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IGCSE Biology 0610 · Chapter 10 notes · ~19 min read

Gas exchange, respiration & excretion

How the lungs work and why the alveoli are perfect for gas exchange, what happens when you breathe, how cells release energy with and without oxygen, and how the kidneys and liver remove waste.

Lungs & alveoliBreathingAerobic & anaerobic respirationKidneysLiver
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Gas exchange happens in the alveoli. Oxygen diffuses into the blood and carbon dioxide diffuses out. The surface is large, thin, moist, well supplied with blood and ventilated.
  • Respiration is the chemical reaction in every living cell that releases energy from glucose. Aerobic uses oxygen (lots of energy). Anaerobic does not (little energy): lactic acid in muscle, ethanol + CO₂ in yeast.
  • Excretion removes waste made by the body's chemistry. The lungs remove CO₂; the kidneys remove urea, excess water and ions; the liver makes the urea. EXT

By the end you can

  • Name the parts of the breathing system and describe how the alveoli are adapted for gas exchange.
  • Compare inspired and expired air and describe the limewater test for carbon dioxide.
  • Describe how exercise changes breathing rate and depth, and why.
  • State the uses of energy in living organisms.
  • Define aerobic and anaerobic respiration and write the word equations.
  • Describe an investigation into the effect of temperature on respiration in yeast.
  • Identify the kidneys, ureters, bladder and urethra, and say what each does.
  • Explain how ribs, intercostal muscles and the diaphragm ventilate the lungs. EXT
  • Explain the roles of cartilage, goblet cells, mucus and cilia in the airways. EXT
  • Write balanced equations and explain lactic acid and oxygen debt. EXT
  • Describe the kidney, nephron (filtration and reabsorption) and the liver's role in making urea. EXT

1Gas exchange in humans

The breathing system

nose / mouthtrachea(windpipe)cartilageringsbronchusbronchiolesalveoli(air sacs)lungribintercostalmusclesdiaphragm
Air travels nose or mouth → trachea → bronchi → bronchioles → alveoli.
  • Path of air: nose or mouth → trachea (windpipe) → two bronchi → many bronchioles → alveoli.
  • The ribs, intercostal muscles and diaphragm make the air move in and out (see ventilation below).
  • EXT The trachea has C-shaped rings of cartilage that keep it open when you breathe in, but the gap at the back lets food pass down the oesophagus.
Memory trickTrachea → Bronchi → Bronchioles → Alveoli = "The Big Bad Air" (T-B-B-A).

The gas exchange surface

O₂CO₂air inalveolusblood arrives: low O₂blood leaves: high O₂wall: onecell thickcapillary(thin wall)Big areamillions oftiny alveoli= huge areaThin wallone cell thick:short diffusiondistanceMoist lininggases dissolvein the waterfilm firstBlood supplyblood keepsthe gradientsteepVentilationbreathing bringsfresh air andremoves CO₂
Each alveolus is wrapped in capillaries. Oxygen and CO₂ swap places by diffusion.
  • There are millions of alveoli in the lungs. Together they give a huge surface area of about 70 m², roughly 40 times the area of your skin.
  • Oxygen is at a higher concentration in the alveolus than in the blood, so it diffuses into the blood (and is picked up by red blood cells). Carbon dioxide is higher in the blood, so it diffuses out into the alveolus and is breathed out (diffusion).
  • Why it works well: a large surface area, a thin wall (one cell thick), a moist lining, a good blood supply and ventilation. Blood and breathing keep the concentration gradient steep.
Exam tipIn an exam always link a feature + what it does: "thin wall, so the diffusion distance is short", "good blood supply, so oxygen is carried away and the gradient stays steep".
Common mistake"Oxygen goes in and CO₂ comes out because the lungs breathe." No. The gases move by diffusion along concentration gradients. Breathing keeps the gradients steep.

Ventilation: how breathing worksEXTENDED

INHALING (breathing in)EXHALING (breathing out)ribsribsair indiaphragmribsribsair outdiaphragm1 intercostal muscles contract2 ribs move up and out3 diaphragm contracts and flattens4 chest volume increases5 pressure falls: air rushes in1 intercostal muscles relax2 ribs move down and in3 diaphragm relaxes and domes up4 chest volume decreases5 pressure rises: air pushed outAir always moves from HIGH pressure to LOW pressure.
EXT Muscles change the volume of the chest. Pressure changes make air flow.
  • EXT Breathing in: the external intercostal muscles contract (ribs up and out) and the diaphragm contracts and flattens. Chest volume increases, pressure falls below atmospheric pressure, so air rushes in.
  • EXT Breathing out: the intercostal muscles relax (ribs down and in) and the diaphragm relaxes and domes up. Volume decreases, pressure rises, so air is pushed out.
Memory trickVolume UP → pressure DOWN → air IN. Volume DOWN → pressure UP → air OUT.
Common mistake"The lungs suck the air in." The lungs have no muscles. They are pushed open by the change in air pressure caused by the muscles around them.

Inspired and expired air

051015202521%16%0.04%4%oxygencarbon dioxide% of the airinspired (in)expired (out)nitrogen:about 78%in both(no change)water vapour:MORE inexpired air
Cells use oxygen and make CO₂, so expired air has less O₂ and more CO₂.
GasInspired (in)Expired (out)
oxygenabout 21%about 16%
carbon dioxideabout 0.04%about 4%
nitrogenabout 78%about 78% (no change)
water vapourvariable (usually less)more (lungs are moist)
  • Why oxygen falls: cells use it for respiration. Why CO₂ rises: cells make it in respiration. Why water vapour rises: water evaporates from the moist lining of the lungs.
limewaterMILKYlots of CO₂ in expired airlimewaterSTAYS CLEARvery little CO₂ in inspired airbreathe outroom airCO₂ turns limewater milky. Expired air has about 100× more CO₂ than inspired air.
The limewater test for carbon dioxide.
  • Limewater test: bubble the gas through limewater. It turns milky if CO₂ is present. Air breathed out turns it milky quickly; air breathed in takes much longer.
Worked example

Inspired air has 21% oxygen and expired air has 16%. Inspired CO₂ is 0.04% and expired is 4%. Find the percentage of the oxygen removed and how many times CO₂ increases.

  1. oxygen removed = 21 − 16 = 5 percentage points
  2. percentage of the oxygen = 5 ÷ 21 × 100 = 23.8% (about 24%)
  3. CO₂ increase = 4 ÷ 0.04 = 100 times greater

Exercise and breathing

muscles contract more andrespire fastermore CO₂ made;more O₂ needed(Extended) the brain detectsmore CO₂ in the bloodbreathing gets fasterand deepermore O₂ in and more CO₂out, to match the demand15300.51.5restexerciserestexercisebreathing rate(breaths/min)volume per breath(dm³)air per minute = rate × volume7.5 → 45 dm³/min (6 times more)
During exercise you breathe faster (rate) and deeper (volume per breath).
  • During exercise both breathing rate and depth increase. The muscles respire faster, so they need more oxygen and make more CO₂ that must be removed.
  • EXT Control: the brain detects the rise in CO₂ in the blood and sends signals that make the intercostal muscles and diaphragm work harder.
  • To investigate: count the breaths in one minute at rest, then after 2 minutes of exercise (for example step-ups), and again as you recover. Change one factor only.
Worked example

At rest a person takes 15 breaths per minute, 0.5 dm³ each. During exercise: 30 breaths per minute, 1.5 dm³ each. Find the volume of air per minute each time and the percentage increase.

  1. volume per minute = rate × volume per breath
  2. rest: 15 × 0.5 = 7.5 dm³/min
  3. exercise: 30 × 1.5 = 45 dm³/min
  4. increase = (45 − 7.5) ÷ 7.5 × 100 = 500% (six times as much)

Keeping the airways cleanEXTENDED

mucus is swept up to the throatmucus: trapsdust, bacteriagoblet cellmakes mucuscartilage ringkeeps airwayopencilia sweepmucus upwardciliatedcellbloodcapillarySmoking damages cilia, so mucus and microbes stay in the lungs.
EXT Mucus catches dust and microbes; cilia sweep the mucus up to the throat.
  • EXT Goblet cells in the lining make mucus, which traps dust and bacteria.
  • EXT Ciliated cells have cilia that beat and move the mucus upwards to the throat, where it is swallowed. This is part of the body's first line of defence (body defences).
  • EXT Cigarette smoke damages cilia, so mucus and microbes stay in the lungs, causing coughing and infections.

2Respiration

Why do living things need energy?

ENERGYfrom glucosemusclecontractioncelldivisionproteinsynthesisactivetransportgrowthnerveimpulseskeeping bodytemperature constant
Energy released by respiration is used by every cell.

Respiration — the chemical reactions in all living cells that release energy from glucose.

  • Uses of energy: muscle contraction, protein synthesis, cell division, active transport, growth, passage of nerve impulses and maintaining a constant body temperature (in mammals and birds).
  • Most of the reactions happen in mitochondria. Some of the energy is released as heat.
Common mistake"Respiration is breathing." No. Breathing (ventilation) moves air in and out. Respiration is the chemical reaction in cells that releases energy. All living things respire, even plants.
Common mistake"Respiration makes energy." Energy cannot be made. It is released from the chemical energy stored in glucose.

Aerobic and anaerobic respiration

AEROBIC respirationneeds oxygen: in mitochondriaglucose+oxygencarbon dioxide+waterC₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (balanced: Extended)LOTS of energyANAEROBIC: in musclesno oxygen: during hard exerciseglucoselactic acidlactic acid builds up:muscle fatigue(oxygen debt: Extended)LITTLE energyANAEROBIC: in yeastno oxygen: fermentationglucoseethanol+carbon dioxideC₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (balanced: Extended)LITTLE energy
Three equations to learn. Anaerobic respiration releases far less energy.

Aerobic respiration — the chemical reactions in cells that use oxygen to break down nutrient molecules (glucose) to release a relatively large amount of energy.

Anaerobic respiration — the chemical reactions in cells that break down nutrient molecules to release a relatively small amount of energy, without oxygen.

glucose + oxygen → carbon dioxide + water
aerobic respiration (word equation)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
EXT balanced equation, aerobic respiration
glucose → lactic acid
anaerobic respiration in muscles
glucose → ethanol + carbon dioxide
anaerobic respiration in yeast (fermentation)
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
EXT balanced equation, yeast
  • Why anaerobic yields less energy: the glucose is only partly broken down; lactic acid and ethanol still hold a lot of chemical energy.
  • Uses of yeast: the CO₂ makes bread rise; the ethanol is the alcohol in brewing.
AerobicAnaerobic
oxygen needed?yesno
energy releasedlarge amountsmall amount
products (muscle)CO₂ + waterlactic acid
products (yeast)CO₂ + waterethanol + CO₂
wheremitochondria (mostly)cytoplasm
Memory trickAnaerobic = "without air". An-aerobic: the "an" means not.

Respiration and photosynthesis

FeatureRespirationPhotosynthesis
energyreleases energystores energy (from light)
usesglucose + oxygenCO₂ + water
makesCO₂ + waterglucose + oxygen
happens inall living cells, all the timecells with chloroplasts, in the light
  • Many exam questions ask you to compare them. They are opposite reactions. Link: photosynthesis.

Investigating respiration in yeast

CO₂ gasyeast +glucosewaterbaththermometerlimewatercount the bubbles per minuteat each temperature010203040506070temperature (°C)rate of CO₂ productionoptimum 40°Cenzymes workfaster as itwarms upenzymesdenature
Yeast respiration speeds up with temperature, then falls when enzymes denature.
  • Method: mix yeast and glucose solution in tubes. Put each in a water bath at a different temperature (for example 10, 20, 30, 40, 50, 60 °C). Count the bubbles of CO₂ per minute (or collect the gas).
  • Control variables: same mass of yeast, same volume and concentration of glucose, same time, same apparatus. The independent variable is temperature. The dependent variable is the rate of CO₂ production.
  • Result: the rate rises with temperature up to about 40 °C (the optimum), then falls sharply because the enzymes denature (enzymes).
  • Reliability: repeat each temperature three times and calculate the mean.
Worked example

A student counts bubbles per minute. At 40 °C three counts are 38, 41 and 41. At 20 °C the mean is 14 and at 30 °C it is 28. Find the mean at 40 °C and the percentage increase from 20 °C to 30 °C.

  1. mean at 40 °C = (38 + 41 + 41) ÷ 3 = 120 ÷ 3 = 40 bubbles/min
  2. increase = 28 − 14 = 14
  3. percentage = 14 ÷ 14 × 100 = 100% (the rate doubled)
  4. enzyme activity rises with temperature until the optimum

Oxygen debtEXTENDED

0123402468101214time (minutes)oxygen (dm³ / min)restexerciserecoveryoxygen NEEDEDoxygen TAKEN INdebt builds(lactic acid)extra O₂ taken inafter exercise =OXYGEN DEBTbeing repaid
EXT During hard exercise the muscles run short of oxygen and make lactic acid. Afterwards you pay it back.
  • EXT During vigorous exercise the muscles need energy faster than the blood can bring oxygen, so they also respire anaerobically and lactic acid builds up. It causes muscle fatigue.
  • EXT After the exercise you keep breathing hard and your heart beats fast. The extra oxygen breaks down the lactic acid (in the liver and muscles). This is removing the oxygen debt.
  • EXT Oxygen debt = the extra oxygen needed after exercise to remove the lactic acid made by anaerobic respiration.
Exam tipFitter people recover faster (heart rate recovery): their circulation delivers oxygen better, so they build up a smaller debt.

3Excretion in humans

What is excretion?

Excretion — the removal from organisms of the waste products of metabolism (chemical reactions in cells, including respiration), toxic materials and substances in excess of requirements.

WasteWhere it is madeExcreted by
carbon dioxiderespiration in all cellsthe lungs (breathed out)
ureathe liver (from excess amino acids)the kidneys (in urine)
excess water and mineral ionstaken in with food and drinkthe kidneys (in urine)
Common mistake"Faeces are excreted." No. Faeces are undigested food that never entered the cells, so removing them is egestion, not excretion.

The urinary system

vena cavaaortakidneyrenal artery(blood in)renal vein(blood out)ureter(carries urine to bladder)bladder(stores urine)urethra(urine out of body)
Urine is made in the kidneys and leaves through the ureters, bladder and urethra.
  • Kidneys: filter the blood and make urine (urea, excess water, excess ions).
  • Ureters: tubes that carry urine to the bladder.
  • Bladder: stores urine until you urinate.
  • Urethra: tube that carries urine out of the body.
Memory trickKidneys Keep blood clean → Ureters Urine → Bladder Bags it → Urethra Uses the exit.

Inside the kidneyEXTENDED

renalarteryrenalveinuretercortexmedulla(pyramids)pelviscapsule
EXT The cortex is the outer layer. The medulla holds the pyramids. Urine collects in the pelvis.
  • EXT The outer cortex is where the blood is filtered. The inner medulla (the pyramids) contains the tubes that carry the filtrate. Urine collects in the pelvis and drains down the ureter.
  • EXT Blood arrives in the renal artery (with urea) and leaves in the renal vein (with much less urea).

The nephron: filtering and reabsorbingEXTENDED

CORTEXMEDULLAblood in (wide)12345to pelvis1glomerulus +Bowman’s capsule:ULTRAFILTRATION2proximal tubule:ALL glucose + amino acids,most water, many ions back3loop of Henle:more water and ionsreabsorbed4distal tubule:final adjustment ofwater and ions5collecting duct:urine flows tothe kidney pelvis
EXT Each kidney has about a million nephrons. Follow the numbers.
afferent arteriole: WIDEefferent: NARROWBowman’s capsule: the filtrate collects herefilter wallPASS THROUGHwater, glucose, aminoacids, urea, mineral ionsSTAY IN THE BLOODblood cellsproteins (too big)HIGH PRESSUREforces small moleculesout of the blood(ultrafiltration)Filtrate = blood plasma without the blood cells and big proteins
EXT Ultrafiltration: high pressure in the glomerulus forces small molecules out through the filter wall.
  • EXT 1 Ultrafiltration: blood enters the glomerulus in a wide vessel and leaves in a narrow one, so the pressure is high. This forces small molecules (water, glucose, amino acids, urea, ions) into the Bowman's capsule. Blood cells and large proteins are too big and stay in the blood.
  • EXT 2 Selective reabsorption: useful substances are taken back into the blood. All the glucose and amino acids are reabsorbed in the proximal tubule (using active transport), along with most of the water and many ions. More water and ions are reabsorbed in the loop of Henle.
  • EXT 3 Urine formation: what is left (urea, excess water and excess ions) flows through the collecting duct to the pelvis. The amount of water and ions reabsorbed depends on what the body needs.
Substance (g per 100 cm³)Blood plasmaFiltrateUrine
protein8.0000
glucose0.100.100
urea0.030.032.00
  • EXT Reading the table: protein is absent from the filtrate (too big to be filtered). Glucose is in the filtrate but not in urine (it was all reabsorbed). Urea is much more concentrated in urine because water was reabsorbed but the urea was not.
  • EXT Glucose in urine is a sign of diabetes (blood glucose control).
Worked example

EXT The kidneys filter about 180 dm³ of fluid a day but produce about 1.5 dm³ of urine. The urea is 0.03 g per 100 cm³ in plasma and 2.00 in urine. Find the percentage of the filtrate reabsorbed and how many times more concentrated urea is in urine.

  1. reabsorbed = 180 − 1.5 = 178.5 dm³
  2. percentage = 178.5 ÷ 180 × 100 = 99.2% (about 99%)
  3. concentration factor = 2.00 ÷ 0.03 = 66.7 times (about 67)
  4. water is reabsorbed but urea is not, so urea becomes concentrated

The liver and ureaEXTENDED

amino acids from digested proteinLIVERAMINO ACIDS NEEDEDbuilt into PROTEINS(for example the proteinsin blood plasma)EXCESS amino acids: DEAMINATIONUREA(toxic if it builds up)carbohydrate(for energy)urea in the blood → KIDNEYSexcreted in URINEUrea is TOXIC if itbuilds up, so the livermakes it and thekidneys remove it.
EXT The liver sorts amino acids. Needed ones make proteins. Excess ones are deaminated and the nitrogen leaves as urea.
  • EXT Amino acids absorbed from the gut go to the liver. Some are used to make proteins (for example the proteins in blood plasma).
  • EXT The body cannot store excess amino acids. The liver removes the amino group (deamination) and turns it into urea. The rest of the molecule becomes carbohydrate, used for energy.
  • EXT Urea is toxic if it builds up. It travels in the blood to the kidneys and leaves in urine.
Memory trickLiver makes urea; kidneys remove urea.

4Test yourself

Test yourself — cover the answers first

Quick check

  1. Name the tubes that carry air from the throat to the alveoli.
  2. Give two features of the gas exchange surface.
  3. What happens to carbon dioxide in the alveoli?
  4. How do you test a gas for carbon dioxide?
  5. Which gas is higher in expired air than in inspired air?
  6. Why do you breathe faster during exercise?
  7. Give two uses of energy in the body.
  8. Write the word equation for aerobic respiration.
  9. What is made by anaerobic respiration in muscle?
  10. What is made by anaerobic respiration in yeast?
  11. Which organ makes urea?
  12. What do the kidneys remove from the blood?
  13. EXT What do the intercostal muscles do when you breathe in?
  14. EXT What is the job of cilia in the trachea?
  15. EXT What is deamination?

Exam-style

  1. Describe how the alveoli are adapted for gas exchange. [4]
  2. Describe how you could investigate the effect of temperature on respiration in yeast. [4]
  3. EXT Explain how the ribs, intercostal muscles and diaphragm cause air to enter the lungs. [4]
  4. EXT Explain why muscles produce lactic acid during exercise and what happens afterwards. [4]
  5. EXT Explain why glucose is found in the filtrate but not in urine. [3]
Show answers
  1. trachea, bronchi, bronchioles
  2. large surface area, thin wall, good blood supply, moist, ventilated
  3. it diffuses from the blood into the alveolus and is breathed out
  4. bubble it through limewater, which turns milky
  5. carbon dioxide
  6. muscles respire faster, so more oxygen is needed and more CO₂ must be removed
  7. muscle contraction, growth, protein synthesis, cell division, active transport, nerve impulses, keeping warm
  8. glucose + oxygen → carbon dioxide + water
  9. lactic acid
  10. ethanol and carbon dioxide
  11. the liver
  12. urea, excess water and excess mineral ions
  13. contract, pulling the ribs up and out
  14. to move mucus (with trapped dust and microbes) up to the throat
  15. removal of the amino group from excess amino acids, forming urea
  16. large surface area from millions of alveoli
    thin wall, one cell thick, short diffusion distance
    good blood supply keeps concentration gradient steep
    ventilation brings fresh air
  17. mix yeast and glucose solution
    put in water baths at different temperatures
    count bubbles of CO₂ per minute
    keep other variables the same and repeat
  18. intercostal muscles contract, ribs move up and out
    diaphragm contracts and flattens
    chest volume increases
    pressure falls so air is pushed in from outside
  19. oxygen cannot be delivered fast enough
    muscles respire anaerobically
    lactic acid builds up causing fatigue
    after exercise extra oxygen breaks it down (oxygen debt repaid)
  20. small molecules pass into the filtrate
    all the glucose is reabsorbed in the proximal tubule
    by active transport back into the blood
Practise this chapter
Gas exchangeRespirationExcretion
Next chapterCoordination & response →
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