iGCSE Biology Notes Excretion in humans

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[h] iGCSE Biology Notes Excretion in humans

• Excretion is the removal of (often toxic) waste products from the body.
• Mainly from metabolic reactions.
• How do we remove carbon dioxide produced by aerobic respiration?
• We breath it out, it diffuses out of the blood into the air in the lungs.

[q] What is the urinary system?

What organs make up the urinary system?

[a] 

• On the right is a diagram of the urinary system.
• It is made up of the two kidneys, two ureters, the bladder and the urethra.
• The renal arteries provide the blood to the kidneys from the aorta, and renal veins take blood away from the kidneys into the vena cava.
• The job of the kidneys is the remove urea, excess ions, and excess water.
• The kidney works by filtering out most substances with high pressure, then reabsorbing any useful things.
• The waste is urine and goes to the bladder before exiting the body through the urethra.

[q] What is urea? (supplement) 

Where does urea come from?

[a] 

• Can anyone remember what proteins are broken down into?
• They’re broken down into amino acids, and we often use these to make new proteins, so they have been assimilated. This happens in the liver.
• However, any amino acids not used to make new proteins are broken down.
• The amino group of the amino acid is removed, which is called deamination, and this amino group is sued to produce urea, which is toxic and must be removed.
• Deamination is the removal of the nitrogen-containing portion of amino acids to produce urea.

[q] Let’s do some more labelling! 

Label the urinary system again

[a]

[q] Let’s do some more labelling! (supplement) 

Let’s look more closely at the kidney itself…

[a]

[q] What is ultrafiltration? (supplement) 

What is ultrafiltration? (supplement) 

[a] 

• A kidney filters most substances out of the blood using high pressure.
• First, blood comes from the renal artery through the afferent arteriole.
• The blood then enters a tangled mess of capillaries called the glomerulus.
• This happens at very high pressure, so most substances are squeezed out of the glomerulus and into the Bowman’s capsule.
• The rest of the blood exits via the efferent arteriole and is more concentrated now since much of the liquid has been lost.

[q] What is ultrafiltration? (supplement) 

How is the high pressure built? 

[a] 

• The afferent arteriole has a much wider lumen than the efferent arteriole.
• This means that when blood flows into the glomerulus, it’s harder to get out through the smaller lumen of the efferent arteriole.
• This builds a high pressure in the glomerulus which pushes most substances (not big molecules or blood cells) into the Bowman’s capsule.
• This part of the kidney is always is always in the cortex. There are thousands of thee structures in each kidney

[q] What is selective reabsorption? (supplement) 

How does reabsorption of useful substances happen? 

[a] 

• After ultrafiltration, the fluid flows through the rest of the nephron, as seen on the right.
• All glucose is reabsorbed via active transport.
• Non-excess ions are reabsorbed.
• Most water is reabsorbed via osmosis, which makes the fluid more concentrated with urea.

[q] Where does the waste go? (supplement) 

What about the substances that aren’t reabsorbed? 

[a] 

• After selective reabsorption, all the fluid that is not reabsorbs flows into a collecting duct.
• This leads to the ureter, then into the bladder.

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