Gas Exchange in Animals and Plants

Lessons

Senior 5 · Biology

Gas Exchange in Animals and Plants

5 lessons

Lesson 2 of 5

The human gas-exchange system, ventilation and lung volumes

25 min

Blow as hard as you can

At a hospital clinic, a patient with a long-lasting cough is asked to take the deepest possible breath and then blow it all out, as hard and as long as possible, into a tube connected to a machine. The machine, a spirometer, measures how much air the lungs can move and how quickly. Doctors use it to diagnose asthma and chronic obstructive pulmonary disease, which you will meet in lesson 4. To read its results, you need to know how the lungs are built, how air is moved in and out, and what the different lung volumes mean.

Definition

Alveolus (plural alveoli): a tiny air sac at the end of a bronchiole, where gas exchange happens.

Tidal volume (TV): the volume of air breathed in, or out, in one breath at rest, about $0.5\,\mathrm{dm^3}$ in an adult.

Vital capacity (VC): the largest volume of air that can be breathed out after the deepest possible breath in.

Residual volume (RV): the volume of air that is always left in the lungs, even after the most forceful breath out.

Pulmonary ventilation: the volume of air moved into the lungs in one minute. $\text{pulmonary ventilation} = \text{tidal volume} \times \text{breathing rate}$

The gas-exchange system

Air passes from the nose and mouth through the trachea, which divides into two bronchi, one to each lung. These branch again and again into narrower bronchioles, which end in clusters of alveoli. Each lung contains hundreds of millions of alveoli, giving a total gas-exchange surface estimated at roughly $70\,\mathrm{m^2}$ (estimates range from about 50 to over 100 m²).

Tissue	Trachea	Bronchi	Bronchioles	Alveoli	Function
Cartilage	C-shaped rings	Irregular plates	None	None	Holds the airway open when air pressure inside falls
Ciliated epithelium	Yes	Yes	Larger ones	None	Cilia beat to sweep mucus up to the throat
Goblet cells	Yes	Yes	Few	None	Secrete mucus that traps dust and microorganisms
Smooth muscle	Yes	Yes	Yes, relatively most	None	Contracts to narrow the airway; relaxes to widen it
Elastic fibres	Yes	Yes	Yes	Yes	Stretch on breathing in and recoil to help push air out

How the alveoli fit their job

Huge surface area from hundreds of millions of alveoli.
Short diffusion path: the alveolar wall is made of one layer of flat cells, pressed against a capillary wall that is also one cell thick; the barrier is less than 1 µm thick.
Steep gradients: ventilation keeps bringing fresh air in, and blood flow keeps carrying oxygen away and bringing carbon dioxide.
Moist lining with surfactant, a detergent-like secretion that lowers surface tension so the alveoli do not collapse when you breathe out.
Elastic fibres around each alveolus let it stretch and then recoil.

Ventilation: changing the volume of the chest

Air moves from high to low pressure. The lungs have no muscles of their own; breathing works by changing the volume of the thorax, which changes the pressure inside it (for a fixed amount of gas, pressure falls when volume rises).

	Inspiration (breathing in)	Expiration at rest (breathing out)
External intercostal muscles	Contract: ribs move up and out	Relax: ribs move down and in
Diaphragm	Contracts and flattens	Relaxes and domes upwards
Volume of thorax	Increases	Decreases
Pressure in lungs	Falls below atmospheric	Rises above atmospheric
Air movement	Into the lungs	Out of the lungs

Quiet expiration is mostly passive: the muscles relax and the stretched elastic fibres recoil. In forced expiration, as when coughing or blowing into a spirometer, the internal intercostal muscles contract to pull the ribs down, and the abdominal muscles push the diaphragm up.

The spirometer and lung volumes

A simple spirometer is a chamber of oxygen floating on water. A person breathes in and out of it through a mouthpiece, wearing a nose clip, and the chamber rises and falls. A pen, or a sensor, records the movement as a trace (Figure 3). Breathed-out carbon dioxide is absorbed by soda lime in the circuit.

Two volumes add together to make a capacity: vital capacity $= \text{TV} + \text{IRV} + \text{ERV}$ , and total lung capacity $= \text{VC} + \text{RV}$ . IRV and ERV are the inspiratory and expiratory reserve volumes: the extra air that can be breathed in, or out, beyond a normal breath. The residual volume cannot be measured with a spirometer, because it never leaves the lungs. Vital capacity is larger in taller people, in men than in women of the same height, and in trained athletes; it falls with age and with lung disease.

Worked example 1: pulmonary ventilation at rest and in exercise

At rest, a student's spirometer trace shows a tidal volume of $0.50\,\mathrm{dm^3}$ and 4 breaths in 20 seconds. During exercise, the tidal volume is $2.2\,\mathrm{dm^3}$ and the breathing rate is 30 breaths per minute. Calculate the pulmonary ventilation in each case, and the factor by which it rises.

Resting rate: $4$ breaths in 20 s $= \frac{4 \times 60}{20} = 12$ breaths per minute.
Rest: $0.50 \times 12 = 6.0\,\mathrm{dm^3\,min^{-1}}$ .
Exercise: $2.2 \times 30 = 66\,\mathrm{dm^3\,min^{-1}}$ .
Factor: $\frac{66}{6.0} = 11$ .

Note: both deeper and faster breathing contribute, but here the tidal volume rose 4.4 times and the rate only 2.5 times.

Worked example 2: oxygen uptake from a spirometer trace

In a spirometer with soda lime, the troughs of a resting trace fall steadily by $0.45\,\mathrm{dm^3}$ over 1.5 minutes. Explain why the trace drifts down, and calculate the rate of oxygen uptake in $\mathrm{cm^3\,min^{-1}}$ .

Oxygen is taken from the chamber and used in respiration. The carbon dioxide breathed out is absorbed by the soda lime, so it does not replace the oxygen; the total volume of gas in the chamber falls.
$\text{rate} = \frac{0.45\,\mathrm{dm^3}}{1.5\,\mathrm{min}} = 0.30\,\mathrm{dm^3\,min^{-1}} = 300\,\mathrm{cm^3\,min^{-1}}$

Check: without the soda lime, the volume would barely change, because about as much carbon dioxide would be breathed out as oxygen was taken in.

Warning

Common mistakes and safety

"Air rushes in, so the chest expands." It is the other way round: the chest expands first, the pressure falls, and then air flows in.
"The lungs empty completely." The residual volume always remains, so vital capacity is less than total lung capacity.
"The diaphragm relaxes when you breathe in." It contracts and flattens when you breathe in.
Safety with a spirometer: use a new or sterilised mouthpiece for each person; the person should be sitting, should stop if dizzy, and should not be tested during an asthma attack. The chamber holds medical oxygen, so keep flames away. Soda lime is corrosive: do not touch it.

Byte Size Med: "Lung Volumes and Capacities | Spirogram". Watch for how each volume and capacity is read from the trace, and why the residual volume cannot be measured by spirometry; compare the definitions with Figure 3.

Tip

Memory hook: a capacity is always made of two or more volumes. Read a trace from the bottom up: RV, ERV, TV, IRV: "Rwanda's Exams Test Intelligence". VC skips the RV at the bottom; TLC includes everything.

✦

Practice: human gas exchange and ventilation

State the function of (a) cartilage in the trachea, (b) goblet cells, (c) elastic fibres in the alveolar walls.
Describe the changes that occur in the thorax during inspiration.
Explain why expiration at rest needs no muscle contraction, but forced expiration does.
From Figure 3, calculate the vital capacity and the total lung capacity.
A man has a tidal volume of $0.45\,\mathrm{dm^3}$ and breathes 16 times a minute. Calculate his pulmonary ventilation.
A spirometer trace (with soda lime) falls by 0.84 dm³ in 3.0 minutes. Calculate the oxygen uptake in cm³ min⁻¹.
Explain why the residual volume is useful, even though it is never breathed out.
Suggest why a spirometer test of vital capacity should be repeated three times and the highest value recorded, not the mean.

Show answersHide answers

(a) Holds the trachea open so it does not collapse when pressure inside falls; (b) secrete mucus to trap dust and microorganisms; (c) stretch when the alveoli fill and recoil to help push air out.
The external intercostal muscles contract, moving the ribs up and out; the diaphragm contracts and flattens; the volume of the thorax rises and the pressure falls below atmospheric, so air flows in.
At rest, the stretched elastic fibres recoil and the relaxing muscles let the thorax return to its smaller volume. Forcing out more air needs the internal intercostals to pull the ribs down and the abdominal muscles to push the diaphragm up.
VC $= 6.0 - 1.2 = 4.8\,\mathrm{dm^3}$ (or $0.5 + 3.0 + 1.3$ ); TLC $= 4.8 + 1.2 = 6.0\,\mathrm{dm^3}$ .
$0.45 \times 16 = 7.2\,\mathrm{dm^3\,min^{-1}}$ .
$\frac{0.84}{3.0} = 0.28\,\mathrm{dm^3\,min^{-1}} = 280\,\mathrm{cm^3\,min^{-1}}$ .
It keeps the alveoli from collapsing completely and lets gas exchange continue between breaths, so the composition of alveolar air, and of the blood, stays steady.
Vital capacity is a maximum: any attempt can only fall short of it, through poor effort or technique, not exceed it, so the best attempt is closest to the true value.

Key points

Air passes trachea → bronchi → bronchioles → alveoli; cartilage, cilia, goblet cells, smooth muscle and elastic fibres are spread differently along the way.
Alveoli: huge area, wall one cell thick (barrier under 1 µm), moist with surfactant, ventilated and well supplied with blood.
Inspiration: external intercostals and diaphragm contract, volume rises, pressure falls, air flows in.
VC = TV + IRV + ERV; TLC = VC + RV; pulmonary ventilation = TV × breathing rate.

Sources and credits

Written by Ganzaa. Facts were checked against 39.3 Breathing and 39.2 Gas Exchange across Respiratory Surfaces, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax, licensed under CC BY 4.0. No text was copied.
Photo: Normal lung, Yale Rosen, CC BY-SA 2.0, via Wikimedia Commons.
Video: Lung Volumes and Capacities, Spirogram, Byte Size Med, embedded from YouTube.

Figures 3 and 4, the tables, the worked examples and the practice questions are by Ganzaa. The volumes in Figure 3 are illustrative.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Blow as hard as you can

2The gas-exchange system

3How the alveoli fit their job

4Ventilation: changing the volume of the chest

5The spirometer and lung volumes

Blow as hard as you can

The gas-exchange system

How the alveoli fit their job

Ventilation: changing the volume of the chest

The spirometer and lung volumes