Biology Year 11 · Module 2

Gas Exchange Between Internal and External Environments

Before we explain the mechanism, look at the data. The numbers tell a story about gradients, distances, and surfaces, and if you read them right, you'll understand the mechanism before you've been taught it.

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Orient and predict

Pick up the worksheets, then commit to your own answer before you read.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions

Think First

Your lungs contain millions of tiny air sacs. Before reading on, what do you think makes them so effective at getting oxygen into your blood? Think about the features that help a gas cross from air into a liquid, what would matter most?

Next up

Frame the learning

Set the intentions and clear the misconceptions that trip most students here.

Know

Define partial pressure and explain how it drives gas exchange
Trace O₂ and CO₂ movements from external air to mitochondria
Explain the role of the cardiovascular system in maintaining gradients
Compare gas exchange efficiency across different respiratory structures
Explain the four universal features of gas exchange surfaces

Understand

Explain gas exchange between the internal and external environments
Relate surface area, membrane thickness, and concentration gradient to exchange rate
Connect: L10 (respiratory structures), L13 (blood), L14 (cardiovascular circuit)
IQ3: how transport medium composition changes

Can Do

State the partial pressure of O₂ and CO₂ at five locations in the gas exchange pathway
Explain why blood flow direction matters for maintaining concentration gradients
Apply Fick's law to predict how changes affect gas exchange rate
Explain why the alveolar surface area must be ~250m² and what happens if it falls
Write a Band 6 response linking structure to function in alveolar gas exchange

HSC Exam Relevance

Content from this lesson that appears directly in HSC Biology exams

High Priority

Alveolar gas exchange, structure to function

Explaining how alveolar structure enables efficient gas exchange is tested in nearly every HSC paper, 4–6 marks in Section II. Must link: large SA, thin membrane, moist surface, maintained gradient (blood flow + ventilation) to each structural feature of the alveolus.

High Priority

Partial pressure gradients, data interpretation

Graph or table showing partial pressure of O₂ and CO₂ across locations (atmospheric → alveoli → blood → tissues → mitochondria). Tested as 3–4 mark data interpretation in Section I or Section II, must use "partial pressure gradient" language and explain direction of diffusion.

Medium Priority

Fick's law, applying the three variables

Rate of diffusion is proportional to SA × concentration gradient / membrane thickness. Tested as 2–3 mark application question, "explain how condition X affects gas exchange rate." Must name the Fick variable affected and describe the directional effect.

Medium Priority

Maintaining gradients, role of ventilation and circulation

Explaining why continuous ventilation (refreshing alveolar air) and blood flow (removing O₂ and delivering CO₂) are essential to maintain the partial pressure gradients that drive diffusion. Tested as 2–3 mark mechanism questions.

Key Terms

partial pressureThe pressure exerted by one gas in a mixture, directly proportional to its concentration and determining the direction of diffusion.

diffusion gradientThe difference in concentration (or partial pressure) of a substance across a membrane that drives its net movement by diffusion.

Fick's lawThe principle that the rate of diffusion is proportional to surface area and concentration gradient, and inversely proportional to membrane thickness.

alveolusA tiny air sac in the mammalian lung (~500 million per lung) that provides the large, moist, thin surface for external gas exchange.

ventilationThe movement of air into and out of the lungs by breathing, which continuously refreshes alveolar air to maintain the diffusion gradient.

internal gas exchangeThe exchange of oxygen and carbon dioxide between the blood and body tissue cells at the capillary beds.

external gas exchangeThe exchange of oxygen and carbon dioxide between the air in alveoli and the blood in surrounding capillaries.

haemoglobinAn iron-containing protein in red blood cells that binds oxygen reversibly, greatly increasing the blood's oxygen-carrying capacity.

Start with the Data

Misconceptions to Fix

✗

Wrong: Oxygen diffuses into blood because blood has low oxygen content.

✓

Right: Diffusion is driven by partial pressure gradients, not absolute amounts. Oxygen diffuses from alveoli (PO₂ ≈ 100 mmHg) into arterial blood (PO₂ ≈ 95 mmHg) because of the small gradient. Even oxygenated blood has a lower PO₂ than alveolar air, so diffusion continues. The key is the gradient, not the total oxygen content.

Next up

Read the data first

Numbers before explanation. See what needs accounting for.

The Data, Before the Theory

Read the numbers. Spot the patterns. Then we'll explain what you're seeing.

Data-First Inquiry

Where does oxygen go, and what drives it there?

The table below shows the partial pressure of O₂ and CO₂ at five locations along the gas exchange pathway, from the air you breathe to the mitochondria inside your cells. Partial pressure is the pressure exerted by one gas in a mixture; it is directly proportional to concentration and determines the direction gases move by diffusion.

Before reading further: Look at the O₂ column. What trend do you notice from top to bottom? What trend do you notice in CO₂? What does this tell you about the direction gases must be moving at each step? Write your prediction in the activity box at the end of this card.

Location	O₂ partial pressure (mmHg)	CO₂ partial pressure (mmHg)	Where in the body
Atmospheric air	159	0.3	Outside the body, inhaled air
Alveolar air	100	40	Inside the lung air sacs, after mixing with dead-space air
Arterial blood (pulmonary vein)	95	40	Blood leaving lungs, just loaded with O₂
Venous blood (at rest)	40	45	Blood returning from body tissues
Tissue cells / mitochondria	20–30	50+	Inside metabolically active cells

O₂: High → Low (159 → 20 mmHg) Drives O₂ inward →

Atmosphere 159 Alveoli 100 Blood 95→40 Cells 20–30

CO₂: Low → High (0.3 → 50+ mmHg) ← Drives CO₂ outward

Atmosphere 0.3 Alveoli 40 Blood 40→45 Cells 50+

Figure: Partial pressure of O₂ (red bars, decreasing) and CO₂ (blue bars, increasing) at each location. Bar length is proportional to partial pressure; O₂ scale: 159 mmHg = full bar; CO₂ scale: 50 mmHg = full bar.

Your prediction, before reading on: What pattern do you see? What direction is each gas moving at each step, and why?

Check Your Understanding

What trend or pattern would you expect to see in data about this topic?

The Mechanism

Next up

Apply partial pressure and Fick's law

Turn the gradient into a rate, in real clinical scenarios.

Reading the Data, Partial Pressure and Fick's Law

The numbers reveal the mechanism. Here is what they mean.

If you noticed that O₂ partial pressure consistently decreases from atmosphere to mitochondria, and CO₂ consistently increases in the opposite direction, you have already identified the fundamental mechanism of gas exchange. Gases always diffuse from high to low partial pressure. The size of the difference at any step determines how fast the gas moves.

Oxygen journey from atmosphere to mitochondrion showing partial pressure changes at each step

The oxygen journey, each step has a lower partial pressure, driving diffusion forward

Anatomical context illustration of an alveolus closely wrapped by a red-blood-cell-filled capillary

An alveolus and its close capillary network. The deterministic oxygen-journey visual above owns the exchange direction and partial-pressure data.

What Each Transition Tells Us

Atmosphere (159) → Alveoli (100): O₂ drops significantly because inhaled air mixes with residual air already in the lungs (dead space air) that hasn't been exhaled yet, this dilutes the incoming O₂. Alveolar O₂ is never as high as atmospheric O₂ for this reason.

Alveoli (100) → Arterial blood (95): A small 5 mmHg drop drives O₂ across the alveolar membrane into blood. This gradient is small but the alveolar surface is enormous (~250m²) and the membrane is extremely thin (~0.5 μm), so diffusion rate is high despite the modest gradient.

Arterial blood (95) → Venous blood at rest (40): A dramatic drop of 55 mmHg as blood delivers O₂ to body tissues over the systemic circuit. Tissues consuming O₂ maintain a low partial pressure in cells, this keeps the gradient between blood and tissues that drives O₂ into cells.

Venous blood (40) → Tissue cells (20–30): Blood still contains significant O₂ even after the systemic circuit, haemoglobin is never fully depleted at rest. During exercise, this venous O₂ drops much lower as demand increases.

This is Fick's Law of Diffusion in action, the rate of diffusion is proportional to surface area and concentration gradient, and inversely proportional to membrane thickness:

Rate of diffusion ∝ (Surface Area × Concentration Gradient) / Membrane Thickness

Every adaptation of the alveolus can be mapped onto one of these three variables

Alveolar Adaptation

~500 million alveoli · total SA ~250m²

Continuous ventilation refreshes alveolar air · continuous blood flow removes loaded O₂

Alveolar wall 1 cell thick · capillary wall 1 cell thick · combined ~0.5 μm total

How It Maximises Gas Exchange Rate

More surface available simultaneously, rate increases proportionally

Gradient maintained by never letting alveolar O₂ fall or blood O₂ rise, diffusion continues at maximum rate

Minimum possible diffusion distance, gases cross in milliseconds

ApplyBand 3

Activity 01

Fick's Law Application, Clinical Scenarios

Apply Fick's law to predict the effect of disease conditions on gas exchange rate.

For each condition, identify which Fick variable is affected, state the direction of change, and predict the overall effect on gas exchange rate.

Condition	Fick Variable Affected	Direction of Change	Effect on Gas Exchange Rate
Emphysema, alveolar walls break down, many alveoli merge into fewer, larger spaces
Pulmonary oedema, fluid accumulates in the space between alveoli and capillaries
Anaemia, reduced haemoglobin concentration in blood
Exercise training, lungs develop increased alveolar density and capillary network

Next up

Trace oxygen to the mitochondrion

Air to alveolus to blood to cell, without skipping a step.

The Complete Gas Exchange Pathway, O₂ from Air to Mitochondrion

Six steps, six concentration gradients, one continuous downhill journey

Gas exchange is not a single event, it is a series of diffusion steps, each driven by a concentration gradient maintained by the previous step. Break one step and the whole chain fails. The cardiovascular system's role is to be the active transporter between the two passive diffusion zones (lungs and tissues).

Figure: Six-step gas exchange pathway. Steps 1 and 4 = bulk flow (long distances); Steps 2 and 5 = diffusion (short distances at exchange surfaces).

Ventilation, Bulk Flow to Alveoli (Not diffusion)

Air moves into lungs by bulk flow driven by pressure differences created by the diaphragm and intercostal muscles expanding the thorax, not by diffusion. Breathing is essential because diffusion alone is too slow to move gases over the distances from mouth to alveolus (~30 cm). Ventilation continuously delivers fresh high-O₂ / low-CO₂ air to the alveoli and removes CO₂-enriched expired air, maintaining alveolar partial pressures.

Alveolar → Blood Diffusion (External gas exchange)

O₂ diffuses from alveolar air (pO₂ 100 mmHg) across the alveolar epithelium and capillary endothelium (combined ~0.5 μm) into pulmonary capillary blood (pO₂ ~40 mmHg arriving, leaves at ~95 mmHg). CO₂ moves in the opposite direction simultaneously. This is "external gas exchange", between the external environment (alveolar air) and the blood. The ~60 mmHg O₂ gradient is maintained by ventilation on one side and blood flow on the other.

O₂ Binding to Haemoglobin in Red Blood Cells

O₂ that has diffused into plasma rapidly binds haemoglobin inside red blood cells, forming oxyhaemoglobin. This binding removes free O₂ from solution, keeping plasma pO₂ low, which maintains the diffusion gradient from alveoli into blood. Without haemoglobin, blood could carry only ~3 mL O₂ per 100 mL; with haemoglobin, it carries ~20 mL O₂ per 100 mL, a 70-fold increase in capacity.

Cardiovascular Transport, Bulk Flow Through Vessels

Oxygenated blood is pumped by the left ventricle through arteries to systemic capillaries, again bulk flow, not diffusion. The cardiovascular system delivers fully loaded haemoglobin to within diffusion distance of every cell in seconds. Without circulation, O₂ would exhaust the tissue-adjacent blood and the gradient would collapse. The heart is gradient maintenance.

Blood → Tissue Diffusion (Internal gas exchange)

At systemic capillaries, O₂ diffuses from capillary blood (pO₂ ~95 mmHg arriving) into tissue cells (pO₂ ~20–30 mmHg) down the gradient. CO₂ diffuses from tissue cells (pCO₂ ~50+ mmHg) into blood (pCO₂ ~40 mmHg). This is "internal gas exchange", between blood and the internal tissues. The tissue pO₂ is kept low by continuous cellular respiration consuming O₂; without metabolism, the gradient would disappear.

O₂ → Mitochondria: Cellular Respiration

O₂ diffuses from cytoplasm into mitochondria where it is used as the terminal electron acceptor in oxidative phosphorylation, producing ATP. CO₂ is produced as a byproduct, diffusing from mitochondria into cytoplasm, then into capillary blood. The mitochondria maintain the lowest pO₂ in the entire pathway, ensuring the gradient from air to mitochondria is always downhill for O₂, and from mitochondria to air is always downhill for CO₂.

Bulk Flow vs Diffusion, The Key Distinction

Gas exchange uses two different transport mechanisms working in series. Diffusion (Steps 2 and 5) occurs across short distances at gas exchange surfaces, it is passive and fast over micrometres. Bulk flow (Steps 1 and 4) moves gases over long distances, breathing moves air metres; circulation moves blood metres per second. Diffusion could never cover these distances in time. The system is brilliant because each mechanism is used exactly where it is most efficient.

EvaluateBand 6

Activity 02

Extended Response, Alveolar Structure and Gas Exchange

Practise the high-mark structure-to-function question format used in HSC Section II.

"Explain how the structure of the alveolus is adapted to enable efficient gas exchange. In your answer, refer to at least three structural features and explain how each contributes to gas exchange efficiency." (6 marks)

Band 6 structure: Feature → structural description → Fick variable → functional consequence. Three features × 2 marks each. Ventilation/blood flow as gradient maintenance is your fourth point for full marks.

Next up

Maintain the gradients

Why breathing and blood flow never stop, in gradient terms.

Maintaining the Gradients, Why Breathing and Blood Flow Never Stop

If either stops, gradients collapse, and so does gas exchange

The partial pressure gradients that drive gas exchange are not passive permanent features, they must be actively maintained every second. Ventilation and blood flow are the two systems that do this. Understanding what happens when either fails reveals why they are both indispensable.

Emphysema, A Fick's Law Disaster

Emphysema is a lung disease (typically from smoking) where alveolar walls break down, many small alveoli merge into fewer, larger air spaces. The biology is a perfect Fick's law problem:

Surface area: ↓↓ dramatically (fewer alveoli, less total surface area, may fall from 250m² to as low as 30m²)
Membrane thickness: ↑ (inflammation, mucus, thickening of remaining walls)
Concentration gradient: also impaired (reduced ventilation in damaged regions)

All three Fick variables move in the wrong direction simultaneously. Gas exchange rate collapses. Patients breathe room air but cannot get enough O₂, they need supplemental O₂ therapy. This is why smoking is so directly damaging: it destroys the structural basis of Fick's law in your lungs.

Check Your Understanding

Explain the key cause-and-effect relationship from this section in one sentence.

Last learn step

Generalise the four features, then record

What every exchange surface shares, then fix it in your book.

Four Universal Features of All Gas Exchange Surfaces

Every respiratory surface, alveoli, gills, tracheoles, shares these four properties

Whether in a fish gill, an insect tracheal system, or a human alveolus, every effective gas exchange surface shares four structural properties. These are not arbitrary, each maps directly onto a variable in Fick's law or a requirement for effective diffusion. In HSC exam language, these are the features you must explain.

Large surface area

Why It's Required: More area = more simultaneous diffusion events = higher total exchange rate (Fick: SA ↑)

What Happens Without It: Rate too slow to meet O₂ demand, cells starve of O₂ despite adequate breathing

Alveolus Example: ~500 million alveoli · ~250m² total · three levels of folding in small intestine (same principle)

Thin membrane

Why It's Required: Short diffusion distance = fast exchange (Fick: membrane thickness ↓ = rate ↑)

What Happens Without It: Gases too slow to cross, inflammation (e.g. pneumonia) thickens the membrane, slowing exchange even when SA is normal

Alveolus Example: Alveolar epithelium (type I cells, ~0.2 μm) + capillary endothelium = combined ~0.5 μm total diffusion distance

Moist surface

Why It's Required: Gases dissolve in the water layer before diffusing through the membrane, O₂ and CO₂ are not directly membrane-permeable in gaseous form

What Happens Without It: Dry surfaces → gas exchange impossible (insects that dry out suffocate; fish removed from water suffocate partly for this reason)

Alveolus Example: Alveolar lining fluid (surfactant + water) · surfactant also reduces surface tension preventing alveolar collapse

Maintained concentration gradient

Why It's Required: Diffusion rate proportional to gradient (Fick: concentration gradient ↑). Without maintenance, gradient equilibrates and diffusion stops

What Happens Without It: Without ventilation: alveolar O₂ falls, CO₂ rises → gradient collapses. Without circulation: tissue-side gradient collapses

Alveolus Example: Ventilation refreshes alveolar air · Blood flow continuously removes O₂ from blood (delivers to tissues) and delivers CO₂ to alveoli

Pattern Recognition for HSC

Any question asking you to "explain how the structure of [respiratory surface] enables efficient gas exchange" is asking you to apply these four features. For each, state the structural adaptation, name the Fick variable it affects, and explain the functional consequence. Band 6 answers do all three for at least three features. Band 3 answers state the feature without explaining the mechanism.

Alveolus + surrounding capillary cross-section, labelled (thin walls, O₂/CO₂ arrows, type I pneumocytes, capillary endothelium, red blood cells)

diagrams/l15-alveolus-capillary.svg

Interactive: Gas Exchange Surface Matcher

Copy into your books

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Partial Pressure Values to Know

Atmospheric O₂: 159 mmHg · CO₂: 0.3 mmHg.
Alveolar O₂: 100 mmHg · CO₂: 40 mmHg.
Arterial blood: O₂ ~95 mmHg · CO₂ ~40 mmHg.
Venous blood (rest): O₂ ~40 mmHg · CO₂ ~45 mmHg.
Tissue cells: O₂ ~20–30 mmHg · CO₂ ~50+ mmHg.

Fick's Law

Rate ∝ (SA × concentration gradient) / membrane thickness.
SA ↑ → more simultaneous diffusion → rate ↑.
Gradient ↑ → steeper driving force → rate ↑.
Thickness ↓ → shorter distance → rate ↑.

4 Universal Gas Exchange Features

Large SA (SA ↑ in Fick's law).
Thin membrane (thickness ↓ in Fick's law).
Moist surface (gases dissolve before diffusing).
Maintained gradient (ventilation + blood flow).

Bulk Flow vs Diffusion

Diffusion: passive, short distances, gas exchange surfaces.
Bulk flow: active/mechanical, long distances, breathing + circulation.
Both required: diffusion can't cover metres; bulk flow can't exchange.

Interactive Tool, Cell Organelles Explorer Open fullscreen ↗

Fick's law states that the rate of gas exchange increases when the exchange surface area:

Activities

Next up

Show what you have learned

Multiple choice first, then the written answers. No notes.

Assessment

Multiple Choice

5 random review questions from a replayable lesson bank

Short Answer

6. Explain the role of blood flow in maintaining efficient gas exchange across the alveolar membrane. In your answer, describe what would happen to gas exchange if blood flow stopped. 3 MARKS

7. Use Fick's law to explain how pulmonary oedema (fluid accumulation between alveoli and capillaries) impairs gas exchange. 3 MARKS

8. Distinguish between external gas exchange and internal gas exchange. In your answer, state where each occurs, which gases move in which direction, and what drives each exchange. 4 MARKS

Two exchange zones × two marks each: location + direction + driving gradient.

Comprehensive Answers

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Multiple Choice

1. C This is the ventilation-perfusion mismatch phenomenon. Not every alveolus receives perfectly matched ventilation and blood flow. Some areas of the lung have slightly impaired ventilation or slight anatomical variations, blood from these areas is slightly less oxygenated. When all pulmonary blood mixes in the pulmonary veins, the average O₂ is fractionally below the ideal alveolar level.

2. B Emphysema destroys alveolar walls, merging many small alveoli into fewer large spaces. Total surface area falls dramatically (potentially from ~250m² to ~30m²). By Fick's law, rate ∝ SA, a massive reduction in SA causes a proportional drop in diffusion rate. The remaining individual alveoli may function normally, but there are far fewer of them.

3. D Ventilation's role is gradient maintenance. Without ventilation, O₂ would be consumed from alveolar air by diffusion into blood but never replaced, so alveolar pO₂ would fall. CO₂ from blood would accumulate in alveolar air, raising alveolar pCO₂. Both changes collapse the partial pressure gradients that drive diffusion. Ventilation does not mechanically push gases across membranes, diffusion is passive and driven by the gradient ventilation maintains.

4. A During intense exercise, muscles dramatically increase cellular respiration rate, consuming O₂ rapidly from capillary blood. Tissue pO₂ drops to ~15 mmHg. This actually increases the gradient from blood to tissue (~95 − 15 = 80 mmHg vs ~95 − 30 = 65 mmHg at rest), accelerating O₂ delivery, a positive feedback loop that meets increased demand. The low venous pO₂ is a consequence of greater O₂ extraction, not reduced loading at the lungs.

5. B Fick's law: rate ∝ (SA × concentration gradient) / membrane thickness. Large SA ↑ rate, thin membrane ↑ rate, maintained gradient ↑ rate. All three must be simultaneously maximised for highest exchange rate, which is exactly what the alveolus achieves.

Q6, Model Answer

Blood flow maintains the concentration gradient driving O₂ from alveolar air into blood by continuously removing O₂-loaded blood from the pulmonary capillaries and replacing it with deoxygenated blood from the systemic circuit. This keeps the pO₂ of blood entering pulmonary capillaries low (~40 mmHg), maintaining a large gradient relative to alveolar air (~100 mmHg). Simultaneously, blood flow delivers CO₂-rich venous blood to the alveoli, maintaining the CO₂ gradient driving CO₂ from blood into alveolar air for exhalation.

If blood flow stopped, blood in the pulmonary capillaries would rapidly equilibrate with alveolar air, pO₂ in blood would rise until it equalled alveolar pO₂ (~100 mmHg) and pCO₂ would equilibrate at ~40 mmHg. With no partial pressure gradient remaining, diffusion would cease entirely. No further O₂ loading or CO₂ unloading would occur. Cardiac arrest produces exactly this outcome, stopping blood flow collapses both alveolar and tissue gas exchange gradients simultaneously.

Q7, Model Answer

Fick's law states that rate of diffusion is inversely proportional to membrane thickness: Rate ∝ (SA × concentration gradient) / membrane thickness. In pulmonary oedema, fluid accumulates in the interstitial space between the alveolar epithelium and pulmonary capillary endothelium. This effectively increases the diffusion distance (membrane thickness) that O₂ and CO₂ must cross, adding a layer of fluid several micrometres thick to the normal ~0.5 μm barrier.

By Fick's law, an increase in membrane thickness causes a proportional decrease in diffusion rate. Less O₂ crosses per unit time despite normal alveolar O₂ levels and normal blood flow. The patient experiences hypoxaemia (low blood O₂) and breathlessness, their lungs contain normal air but cannot transfer it efficiently into the blood.

Q8, Model Answer

External gas exchange occurs at the alveolar surface, the interface between the internal environment (blood in pulmonary capillaries) and the external environment (alveolar air). O₂ diffuses from alveolar air (pO₂ ~100 mmHg) into pulmonary capillary blood (pO₂ ~40 mmHg arriving), driven by a ~60 mmHg partial pressure gradient. CO₂ simultaneously diffuses from blood (pCO₂ ~45 mmHg) into alveolar air (pCO₂ ~40 mmHg), driven by a ~5 mmHg gradient. Both gradients are maintained by ventilation refreshing alveolar air and blood flow cycling venous blood through the capillaries.

Internal gas exchange occurs at systemic capillaries, the interface between blood and body tissues. O₂ diffuses from capillary blood (pO₂ ~95 mmHg) into tissue cells (pO₂ ~20–30 mmHg), driven by a ~65–75 mmHg gradient maintained by continuous cellular respiration consuming O₂. CO₂ diffuses from tissue cells (pCO₂ ~50+ mmHg) into blood (pCO₂ ~40 mmHg), driven by a ~10 mmHg gradient. Both gradients are maintained by tissue metabolism and continuous blood flow through the systemic circuit.

The key distinction is location and what maintains each gradient: external exchange is maintained by ventilation (refreshing alveolar air) and pulmonary blood flow; internal exchange is maintained by cellular metabolism (consuming O₂, producing CO₂) and systemic blood flow.

Last step

Retrieve and reflect

Revisit your first answer, then bank the lesson.

🏎️

Speed Race

Race Through Gas Exchange

Answer questions on respiratory surfaces, diffusion gradients, gills and lungs before your opponents cross the line. Fast answers = faster car.

I have completed this lesson

Revisit Your Thinking

Look back at what you wrote at the start of this lesson. How has your thinking changed? What new connections can you make?

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