10.1Chapter 10: Excretion in Humans

What is excretion? — Alhussan Biology - Stage 9 Revision Notes & Topical Questions

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10.1 What is excretion? — Removing Metabolic Waste Products — Alhussan International Academy — Biology (Stages 7–9) notes: Define excretion, name the main excretory products and organs, and explain how urea is made and why excretion matters.Excretion = the removal of the waste pr…

Carbon dioxide
Protein excess
Deamination
Daily output
Meat eaters
Urine colour

Part 1 of 6What excretion means

Excretion

Excretion the removal of the waste products of metabolism, and of substances in excess of the body's needs, from the body.

Metabolism is the sum of all the chemical reactions in cells, and many produce substances that are harmful if they build up.
Excretion is one of the characteristics of all living organisms.

Egestion

Egestion the removal of undigested food as faeces, which is not excretion.
Faeces are materials taken into the body but never absorbed into the blood, such as plant fibre.

Part 2 of 6Main excretory products and organs

Waste product	Where it comes from	Organ that removes it
Carbon dioxide	Respiration in all cells	Lungs (breathed out)
Urea	Breakdown of excess amino acids in the liver	Kidneys (in urine); some in sweat
Excess water and salts	Food and drink	Kidneys; also skin (sweat) and lungs (water vapour)

Carbon dioxide
- it dissolves to form an acid and lowers the pH of blood, so large amounts are toxic.

Part 3 of 6How urea is made

Protein excess

Protein excess the body can store excess carbohydrate and lipid as fat but cannot store amino acids or protein.

Deamination

Deamination in the liver the amino group is removed from the amino acid and converted into urea.
The remaining carbon-containing part is used for respiration or converted to carbohydrate or fat.

Transport

Transport urea is soluble, so it is carried dissolved in the blood plasma to the kidneys.

Toxicity

Toxicity if the urea concentration in the blood gets too high it damages organs including the brain and can kill.

Daily output

Daily output a healthy adult passes about 1.5 litres of urine containing about 25 g of urea.

Meat eaters

Meat eaters an animal such as a lion eating a lot of protein makes a lot of urea.

Part 4 of 6The kidneys and urine

Kidney

Kidney it is a reddish-brown organ about 10 cm long, against the back at about waist level.
A thick layer of fat protects its delicate structures from damage.

Urea

Urea it forms white crystals that dissolve in water to give a colourless solution.

Urine colour

Urine colour the yellow colour comes from yellow substances excreted along with the urea and water.

Part 5 of 6Why excretion is essential

It prevents the build-up of toxic substances that would damage cells and enzymes.

It keeps the chemical composition of blood and tissue fluid steady, as part of homeostasis.

It controls the water content of the body so that cells do not shrink or swell.

Kidney failure

Kidney failure it lets urea and other wastes accumulate, causing illness and eventually death unless treated.

Part 6 of 6Excretion in plants

1Plants excrete without special organs

Plants excrete without special organs oxygen from photosynthesis leaves through stomata, and for example palms release oxygen into the air.

2Stored wastes

Stored wastes waste products can be stored in leaves that are lost in autumn.

Key terms

Term	Meaning
Excretion	Removal of waste products of metabolism from the body.
Egestion	Removal of undigested food as faeces.
Metabolism	All the chemical reactions in the cells of an organism.
Urea	A toxic waste made in the liver from excess amino acids.
Deamination	Removal of the amino group from amino acids in the liver.
Urine	Liquid containing urea, water and salts, made by the kidneys.