6 narrated scenes. Press play, listen and read along — the points appear as they are explained. Use ‹ Prev / Next › or the scene bar to jump to any scene.
LESSON B12 · EXPLAINER
Excretion and the Kidney
Your kidneys filter about 180 litres of fluid a day, but you only make 1–2 litres of urine. Where does the rest go?
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Step 2 · Read
Key notes
Copy these into your notebook in your own words — that is how they stick.
Excretion
Removal of metabolic wastes (urea, CO₂, excess water/salts).
Egestion
Removal of undigested food (faeces) through the anus.
Diffusion of gases via stomata/lenticels; wastes stored in leaves (leaf fall), bark, dead tissues.
Ultrafiltration
Glomerulus → Bowman’s capsule under high pressure; blood cells and plasma proteins stay in blood.
Selective reabsorption
All glucose and amino acids, most water and salts reabsorbed in proximal tubule.
ADH
Released when blood is too concentrated; collecting duct more permeable → more water reabsorbed → concentrated urine.
Kidney failure
Accumulation of urea and water; treated by dialysis/transplant. Prevented by controlling diabetes & blood pressure.
EXAM TIPS & COMMON MISTAKES
Urine formation answers need all three steps: ULTRAFILTRATION, SELECTIVE REABSORPTION, water reabsorption (ADH).
ADH questions: name the hormone, the organ it acts on (collecting duct), the effect (more water reabsorbed) and the result (less, more concentrated urine).
In kidney disease, blood UREA increases.
Step 2 · Learn from examples
Worked examples & study resources
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Worked example 1
Plasma, filtrate and urine
Concentrations (g per 100 cm³) in a healthy kidney:
Substance
Blood plasma
Filtrate in the Bowman’s capsule
Urine
Protein
8.0
0
0
Glucose
0.1
0.1
0
Urea
0.03
0.03
2.0
Explain the figures for each substance.
Try it in your notebook first, then reveal the solution one step at a time.
Protein: none in the filtrate because protein molecules are too large to pass through the walls of the glomerulus during ultrafiltration.
Glucose: small enough to be filtered (0.1 in filtrate), but none in the urine because ALL of it is taken back into the blood by selective reabsorption in the tubule.
Urea: filtered, then becomes about 67 times more concentrated in urine (2.0 ÷ 0.03) because most of the WATER is reabsorbed while urea is not.
Answer. Protein — too large to be filtered. Glucose — filtered, then completely reabsorbed. Urea — filtered and not reabsorbed; concentrated in urine because water is reabsorbed.
Examiner’s tip. Glucose in urine is a sign of diabetes: blood glucose is so high that the tubules cannot reabsorb it all.
Worked example 2
ADH on a hot day
After playing football on a hot afternoon, Marcus produces a small volume of dark urine. Explain how his body produced this response.
Try it in your notebook first, then reveal the solution one step at a time.
Marcus lost a lot of water in sweat, so the water content of his blood fell (the blood became more concentrated).
This was detected by receptors in the brain (hypothalamus).
The pituitary gland released more ADH (antidiuretic hormone) into the blood.
ADH makes the walls of the collecting ducts more permeable to water, so more water is reabsorbed into the blood.
Result: a small volume of concentrated (dark) urine, conserving water. When he drinks, less ADH is released and more dilute urine is made — an example of negative feedback.
Answer. Water lost by sweating → brain detects concentrated blood → pituitary releases more ADH → collecting ducts reabsorb more water → small volume of concentrated urine.
Examiner’s tip. Name the hormone (ADH), the gland (pituitary) and the target (collecting ducts/tubules). This control of water balance is called osmoregulation.
On the board · Excretory organs and what they remove
Organ
Excretory product
Where it comes from
Lungs
Carbon dioxide, water vapour
Respiration
Kidneys
Urea, excess water and salts (urine)
Urea from breakdown of excess amino acids (deamination) in the liver
Skin
Sweat: water, salts, a little urea
Mainly for temperature control
Liver
Bile pigments (in bile)
Breakdown of old red blood cells
Leaves (plants)
Oxygen (by day), carbon dioxide, water vapour
Photosynthesis and respiration
Leaves and bark (plants)
Waste stored and lost when leaves fall or bark peels
Various metabolic wastes
Excretion removes waste made by the body’s metabolism. Faeces are egested, not excreted.
Key terms
Excretion
Removal of waste products of metabolism from the body.
Egestion
Removal of undigested food (faeces) — not excretion.
Deamination
Removal of the nitrogen part of excess amino acids in the liver, forming urea.
Ultrafiltration
Filtering small molecules out of the blood under high pressure in the glomerulus.
Selective reabsorption
Taking useful substances (all glucose, much water and salts) back into the blood from the tubule.
Osmoregulation
Controlling the water and salt balance of the body.
ADH
Antidiuretic hormone from the pituitary; increases water reabsorption.
Common misconceptions
✗ Passing faeces is excretion.✓ Faeces are mostly undigested food that never entered the body’s cells — removing them is egestion.
✗ The kidneys just make urine.✓ They also control the body’s water and salt balance (osmoregulation).
✗ Sugar in urine is normal after a sweet meal.✓ In a healthy person all glucose is reabsorbed. Glucose in urine suggests diabetes.
Memory aid
ADH = “Anti-Dehydration Hormone”: more ADH → more water kept → less, darker urine.
Real-life link
Diabetes and hypertension are the main causes of kidney failure in the Caribbean. Patients whose kidneys fail need dialysis several times a week or a kidney transplant — controlling blood sugar and blood pressure protects the kidneys.
Try this
For one day, compare the colour of your urine with how much water you drank. Pale straw-coloured urine means you are well hydrated.
Play & practise
Games
4 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
Step 3 · Practise
Paper 1 practice — multiple choice
8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Step 4 · Apply
Paper 2 practice — structured questions
Write full answers, then reveal the mark scheme and tick the points you earned (23 marks in total). Your answers are saved on this device.
Based on CSEC P2 May/June 2021, Question 4
Question 1 [11 marks]
A labelled nephron shows Bowman’s capsule, the proximal tubule, loop of Henle, distal tubule and collecting duct.
(a) Describe how urine is formed. [6]
MARK SCHEME — (a) (max 6)
(b) Explain why there is a high concentration of glucose in the urine of a diabetic. [3]
MARK SCHEME — (b) (max 3)
(c) Suggest TWO ways kidney damage can be prevented in diabetic and hypertensive patients. [2]
MARK SCHEME — (c) (max 2)
Based on CSEC P2 January 2024 Q6(d)–(e) and May/June 2023 Q6
Question 2 [12 marks]
Excretion and homeostasis.
(a) Define “egestion” and “excretion”; list TWO excretory organs with a substance each excretes. [6]
MARK SCHEME — (a) (max 6)
(b) Joy is dehydrated after vomiting. Explain, naming the hormone, how her kidneys help correct the fluid imbalance. [4]
MARK SCHEME — (b) (max 4)
(c) Suggest TWO consequences of kidney failure. [2]
MARK SCHEME — (c) (max 2)
Your progress
Lesson readiness
0% READY
Not started
Video watched (20%)0%
Multiple choice (40%)0%
Structured (40%)0%
Sources
Where this topic appears in CSEC papers
Find these in your own past-paper booklets for extra timed practice. The practice above is adapted from them.