Excretion
Lesson Objectives:
● Describe what excretion is
● Explain the process of excretion in animal and plants
● Define the differences in the processes that ultimately lead to
excretion in animals and plants.
Warm Up:
Which factors can affect the rate of gas exchange?
Excretion
Excretion is the removal of the following substances:
● toxic materials
● waste products of metabolism
● excess substances from organisms
Excretion is not the same as egestion - which is the passing out of
undigested food through the anus as faeces.
Excretion
Plants need to excrete excess carbon dioxide and oxygen. Carbon dioxide is
a waste product of aerobic respiration in plant cells. Oxygen is a waste
product of photosynthesis.
Unlike animals, plants do not have specialised excretory organs. Excess
carbon dioxide and oxygen are excreted from the plant through the stomata in
the leaves.
Excretion
The organs of excretion in humans include the skin, lungs and kidneys.
Skin
Sweat glands in the skin produce sweat. The water in sweat helps to
keep the body cool in hot conditions, and it contains salts and urea.
Lungs
Excess carbon dioxide and some water vapour are removed through the
lungs when humans breathe out.
Kidneys
The kidneys are organs of the urinary system - which removes excess
water, salts and urea.
Urinary System
Blood is brought to the kidney in the renal artery. The
kidneys filter the blood and then reabsorb useful
materials such as glucose. After it has been purified, the
blood returns to the circulation through the renal vein.
Urine is taken from the kidneys to the bladder by the
ureters. The bladder stores the urine until it is
convenient to expel it from the body.
Note that ‘ureter’ differs from the word ‘urethra’. The
ureters are tubes that carry urine from the kidneys to the
bladder, whereas the urethra is the tube that carries
urine out of the body.
Urine contains water, urea and salts. Urea is produced in
the liver when excess amino acids are broken down. It
is the main waste product removed in the urine.
Kidneys
The inner part of the kidney is called the medulla and the outer part is the cortex.
A renal artery carries blood to the kidney and a renal vein carries it away. The ureter
carries urine from the kidney to the bladder.
Blood is filtered at high pressure to remove glucose, water, salts and urea.
All the glucose, and some water and salts, are reabsorbed back into the blood. Note that
urea is not reabsorbed.
Nephron
Urine is produced in microscopic structures in the kidney called nephrons. Each kidney
contains around a million nephrons.
The Bowman’s capsule (renal
capsule):
●
●
●
surrounds a ball of
capillaries called the
glomerulus
the blood is put under
high pressure
ultrafiltration of the
blood happens, in which
water, ions,glucose and
other small molecules
pass into the tubule (but
not proteins or cells)
Nephron
The first convoluted tubule
(proximal convoluted tubule) is
responsible for:
●
the selective reabsorption
of glucose
The collecting duct is responsible
for:
●
●
the selective reabsorption
of water
sending urine to the ureter
ADH
The water content of the blood is controlled by a hormone called ADH (anti-diuretic
hormone). Different amounts of ADH are released into the bloodstream according to the
concentration of the blood plasma.
The diagrams show what happens when there is either too little or too much water in the
blood.
Nephron Summary