Excretion In Humans

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Excretion: Definition & Organs

Excretion is the removal of waste products of metabolic reactions, toxic materials, and substances in excess of requirements.
The main excretory organs are the lungs (excrete CO₂), kidneys (excrete excess water, salts, and urea), and the liver (produces urea via deamination).
Egestion (removal of undigested food via the anus) is not excretion.
CO₂ must be excreted because it dissolves in water to form an acidic solution, lowering pH and reducing enzyme activity.
Urea is toxic at high concentrations and must be removed.

The Urinary System

Kidneys: two bean-shaped organs that filter blood to form urine.
Ureter: tube connecting each kidney to the bladder.
Bladder: stores urine until it is released.
Urethra: tube that carries urine from the bladder to the exterior.
Note: ureter and urethra are different tubes — do not confuse them.

The excretory system in humans

Kidney Function & the Nephron

Kidneys regulate water content of blood (maintains blood pressure) and excrete urea and excess salts.
Each kidney contains about 1 million nephrons (kidney tubules).
Nephrons start in the cortex, loop into the medulla, and return to the cortex; urine drains into the renal pelvis then ureter.

The structure of a human kidney

Ultrafiltration

Blood enters the glomerulus via an afferent arteriole at high pressure.
Capillaries narrow, increasing pressure, forcing small molecules out into Bowman's capsule — this is ultrafiltration.
Filtrate contains: glucose, water, urea, salts. Large molecules like proteins remain in blood.
The filtrate then flows through the nephron for selective reabsorption.

Diagram showing the process of ultrafiltration

Selective Reabsorption

Glucose is reabsorbed by active transport in the proximal convoluted tubule (PCT).
PCT cells have many mitochondria to supply energy for active transport.
In a healthy person, all glucose is reabsorbed; in diabetes, excess glucose remains in urine.
Salts are reabsorbed in the Loop of Henle by diffusion and active transport; water follows by osmosis.
Water reabsorption in the collecting duct varies according to the body's needs.

Diagram showing the reabsorption of glucose

Role of the Liver: Deamination

Excess amino acids cannot be stored and are broken down in the liver by deamination.
The amino group (-NH₂) is removed; the carbon part is converted to glycogen and stored.
The nitrogen part is converted to ammonia (highly toxic), then immediately to urea (less toxic).
Urea dissolves in blood, is carried to the kidneys, and excreted in urine (a small amount in sweat).
High urea levels can cause cell death, reduced insulin response, and deposits in blood vessels.

Amino acid groups

Excretory Products Summary

Lungs: excrete CO₂ (from aerobic respiration).
Kidneys: excrete excess water, salts, and urea.
Liver: produces urea from deamination of excess amino acids; also breaks down alcohol and toxins.
Skin: excretes small amounts of urea and salts in sweat.

Waste substances

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Practice questions

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1.What is urea produced from?
Easy
- AAmino acids
- BFatty acids
- CGlucose
- DGlycerol
2.Which of the following is a correct example of excretion in mammals?
Easy
- AThe removal of excess salts from the blood.
- BThe production of carbon dioxide from aerobic respiration.
- CThe removal of undigested food via the anus.
- DThe release of hormones from glands.
3.Which of the following is not a main function of the kidneys in a healthy person?
Medium
- ATo breakdown toxins.
- BTo reabsorb all glucose.
- CTo regulate blood pressure.
- DTo eliminate excess sodium and potassium ions.
4.Which of the following substances would be reabsorbed by the kidneys?
Medium
- ASalts and glucose
- BSalts, urea and glycogen
- CSalts and glycogen
- DUrea and glucose
5.Which statement about urea is correct?
Easy
- AUrea is formed from excess amino acids in the liver and excreted by the kidneys.
- BUrea is formed from excess amino acids in the kidneys and excreted by the liver.
- CUrea is formed from excess glucose in the liver and egested by the kidneys.
- DUrea is formed from excess glucose in the kidneys and egested by the liver.
6.The table shows the composition of blood entering the kidneys. | Substance | Blood (%) | |---|---| | Glucose | 0.20 | | Urea | 0.01 | | Protein | 7.00 | Which row shows the composition of urine in a healthy person?
Medium
- Aglucose 0.00%, urea 2.50%, protein 0.00%
- Bglucose 0.00%, urea 2.50%, protein 3.50%
- Cglucose 2.00%, urea 0.00%, protein 7.00%
- Dglucose 0.20%, urea 0.01%, protein 0.00%
7.A person produced 1.75 dm³ of urine on a cool day. How will the volume and concentration of urine change on a hot day, if fluid intake is the same?
Medium
- Avolume decreases, concentration increases
- Bvolume increases, concentration decreases
- Cvolume decreases, concentration decreases
- Dvolume increases, concentration increases
8.Which of the following organs are responsible for removing excretory products from the blood?
Easy
- AKidneys and lungs
- BBladder and kidneys
- CLungs and liver
- DBladder and liver

More questions

9.Which of the following correctly shows where filtration and selective reabsorption occur in the nephron?
Medium
- AFiltration in the glomerulus, reabsorption in the collecting duct
- BFiltration in the Bowman's capsule, reabsorption in the loop of Henle
- CFiltration in the Bowman's capsule, reabsorption in the distal convoluted tubule
- DFiltration in the glomerulus, reabsorption in the proximal convoluted tubule
10.The table shows the composition of a solution found in the body. | Substance | Concentration (g/100 cm³) | |---|---| | Glucose | 0.00 | | Urea | 4.00 | | Protein | 0.00 | | Salts | 3.00 | | Amino acids | 0.00 | In a healthy person, which structure would contain this solution?
Hard
- ACollecting duct
- BGlomerulus
- CBowman's capsule
- DProximal convoluted tubule
11.What is the definition of excretion?
Easy
- ARemoval of waste products of metabolic reactions, toxic materials, and substances in excess of requirements
- BRemoval of undigested food from the anus
- CRelease of hormones from glands
- DProduction of carbon dioxide in cells
12.Which row correctly shows the roles of kidneys, liver, and lungs in waste processing?
Medium
- Akidneys excrete urea, liver excrete amino acids, lungs remove CO₂
- Bkidneys remove CO₂, liver deamination of amino acids, lungs excrete urea
- Ckidneys remove urea from blood, liver deamination of amino acids, lungs remove CO₂
- Dkidneys excrete urine, liver deamination of urea, lungs remove CO₂
13.The table shows the composition of blood plasma entering the kidney. | Substance | Plasma (%) | |---|---| | Glucose | 0.10 | | Urea | 0.03 | | Protein | 7.00 | Which row shows the correct composition of the urine produced, compared with the plasma, for a healthy person?
Medium
- Aglucose 0.00%, urea 2.00%, protein 0.00%
- Bglucose 0.10%, urea 0.03%, protein 0.00%
- Cglucose 0.00%, urea 0.03%, protein 7.00%
- Dglucose 0.10%, urea 2.00%, protein 7.00%
14.A patient produces 2.5 dm³ of urine per day with a urea concentration of 0.8 g/dm³. What is the total mass of urea excreted per day?
Medium
- A2.0 g
- B3.125 g
- C0.32 g
- D1.7 g
15.After a high-protein meal, the concentration of urea in the blood rises and then gradually falls back towards normal over the following hours. Which statement explains this change?
Medium
- AUrea concentration increases because excess amino acids are deaminated in the liver.
- BUrea concentration increases because more urea is reabsorbed by the kidneys.
- CUrea concentration decreases because the kidneys excrete more urea immediately.
- DUrea concentration decreases because the liver stores amino acids.
16.Which of the following is the correct definition of excretion?
Easy
- AThe removal of waste products of metabolism from the body.
- BThe removal of undigested food from the body.
- CThe removal of any substance from the body.
- DThe production of waste products by cells.
17.The diagram shows a nephron. Which part is the site of ultrafiltration?
Easy
- AGlomerulus
- BProximal convoluted tubule
- CLoop of Henle
- DCollecting duct
18.A person drinks a large volume of water. How does the kidney respond to maintain water balance?
Medium
- ALess water is reabsorbed from the collecting duct, producing more dilute urine.
- BMore water is reabsorbed from the collecting duct, producing more concentrated urine.
- CLess urea is excreted to conserve water.
- DMore salts are reabsorbed to increase blood osmolarity.
19.Which substances are normally present in the glomerular filtrate of a healthy person?
Medium
- AGlucose, urea, water, salts
- BGlucose, protein, water, salts
- CUrea, water, salts, protein
- DGlucose, urea, water, protein
20.Which organ is responsible for the deamination of excess amino acids?
Easy
- ALiver
- BKidney
- CLung
- DBladder
21.In an experiment, the concentration of glucose in the blood entering the kidney is 1.2 g/dm³ and in the urine is 0.0 g/dm³. Which process explains this difference?
Medium
- ASelective reabsorption by active transport in the proximal convoluted tubule.
- BDiffusion of glucose out of the collecting duct.
- CFiltration of glucose in the glomerulus and secretion into the urine.
- DGlucose is converted to urea in the kidney.
22.The diagram shows the human urinary system. Which structure is the ureter?
Easy
- ATube connecting kidney to bladder
- BTube connecting bladder to exterior
- COrgan that stores urine
- DOrgan that filters blood
23.Which of the following is an excretory product of the lungs?
Easy
- ACarbon dioxide
- BUrea
- CWater
- DSalts
24.A student measures the volume of urine produced over 24 hours on a cool day (1.5 dm³) and on a hot day (0.8 dm³), with same fluid intake. Which statement explains the difference?
Medium
- AOn a hot day, more water is lost through sweating, so less urine is produced.
- BOn a hot day, the kidneys reabsorb less water, producing more concentrated urine.
- COn a hot day, the body produces more urea, increasing urine volume.
- DOn a hot day, the bladder stores more urine, so volume measured is lower.
25.A student collects a sample of urine from a healthy person and tests it with Benedict's solution. What would be the expected result and why?
Medium
- ABlue colour, because all glucose is reabsorbed in the proximal convoluted tubule.
- BBrick-red precipitate, because glucose is normally present in urine.
- CBlue colour, because glucose is not filtered at the glomerulus.
- DBrick-red precipitate, because glucose is actively secreted into the urine.
26.In an experiment, a sample of blood from the renal vein of a healthy person is compared to a sample from the renal artery. Which substance would show the largest decrease in concentration in the renal vein?
Medium
- AUrea
- BGlucose
- COxygen
- DCarbon dioxide
27.The diagram shows a nephron. Which labelled part is the cup-shaped structure that collects the glomerular filtrate?
Easy
- ABowman's capsule
- BGlomerulus
- CLoop of Henle
- DCollecting duct
28.A person eats a high-protein meal. Which statement correctly describes the change in urea concentration in the blood over the next few hours?
Medium
- AA steady increase then a gradual decrease back to normal.
- BA steady decrease then a gradual increase back to normal.
- CNo change because urea is not produced from protein.
- DA sharp increase and then remains high.
29.Which of the following correctly describes the process of deamination?
Easy
- ARemoval of the amino group from an amino acid in the liver.
- BRemoval of the carboxyl group from an amino acid in the kidneys.
- CConversion of glucose into glycogen in the liver.
- DBreakdown of urea into ammonia in the kidneys.
30.What is the main function of the ureter?
Easy
- ATo carry urine from the kidney to the bladder.
- BTo carry urine from the bladder to the outside of the body.
- CTo filter blood in the kidney.
- DTo store urine until it is released.
31.A student investigates the effect of temperature on urine production. On a hot day (30°C) with water intake of 1.5 dm³, urine volume is 0.8 dm³. On a cool day (15°C) with same water intake, urine volume is 1.5 dm³. What is the most likely explanation?
Hard
- AMore water is lost as sweat on the hot day, so less urine is produced.
- BThe kidneys produce less urine on the cool day to conserve heat.
- CMore water is reabsorbed in the collecting duct on the cool day.
- DThe bladder stores more urine on the hot day.
32.Which of the following substances is NOT normally present in the glomerular filtrate?
Medium
- AProtein
- BGlucose
- CUrea
- DSalts
33.A patient's urine contains glucose. Which of the following is a possible cause?
Medium
- AThe patient has diabetes and their blood glucose is too high for all glucose to be reabsorbed.
- BThe patient's kidneys are not filtering glucose at the glomerulus.
- CThe patient is excreting excess glucose because they ate too much carbohydrate.
- DThe patient's liver is converting protein into glucose.
34.Which row correctly matches the organ to its excretory function?
Medium
- ALungs – excrete carbon dioxide; Kidneys – excrete urea; Liver – deamination of amino acids.
- BLungs – excrete urea; Kidneys – excrete carbon dioxide; Liver – produces bile.
- CLungs – excrete oxygen; Kidneys – excrete water; Liver – stores glycogen.
- DLungs – excrete water vapour; Kidneys – excrete glucose; Liver – detoxifies alcohol.
35.In a healthy person, which of the following is reabsorbed by the kidneys?
Medium
- AUrea
- BGlucose
- CSalts only
- DGlucose and salts
36.A person produces 2.0 dm³ of urine per day with a urea concentration of 0.9 g/dm³. What is the total mass of urea excreted per day?
Medium
- A1.8 g
- B2.0 g
- C0.9 g
- D2.9 g
37.The table shows the composition of blood entering the kidney. | Substance | Blood (g/100 cm³) | |---|---| | Glucose | 0.10 | | Urea | 0.03 | | Protein | 7.0 | Which row is correct for the urine produced by a healthy person?
Hard
- AUrine: glucose 0.00, urea 2.0, protein 0.00
- BUrine: glucose 0.10, urea 0.03, protein 7.0
- CUrine: glucose 0.00, urea 0.03, protein 0.00
- DUrine: glucose 0.10, urea 2.0, protein 7.0
38.Which of the following factors affect the volume and concentration of urine produced? 1. Exercise, 2. Temperature, 3. Water intake
Medium
- A1 and 3 only
- B2 and 3 only
- C3 only
- D1, 2 and 3
39.The concentration of urea in the blood entering the kidney is 0.01 g/100 cm³. The urine produced has a urea concentration of 2.5 g/100 cm³. If the volume of urine produced per day is 1.5 dm³, what is the mass of urea excreted per day?
Medium
- A0.015 g
- B0.15 g
- C3.75 g
- D37.5 g
40.Each row below shows a possible composition of blood plasma entering the kidney and the urine produced. Which row is correct for a healthy person? (concentrations in g/100 cm³)
Hard
- ARow A: glucose 0.10, urea 0.03, protein 7.0 in plasma; glucose 0.00, urea 2.0, protein 0.0 in urine
- BRow B: glucose 0.10, urea 0.03, protein 7.0 in plasma; glucose 0.10, urea 0.03, protein 7.0 in urine
- CRow C: glucose 0.00, urea 0.03, protein 0.0 in plasma; glucose 0.00, urea 2.0, protein 0.0 in urine
- DRow D: glucose 0.10, urea 0.03, protein 7.0 in plasma; glucose 0.00, urea 2.0, protein 7.0 in urine
41.Urea is produced in the kidneys.
Easy
True or false?
42.Which of the following is a correct example of excretion?
Medium
- ARemoval of faeces from the anus
- BRemoval of carbon dioxide from the lungs during exhalation
- CRemoval of sweat from the skin
- DRemoval of urine from the bladder
43.Which substances are forced out of the glomerular capillaries during ultrafiltration?
Medium
- AGlucose, water, urea, salts
- BGlucose, water, urea, proteins
- CWater, urea, salts, proteins
- DGlucose, water, salts, red blood cells
44.Where in the nephron does selective reabsorption of glucose occur?
Easy
- ABowman's capsule
- BProximal convoluted tubule
- CLoop of Henle
- DCollecting duct
45.In a healthy person, which of the following would you expect to find in the glomerular filtrate but NOT in urine?
Medium
- AUrea
- BGlucose
- CSalts
- DWater
46.Match each organ to its excretory product.
Medium
- Lungs
- Kidneys
- Liver
- Carbon dioxide
- Urea
- Excess water and salts
47.Arrange the following structures in the order that urine passes through them after being formed in the nephron: 1. Urethra, 2. Bladder, 3. Ureter, 4. Collecting duct.
Easy
- Collecting duct
- Ureter
- Bladder
- Urethra
48.Match each organ to its main excretory product.
Medium
- Lungs
- Kidneys
- Liver
- Carbon dioxide
- Urea (produced from excess amino acids)
- Excess water, salts, and urea
49.Arrange the following structures in the order that urine passes through them after being formed in the nephron.
Easy
- Collecting duct
- Ureter
- Bladder
- Urethra
50.Which of the following substances are forced out of the glomerular capillaries during ultrafiltration? (Select all that apply.)
Medium
- AGlucose
- BRed blood cells
- CUrea
- DProteins
- EWater
- FSalts
51.Name the process by which excess amino acids are broken down in the liver.
Easy
52.Complete the sentence about the excretory system.
Easy
The tube that connects the kidney to the bladder is the ____, and the tube that carries urine out of the body is the ____.
53.A person produces 1.5 dm³ of urine per day with a urea concentration of 0.8 g/dm³. What is the total mass of urea excreted per day? Give your answer in grams.
Medium
54.A person produces 1.8 dm³ of urine per day with a urea concentration of 0.9 g/dm³. Calculate the total mass of urea excreted per day. Give your answer in grams.
Medium
55.Explain why glucose is normally absent from the urine of a healthy person.
Medium
56.A person produces 1.5 dm³ of urine per day with a urea concentration of 0.8 g/dm³. What is the total mass of urea excreted per day? Give your answer in grams.
Medium
- A2.2 g
- B0.2 g
- C3.2 g
- D1.2 g
57.A person produces 1.8 dm³ of urine per day with a urea concentration of 0.9 g/dm³. Calculate the total mass of urea excreted per day. Give your answer in grams.
Medium
- A0.62 g
- B2.62 g
- C1.62 g
- D3.62 g

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