Ch 1 · Excretion in Humans · O Level Biology 5090
Topic 1 of 1 in Excretion in Humans — O Level Biology 5090 — 48 lessons.
Introduction
Metabolism produces waste. Respiration makes carbon dioxide, and breaking down surplus amino acids makes urea, which is poisonous if it accumulates. Excretion removes these toxic products of metabolism and substances present in excess. This lesson covers the lungs and kidneys, the part played by the liver and, for candidates, how a nephron makes urine and how dialysis substitutes for failed kidneys.
Key ideas — What is excreted, and where
Be careful with the word "excess": water and salts are essential, and only the surplus is removed. Faeces are not an excretory product, because undigested food was never part of the body's metabolism.
🧑🏫 Teacher's explanation
Think of your body as a busy city with a waste management system. Every cell is a factory producing carbon dioxide as a by-product of energy release. This gas is swept away by the blood and released through the lungs, much like smoke leaving a chimney. This is the first major excretory route.
📖 Definitions
🔍 Worked example — Easy
List the three main excretory products in humans and state the organ responsible for removing each.
🔍 Worked example — Standard
Explain why the liver is essential for the excretion of urea, and why urea must be removed from the blood.
🔍 Worked example — Exam-style
Describe the structure of a nephron and explain how it functions to produce urine, specifically mentioning filtration and reabsorption.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(b) Explain why the liver is involved in the excretion of urea. (2 marks) (c) Describe the role of the kidneys in the excretion of urea and excess water. (2 marks) [5 marks]
Key ideas — The urinary system and the liver
On a diagram of the urinary system, identify:
🧑🏫 Teacher's explanation
Think of your body as a busy city with a waste management system. The liver is the recycling plant: it takes in surplus amino acids from your food, breaks them down, and packages the toxic nitrogen waste into urea. The kidneys are the sewage treatment plants: they filter this urea out of the blood, along with excess water and salts, to keep the internal environment stable.
📖 Definitions
🔍 Worked example — Easy
Identify the four main organs of the urinary system on a diagram and state the specific function of the bladder.
🔍 Worked example — Standard
Explain why the liver is essential for the excretion of nitrogenous waste, referencing the fate of excess amino acids.
🔍 Worked example — Exam-style
A patient with kidney failure has a blood urea concentration of 8.0 mmol/dm^3. Normal blood urea is 2.0 mmol/dm^3. Explain why this patient requires dialysis and describe the principle of how dialysis removes urea.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(b) Explain the role of the liver in the excretion of nitrogenous waste. (3 marks) (c) A patient with kidney failure undergoes dialysis. Explain how the dialysis machine removes urea from the blood, referring to the movement of substances across a membrane. (3 marks) [9 marks]
Key ideas — Kidney structure and the nephron
how the kidney makes urine. A kidney cut lengthways shows an outer cortex, an inner medulla, and the ureter leaving from the centre. Each kidney contains about a million microscopic tubes called nephrons.
🧑🏫 Teacher's explanation
Think of the kidney as a high-pressure water filter. Just like a coffee filter catches the grounds but lets the liquid through, the glomerulus uses high blood pressure to push small molecules out of the blood while keeping large cells and proteins behind. This initial liquid is called filtrate.
📖 Definitions
🔍 Worked example — Easy
Identify the three main parts of the nephron shown in a diagram and state the specific function of the glomerulus.
🔍 Worked example — Standard
Explain why glucose is present in the filtrate in Bowman's capsule but absent in the urine leaving the kidney.
🔍 Worked example — Exam-style
A patient with kidney failure has a high concentration of urea in their blood. Explain why this is dangerous and how dialysis removes it.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(b) Explain why glucose is present in the filtrate but not in the urine. (c) State one reason why urea must be removed from the blood. [6 marks]
Key ideas — Dialysis
the dialysis principle. When kidneys fail, urea and excess ions accumulate in the blood and must be removed artificially. In a dialysis machine, the patient's blood flows through tubing made of a partially permeable membrane, surrounded by dialysis fluid. The fluid contains glucose and ions at the same concentration as healthy blood, but no urea. Small molecules can cross the membrane; blood cells and proteins cannot.
🧑🏫 Teacher's explanation
Think of your blood as a crowded bus packed with passengers (glucose, ions, proteins) and a few very smelly, toxic passengers (urea). Normally, your kidneys are the bus stop where the toxic passengers get off. If the kidneys fail, the toxic passengers stay on, making the air inside the bus (your body) poisonous. Dialysis is an external bus stop. It uses a special window (a partially permeable membrane) that lets small, smelly molecules escape but keeps the large, essential passengers inside.
📖 Definitions
🔍 Worked example — Easy
A patient with kidney failure has blood with a urea concentration of 100 mg/dL and a glucose concentration of 100 mg/dL. The dialysis fluid has a urea concentration of 0 mg/dL and a glucose concentration of 100 mg/dL. Describe the net movement of urea and glucose across the dialysis membrane.
🔍 Worked example — Standard
Explain why the dialysis fluid must contain glucose and ions at the same concentration as healthy blood, but must not contain urea.
🔍 Worked example — Exam-style
A student claims: 'Dialysis works because the blood cells are filtered out of the blood into the dialysis fluid.' Evaluate this statement using knowledge of the structure of the nephron and dialysis membranes.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
🔍 Worked example — Easy
State why excess amino acids cannot be stored in the human body.
🔍 Worked example — Standard
Describe the role of the liver in the assimilation of amino acids and the formation of urea.
🔍 Worked example — Exam-style
Explain why a build-up of urea in the blood is harmful and how dialysis removes it.
⚠️ Common errors
Common mistakes
Practice
Answers