Learning outcomes:
2.67
understand the origin of carbon dioxide and oxygen as waste products of metabolism and their loss from the stomata of a leaf
2.68
recall that the lungs, kidneys and skin are organs of excretion
2.69
understand how the kidney carries out its roles of excretion and osmoregulation - b
2.70
describe the structure of the urinary system, including the kidneys, ureters, bladder and urethra
2.71
describe the structure of a nephron, to include Bowman’s capsule and glomerulus, convoluted tubules, loop of Henlé and collecting duct
2.72
describe ultrafiltration in the Bowman’s capsule and the composition of the glomerular filtrate
2.73
understand that water is reabsorbed into the blood from the collecting duct
2.74
understand that selective reabsorption of glucose occurs at the proximal convoluted tubule
2.75
describe the role of ADH in regulating the water content of the blood.
2.76
understand that urine contains water, urea and salts.
Excretion: the removal of waste products of metabolism from living organisms

Excretion in Flowering plants:
CO2 and O2 are excreted by leaves via the stomata. O2 is excreted during photosynthesis and CO2 is excreted during respiration

Excretion in Humans:
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.

Extension - what’s urea? (not technically on syllabus)

We need to have a certain amount of protein in our diet to supply the amino acids we need to make our own body proteins. However, we usually eat far more that we need, so we must excrete the rest.
Problem: when amino acids are broken down they make ammonia, which is very toxic.
Solution: the liver turns the ammonia into urea, which is harmless.
Therefore urea is a product of the metabolism of amino acids.

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”
Osmoregulation - balancing the water level of the body (water homeostasis)

A Nephron:

nephron.GIF

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)
6. The PCT leads to the Loop of Henlé, which re-absorbs the water my 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.

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)

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

When water levels are too high exactly the opposite happens (i.e. the pituitary releases less ADH)

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