Edexcell ppt Biology 2.68 - 2.76 Used in lessons to scaffold class teaching and as a revision resource for students

1. Excretion in Humans (2.68 – 2.76)
2.68 recall that the lungs, kidneys and skin are organs of excretion
Humans have 3 main excretory organs
1) Lungs – excrete CO2 and H2O
2) Skin – excretes H2O
3) Kidneys – excrete H2O, urea, excess
minerals and other wastes. Parts
game

2. UREA
2.68 recall that the lungs, kidneys and skin are organs of excretion
What’s urea? (not technically on syllabus)
• All organisms produce ammonia as they metabolize
nutrients (protein digestion/amino acids)
• Ammonia is a nitrogenous waste that is toxic and must
be removed from the body
Solution: the liver turns the ammonia into urea, which is
harmless.
Therefore urea is a product of the metabolism of amino
acids.

3. UREA
2.68 recall that the lungs, kidneys and skin are organs of excretion
Many land animals and some bony fish (amphibians,
mammals) dilute the toxic ammonia with water.
This substance is called Urea & is filtered out by the
kidneys.
- The problem is they do lose water in the process
- Requires Energy

4. Excretion & Osmoregulation
2.69 understand how the kidney carries out its roles of excretion and osmoregulation
The Kidney:
The functional unit of the kidney is the nephron. There are
millions of nephrons in a single kidney.
Nephrons have 2 jobs;
Excretion - filtering the blood
and reclaiming the “good bits”
& removing waste
Osmoregulation - balancing the
water level of the body (water
homeostasis)

5. Excretion System
2.70 describe the structure of the urinary system, including the kidneys, ureters, bladder and urethra

6. Excretion System
2.70 describe the structure of the urinary system, including the kidneys, ureters, bladder and urethra

7. Kidney
Nephron and the Kidney
2.71 describe the structure of a nephron, to include Bowmancs capsule and glomerulus, convoluted tubules, loop of Henlé and collecting duct

8. Nephron and capilliaries
2.71 describe the structure of a nephron, to include Bowmancs capsule and glomerulus, convoluted tubules, loop of Henlé and collecting duct

9. How the Nephron works
2.71 describe the structure of a nephron, to include Bowmancs capsule and glomerulus, convoluted tubules, loop of Henlé and collecting duct
2.74 understand that selective reabsorption of glucose occurs at the proximal convoluted tubule
How the nephron works:
1) Dirty blood enters the kidney via the afferent artery
2) The artery splits up into a ball of capillaries, called the glomerulus
3) The blood is under high pressure, so all small substances are forced
out of the holes in the capillary walls. Only large proteins and cells stay
behind.
4) The small substances (glucose, minerals, urea, water etc) move into
the bowman’s capsule, which wraps around the glomerulus
5) The capsule leads into the PCT, which re-absorbs all glucose via active
transport (i.e. it selectively removes the glucose from the nephron and
returns it to the blood)

10. How the Nephron works
2.71 describe the structure of a nephron, to include Bowmancs capsule and glomerulus, convoluted tubules, loop of Henlé and collecting duct
6) The PCT leads to the Loop of Henlé, which re-absorbs the water by osmosis
7) The Loop leads to the DCT, which re-absorbs all minerals, amino acids and other
“useful” substances by active transport
8) The remaining fluid (containing excess water, excess minerals and urea) passes into
the collecting duct
9) The collecting ducts from other nephrons join and form the ureter, which leads to
the bladder
10) The fluid is now called urine and is stored in the bladder for excretion
11) The bladder takes the urine to the outside world via the urethra
This is the first role of the nephron (it’s role in excretion).
Remember, the nephron has a second role in osmoregulation.

11. Excretion in Humans (2.68 – 2.76)
2.71 describe the structure of a nephron, to include Bowmancs capsule and glomerulus, convoluted tubules, loop of Henlé and collecting duct
Kidney

12. Excretion in Humans (2.68 – 2.76)
2.72 describe ultrafiltration in the Bowmancs capsule and the composition of the glomerular filtrate
The glomerulus
filters blood and
produces glomerular
filtrate.
This filtrate contains:
water, glucose, salts
and urea (amino
acids).
(Large molecules such as
protein are too large to
fit through the blood
capillary walls.)

13. Excretion in Humans (2.68 – 2.76)
2.73 understand that water is reabsorbed into the blood from the collecting duct
Teach
Blood water levels are sensed by the hypothalamus in the brain.
When water levels are too low, the hypothalamus tells the pituitary
gland (also in the brain) to release the hormone Anti-Diuretic Hormone
(ADH)

14. Excretion in Humans (2.68 – 2.76)
2.73 understand that water is reabsorbed into the blood from the collecting duct
2.75 describe the role of ADH in regulating the water content of the blood
When blood water levels are too low;
1) Hypothalamus detects
2) Pituitary gland releases ADH into bloodstream
3) ADH travels all over the body
4) Only the cells in the collecting duct of the nephrons of the kidney have
receptors for ADH, so only they respond to the hormone
5) The collecting duct becomes more permeable
6) Water is draw out of the collecting duct back into the blood
7) Water levels return to normal
BBC

15. Review of Urine
2.76 understand that urine contains water, urea and salts.
The waste, consisting of:
• excess water
• excess salts
• urea
is urine.
This process can be summarized in three important steps:
Revision
1) Ultra-Filtration - where lots of water, ions, urea and sugar are squeezed from the
blood into the tubules.
2) Selective reabsorption – the useful substances (ions and sugars) are reabsorbed
back into the blood from the tubules. The amount of water in the blood is regulated
here to maintain it at a constant rate. This is known as ‘osmoregulation’.
3) Excretion of waste - urea and excess water and ions travel to the bladder as urine,
to be released from the body.