Gas exchange

In a nutshell

Every organism has to take in oxygen and remove carbon dioxide, and the bigger and more active it is, the harder that becomes.

This subtopic is about the gas exchange surfaces of different organisms (a single-celled organism, an insect, a fish, a leaf and the human lung), the trade-off between efficient exchange and water loss on land, and how we ventilate our lungs.

Assumed knowledge: Surface area to volume ratio, Transport across cell membranes.

Core content

What makes a good gas exchange surface

Almost every mark in this topic comes back to the same short list. A surface is good at gas exchange when it has:

• a large surface area, so more diffusion happens at once,
• a thin exchange surface (a short diffusion pathway), so gases cross quickly,
• a steep concentration (diffusion) gradient that is maintained, usually by a good blood supply and by ventilation,
• a permeable surface that gases can dissolve in and cross.

Learn these four as a set. For any organism the exam question is really "name the feature, then say which of these it provides, and how".

Gas exchange in a single-celled organism

A single-celled organism (such as an Amoeba) exchanges gases straight across its body surface (its cell-surface membrane) by diffusion. It needs no specialised system.

• It has a large surface area to volume ratio, so its surface can supply the whole small volume.
• The diffusion distance to any point inside is very short.
• Its low metabolic demand is met by diffusion alone.

Gas exchange in an insect: the tracheal system

Insects have a tracheal system: a network of air-filled tubes that delivers oxygen directly to the respiring cells, so insects do not use blood to transport oxygen.

The parts, from outside in:

• Spiracles: pores in the exoskeleton that open and close; the way air enters and leaves, and the main site of water loss.
• Tracheae: large tubes carrying air into the body.
• Tracheoles: fine, highly branched tubes that reach individual cells; the actual gas exchange surface.

How the system stays efficient:

• Large surface area: there are many highly branched tracheoles, giving a large surface area for diffusion.
• Short diffusion distance: tracheoles are thin-walled and run directly to the cells.
• Steep, maintained gradient: respiring cells use oxygen and make carbon dioxide, keeping oxygen low and carbon dioxide high in the tissue, so gases diffuse down their gradients continuously.
• Ventilation in active insects: abdominal pumping (rhythmic body movements) changes the pressure in the tracheae, drawing air in and forcing it out, which maintains the gradient.
• Fluid at the tracheole ends: during activity, lactate builds up in the muscles, water moves out of the tracheole tips into the tissues, and air is drawn further along the tracheoles. Gases now diffuse through air rather than water (faster) and over a larger exposed surface, speeding up exchange.

Gas exchange across the gills of a fish

Water holds far less oxygen than air, so fish gills are built to extract as much as possible.

The gill structure gives the first two features:

• Each gill has many gill filaments, and each filament carries many thin gill lamellae: together a very large surface area.
• The lamellae have a thin epithelium and many capillaries: a short diffusion pathway and a good blood supply.

The gradient is kept steep by the counter-current principle:

• Blood and water flow in opposite directions across the lamella.
• This means water with a higher oxygen concentration always meets blood with a lower oxygen concentration, so a diffusion gradient is maintained along the whole length of the gill.
• Blood therefore keeps taking up oxygen right along the lamella (equilibrium is never reached).

The water line stays above the blood line at every point, so oxygen keeps diffusing from water to blood the whole way along. Blood can leave the gill about 80% saturated.

Still don't get it? · why counter-current works and parallel flow does not

Picture two moving walkways side by side carrying buckets. On one walkway the buckets are full of water (oxygen); on the other they are empty (blood). At every point where a full bucket passes an empty one, water tips across into the empty bucket.

If the two walkways move in the SAME direction (parallel flow), a full bucket and an empty bucket start together, water tips across until both are half full, and then nothing more happens: they have reached equilibrium and travel the rest of the way as a useless pair.

If the walkways move in OPPOSITE directions (counter-current), an empty bucket that is nearly finished meets fresh full buckets just arriving, and a full bucket that has already given a lot away meets buckets that are still fairly empty. So at every point along the line the full bucket is a bit fuller than the empty one beside it, and water keeps tipping across the whole way.

Here is the same idea as a graph. In parallel flow the two lines meet and flatten: the gradient collapses at equilibrium and the blood only reaches about half saturation.

The exam version: in counter-current flow blood and water flow in opposite directions, so a concentration (diffusion) gradient is maintained along the whole length of the gill, and the blood becomes far more highly saturated with oxygen than parallel flow would allow. The mark is in the word "maintained" and the phrase "whole length", not just "opposite directions".

Fish also ventilate their gills to keep fresh water flowing over them: lowering the floor of the buccal cavity (mouth) draws water in, then raising it forces water out over the gills and past the operculum (gill cover), giving a continuous one-way flow.

Gas exchange in a leaf (dicotyledonous plant)

Leaves exchange gases with the air for both photosynthesis and respiration. Gases move in and out through the stomata by diffusion; the leaf has no specialised transport system for gases.

• Stomata (pores mainly in the lower epidermis), each controlled by two guard cells, let carbon dioxide diffuse in and oxygen diffuse out. They can close to limit water loss.
• The spongy mesophyll has interconnecting air spaces, giving a large surface area for gases to reach the cells and letting gases diffuse freely inside the leaf.
• The leaf is thin and flat, giving a short diffusion distance and a large surface area.

A leaf does not need a transport system for gases because no cell is far from the air, gases diffuse quickly in air, and a plant's metabolic rate (and so its gas demand) is low.

The compromise: efficient gas exchange versus water loss

Here is the catch for organisms on land. The very features that make a surface good at gas exchange, being thin, permeable and large in area, also make it good at losing water. Terrestrial organisms cannot have one without risking the other, so they compromise.

Still don't get it? · why a good gas exchange surface always loses water

Think of the exchange surface as an open window. A wide-open window lets plenty of fresh air (oxygen) in, which is exactly what you want. But the same open window also lets the warmth (water) escape. You cannot open it for the air without also losing heat.

Step by step: gas exchange needs a thin, moist, permeable surface with a large area and a gradient, so gases move across fast. Water evaporates from that same moist surface and diffuses out down its own water potential gradient, and a big, thin, permeable surface loses it fast. So every land organism sits on a see-saw: tip toward better gas exchange (open the window) and you lose more water; tip toward saving water (close it) and gas exchange slows.

The exam version: land organisms show structural and functional compromises between the need for efficient gas exchange and the need to limit water loss. A good answer names a feature, says whether it helps gas exchange or saves water, and, for water-saving features, says it reduces the water potential gradient or reduces the surface area for evaporation.

Terrestrial insects limit water loss by:

• closing the spiracles when gas exchange is not needed (valves shut the pores),
• hairs around the spiracles that trap water vapour, reducing the water potential gradient,
• a small surface area to volume ratio and a waterproof, waxy exoskeleton.

Xerophytes are plants adapted to dry or windy habitats (for example cacti and marram grass). Their leaf adaptations all either reduce the surface area for evaporation or reduce the water potential gradient that drives it:

The human gas exchange system

Air reaches the exchange surface through a branching set of airways.

• Trachea: the main airway. It is held open by C-shaped rings of cartilage and lined with ciliated epithelium and goblet cells (mucus traps particles; cilia sweep it up).
• Bronchi: two branches, one to each lung, also supported by cartilage.
• Bronchioles: finer branches with no cartilage, leading to the alveoli.
• Alveoli: tiny air sacs, the actual gas exchange surface.
• Lungs: the pair of organs the airways and alveoli sit inside.

The alveolar epithelium as an exchange surface

Oxygen diffuses across the alveolar epithelium and across the capillary endothelium into the blood; carbon dioxide diffuses the opposite way. Both move by simple diffusion, down a concentration gradient.

The alveolar epithelium is a good exchange surface because it is:

• thin: the alveolar epithelium is one cell thick and the cells are flattened, giving a short diffusion pathway,
• large in area: there are millions of alveoli, giving a very large total surface area,
• well supplied with blood: a dense capillary network carries oxygenated blood away and brings deoxygenated blood in, so the concentration gradient stays steep,
• ventilated: breathing constantly refreshes the air in the alveoli, which also maintains the gradient.

Be precise: it is the alveolar epithelium (the wall of the alveolus) that is one cell thick, not "the alveolus" and not "a membrane".

Ventilation: the mechanism of breathing

Ventilation moves air in and out of the lungs. It works by changing the volume of the thoracic cavity, which changes the pressure inside, so air flows down a pressure gradient. The diaphragm and the antagonistic external and internal intercostal muscles drive these changes.

The external and internal intercostal muscles are antagonistic: when the external set contracts (breathing in) the internal set relaxes, and when the internal set contracts (forced breathing out) the external set relaxes.

Pulmonary ventilation rate

Pulmonary ventilation rate (PVR) is the total volume of air moved into (or out of) the lungs per minute. It is found from:

$PVR = tidal\ volume \times breathing\ rate$

• Tidal volume is the volume of air breathed in or out in one normal breath (in dm³).
• Breathing rate is the number of breaths per minute.
• PVR comes out in dm³ min^-1 (worked in full below).

Lung disease, risk factors and interpreting data

You are expected to interpret data on how lung disease, smoking and pollution affect gas exchange and ventilation, and to judge risk factors and correlations, not to recall named diseases in detail.

Lung diseases reduce gas exchange or ventilation by attacking the features above, for example:

• thickening the diffusion pathway (fibrosis lays down scar tissue), so gases cross more slowly,
• reducing the surface area for gas exchange (alveolar walls break down), so less exchange happens,
• reducing the elasticity of the lung tissue, so less air is forced out,
• narrowing the airways (inflammation or extra mucus), so airflow falls.

Two ideas the exam keeps testing:

• A risk factor is anything that increases the chance (probability) of getting a disease. It does not, on its own, mean the factor causes the disease.
• A correlation (two variables changing together) does not by itself prove causation. To argue causation you need a plausible mechanism and to rule out other factors; to argue significance you look at whether error bars or standard deviations overlap.

Worked examples

Calculation: pulmonary ventilation rate (and rearranging it).

A person has a tidal volume of 0.45 dm³ and takes 15 breaths per minute. Find their pulmonary ventilation rate.

$PVR = tidal\ volume \times breathing\ rate$ $PVR = 0.45 \times 15 = 6.75 \text{ dm}^3\ \text{min}^{-1}$

Now suppose you are told the PVR is 7.2 dm³ min^-1 and the breathing rate is 16 breaths per minute, and asked for the tidal volume. Rearrange the same equation:

$tidal\ volume = \frac{PVR}{breathing\ rate} = \frac{7.2}{16} = 0.45 \text{ dm}^3$

Always carry the units through: the volume marks and the unit mark are separate, so an answer without dm³ min^-1 can drop a mark even when the number is right.

Model 4-mark answer: "Explain how a fish gill is adapted for efficient gas exchange."

A "how is it adapted" answer needs a feature plus what it does, one linked pair per mark:

1. Many gill filaments and lamellae give a large surface area for diffusion.
2. The lamellae have a thin epithelium, giving a short diffusion pathway.
3. Blood and water flow in opposite directions (counter-current).
4. This maintains the concentration gradient along the whole length of the gill, so blood keeps absorbing oxygen (equilibrium is not reached).

Notice that naming "counter-current" earns nothing on its own. The mark is the consequence: the gradient is maintained along the whole length of the gill.

Common exam mistakes

• Giving "a thin membrane", "one-cell-thick membrane" or "thin lining" as a standalone feature of an alveolus. The credited point is a reduced diffusion pathway, and it is the alveolar epithelium (one cell thick, flattened) that is thin, not "a membrane".
• Naming a feature without explaining it. "Large surface area" or "highly branched tracheoles" alone is not enough; you must link the feature to faster diffusion or a shorter pathway.
• Saying the insect spiracles are an adaptation for efficient gas exchange. Spiracles are for limiting water loss; they are not credited as a feature that improves exchange.
• Describing the counter-current as just "opposite directions". The mark needs the gradient to be maintained along the whole length of the gill, so equilibrium is never reached.
• Writing that gases diffuse "across", "along" or "to" the gradient. Diffusion is down a concentration gradient.
• Saying air enters the alveoli "by diffusion". Air moves in and out of the lungs by mass flow, down a pressure gradient; it is not "sucked" in and it does not move by diffusion. Only the gas exchange in the alveoli is diffusion.
• Confusing the muscles or the diaphragm. The external intercostals contract for breathing in; the internal intercostals contract for forced breathing out. The diaphragm contracts to flatten (breathing in) and relaxes to dome up (breathing out): state the contraction or relaxation, not just the shape.
• Using "water concentration" instead of water potential for osmosis and xerophytes, which is a lower-level answer and is not credited.
• For xerophytes, saying hairs "trap water" rather than water vapour, or leaving out the water potential gradient. Sunken stomata, hairs and rolled leaves all work by reducing the water potential gradient.
• Claiming small or needle-shaped leaves "reduce the surface area to volume ratio". Spines and needles reduce the surface area for water loss.
• For a single-celled organism, writing "large surface area" instead of a large surface area to volume ratio.
• Defining a risk factor as "a factor that affects risk", or answering data questions with stock lines ("no repeats", "sample too small", "we do not know"). A risk factor increases the chance of getting a disease, and data answers must use the actual data provided.

Key definitions

• Counter-current principle: blood and water flow in opposite directions across the gill, so a concentration (diffusion) gradient for oxygen is maintained along the whole length of the gill.
• Ventilation: the movement of air into and out of the lungs.
• Tidal volume: the volume of air breathed in or out in one (normal) breath.
• Pulmonary ventilation rate: the total volume of air moved into the lungs per minute; pulmonary ventilation rate = tidal volume × breathing rate.
• Antagonistic muscles: a pair of muscles in which the contraction of one is accompanied by the relaxation of the other, so they produce opposite effects.
• Risk factor: something that increases the chance (probability) of getting a disease.
• Correlation: an association between two variables, which does not on its own show that one causes the other.

Specification

• I can explain gas exchange across the body surface of a single-celled organism.
• I can describe the insect tracheal system (tracheae, tracheoles and spiracles) and explain how it is adapted for efficient gas exchange.
• I can describe gas exchange across fish gills (gill lamellae and filaments) and explain the counter-current principle.
• I can explain gas exchange by the leaves of dicotyledonous plants, including the roles of the mesophyll and stomata.
• I can explain the structural and functional compromises between efficient gas exchange and limiting water loss in terrestrial insects and in xerophytic plants.
• I can describe the gross structure of the human gas exchange system: the alveoli, bronchioles, bronchi, trachea and lungs.
• I can describe the essential features of the alveolar epithelium as a gas exchange surface.
• I can explain the mechanism of breathing, including the role of the diaphragm and the antagonistic external and internal intercostal muscles in changing the pressure in the thoracic cavity.
• I can calculate pulmonary ventilation rate from tidal volume and breathing rate, and rearrange the equation.
• I can interpret data on the effects of lung disease, pollution and smoking on gas exchange and ventilation, and recognise the difference between correlation and causation.

Related notes

• Surface area to volume ratio
• Digestion and absorption
• Mass transport
• Transport across cell membranes