Cambridge IGCSE • Year 11 • Biology
Excretion in Humans
Kidney function, excretion and homeostasis links.
Chapter 13
Verified Curriculum Topic
What is Excretion in Humans?
Kidney function, excretion and homeostasis links.
Excretion in Humans matters because it helps students explain living systems with precise vocabulary and clear cause-and-effect reasoning. At Year 11 level, strong performance usually depends on understanding processes, structures, functions, and diagram-based explanations.
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Summary
The One Thing
The kidneys maintain homeostasis by filtering the blood, removing urea and excess substances, selectively reabsorbing useful materials, and adjusting water reabsorption under the control of antidiuretic hormone (ADH).
Reactions, Processes and Experiments
| What happens | Equation or process | What you observe | Type |
|---|---|---|---|
| Excess amino acids are broken down in the liver; their amino groups are converted into urea, while the remaining parts may be used in respiration or converted to carbohydrate or fat. | Deamination of excess amino acids in the liver | Urea is produced and transported in the blood to the kidneys. | Metabolic process |
| Carbon dioxide is removed from the body by the lungs. | Excretion of carbon dioxide by the lungs | Carbon dioxide leaves the body during exhalation. | Excretion |
| Blood enters each kidney through the renal artery and leaves through the renal vein. | Blood flow: renal artery → kidney → renal vein | Blood is supplied to and removed from the kidney. | Transport process |
| Blood is filtered in the glomerulus, and small molecules pass into the Bowman's capsule. | Ultrafiltration: water, glucose, amino acids, urea, and mineral ions pass into the Bowman's capsule; large plasma proteins and blood cells remain in the blood. | Filtrate collects in the Bowman's capsule, while blood cells and large plasma proteins remain in the bloodstream. | Ultrafiltration |
| High pressure is generated in the glomerulus because the afferent arteriole is wider than the efferent arteriole. | Afferent arteriole wider than efferent arteriole | High blood pressure drives filtration in the glomerulus. | Pressure-driven filtration |
| Useful substances are returned from the filtrate to the blood, mainly in the proximal convoluted tubule. | Selective reabsorption | All glucose and amino acids, most water, and some mineral ions are reabsorbed. | Selective reabsorption |
| Glucose is returned to the blood using energy supplied by respiration. | Active transport of glucose during selective reabsorption | Glucose is normally absent from urine; glucose in urine may indicate excessive blood glucose concentration or impaired reabsorption. | Active transport |
| The loop of Henle and associated blood vessels establish conditions that allow water to be reabsorbed from the collecting duct. | Creation of a concentration gradient by the loop of Henle | The collecting duct can reabsorb more water. | Concentration-gradient process |
| ADH increases the permeability of collecting ducts to water when blood water potential falls. | ADH released when blood water potential falls → collecting ducts become more permeable to water → more water returns to the blood | Urine becomes more concentrated and its volume decreases. | Hormonal control; negative feedback |
| Less ADH is released when blood water potential rises. | Blood water potential rises → less ADH released → less water reabsorbed | A larger volume of more dilute urine is produced. | Hormonal control; negative feedback |
| Filtrate passes through the collecting duct and leaves the kidney as urine. | Urine leaves through the ureter, is stored in the bladder, and exits through the urethra. | Urine containing water, urea, and variable amounts of mineral ions and other wastes is eliminated. | Excretion and elimination |
| Water balance is maintained when body water content remains constant. | water intake = water lost in urine + water lost in faeces + water lost through the skin and lungs | Total water input equals total water loss. | Homeostatic relationship |
| Dialysis removes urea and adjusts the concentration of useful substances in the blood. | Dialysis fluid contains a normal concentration of glucose and mineral ions but no urea; it is continually refreshed. | Urea diffuses out of the blood, while useful substances are maintained at appropriate concentrations. | Artificial filtration |
| A kidney transplant treats permanent kidney failure by placing a healthy kidney into the patient. | Surgical placement of a healthy kidney into the patient | More normal kidney function may be restored, but rejection and the need for immunosuppressant drugs are possible. | Transplant treatment |
Key Terms
- Excretion: The removal from the body of toxic materials, substances in excess of requirements, and waste products of metabolism.
- Metabolic waste: A waste substance produced by chemical reactions in cells, such as carbon dioxide and urea.
- Urea: A nitrogenous waste product made in the liver when excess amino acids are broken down; it is transported in the blood to the kidneys.
- Kidney: An organ that filters blood, removes urea, and regulates the water and ion content of the body.
- Nephron: The functional unit of the kidney where filtration, reabsorption, and urine formation occur.
- Glomerulus: A network of capillaries inside the Bowman's capsule where high-pressure ultrafiltration takes place.
- Bowman's capsule: A cup-shaped structure surrounding the glomerulus that collects the filtrate.
- Ultrafiltration: The filtration of small molecules from the blood into the Bowman's capsule because of high blood pressure in the glomerulus.
- Selective reabsorption: The movement of useful substances from the filtrate back into the blood, mainly in the proximal convoluted tubule.
- Proximal convoluted tubule: The nephron region where all glucose, amino acids, and much of the water and ions are reabsorbed.
- Loop of Henle: A nephron region that helps establish a concentration gradient, allowing more water to be reabsorbed from the collecting duct.
- Collecting duct: A tube that carries filtrate through the kidney and whose water permeability is controlled by antidiuretic hormone.
- Antidiuretic hormone: A hormone released from the pituitary gland that increases the permeability of collecting ducts to water, reducing water loss in urine.
- Homeostasis: The maintenance of a constant internal environment despite changes inside or outside the body.
- Osmoregulation: The control of water potential and ion concentration in body fluids.
- Urine: A liquid containing water, urea, and variable amounts of mineral ions and other wastes that is produced by the kidneys.
- Dialysis: A treatment for kidney failure in which a machine removes urea and adjusts the concentration of useful substances in the blood.
- Kidney transplant: A treatment for permanent kidney failure in which a healthy kidney is surgically placed into the patient.
Easily Confused
- Excretion versus egestion: Excretion removes metabolic waste products, whereas egestion is not described in this material and concerns the removal of undigested food.
- Ultrafiltration versus selective reabsorption: Ultrafiltration moves small molecules from the blood into the Bowman's capsule, whereas selective reabsorption returns useful substances from the filtrate to the blood.
- Glomerulus versus Bowman's capsule: The glomerulus is the capillary network where filtration occurs, whereas the Bowman's capsule collects the filtrate.
- Renal artery versus renal vein: The renal artery carries blood into the kidney, whereas the renal vein carries blood away from it.
- Ureter versus urethra: The ureter carries urine from the kidney to the bladder, whereas the urethra carries urine out of the body.
- Dialysis versus kidney transplant: Dialysis artificially removes wastes and usually requires repeated treatment, whereas a kidney transplant surgically places a healthy kidney into the patient.
- High versus low blood water potential: A fall in blood water potential causes more ADH release and more water reabsorption, whereas a rise causes less ADH release and the production of more dilute urine.
- Ultrafiltration versus active transport: Ultrafiltration is driven by high blood pressure, whereas glucose reabsorption requires energy from respiration.
What Gets Asked
- Identify the main human excretory products and their routes of removal: carbon dioxide is excreted by the lungs, while urea, excess water, and excess mineral ions are mainly excreted by the kidneys.
- Explain how urea is formed: excess amino acids undergo deamination in the liver, and the amino group is converted into urea.
- Describe ultrafiltration and distinguish substances that enter the filtrate from those that remain in the blood. The specific mark-losing error is stating that blood cells and large plasma proteins pass into the Bowman's capsule.
- Explain selective reabsorption in the proximal convoluted tubule. The essential details are that all glucose and amino acids are normally reabsorbed, and that glucose reabsorption occurs by active transport.
- Explain ADH control of water balance using negative feedback. A fall in blood water potential must be linked to increased ADH release, increased collecting-duct permeability, increased water reabsorption, and more concentrated urine.
- Compare dialysis with kidney transplantation, including the continually refreshed dialysis fluid, repeated treatment, transplant rejection, and the need for immunosuppressant drugs.
Flashcards
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What is the main purpose of excretion in humans?
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Sign up free — save & unlock everythingLearning objectives
- 13.1Define excretion as the removal of the waste products of metabolism, toxic materials, and substances in excess.
- 13.2Describe the structure of the urinary system, naming the kidneys, ureters, bladder, and urethra.
- 13.3State that the kidney removes urea, excess water, and excess ions from the blood to form urine.
- 13.4Explain the role of the kidney in osmoregulation, controlling the water content of the blood.extended
- 13.5Describe the source of urea and explain why it must be removed from the body.
- 13.6Describe the principles of dialysis as a treatment for kidney failure.extended
Practice questions
Q1. Urea is produced in the body mainly by the breakdown of:1 mark · core
- A. Excess glucose
- B. Excess amino acids
- C. Excess fats
- D. Excess water
Answer: B
- • 1 mark for selecting B
Urea is produced in the liver by deamination of excess amino acids, which cannot be stored by the body.
Q2. State three substances removed from the blood by the kidney to form urine.3 marks · core
Answer: Urea, excess water, excess ions (salts).
- • 1 mark for each correct substance, up to 3: urea / excess water / excess ions or salts
Q3. Explain how the kidney helps to control the water content of the blood.3 marks · extended
Answer: If blood water content is too high, the kidney reabsorbs less water, producing more dilute urine; if too low, the kidney reabsorbs more water, producing more concentrated urine — keeping blood water content stable.
- • 1 mark: if blood is too dilute, less water is reabsorbed, producing dilute urine
- • 1 mark: if blood is too concentrated, more water is reabsorbed, producing concentrated urine
- • 1 mark: this keeps the water content of the blood within a stable range (homeostasis)
Q4. Define excretion.2 marks · core
Answer: The removal of waste products of metabolism, toxic materials, and substances in excess from the body.
- • 1 mark: removal of metabolic waste products / toxic materials
- • 1 mark: and substances present in excess, from the body
Key ideas to master
- Master the important terms, labelled structures, and process sequences in Excretion in Humans.
- Explain how the system works step by step using accurate biological vocabulary.
- Practise diagram-based recall, comparisons, and function-based questions.
- Focus on causes, effects, and interactions rather than memorising isolated points.
Common exam prompts
- Describe the process or structure in Excretion in Humans in the correct sequence.
- Label or explain a likely diagram-based question from this topic.
- Compare related systems, tissues, organs, or processes where the chapter requires it.
- Summarise the functional importance of Excretion in Humans in concise exam language.
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Kidney function, excretion and homeostasis links.
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