Gas exchange in humans

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What is gas exchange?

Gas exchange is the process of taking in **oxygen** from the air and removing **carbon dioxide** from the bloodstream.
It occurs in tiny air sacs called **alveoli** (singular: alveolus).
Oxygen is needed for **cellular respiration**; carbon dioxide is a waste product.

The pathway of air

Air enters through the nose or mouth and passes through the **trachea**.
The trachea splits into two **bronchi** (one to each lung).
Bronchi branch into smaller tubes called **bronchioles**.
Bronchioles lead to the **alveoli**, where gas exchange happens.

Diffusion in the alveoli

**Diffusion** is the movement of particles from a high concentration to a low concentration.
Oxygen diffuses from the alveoli (high concentration) into the blood in **capillaries** (low concentration).
Carbon dioxide diffuses from the blood (high concentration) into the alveoli (low concentration).
Capillaries are small blood vessels that wrap around each alveolus.

How alveoli maximise diffusion

They give the lungs a **large surface area**.
They have a **good blood supply** with thin-walled capillaries.
They are **moist**, so gases dissolve quickly.

Exhalation of carbon dioxide

Carbon dioxide moves from the alveoli into the **bronchioles**.
Then through the **bronchi** and **trachea**.
Finally, it is expelled through the nose or mouth.

Summary of gas exchange

Gas exchange provides **oxygen** to the body's cells and removes **carbon dioxide**.
It relies on diffusion across the thin walls of alveoli and capillaries.
The structure of alveoli is adapted for efficient diffusion.

Gas exchange in the alveoli: oxygen diffuses into the blood, carbon dioxide diffuses out.

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Practice questions

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1.Which of the following is a waste product of cellular respiration that is removed from the body through the lungs?
Easy
- ACarbon dioxide
- BOxygen
- CNitrogen
- DWater vapour
2.Diffusion is the movement of particles from an area of high concentration to an area of low concentration. Which of the following best describes this process?
Easy
- AMovement from high to low concentration
- BMovement from low to high concentration
- CMovement only when there is a membrane
- DMovement only in liquids
3.Which of the following is NOT a way that alveoli maximise diffusion?
Easy
- AThey have a good blood supply
- BThey are moist
- CThey have thick walls
- DThey provide a large surface area
4.What is the correct order of structures that air passes through when moving from the trachea to the alveoli?
Medium
- ATrachea → bronchi → bronchioles → alveoli
- BTrachea → bronchioles → bronchi → alveoli
- CBronchi → trachea → bronchioles → alveoli
- DBronchioles → bronchi → trachea → alveoli
5.Which of the following best describes the concentration of oxygen in the blood before and after it passes through the alveoli?
Medium
- AOxygen concentration is lower before and higher after
- BOxygen concentration is higher before and lower after
- COxygen concentration remains the same
- DOxygen is absent before and present after
6.Which of the following is the correct route that carbon dioxide takes when it is exhaled?
Medium
- AAlveoli → bronchioles → bronchi → trachea → nose/mouth
- BAlveoli → bronchi → bronchioles → trachea → nose/mouth
- CTrachea → bronchi → bronchioles → alveoli → nose/mouth
- DBronchioles → alveoli → bronchi → trachea → nose/mouth
7.Alveoli are surrounded by capillaries. Which of the following statements about these blood vessels is correct?
Hard
- ACapillaries have thin walls to allow diffusion
- BCapillaries have thick walls to withstand pressure
- CCapillaries carry blood away from the lungs only
- DCapillaries are the same size as alveoli
8.Which of the following best describes the function of the alveoli?
Easy
- ASite of gas exchange between air and blood
- BTubes that carry air to the lungs
- CMuscles that help with breathing
- DStructures that produce mucus

More questions

9.Which of the following is NOT a feature that helps alveoli maximise diffusion?
Medium
- ALarge surface area
- BRich blood supply
- CMoist lining
- DThick muscular walls
10.A student observes a diagram of an alveolus and its surrounding capillary. Which of the following correctly describes the direction of diffusion for oxygen and carbon dioxide?
Hard
- AOxygen from alveolus to blood; carbon dioxide from blood to alveolus
- BOxygen from blood to alveolus; carbon dioxide from alveolus to blood
- CBoth gases diffuse from alveolus to blood
- DBoth gases diffuse from blood to alveolus
11.The diagram shows an alveolus and a surrounding capillary. Which labelled part is the site where oxygen and carbon dioxide are exchanged?
Medium
- APart A (alveolus wall)
- BPart B (capillary wall)
- CPart C (air space)
- DPart D (blood)
12.The diagram shows a cross-section of an alveolus. Which structure is labelled X?
Hard
- ACapillary
- BBronchiole
- CAlveolus
- DTrachea
13.The diagram shows a cross-section of an alveolus and a surrounding capillary. Which labelled part is the site where oxygen and carbon dioxide are exchanged?
Easy
- AThe wall of the alveolus
- BThe lumen of the capillary
- CThe cavity of the alveolus
- DThe wall of the bronchus
14.The diagram shows the internal structure of a leaf. Which labelled part is the site of gas exchange?
Medium
- AStoma
- BPalisade mesophyll
- CSpongy mesophyll
- DCuticle
15.A student measures the oxygen concentration in the air entering the alveoli and in the blood leaving the alveoli. Which of the following best describes the expected result?
Hard
- AOxygen concentration is higher in the air than in the blood leaving the alveoli.
- BOxygen concentration is higher in the blood leaving the alveoli than in the air.
- COxygen concentration is equal in the air and the blood leaving the alveoli.
- DOxygen concentration is lower in the air than in the blood entering the alveoli.
16.The table shows the percentage of gases in inhaled and exhaled air. Which row is correct?
Medium
- AOxygen: inhaled 21%, exhaled 16%; Carbon dioxide: inhaled 0.04%, exhaled 4%
- BOxygen: inhaled 16%, exhaled 21%; Carbon dioxide: inhaled 4%, exhaled 0.04%
- COxygen: inhaled 21%, exhaled 21%; Carbon dioxide: inhaled 0.04%, exhaled 0.04%
- DOxygen: inhaled 16%, exhaled 16%; Carbon dioxide: inhaled 4%, exhaled 4%
17.A patient has a lung disease that reduces the surface area of the alveoli. Which of the following would be a direct consequence?
Hard
- ALess oxygen diffuses into the blood per breath.
- BMore carbon dioxide diffuses into the blood.
- CThe rate of breathing decreases.
- DThe heart pumps more slowly.
18.During exercise, the breathing rate increases. Which of the following explains why this is necessary?
Medium
- ATo supply more oxygen to the muscles and remove more carbon dioxide.
- BTo supply more carbon dioxide to the muscles and remove more oxygen.
- CTo cool down the body by evaporating water from the lungs.
- DTo increase the concentration of oxygen in the alveoli.
19.A scientist measures the rate of diffusion of oxygen across an alveolus. Which of the following changes would increase the rate of diffusion?
Hard
- AIncreasing the surface area of the alveolus.
- BDecreasing the concentration of oxygen in the air.
- CIncreasing the thickness of the alveolar wall.
- DDecreasing the blood supply to the alveolus.
20.Which of the following correctly describes the direction of diffusion of carbon dioxide at the alveoli?
Medium
- AFrom the blood into the alveolus.
- BFrom the alveolus into the blood.
- CFrom the bronchus into the alveolus.
- DFrom the alveolus into the bronchus.
21.In an experiment, a model lung is made using a balloon inside a jar. When the rubber sheet at the bottom is pulled down, the balloon inflates. Which part of the real respiratory system does the rubber sheet represent?
Hard
- AThe diaphragm
- BThe rib cage
- CThe trachea
- DThe alveoli
22.A person breathes in and out. Which of the following shows the correct order of structures that air passes through during inhalation?
Medium
- ATrachea → bronchi → bronchioles → alveoli
- BAlveoli → bronchioles → bronchi → trachea
- CBronchi → trachea → bronchioles → alveoli
- DTrachea → bronchioles → bronchi → alveoli
23.In an experiment to model gas exchange, a student sets up a jar with a balloon inside to represent a lung. The balloon is attached to a glass tube that goes through the lid. The bottom of the jar is covered with a rubber sheet. When the student pulls the rubber sheet downward, the balloon inflates. Which part of the model represents the diaphragm?
Medium
- AThe rubber sheet
- BThe balloon
- CThe glass tube
- DThe jar
24.A student investigates the effect of exercise on breathing rate. She counts the number of breaths per minute at rest, then after 1 minute of running on the spot, and again after 2 minutes of rest. Which of the following is the most likely explanation for the change in breathing rate after exercise?
Hard
- AIncreased carbon dioxide levels in the blood stimulate the brain to increase breathing rate
- BDecreased oxygen levels in the blood directly cause the lungs to work harder
- CThe muscles produce more oxygen during exercise, which must be expelled
- DThe heart rate increases, which automatically increases breathing rate
25.A student sets up an experiment using a beaker of water and a glass tube to model diffusion. She places a drop of red food coloring at the bottom of the beaker and observes that the color spreads throughout the water over time. Which of the following changes would increase the rate of diffusion of the food coloring?
Medium
- AHeating the water
- BUsing a larger beaker
- CAdding more water
- DUsing a darker color of food coloring
26.Which of the following is the correct order of structures that air passes through during exhalation?
Medium
- AAlveoli → bronchioles → bronchi → trachea → mouth/nose
- BMouth/nose → trachea → bronchi → bronchioles → alveoli
- CAlveoli → bronchi → bronchioles → trachea → mouth/nose
- DTrachea → bronchi → bronchioles → alveoli
27.A scientist uses a spirometer to measure the volume of air a person inhales and exhales. The person breathes normally for 2 minutes, then breathes as deeply as possible. Which of the following can be calculated from these measurements?
Hard
- AThe vital capacity of the lungs
- BThe rate of diffusion of oxygen
- CThe concentration of carbon dioxide in exhaled air
- DThe number of alveoli in the lungs
28.A student builds a model of the respiratory system using a plastic bottle, a balloon, and a rubber sheet. Which part of the model corresponds to the alveoli?
Medium
- AThe balloon inside the bottle
- BThe rubber sheet at the bottom
- CThe plastic bottle itself
- DThe neck of the bottle
29.A student investigates the effect of surface area on the rate of diffusion. She uses two cubes of agar jelly containing a pH indicator: one cube with side length 1 cm and another with side length 2 cm. She places both cubes in a beaker of acid and times how long it takes for the indicator to change color completely. Which cube will change color first, and why?
Hard
- AThe 1 cm cube, because it has a larger surface area to volume ratio
- BThe 2 cm cube, because it has a larger surface area
- CThe 1 cm cube, because it has a smaller volume
- DThe 2 cm cube, because it has a smaller surface area to volume ratio
30.Which of the following statements about diffusion is correct?
Easy
- ADiffusion is the movement of particles from an area of high concentration to an area of low concentration
- BDiffusion requires energy from the cell
- CDiffusion only occurs in liquids
- DDiffusion moves particles only through membranes
31.A student uses a respirometer to measure the rate of oxygen consumption in germinating seeds. The apparatus includes a sealed container with the seeds, a manometer, and a chemical that absorbs carbon dioxide. Which of the following best explains why the carbon dioxide absorbent is necessary?
Hard
- ATo ensure that any change in gas volume is due to oxygen consumption, not carbon dioxide production
- BTo provide oxygen for the seeds to respire
- CTo prevent the seeds from drying out
- DTo measure the amount of carbon dioxide produced
32.In an experiment, a model lung is made using a balloon inside a jar. When the rubber sheet at the bottom is pulled down, the balloon inflates. Which part of the model represents the diaphragm?
Hard
- AThe balloon
- BThe jar
- CThe rubber sheet
- DThe glass tube
33.A student investigates the effect of surface area on the rate of diffusion. She uses two cubes of agar jelly containing a pH indicator: one cube with side length 1 cm and one with side length 2 cm. She places each in a beaker of acid and measures the time for the colour to change. Which cube will change colour faster and why?
Hard
- AThe 1 cm cube, because it has a larger surface area to volume ratio
- BThe 2 cm cube, because it has a larger surface area
- CThe 1 cm cube, because it has a smaller volume
- DBoth cubes change colour at the same rate
34.A student uses a respirometer to measure the rate of oxygen consumption in germinating seeds. The apparatus includes a sealed container with the seeds, a manometer, and a substance to absorb carbon dioxide. Which of the following would cause the coloured liquid in the manometer to move towards the seeds?
Medium
- AOxygen is consumed, reducing pressure inside the container
- BCarbon dioxide is produced, increasing pressure inside the container
- CThe seeds release heat, expanding the air
- DThe absorbent releases carbon dioxide
35.A student sets up a model lung using a plastic bottle, a Y-shaped tube, two balloons (one inside the bottle to represent a lung, one attached to the bottom to represent the diaphragm), and a rubber sheet. When the rubber sheet is pulled down, the balloon inside inflates. Which part of the model corresponds to the alveoli?
Medium
- AThe balloon inside the bottle
- BThe Y-shaped tube
- CThe rubber sheet
- DThe balloon attached to the bottom
36.A student uses a respirometer to measure the rate of oxygen consumption in germinating seeds. The apparatus includes a sealed container with the seeds, a manometer, and a syringe. Which of the following would cause the coloured liquid in the manometer to move towards the seeds?
Medium
- AThe seeds respire aerobically, using up oxygen and producing carbon dioxide which is absorbed by soda lime
- BThe seeds photosynthesise, producing oxygen
- CThe seeds respire anaerobically, producing ethanol
- DThe temperature of the surroundings increases
37.In an experiment to investigate the effect of surface area on the rate of diffusion, a student prepares two agar cubes of different sizes, each containing a pH indicator that changes colour when carbon dioxide diffuses in. The cubes are placed in a solution of carbon dioxide. Which of the following would be the most appropriate way to measure the rate of diffusion?
Hard
- AMeasure the time taken for the indicator to change colour completely throughout the cube
- BMeasure the volume of carbon dioxide solution absorbed by the cube
- CMeasure the change in mass of the cube over time
- DMeasure the distance the carbon dioxide has diffused into the cube after a fixed time
38.A scientist uses a spirometer to measure lung volumes. The trace shows tidal volume and vital capacity. Which of the following best defines vital capacity?
Medium
- AThe maximum volume of air that can be exhaled after a maximum inhalation
- BThe volume of air inhaled or exhaled during normal breathing
- CThe volume of air remaining in the lungs after a maximum exhalation
- DThe total volume of air the lungs can hold
39.A student investigates the effect of exercise on breathing rate and depth. She records the number of breaths per minute and the volume of each breath at rest and after exercise. Which of the following combinations of changes would most likely be observed after exercise?
Hard
- ABreathing rate increases, tidal volume increases
- BBreathing rate increases, tidal volume decreases
- CBreathing rate decreases, tidal volume increases
- DBreathing rate decreases, tidal volume decreases
40.Which of the following is a waste product of respiration that is removed from the body through the lungs?
Easy
- Aoxygen
- Bcarbon dioxide
- Cwater
- Dglucose

Revision notes

## Gas Exchange in Humans Gas exchange is vital: we take in oxygen (O₂) and remove carbon dioxide (CO₂). It happens in tiny air sacs called **alveoli** (singular: alveolus). ### Path of Air - Inhale: air → trachea → bronchi → bronchioles → alveoli. - Exhale: CO₂ travels back: alveoli → bronchioles → bronchi → trachea → nose/mouth. ### Diffusion - **Definition**: movement of particles from high concentration to low concentration. - O₂ diffuses from alveoli (high O₂) into blood capillaries (low O₂). - CO₂ diffuses from blood (high CO₂) into alveoli (low CO₂). ### How Alveoli Maximise Diffusion 1. **Large surface area** – many alveoli increase area for exchange. 2. **Good blood supply** – thin-walled capillaries wrap around each alveolus. 3. **Moist surfaces** – gases dissolve quickly, speeding diffusion. ### Key Points - CO₂ is a waste product of cellular respiration. - Gas exchange provides O₂ to body cells and removes CO₂. - After passing alveoli, blood has **more O₂** and **less CO₂**. ### Quick Check (True/False from source) - Carbon dioxide is a waste product of respiration. **True** - Diffusion is movement from low to high concentration. **False** - Alveoli give lungs a small surface area. **False** - Capillaries wrap around each alveolus. **True** - Blood capillaries have thick walls. **False** - Gas exchange provides oxygen to cells. **True**

Alveoli: Tiny air sacs in the lungs where gas exchange occurs.
Alveolus: Singular of alveoli.
Diffusion: Movement of particles from a high concentration to a lower concentration.
Capillaries: Small blood vessels that wrap around each alveolus, with thin walls for efficient diffusion.
Trachea: Windpipe; air passes through it to the bronchi.
Bronchi: Two main branches from the trachea leading to the lungs.
Bronchioles: Smaller tubes branching from the bronchi, leading to alveoli.
Cellular respiration: Process that produces carbon dioxide as a waste product.

Flashcards

What is gas exchange in humans?

Gas exchange is the process of taking in oxygen from the air and removing carbon dioxide from the bloodstream.

Where does gas exchange take place in the lungs?

Gas exchange takes place in tiny air sacs called alveoli (singular: alveolus).

What is the singular term for alveoli?

Alveolus.

What is the path of air from inhalation to the alveoli?

Air travels through the trachea, then the bronchi, then bronchioles, and finally enters the alveoli.

How does oxygen move from alveoli into the blood?

Oxygen diffuses through the thin walls of the alveoli and into the surrounding capillaries.

What are capillaries?

Capillaries are small blood vessels that wrap around each alveolus.

Define diffusion.

Diffusion is the movement of particles from a high concentration to a lower concentration.

What is the waste product of cellular respiration that is removed by the lungs?

Carbon dioxide (CO2).

How does carbon dioxide move from blood to alveoli?

Carbon dioxide diffuses from the blood into the alveoli.

What is the route of carbon dioxide during exhalation?

Carbon dioxide travels from alveoli through bronchioles, bronchi, trachea, and out through the nose or mouth.

List three ways alveoli maximise diffusion.

Large surface area, good blood supply with thin-walled capillaries, and moist surfaces for quick gas dissolution.

Why do alveoli have a large surface area?

To maximise the rate of diffusion of oxygen and carbon dioxide.

Why is a good blood supply important for gas exchange?

It maintains a concentration gradient for diffusion by quickly carrying away oxygen and bringing carbon dioxide.

Why are the walls of alveoli and capillaries thin?

Thin walls reduce the distance for diffusion, making gas exchange more efficient.

Why are alveoli moist?

Moisture allows gases to dissolve quickly, which is necessary for diffusion across the membrane.

True or false: Carbon dioxide is a waste product of respiration.

True.

True or false: Diffusion is the movement of particles from a low concentration to a higher concentration.

False. Diffusion is from high to low concentration.

True or false: Alveoli give the lungs a small surface area.

False. They give a large surface area.

True or false: There are small blood vessels called capillaries wrapped around each alveolus.

True.

True or false: Blood capillaries have thick walls to protect them.

False. They have thin walls to allow diffusion.

True or false: Gas exchange is necessary for providing oxygen to the body’s cells.

True.

Compare oxygen concentration in blood before and after passing through alveoli.

Before: low oxygen concentration. After: high oxygen concentration (oxygen diffuses into blood).

Compare carbon dioxide concentration in blood before and after passing through alveoli.

Before: high carbon dioxide concentration. After: low carbon dioxide concentration (CO2 diffuses out of blood).

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