Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

Gas exchange

Gas exchange surfaces

Organisms usually need to take in oxygen and give out carbon dioxide.

This is because oxygen is used in aerobic respiration, and carbon dioxide is produced, so these substances must move in different directions. Then there is the less efficient anaerobic respiration, which in animals does not require oxygen nor produce carbon dioxide.

All plant cells also respire so they need oxygen and give out carbon dioxide. Photosynthesing cells are not really an exception; they just carry out photosynthesis as well as respiration, and so they take in more carbon dioxide than they give out, as well as giving out more oxygen than they take in.
This is still gas exchange, but in the opposite direction.

Each of these cases can be described as gas exchange because the two gases are moving in different directions (and usually in set mathematical ratios).

The place where gases enter and leave is called a respiratory surface.

These tend to have the following features:
a large surface area, for absorbing or getting rid of respiratory gases,
thin, to minimise the diffusion path,
moist, to allow gases to enter or leave solution.

Most multicellular animals need a reliable supply of energy derived from respiration.
They can maximise this by having a blood circulatory system to move the (dissolved) gases between the respiratory surface and cells of the rest of the body.
They can also have have a mechanical means of ventilating their respiratory surface (breathing).

Organisms exchanging gases with atmospheric air - 'terrestrial organisms' - have an associated problem: the loss of water from the respiratory surface.

Gases must dissolve before they can be exchanged

Gas exchange relies on molecules of gases dissolving in liquid, generally based on water.

Factors affecting solubility of gases (in water):

solubility of gases is reduced at higher temperatures - gases come out of solution when liquid is heated
(and living organisms exist in a fairly narrow temperature range)

percentage of gas in mixture
air is 20% oxygen, so solubility of oxygen in water exposed to pure oxygen is 5 times the solubility of oxygen in water exposed to ordinary air

other solutes
less oxygen dissolves in seawater than fresh water

Gases move slowly by diffusion, and dissolved gases move even more slowly

Gases move down a concentration gradient. If there is no difference in concentration (between two points), there is no effective movement of gases.

If gases are moved as a result of pressure by pumping processes (e.g. breathing) then a concentration gradient can be established, and movement of gases will be maintained. Similarly, currents in water improve solubility of gases like oxygen and deliver it more efficiently.

Fick's law

This explains the relationship between the three factors that control the rate of diffusion (across a respiratory surface). The rate of diffusion is proportional to:

surface area x concentration difference

The (body?) surface of a single-celled organism

The outer surface of a single-celled organism - such as a protoctistan - is usually the cell membrane. Bacteria and algae can have a cell wall on the outside of this but they are usually not a physical barrier to small molecules like oxygen and carbon dioxide.

And single-celled organisms are generally in an aqueous environment, which is in contact with air containing oxygen and carbon dioxide. So they exchange gases in a dissolved form. The same considerations apply to single-celled parasites living in the body fluids of their host.
Amoeba proteus amoebaproteus450 (24K) A number of vacuoles can be seen in the central endoplasm

Biology teachers and students like to start by thinking about Amoeba, which is a microscopic animal.

Because it is quite transparent, we can easily see what is going on, and we can make educated guesses about their biological processes. OK it's not so easy to see oxygen and carbon dioxide.

The cell membrane is the gas exchange surface, and (dissolved) oxygen diffuses in whilst (dissolved) carbon dioxide diffuses out over the whole of this outer surface.

The tracheal system of an insect

The respiratory system of an insect

insect_tracheoles (34K) There are in fact a large number of tracheoles, forming a white fluffy mass inside the body.
The insect respiratory system delivers air directly to the cells of the body.

Air passes in via openings called spiracles - perhaps 2 per body segment. These have valves which can be closed to reduce loss of water (as vapour) when the insect is inactive.

Body movements can draw air in by acting like a set of bellows, especially in flying insects, but most of the gas movement is passive - by gaseous diffusion.

Gaseous air passes into tubes (tracheae) which have rings of chitin to keep them open, and these branch repeatedly. Gas exchange takes place at the surface of the fluid (the 'fluid/gas interface') at the ends of the finer tubes - tracheoles - where oxygen dissolves in the fluid and CO2 leaves it. The fluid is in contact with a thin layer of liquid covering the internal muscles etc. and dissolved oxygen is passed to them by diffusion.

The following features maximise gas exchange:

Tracheoles are highly branched and there is a large number of them, so they present a large surface area (for gas exchange).

They also have thin walls so there is only a short diffusion distance to cells.

Tracheae provide tubes full of air so there is fast diffusion (into insect tissues). Diffusion is faster in gases than liquids.

The fluid in the ends of the tracheoles moves out (into tissues) during exercise so there is a larger surface area (for gas exchange) and diffusion through air is faster. Active muscle cells may respire anaerobically resulting in lactate formation, causing water to be drawn out of the ends of the tracheoles and improving delivery of oxygen, so more efficient aerobic respiration can take place.
Do not confuse names of insect airways with their mammalian counterparts - there are no bronchioles

Back then, things were different

Fossilised insect shows how things could be meganeura (109K) This Meganeura had a wingspan of 70cm and a head to body length of 43cm. It lived about 300 million years ago.
It is thought that the body size of insects is limited by the efficiency of transmission of oxygen along tracheoles and into their body.

In the carboniferous era atmospheric oxygen contained about 35% oxygen. This allowed insects to be quite a lot larger than today.

Meganisopterans or 'griffinflies' are ancestors of the modern insect group Isoptera - dragonflies and damselflies.

The largest comparable species today is a fairly narrow-bodied damselfly Megaloprepus caerulatus - wingspan 19 cm.

dragonfly (7K) Modern dragonflies have a lighter network of veins in their wings

The gills of fish

The breathing cycle of a fish

../pot/animals/fish0.gif Continuous animation
Most fish use gills to absorb dissolved oxygen from the water around them, and at the same time to get rid of carbon dioxide produced in the body of the fish.

They draw in water through their mouth as a result of lowering the floor of their mouth, then close their mouth and raise the floor of the mouth. This causes water to pass across and between the gills. As oxygen is absorbed into the blood in the capillaries of the gills, dissolved carbon dioxide also leaves the blood and passes into the water.

This water then passes out of a slit-shaped opening behind the operculum (gill cover).

Where does the dissolved oxygen get into the water?
> At the surface of the water, where it is in contact with air.
And it may come from submerged plants that are photosynthesising.

What happens to the dissolved carbon dioxide?
> It moves through the water, and leaves the surface of the water, where it is in contact with air.
Of course it may be used by submerged plants for photosynthesis.

Gills of a live fish
(seen by raising the gill cover)
gills (104K) Here the gill rakers, gill arches and the reddish filaments can be seen. The gaps between the 4 gills are also displayed.
Gill rakers protect the gills from particles in the fish's mouth. Each gill arch is supported by a curved bone and contains two blood vessels: the afferent arteriole bringing in deoxygenated blood from the rear of the body, and the efferent arteriole carrying oxygenated blood back to supply the muscles and other organs.

Each of these blood vessels has branches to send blood down and back along the edges of the flattened gill filaments (primary lamellae).

Between them there are hoop-shaped secondary lamellae carrying blood capillaries. These present a large surface area in order to maximise the exchange of oxygen from the water with the blood in the capillaries.

Water and blood flow in the gills of a fish

gill_countercurrent (34K)
↑ % O2 shown above is % of saturation

The counter-current principle

In many biological applications there needs to be an interchange between two fluids, and this exchange is only achieved efficiently if they are flowing in opposite directions.

In the fish's gills, fresh oxygenated water flows across the capillaries in the lamellae so that it meets blood that is leaving the capillary network, and there is a (slight) difference in oxygen concentration between the water and the blood. As a result the water moving over the lamella has a progressively lower oxygen concentation but it is always meeting blood flowing into the capillary network with an even lower oxygen concentration, so oxygen is always transferred into the blood.

There is a similar (but opposite) difference in carbon dioxide concentration between blood in the capillaries and water, and CO2 is always transferred out of the blood.

A lot of fish in this tank!

People used to say you could put an 'inch of fish per gallon' in a fish tank but some say this is crowding them in too much. And how many cm of fish per litre is that?

fishtank (151K)

Oxygen enters the water at the surface, and carbon dioxide passes out of solution here too.

This tank is also aerated with air pumped in and broken into many small bubbles using an 'airstone'.

It still looks like too many fish, and the plants don't look very real, either.

Exceptions to the rule

Siamese Fighting Fish Betta splendens

640px-Betta_fish (61K)

Like many popular tropical fish in the family Anabantidae (labyrinth fishes) - which includes Gouramis, the Siamese Fighting Fish can survive in small volumes of water because they take in gulps of air from the surface and pass it into a coiled organ which extracts the oxygen.

This is significant because in the wild they are often found in polluted water, and when kept in aquarists' shops they can be housed in small jars. This also prevents them from fighting, which is what they were bred for in Thailand.

The leaves of dicotyledonous plants

Dicotyledons (dicots for short) are a very large group of flowering plants, usually with broad leaves, as opposed to monocots which have strap-shaped leaves. Within the seed there are two simple leaves in dicots, and one in monocots.

Within plant leaves the main photosynthetic tissue - mesophyll - is sandwiched between upper and lower epidermis layers, both of which have a waxy cuticle.

The mesophyll consists of an upper layer - the palisade layer - and a lower layer - the spongy layer.

These cells have a thin layer of water (from the xylem) on their surface, and they carry out gas exchange with air in spaces between the spongy cells.

Palisade cells are packed in vertically but they have angled bases which offer access to the gas below. Cells of the spongy layer have all-round exposure to it.

When photosynthesising, mesophyll cells take in carbon dioxide and give out oxygen, and in the dark these gases move in the opposite direction due to respiration.

Gases diffuse into (and out of) the leaf via the pores called stomata, mostly on the underside of the leaf. The top surface has no stomata, and a thicker cuticle.
If water loss (as water vapour) becomes excessive, it can be controlled by the closing of the stomata. Each stoma has two guard cells on the outside.
Stoma in surface view

guard_cells (101K) The inner cell walls are thickened and this causes curvature when the guard cells are turgid

In fact guard cells are the only cells containing chloroplasts in the lower epidermis, and in the light they carry out photosynthesis. As a result, sugars and other soluble substances such as potassium ions (K+) accumulate and lower the water potential in their cytoplasm, causing the entry of water by osmosis from the surrounding cells. This swelling makes the two guard cells peel apart in the centre, opening the stoma when the leaf is illuminated.

If water is in short supply, and in the dark, the process is reversed and the guard cells revert to their previous shape, closing the stoma and reducing the loss of water vapour from the leaves.

Vertical section through a dicot leaf
leafpl (15K)

Within the guard cells, blue light excites phototropin proteins, causing a phosphorylation cascade, which activates H+-ATPase, a pump responsible for pumping H+ ions out of the cell.

This hyperpolarises the guard cell membrane, allowing the ingress of potassium ions (K+) and chloride ions (Cl-), which causes an increase in the solute concentration and a decrease in the water potential within the cytoplasm.

Specialisation for terrestrial environments

Although life evolved in a watery environment, there are a number of advantages to living on land, and one is the availability of gaseous oxygen in the air. This requires adaptations to maximise gas exchange. However in some warmer and drier regions there is a distinct problem - dehydration.

Which environment would you prefer?

The solubility of oxygen in fresh water at standard atmospheric pressure varies from 14.6 (at 0 °C) to 7 mg/L (at 35 °C). In volumetric terms, this is 10.2-5.5 ml/L.

Air contains 21% oxygen by volume, in other words 210ml/L. So oxygen is about twenty to fourty times more abundant in air than in water.

And the solubility of oxygen in sea water is lower still: about 76% of the value for fresh water.

The density of water (1.00 g per cm3) is 800 times greater than the density of air (so water is a good support medium).

The viscosity of water is 100 times greater than air, so it is easier to suck in air than water.

It is said that a soluble compound's diffusion coefficient is about 10,000× as great in air as in water, so we can say that the diffusion rate of oxygen in air is 10,000 times greater than in water.

Terrestrial insects

Insects have a waxy exoskeleton or cuticle composed of chitin which is fairly impermeable to both oxygen and water vapour.

Spot the spiracles
grasshopper (173K)
Air is taken into the body along the system of tracheae and tracheoles described above. Obviously there is a compromise to be made between the intake of oxygen and the loss of water vapour.

Spiracles are the external openings to these tubes, and they can be closed to reduce water loss. Some insects have elastic air sacs between the reinforcing hoops of the tracheae, and these are inflated to provide a supply of air (containing oxygen) when spiracles are closed.
spiracle-001 (361K)

In this view of the spiracle of a Catalpa Hornworm Caterpillar, the opening is largely closed except for a narrow slit down the centre.

Aquatic insects let it all hang out

A mayfly larva mayfly-gills1 (54K) Dragonflylady has helpfully arrowed the gills

Insects living in water, e.g. mayfly larvae often have plate-like or feathery gills to absorb dissolved oxygen from the water. Sometimes thet jiggle them to cause water to flow over them in order to maximise the uptake of oxygen.

Xerophytic plants

Xerophytic plants have a number of morphological and physiological adaptations to enable them to survive in inhospitable conditions. Once again these are compromises between the opposing needs for efficient gas exchange and the limitation of water loss, especially in the context of exposing sufficient photosynthetic area for the absorption of light energy, which is clearly linked with heat.

Succulents store water within their leaves or stems.
cactusareoles (194K) Reducing cactus leaves to spines obviously reduces their surface area to volume ratio but they are really protection for the water-storage tissue in the stem
The most well known examples are cacti which have areoles (specialised sites where spines and flowers form). In fact the spines are modified leaves and it is the stem that performs photosynthesis. This is performed using a slightly different process from that in other plants - the CAM pathway.

Cacti have stomata, but these are small, sunken or recessed into the waxy epidermis tissue and crucially they are closed during the day, in order to conserve water.

During the night, stomata open and carbon dioxide in the air is absorbed and chemically incorporated into organic acids which are stored in the vacuole.
In the daytime the stomata are closed and the processes are reversed, releasing carbon dioxide inside the chloroplasts of the stem of the cactus, to be used in photosynthesis.

Other xerophytic adaptations - plant leaves

Do not refer to water or moisture alone - it is water vapour (or 'moist air') that is moving

Hairy leaves

hairy_leaves (237K)
These soft hairs trap a layer of water vapour around the leaf, preventing wind from blowing it away. This means that the water potential gradient from inside to just outside the leaf is decreased.
The lighter colour may also reflect away heat (in the form of infra-red radiation).

Sunken stomata

sunken_stoma (648K)
Stomata in pits or grooves on the underside of leaves also allow moist air to accumulate in these spaces, again reducing the water potential gradient

Curled leaves

marram grass dune (531K) Marram grass is a coastal species growing on sand dunes. When water is in short supply, the leaves roll up to conserve water.
marram grass leaf (56K) The stomata in marram grass are in sunken pits and the hair-like projections act as baffles to maintain a humid environment.

Leaves that roll or fold up also 'trap' water vapour and so the water potential gradient is decreased.

Waxy cuticle

Water-on-leaves 2-200x200 (61K)
An external waxy layer also functions as a barrier to the movement of water vapour so it reduces evaporation/transpiration.

Crassula ovata (jade plant) jade-plant-care-crassula-ovata-24-1 (47K) This succulent plant gives its name to the CAM pathway

Crassulacean acid metabolism (CAM) photosynthetic pathway

Here carbon dioxide is taken up by PEP (3C) which is carboxylated and converted into OAA (4C) and reduced to malate (4C) which is stored as malic acid in the vacuole.
During daytime these plants export malate to chloroplasts and release CO2 in order to perform photosynthesis when stomata are closed and water conservation mechanisms operate.

Xerophytic root systems

Cactus root system cactus_roots (38K)
Some desert plants (trees and shrubs) grow deep taproots to bring up water from the sunken water table.

Many cacti grow wide-ranging but shallow root systems to take maximum advantage of rainfall which is extremely infrequent. This means that they can absorb water from a wide area, and do so quickly, before the water drains away.

These plants often also have high salt content within the cytoplasm of the cells of the root so water from the soil enters plant roots by osmosis. This makes a steeper concentration gradient between the roots and the damp soil outside.
Water moves from a region with a higher water concentration (higher water potential) to one with a lower water concentration (a lower - more negative - water potential).

The human gas exchange system

Airways within the chest
not quite to scale airways (37K) Pleura are two protective membrane layers around each lung.
Image: Macmillan Cancer Support
The air sacs called alveoli within lungs are the gas exchange surface, and air passes in and out via a branching system of tubes ('airways') with decreased diameters. This movement of air is due to changes in pressure caused by movements of the chest wall and diaphragm. See below.

The (single) trachea or windpipe takes air from the throat into the chest and it divides into two principal bronchi, one into each lung. These divide again into secondary and tertiary bronchi (and further), then into finer tubes called bronchioles which take air into clusters of spherical alveoli with blood capillaries on their outer surface.

The walls of the trachea are reinforced with C-shaped sections of cartilage, so that it does not change in size during breathing. The walls of bronchi are similarly reinforced with hoops of cartilage.

Bronchioles have rings of smooth muscle (involuntary muscle) on the outside, and these can cause bronchodilation or bronchoconstriction - widening or narrowing of the airway.
Do not confuse names of mammals' airways with their insect counterparts - there are no tracheoles

Why does the trachea have C-shaped sections whereas bronchi have full hoops of cartilage? Clue: what is alongside?
> Trachea has oesophagus alongside, and food swallowed causes it to bulge as it goes down, but bronchi do not have anything else alongside.

The bronchial tree and factors affecting flow in the airways

A bronchogram
- radiograph (X-ray) of the bronchial tree after injection of a radiopaque substance bronchogram (35K) Click here for inverted image
and click here to put the image the right way up
The airways in the chest can be compared with the structure of a tree (trunk, branches and twigs), but inverted, and composed of hollow tubes.

Main airways

The trachea and bronchi are lined with ciliated epithelium, interspersed with 'goblet cells' which produce mucus. This mucous membrane has a protective function, trapping particles breathed into the lungs (including dust, bacteria and mould spores), and carrying them upwards to be coughed up or simply taken to the back of the throat and swallowed. Acid conditions in the stomach do the rest!

If mucus production is excessive, perhaps due to infection, airways become narrower and less air can be moved into and out of the lungs. Mucus may not be efficiently moved upwards by the cilia, and it may slide down, blocking airways which are increasingly narrow.
See Bronchitis, below

Minor airways

Bronchioles have a much smaller diameter than the bronchi (about 0.3-0.5 mm). Smooth muscle contraction under stimulation by nerves of the parasympathetic nervous system leads to bronchoconstriction and sympathetic stimulation causes bronchodilation.
See Asthma, Emphysema, Pneumonia, below

Above the chest

If a person cannot breathe normally, this is known as respiratory failure, which may be caused by problems with the chest muscles or associated nervous system, or by blockages due to swelling or cancer in the throat area. It may be treated by a tracheostomy - a surgical procedure in which an opening is created in the front of the neck.

Structure of an alveolus

A single alveolus and its associated blood supply alveolus (97K) Each lining cell is drawn to show its nucleus
The epithelium which forms the wall of each alveolus is composed of a single layer of flattened cells (squamous epithelium).

There is a thin layer of fluid lining the alveolus. This is produced by other cells within the wall of the alveolus. Oxygen dissolves in this, and carbon dioxide passes out of solution here.

On the outside of the alveolus is a network of blood capillaries. The wall of a blood capillary is also composed of a single layer of flattened cells (endothelium).

Deoxygenated blood is supplied from a branch of the pulmonary artery, and oxygenated blood flowing away from the capillary network enters a branch of the pulmonary vein.

The ins and outs of gas exchange

Explain how the structure of an alveolus allows efficient gas exchange to occur.
> wall is only one cell thick ('thin' is not enough)
> so only short diffusion pathway/distance

Gas exchange is a two-stage process.

Click here for the equivalent questions (below) about movement of carbon dioxide

What is the pathway of an oxygen molecule from the air inside an alveolus to a cell in the blood?
> (dissolves in and moves) through the layer of fluid
> through/across the epithelium of the alveolus
> through/across the endothelium of the capillary
> through the blood plasma (and into red blood cell)
What process makes oxygen move along this pathway?
> diffusion (down O2 concentration gradient)
What makes oxygen move into the alveolus?
> (negative) pressure (below atmospheric pressure) resulting from breathing movements

Gas exchange is not 100% efficient

Air that is breathed in contains about 21% oxygen, and gas that is breathed out contains about 17% oxygen.
Similarly the gas that we breathe out is about 4% carbon dioxide, compared to 0.04% carbon dioxide in the outside air.
In other words, only a fraction of the oxygen and carbon dioxide entering the respiratory system is actually exchanged as described above.

Dead space

Also, not all the air breathed in is actually delivered to the alveoli to be used in gas exchange. Some (about 150 cm3 per breath) remains in the airways, and mixes with air that is breathed out. It is thought that this has the advantages of retaining CO2 which is important for bicarbonate buffering of blood, as well as temperature regulation and humidification.

How many alveoli? and How big?

Estimates of the number of alveoli in the human lung vary considerably.

It is sometimes stated that there are about 150 million alveoli in a human lung, 300 million in two, and their total area has been estimated to be 70m2. The diameter of a single alveolus is often given as 200 µm (0.2 mm).

Ochs et al (2003) found that in six single adult human lungs, the mean alveolar number was 240 million per lung (range: 137-395 million; standard deviation 89). Alveolar number was closely related to total lung volume, with larger lungs having considerably more alveoli.
The mean volume of a single alveolus was rather constant at 4.2 x 106 µm3 (range: 3.3-4.8 x 106 µm3, with standard deviation 0.4), irrespective of the lung size.

What can you tell about this data from the standard deviations of the mean?
number of alveoli > varied considerably about the mean - but the donors were obviously different body sizes
volume of alveoli > was much closer to the mean - so this size must be most efficient?

Alveolar fluid and the transfer of gases

The fluid within the alveoli is mostly water, containing pulmonary surfactant, a mixture of lipids and proteins which is secreted by the epithelial type II cells into the alveolar space. Its main function is to reduce the surface tension at the air/liquid interface in the lung.

This means that it can spread out, forming a thin but continuous layer on the inside of the alveoli.

Oxygen gas from the air breathed in dissolves in this fluid, and then diffuses through two layers of cells into the blood in the surrounding capillaries. It then diffuses from the blood plasma into the red blood cells where it is absorbed by haemoglobin.

Carbon dioxide diffuses in the opposite direction, eventually leaving solution to join the gas in the alveolar space. Carbon dioxide is carried in the blood as bicarbonate ions HCO3-, and in alkaline conditions in the lungs the enzyme carbonic anhydrase catalyses the release of CO2.
HCO3- → CO2 + H2O

Alveolar fluid also protects the cells of the epithelial from drying out, but it inevitably leads to water loss from the body as water vapour in the air breathed out.

Taking the first breath

During development in utero the foetus has fluid in its lungs. Immediately after birth it is essential that a baby's lungs are cleared so as to permit gas exchange with the surrounding air. It is always felt that a baby's first cries are a good omen!

It was found that in premature babies epithelial type II cells were not yet producing surfactant, and their alveoli were underdeveloped, so they often suffered from respiratory distress syndrome, which was a major cause of death. However treatment with substitute fluid prevented alveoli from sticking together, and allowed them to expand during the breathing cycle, and this has dramatically increased the chances of survival.

What is the gas exchange surface for a foetus in utero?
> the placenta (and secondarily the mother's lungs)

The mechanism of breathing (how we breathe in and out)

The chest or thoracic cavity is surrounded by the chest wall, composed of ribs and associated muscles (the 'rib cage'), and the upwardly curved muscular diaphragm which forms a barrier between the chest and abdomen. There are no gaps or openings in this cavity, except for the airways leading from the throat into the lungs.

The ribs are loosely attached to the vertebrae at the back and the sternum in the middle of the chest, but different muscles can cause the ribcage to rise or fall, effectively swivelling at the back.

These movements are co-ordinated with movement of the diaphragm, so that the volume of the chest cavity can be increased or decreased.

The lungs within this space are not attached to the chest wall, but they are elastic and respond to changes in (internal) pressure resulting from changes in chest volume.

There are two sets or layers of intercostal muscles between the ribs - extermal and internal. These are antagonistic muscles - they work in opposition to one another and pull the ribs at different angles.

Breathing in (Inspiration)

The edges of the diaphragm remain attached to the lower ribs. It does not move up and down like a piston.
The external intercostal muscles contract, causing the front of the chest to rise and widen. Also, the internal intercostal muscles relax.

At the same time, the diaphragm contracts, causing it to become flatter, effectively pushing the abdomen downwards.

As a result, the volume of thoracic cavity increases, causing internal pressure to decrease below atmospheric pressure. This pressure difference causes air to move in through the nose or mouth, and down the airways (trachea, bronchi, bronchioles) into the lungs. Of course air moves into the alveoli, which are slightly elastic so they become partly inflated.

Breathing out (Expiration)

This is generally a more passive process.

The diaphragm relaxes and becomes curved upwards (in the centre), and the external intercostal muscles relax and so the chest falls, reducing the volume of the thoracic cavity. This causes the pressure inside the thoracic cavity to increase (to atmospheric pressure).

The alveoli (lung tissue) are elastic so they recoil and reduce in size, and gases inside pass into the airways.

Greater movement can be caused by contraction of the internal intercostal muscles, increasing pressure inside the thoracic cavity (above atmospheric) and causing active exhalation so that air is actively blown out.

Cross-sectional views of the chest
ignoring details of airways within the lungs

Mechanism-of-Breathing (78K)

Conditions affecting the functioning of the respiratory system


Asthma is an inflammatory condition that leads to tightening of the muscles around the bronchioles, causing these finest airways to become even more narrow and reducing the flow of air in and out of the lungs. It can be treated using inhalers or atomiser masks, typically providing: Of course the intake of these drugs can be problematic if the airways have already started to become constricted.

An asthma attack is a sudden worsening of asthma symptoms caused by the tightening of muscles around the bronchioles (bronchospasm). During the asthma attack, the lining of the airways also becomes swollen or inflamed and thicker mucus - more than normal - is produced.

peakflowblow (78K)
Asthmatics are encouraged to monitor their condition by measuring their peak flow by blowing into a simple device. Typical values for a healthy male aged 40 are 600 L/min or above, falling to 500 at age 65. For women values are generally lower.


Pulmonary fibrosis scars and thickens the tissue on the outside and between the alveoli and it reduces the volume of the lungs, without affecting the airways. This results in shortness of breath, dry cough and general fatigue.

In clinics, spirometer machines can be used to record the flow of air breathed out over a short period of time, giving a trace like the ones below.
The average volume of air breathed out by three groups of people asthma_and_fibrosis (76K)

On this graphic, group A had healthy lungs.
Groups B and C had different lung conditions that affect breathing, mentioned above.

Use the shape of these traces to answer the questions below.
You should be able to comment using these values:
1 Forced expiratory volume (FEV): the greatest volume of air a person can breathe out in 1 second.
2 Forced vital capacity (FVC): the greatest volume of air a person can breathe out in a single breath.
3 Together with the probable effect caused by the airways

Click here for the equivalent questions (below) about asthma

Which group, B or C, had fibrosis of their lungs?
Group >      B      
1> They breathe out nearly as quickly as the healthy at the start [similar FEV to group A - 67%]
2> The total volume breathed out [FVC] is reduced below normal because deflation of lungs stops at a lower value [62% of A]
3> So bronchioles are not affected as they do not provide much resistance to flow

Chronic obstructive pulmonary disease (COPD)

This term covers a group of lung conditions that cause breathing difficulties.
The following symptoms are common:
increasing breathlessness, particularly after activity
a persistent chesty cough with phlegm (thick mucus) - often described as a "smoker's cough"
frequent chest infections
persistent wheezing
The main causes are long-term exposure to airborne irritants, including tobacco smoke, marijuana smoke, air pollution, and chemical fumes and dust.


emphysema (37K)
is a condition in which the walls between the alveoli break down and enlarge the air spaces. This reduces the area for gas exchange and so an affected person experiences breathing difficulties after exercise. The lung tissue has less elasticity, possibly caused by reduction in the amount of elastin in the connective tissue surrounding the alveoli.


This is long-term inflammation of the main airways: trachea and bronchi. Smoke causes cilia to stop beating, and more mucus is produced, reducing movement of air into the lungs.


is an infection in one or both lungs, possibly caused by bacteria, viruses, or fungi. This causes inflammation of the alveoli, and they fill with fluid, reducing access of oxygen to the alveolar epithelium and making it difficult to breathe.

Coronavirus (Covid 19)

This virus infects the respiratory system, and may cause symptoms of variable seriousness:
a high body temperature
a new, continuous cough

Infected persons may get better after 7 days' isolation, or if symptoms worsen then hospitalisation may be necessary.

oxygenmask (72K)
nasal_tube (48K)

Here, oxygen is often supplied via a simple face mask or nasal tube because ordinary air does not provide enough oxygen when alveolar surfaces are covered with fluid and breathing is laboured.

Further deterioration may be treated using different levels of technology, in an intensive care unit:

Continuous Positive Airway Pressure (CPAP)

cpap-machine1-620x393 (49K)
is a form of non-invasive ventilation (using a firmly attached mask but not 'intubation') that often works best with Covid-19 patients. This technique was developed to treat sleep apnoea at home. It may use resources that are in short supply. Some CPAP machines use 50 litres of oxygen per minute for a single patient.

Mechanical ventilation

is a more serious process, taking over the breathing process using a pump and electronic control system, together with a humidifier. The oxygen is sent into the body via a tube inserted through the mouth and into the airway, and gases leaving the lungs are continuously analysed, especially for CO2 content.

Mechanical ventilator for positive pressure ventilation

ventilator (67K)
Picture credit: Peter lamb

This generally requires sedation (induced muscle paralysis, possibly using the 'poison arrow' compound curare which is an inhibitor of acetylcholine esterase) to prevent the body's natural breathing movements, and requires supervision by highly trained staff.

Unfortunately these procedures may be accompanied by other health problems: Ventilator-induced lung injury (VILI) and ventilator-associated lung injury (VALI).

Barotrauma is a form of VILI that is associated with alveolar rupture due to the associated pressure;

Lung cancer

comes in many forms.

Squamous cell carcinoma

begins in the tissues that line the larger bronchi, but it can spread to other parts of the body.
It is especially linked to exposure to cigarette smoke, but it has also been found that the second most significant risk factor is the radioactive gas radon which is naturally released from rocks like granite in some parts of the country. It is thought that improvements in building construction including double glazing have caused less draughty houses in which the gas can accumulate.


is a form of cancer that mainly affects the lining of the lungs (pleura), altough it can affect other parts of the body. It is caused by long-term exposure to irritant substances such as asbestos fibres from roofing and walling products (asbestosis), or stone dust (silicosis). Presumably they are breathed in and pass through the walls of the alveoli and into the pleura on the outside of the lung.

Pulmonary ventilation rate (PVR), tidal volume and breathing rate

In order to match the supply of oxygen to (and removal of carbon dioxide from) the body for (internal) respiration at different levels of activity, we can vary the frequency and depth of our breathing.

Pulmonary ventilation is the process of air flowing into the lungs during inspiration (inhalation) and out of the lungs during expiration (exhalation). This is basically breathing, but its rate is often expressed in terms of the volume of air moved per unit time: units dm3 minute -1

The pattern of breathing in a person sitting at rest tidal_volume (22K)
How can you tell that the person is at rest?
> uniform small variations (breaths): no change in amplitude/frequency
so steady low activity - no need to breathe more deeply/faster
The tidal volume is the volume of air moved into and out of the lungs during each ventilation cycle (each breath). In a healthy, young human adult, tidal volume is approximately 0.5 dm3 per inspiration or 7 cm3 per kg of body mass

Breathing rate is the number of (muscular) breathing movements performed by the body per unit time. It is typically 12-20 per minute for adults, but higher for children. During exercise, the rate can rise to 45-50 per minute.
It is rather difficult to measure one's own breathing rate as breathing is at least partially under conscious control.
See opposite

These factors are correlated by this reaction:

PVR = tidal volume breathing rate

Control of breathing

Breathing is under conscious control as well as "automatic" control by the body. For the purposes of speech and singing, nervous impulses generated in the cerebral cortex are passed to the muscles concerned with breathing, especially the diaphragm.

The way in which muscles causing breathing are controlled breathingcycle (32K)

It is said that breathing rate is primarily regulated by neural and chemical mechanisms, in a similar way that heart rate is controlled.

Chemoreceptors in carotid and aortic bodies sense the amount of oxygen, carbon dioxide and acid present in the blood and modulate the rate of breathing by varying the frequency of impulses passed to a respiratory centre in the medulla oblongata.

This sends an increased number or frequency of nervous impulses down the spinal cord and out along the (left and right) phrenic nerves to the diaphragm and intercostal muscles, resulting in a greater rate of muscular contractions causing faster breathing rate.

Respiratory quotient (RQ) (from the sublime to the ridiculous)

This is in fact the ratio of carbon dioxide given off to oxygen taken in for aerobic respiration by an organism:
volume of CO2
volume of O2

In fact the volumes of these gases are directly proportional to the numbers of molecules involved in the reaction, as shown by equations.

For an organism using carbohydrate e.g. glucose as a respiratory substrate:

C6H12O6 + 6 O2 6 CO2 + 6 H2O
the ratio is 6/6, i.e. 1.0, and the accepted RQ value for carbohydrates is usually taken as 1.0.
This does NOT apply directly to polysaccharides such as starch and cellulose, which need to be digested before being respired.

For an organism using fat e.g. glyceryl tristearate as a respiratory substrate:

C57H110O6 + 82.5 O2 57 CO2 + 55 H2O
the ratio is 57/82.5, i.e. 0.69, and the accepted RQ value for lipids is usually taken as 0.7.

For an organism using protein as a respiratory substrate [using a somewhat prehistoric empirical formula from the Dutch chemist Gerardus Johannes Mulder (1802-1880):]

C400H620[N100O120P1S1] + (555) O2 400 CO2 + 310 H2O
the ratio is 400/555, i.e. 0.88, and the accepted RQ value for proteins is usually taken as 0.8-0.9.

Other respiratory substrates give different respiratory quotients, but the general principle is that by calculating RQ we can see what a living organism is using to fuel its metabolism.

The use of a simple respirometer to measure volumes of gases involved in gas exchange

This apparatus was used to determine the rate of respiration and the respiratory quotient (RQ) of some blowfly larvae.

maggotrespirometer (23K) Sodium hydroxide absorbs carbon dioxide from the air. In the first experiment, 5 cm3 of concentrated sodium hydroxide solution was placed in the test tube and 10 blowfly larvae were placed in the wire cage. The assembled apparatus was placed in a water bath at 20 °C, with the tap left open. After 10 minutes, the tap was closed and the position of the bead of liquid was recorded at 1-minute intervals. A second experiment was carried out using 5 cm3 of water instead of the sodium hydroxide solution.

Why did the bead of liquid move along the scale in the first experiment but not in the second experiment?
[First experiment]
>Oxygen is consumed by the animals/blowfly larvae AND
>CO2 is given out/released but absorbed by NaOH CAUSING
> a reduction in volume / pressure
[Second experiment]
>(Volume of) O2 consumed = (volume of) CO2 produced /because RQ = 1.0

Results from these experiments
Position of bead/mm
1st expt 2nd expt
0 11 40
1 23 40
2 34 40
3 47 40
4 60 40
5 71 40

The cross-sectional area of the capillary tube was 1 mm2, and the 10 blowfly larvae had a total mass of 0.5 g.
What is the rate of oxygen consumption of the blowfly larvae, in mm3 g-1 hour-1?
> 1440 - This is (71-11) x 1 x 1/0.5 x 60/5

Three-way taps and simple respirometers to measure volumes of air (?) involved in gas exchange

I felt inspired to correct an exam question that I thought was not very practical. respirometerplussyringe (17K)
The original version of the graphic did not have 2 tubes through the bung, and just had a single right-angled glass tube, which continued into the capillary.

It was said that a student positioned the flask in a water bath so that the yeast culture reached a constant temperature, and then left the apparatus for one hour before starting the investigation.

Since alkaline pyrogallol absorbs oxygen (and to a certain extent carbon dioxide) that would have meant that the volume of gas inside the flask would have been reduced by about 20%. That surely would have sucked the coloured liquid back along the tube, and dropped it into the flask, so it would not be available in the second part of the experiment.

My recommendation is that the 3-way valve should be set to allow air to enter the flask (handle pointing 'upwards') so as to offset the reduction in volume. This would not be ideal as it would still contain oxygen, which would be absorbed, so more air would be sucked in. Anyway, the air in the flask would be mostly nitrogen, so the yeast would stop respiring aerobically and only respire anaerobically. This produces carbon dioxide and ethanol (alcohol).

After the hour for absorption of oxygen [as well as to reach a constant temperature], it would be time to switch the 3-way tap towards the syringe (handle pointing towards vent) and adjust the position of the coloured liquid to zero (at the left-hand side of the scale. Then perhaps the valve should be shut completely (handle pointing downwards).

I think the student was quite lucky that the liquid took a whole 24 hours to reach the other side of the capillary tube (and it didn't 'run out'), before taking a reading and calculating the volume of gas given off.

Other related topics on this site

(also accessible from the drop-down menu above)
Similar level
Exchanges with the environment
Surface area to volume ratio
Digestion and absorption
Mass transport in animals

Control of heart rate

Simpler level
The structure of the leaf
Transport and support in plants
The human respiratory system
Experiments to compare carbon dioxide content of inhaled and exhaled air

Interactive 3-D molecular graphic models on this site

(also accessible from the drop-down menu above)
The salbutamol, terbutamine, adrenaline and triamcinolone molecules - rotatable in 3 dimensions

Extension work (from dependable sources)

COVID worksheets

Mark Levesley (UK-based science education writer, with an interest in Key Stage 3 and GCSE science education) has put together a series of KS3/4 worksheets (designed for more independent study at home) with a COVID-19 theme.
These cover several related subjects, all with a similar theme to some of my most recent topics.
First few: Lungs - Respiration - Ventilation - Surface area - Diffusion - and loads more
The links download as pdfs.
Each comes as a 2-page worksheet, as well as a third page with notes for home educators, including answers!
In fact Mark has been adding to these at a cracking rate so I have been having problems keeping up with the total - but they are now all available from one page - link below.
COVID-19 linked series of worksheets Mark Levesley

Practical activities

Core practical 9: Investigate factors affecting the rate of aerobic respiration using a respirometer from a Biology Blog
Measuring respiratory quotient from the Royal Society of Biology

Web references

The Number of Alveoli in the Human Lung image analysis of 6 untransplanted lungs

Role of pulmonary surfactant components in surface film formation and dynamics

Pulmonary surfactant in health and human lung diseases: state of the art.

Oxygen - Solubility in Fresh Water and Seawater - from Engineering ToolBox

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