Biology · Lesson 11

Excretion & homeostasis

JAMBWAECNECO

What the syllabus asks

JAMB: excretory systems and mechanisms, homeostasis, the kidney, the skin, the liver, excretion in plants

WAEC: excretion in unicellular organisms, excretory organs of animals (contractile vacuole, flame cells, nephridia, Malpighian tubules, kidneys, skin, lungs, liver), structure and function of the mammalian kidney and nephron, excretion in plants, homeostasis: regulation of body temperature, blood sugar, water and salt balance, the liver, kidney diseases

NECO: excretion in unicellular organisms, excretory organs of animals (contractile vacuole, flame cells, nephridia, Malpighian tubules, kidneys, skin, lungs, liver), structure and function of the mammalian kidney and nephron, excretion in plants, homeostasis: regulation of body temperature, blood sugar, water and salt balance, the liver, kidney diseases

What you will learn

Define excretion and list the main excretory products and organs.
Describe excretion in Amoeba, flatworms, earthworms and insects.
Describe the structure of the mammalian kidney and nephron and explain how urine is formed.
Explain the excretory roles of the skin, lungs and liver.
Describe excretion in plants.
Explain homeostasis and negative feedback using temperature, water and blood glucose.
Describe common kidney disorders and their management.

Key points

Excretion removes metabolic wastes: CO₂ (lungs), urea, water and salts (kidneys, skin), bile pigments (liver).
Urea is made in the liver by deamination of excess amino acids.
Amoeba: diffusion + contractile vacuole; flatworms: flame cells; earthworm: nephridia; insects: Malpighian tubules.
Urinary system: kidney → ureter → bladder → urethra; renal vein blood has least urea.
Nephron: ultrafiltration in glomerulus/Bowman's capsule, selective reabsorption in PCT, water reabsorption in collecting duct.
ADH from the pituitary increases water reabsorption, giving little concentrated urine.
Skin cools by sweating and vasodilation, conserves heat by vasoconstriction, shivering and raised hairs.
Insulin lowers and glucagon raises blood glucose, both acting on the liver.
Homeostasis uses receptor, control centre, effector and negative feedback.
Plants excrete by diffusion, leaf fall, storage in bark and crystals, and exudation of gums and latex.

Start here: Why excretion is necessary

Your body is like a busy factory in Aba. Raw materials come in (food, oxygen, water), products are made (energy, new cells, enzymes), and waste is produced. If the waste is not cleared, the factory chokes. In the body, those wastes are poisonous.

Excretion is the removal from the body of the waste products of metabolism (the chemical reactions in cells), together with substances in excess of the body's needs.

Waste	Where it comes from	Why it is harmful	Removed by
Carbon dioxide	Respiration	Forms carbonic acid, lowers blood pH	Lungs
Urea	Breakdown of excess amino acids in liver	Toxic at high levels	Kidneys (some in sweat)
Excess water	Food, drink, respiration	Dilutes blood, cells swell	Kidneys, skin, lungs
Excess salts	Diet	Upsets osmotic balance	Kidneys, skin
Bile pigments	Breakdown of old red blood cells	Cause jaundice if they build up	Liver (into bile, out with faeces)

Why are amino acids a special problem? The body cannot store protein. When you eat more beans, fish or meat than you need, the extra amino acids are broken down in the liver. The nitrogen-containing amino group is removed (deamination) and turned first into ammonia, which is very poisonous, and then into the less harmful urea, which the blood carries to the kidneys.

Worked example 11.1

Explain why a person who eats a very high-protein diet produces more urea.

Protein is digested into amino acids. Amino acids beyond the body's needs cannot be stored.
The excess is deaminated in the liver; each amino group becomes ammonia, then urea.
More excess amino acids means more deamination, so more urea.

More surplus amino acids → more deamination → more urea

⚠️ Common mistake: saying urea is made in the kidney. Urea is made in the liver; the kidney only removes it.

Try it yourself. Name the excretory product of the lungs and the process that makes it.

Show solution

Carbon dioxide (and water vapour), made by respiration.

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Practice questions

1. The functional unit of the kidney is the

nephron
neuron
alveolus
villus

Show answer

A: nephron
Each kidney has about a million nephrons.

2. Urea is formed in the

kidney
bladder
pancreas
liver

Show answer

D: liver
Deamination of excess amino acids in the liver produces urea; the kidney only removes it.

3. Plants excrete excess oxygen mainly through

flowers
xylem
stomata
roots

Show answer

C: stomata
Gases leave the leaf through the stomata (and lenticels in stems).

4. Which structure in the nephron is a knot of capillaries?

Ureter
Glomerulus
Loop of Henle
Collecting duct

Show answer

B: Glomerulus
It sits inside Bowman's capsule.

5. The breakdown of excess amino acids to form urea is called

deamination
ultrafiltration
assimilation
emulsification

Show answer

A: deamination
It takes place in the liver.

Key words

Excretion: Removal from the body of waste products of metabolism.
Urea: Nitrogenous waste made in the liver from excess amino acids.
Deamination: Removal of the amino (–NH₂) group from amino acids in the liver.
Nephron: The functional unit of the kidney.
Glomerulus: A knot of capillaries in the Bowman's capsule where blood is filtered.
Ultrafiltration: Filtration under high pressure that forces small molecules out of the blood.
Selective reabsorption: Taking useful substances back from the filtrate into the blood.
ADH: Antidiuretic hormone from the pituitary; makes collecting ducts more permeable to water.
Homeostasis: Keeping the internal environment of the body nearly constant.
Negative feedback: A change triggers a response that reverses the change.
Osmoregulation: Control of the water and salt balance of the body.
Vasodilation: Widening of blood vessels near the skin surface.

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