtrate (glomerular filtrate is a mixture of urea, water,
glucose and mineral ions that diffused from the blood
to the nephrone).

12. Reabsorption:
The glomerular filtrate moves in the nephrone till it reaches
the loop of henle, which is surrounded by a dense network
of blood capillaries of the renal vein. Here there is a
concentration gradient of the content of the nephrone
between the nephrone and blood. Both diffusion and active
transport occur to ensure the complete reabsorption of
valuable substances from the glomerular filtrate back to the
blood; these substances are glucose and amino acids. Some
water also moves by osmosis to the blood, as well as
minerals.
That leaves urea, excess water and minerals to continue in
the nephrone till it reaches the collecting duct and the
pelvis. This mixture is called urine. Urine is transported from
the pelvis to the urinary bladder by the ureters. It is them
secreted out of the body through the urethra.