Lesson overview
Excretion and the kidneys: How do your kidneys clean your blood?
Send blood through a model kidney and count the urea it removes, then follow the urine out of the body. Inside one nephron, watch the blood being filtered and the useful substances taken back. Then see a liver cell make urea from amino acids the body cannot store.
Suggested sequence
- Lesson 1 · Getting rid of waste. Excretion of carbon dioxide by the lungs and of urea, excess water and excess ions by the kidneys; the kidneys, ureters, bladder and urethra; the cortex and medulla of the kidney (Supplement). Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Inside a nephron. Supplement: one nephron and its blood vessels; filtration in the glomerulus, reabsorption back into the blood, and what urine contains. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Where urea comes from. Supplement: the liver builds proteins from amino acids and makes urea from the excess by deamination; why urea must be excreted. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 9 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 10 questions, 15 min, lesson closed.
Learning goals
Part 1: Getting rid of waste
- State that carbon dioxide is excreted through the lungs (13.1.1)
- State that the kidneys excrete urea and excess water and ions (13.1.2)
- Identify in diagrams and images the kidneys, ureters, bladder and urethra (13.1.3)
- Supplement: identify in diagrams and images the structure of the kidney, limited to the cortex and medulla (S13.1.4)
Part 2: Inside a nephron
- Supplement: outline the role of the glomerulus in the filtration from the blood of water, glucose, urea and ions (S13.1.5)
- Supplement: outline the role of the nephron in the reabsorption of all of the glucose, some of the ions and most of the water back into the blood (S13.1.5)
- Supplement: outline the formation of urine containing urea, excess water and excess ions (S13.1.5)
Part 3: Where urea comes from
- Supplement: describe the role of the liver in the assimilation of amino acids by converting them to proteins (S13.1.6)
- Supplement: state that urea is formed in the liver from excess amino acids, and describe deamination as the removal of the nitrogen-containing part of amino acids to form urea (S13.1.7, S13.1.8)
- Supplement: explain the importance of excretion, limited to the toxicity of urea (S13.1.9)
Misconceptions to listen for
- “S is the urethra.” — S runs from a kidney to the bladder: it is a ureter. The urethra (U) leads from the bladder to the outside.
- “Q is the renal vein and R is the renal artery.” — The renal artery (Q) comes from the aorta and brings blood to the kidney. The renal vein (R) takes blood to the vena cava.
- “U is a ureter.” — U leads out of the bladder to the outside, so it is the urethra.
- “The kidneys.” — The kidneys excrete urea, excess water and excess ions in urine. Carbon dioxide leaves through the lungs.
- “The bladder.” — The bladder only stores urine until it is passed. Carbon dioxide leaves in the air breathed out.
- “The liver.” — The liver does not remove carbon dioxide from the body. The blood carries it to the lungs.
- “Glucose.” — Normal urine has no glucose: the body keeps it.
- “Carbon dioxide.” — Carbon dioxide leaves through the lungs, not in urine.
- “Protein.” — Protein stays in the blood: normal urine has none.
- “Breathing out carbon dioxide made by muscle cells.” — Carbon dioxide is a waste product of respiration in the muscle cells, so breathing it out is excretion.
Scope
Excretion of carbon dioxide through the lungs (13.1.1) and of urea, excess water and excess ions by the kidneys (13.1.2); the kidneys, ureters, bladder and urethra (13.1.3); Supplement: the cortex and medulla of the kidney (13.1.4); the structure and function of a nephron and its blood vessels: filtration in the glomerulus, reabsorption of all of the glucose, some of the ions and most of the water, and urine (13.1.5); the liver assimilates amino acids by converting them to proteins (13.1.6); urea forms in the liver from excess amino acids by deamination (13.1.7, 13.1.8); the importance of excretion, limited to the toxicity of urea (13.1.9). The control of water and of body temperature is in Homeostasis; the detailed mechanisms of reabsorption, dialysis and transplants are not part of the syllabus. The numbers in the experiment are a model with far fewer particles than a real nephron.