Gas exchange in single-celled organisms

A single-celled organism exchanges gases directly with its surroundings across its cell surface membrane. The gases move by diffusion: their particles move from a region of higher concentration to a region of lower concentration.

Oxygen diffuses into the cell when its concentration is higher outside than inside. Carbon dioxide diffuses out when its concentration is higher inside than outside. Each gas therefore moves down its own concentration gradient.

Why the cell surface is sufficient

A single-celled organism has a large surface area relative to its volume. This means there is plenty of membrane surface available for gas exchange compared with the volume of the cell that needs gases exchanged.

The cell is also small, so gases have only short distances to diffuse between the membrane and the parts of the cell where they are used or produced. Together, the large relative surface area and short diffusion distances allow exchange across the cell surface to meet the organism’s needs without a specialised gas exchange system.

Insect tracheal gas exchange

Insects exchange gases through a network of air-filled tubes that reaches throughout the body. Air enters and leaves through openings called spiracles. Each spiracle opens into a larger tube called a trachea, which branches into much finer tubes called tracheoles.

The tracheoles reach close to respiring cells. Their large number provides a large surface area for gas exchange, while their proximity to cells gives gases a short diffusion path.

Gas movement in the tracheal system

Oxygen diffuses from the air in the tracheoles towards respiring cells, where it is used in respiration. Carbon dioxide produced by respiration diffuses in the opposite direction, from the cells into the tracheoles, and can leave through the spiracles.

Body movements can help ventilate the tracheal system by moving air in and out. This helps maintain the concentration gradients needed for diffusion: oxygen remains more concentrated in the air near the cells, while carbon dioxide remains more concentrated in the cells.

Spiracles and water loss

Spiracles can close, reducing the loss of water from the insect's body. However, closed spiracles also restrict the entry of oxygen and the removal of carbon dioxide. The insect therefore faces a compromise between conserving water and exchanging gases.

Fish gill gas exchange

Fish gills exchange gases between water and blood by diffusion. Oxygen diffuses from the water into the blood, while carbon dioxide diffuses from the blood into the water.

Gill filaments and lamellae

The gills contain gill filaments, each carrying many thin structures called lamellae. Water flows over the lamellae, and blood flows through them. The many lamellae provide a large surface area for gas exchange. Their thin exchange surfaces also give gases a short distance to diffuse between the water and the blood.

Counter-current flow in the gills

Water and blood flow in opposite directions along the lamellae. This is called a counter-current arrangement.

As water flows over a lamella, it loses oxygen to the blood. However, because blood flows in the opposite direction, the water at each point meets blood with a lower oxygen concentration than its own. This maintains a difference in oxygen concentration along the exchange surface, so oxygen continues to diffuse from water into blood.

Gas exchange in dicotyledonous leaves

Gases enter and leave a dicotyledonous leaf mainly through small pores called stomata in its outer layer, the epidermis. Inside the leaf, interconnected air spaces run between the mesophyll cells. These spaces provide a route for gases to diffuse between the stomata and the cells.

The air spaces expose a large area of moist mesophyll cell walls to air. Gases dissolve in this moisture before diffusing into cells, or diffuse out of cells into the air spaces. The arrangement also keeps the diffusion distance between the air spaces and mesophyll cells short.

Gas movement during photosynthesis and respiration

Mesophyll cells use carbon dioxide in photosynthesis. Carbon dioxide can therefore diffuse from the outside air, through stomata and air spaces, into the cells. Oxygen produced by photosynthesis can diffuse out along the reverse route.

Cells also carry out respiration, which uses oxygen and produces carbon dioxide. Both processes affect the direction of net gas movement. For example, when photosynthesis uses carbon dioxide faster than respiration produces it, there is net movement of carbon dioxide into the leaf. When respiration produces carbon dioxide faster than photosynthesis uses it, there is net movement out.

Stomata and water loss

Open stomata allow gases to exchange with the outside air, but they also provide a route for water vapour to diffuse out of the leaf. Gas exchange through open stomata is therefore accompanied by water loss.

Gas exchange and water conservation in xerophytes

Xerophytes are plants adapted to dry conditions. Their leaves must retain water while allowing some carbon dioxide to enter for photosynthesis. Water vapour can escape through the same stomata that allow carbon dioxide to enter, so gas exchange creates a risk of water loss.

Reducing water loss from xerophyte leaves

A thick waxy cuticle forms a barrier over the leaf surface, reducing water loss from that surface.

Other adaptations reduce water loss through stomata by retaining humid air around them:

• Sunken stomata lie in pits where water vapour can accumulate.
• Leaf hairs trap a layer of air close to the leaf surface.
• Rolled leaves enclose air around stomata on the inner surface.

The humid air has a higher water vapour concentration than dry outside air. This reduces the difference in water vapour concentration between the inside of the leaf and the air near the stomata, so water vapour diffuses out more slowly.

Stomatal closure and the gas exchange compromise

Closing stomata reduces water loss, but it also restricts the entry of carbon dioxide. If less carbon dioxide enters the leaf, the rate of photosynthesis can be limited.

Xerophyte adaptations therefore balance water conservation against gas exchange. They reduce water loss while retaining some capacity for carbon dioxide uptake; they do not completely separate gas exchange from water loss.

Human gas exchange system structure

The human gas exchange system has two lungs connected to the outside by a system of branching airways. These airways provide routes for air to reach the gas exchange surfaces throughout the lungs.

The trachea is the main airway leading towards the lungs. It divides into two bronchi, one entering each lung. Inside the lungs, the bronchi branch into progressively narrower airways called bronchioles.

The bronchioles lead to small air-filled structures called alveoli. At the alveoli, gases are exchanged between the air and the blood. The branching airways distribute air through the lungs so that it can reach these exchange surfaces.

Alveolar gas exchange

Gas exchange takes place between the air in the alveoli and the blood in the capillaries surrounding them. An alveolus is a tiny air sac in the lungs. Gases cross the alveolar and capillary walls by diffusion: their net movement is from a region of higher concentration to a region of lower concentration.

Oxygen is at a higher concentration in alveolar air than in the blood arriving at the alveoli, so it diffuses into the blood. Carbon dioxide is at a higher concentration in the arriving blood than in alveolar air, so it diffuses in the opposite direction, into the alveoli.

Alveolar adaptations for gas exchange

The lungs contain numerous alveoli, providing a large surface area across which gases can diffuse. The alveolar epithelium is very thin, and the capillary walls are also thin. This gives gases a short diffusion distance between air and blood. The alveolar surface is moist, allowing gases to dissolve before diffusing.

A rich capillary supply continually brings blood to the alveoli and carries it away. This helps maintain the concentration differences needed for oxygen and carbon dioxide to diffuse. Ventilation also maintains these gradients: fresh air replenishes oxygen in the alveoli, while air containing carbon dioxide is breathed out.

Human ventilation mechanics

Ventilation moves air into and out of the lungs. Muscles change the volume of the thorax, the part of the body containing the lungs. This changes the pressure inside the lungs relative to atmospheric pressure: air moves into the lungs when their pressure is lower and out when their pressure is higher.

Inspiration

During inspiration, the diaphragm contracts and flattens. At the same time, the external intercostal muscles between the ribs contract, raising the ribs. These movements increase the volume of the thorax, allowing the lungs to expand.

As the lungs expand, the pressure inside them falls below atmospheric pressure. Air therefore enters the lungs.

Expiration

During expiration, the diaphragm and external intercostal muscles relax. The diaphragm becomes dome-shaped again and the ribs move down. Thoracic volume decreases, and pressure inside the lungs rises above atmospheric pressure, driving air out.

Expiration at rest is largely passive: it occurs as these muscles relax rather than through an active contraction to force air out. During active expiration, the internal intercostal muscles contract and help lower the ribs. They act antagonistically to the external intercostal muscles: the internal intercostals help lower the ribs, while the external intercostals raise them.

Interpreting lung disease effects on gas exchange and ventilation

Lung disease can affect breathing by changing how air moves, how gases diffuse between the alveoli and the blood, or both. To interpret its effects, connect the observed change to the movement of oxygen or carbon dioxide.

Ventilation is the movement of air into and out of the lungs. Gas exchange is the diffusion of oxygen and carbon dioxide between the air in the alveoli and the blood. These are different processes, but ventilation helps gas exchange by continually refreshing alveolar air.

Reduced airflow and concentration gradients

If disease narrows an airway, less air may move into or out of the alveoli. Reduced ventilation means that alveolar air is refreshed less effectively: oxygen is replaced more slowly, and carbon dioxide is removed more slowly.

As a result, the difference in oxygen concentration between alveolar air and the blood may become smaller. This reduces the rate at which oxygen diffuses into the blood. A smaller difference in carbon dioxide concentration between the blood and alveolar air can likewise reduce diffusion of carbon dioxide into the alveoli.

Changes to the gas-exchange surface

Evidence about the alveolar exchange surface can also explain changes in gas exchange:

• Reduced surface area: if less surface is available for diffusion, less gas can cross between the alveoli and blood in a given time.
• Increased diffusion distance: if the barrier between alveolar air and blood becomes thicker, gases have farther to diffuse, reducing the rate of exchange.

When interpreting evidence, identify whether it shows a change in airflow, surface area, diffusion distance or a concentration gradient. Then explain how that change affects oxygen uptake into the blood or carbon dioxide removal from it. A change in ventilation can affect gas exchange, but movement of air should not be described as diffusion between the alveoli and blood.

Lung disease risk-factor data

Data on new cases of lung disease can show whether people with different exposures have different risks of developing disease. For example, researchers might compare incidence among people who smoke and those who do not, or among populations exposed to different levels of pollution.

Incidence concerns new cases arising in a defined population over a stated period. It is different from the total number of people living with the disease, which includes both existing and new cases.

Comparing lung disease incidence

A larger number of new cases does not necessarily mean a greater risk: a larger population might have more cases simply because it contains more people. Comparisons must take account of the population sizes and the periods over which cases were recorded.

The populations and exposure measures must also be comparable. For example, a comparison involving smoking needs to consider how smoking was measured, such as whether people smoked at all or how much they smoked. Pollution comparisons likewise depend on how exposure was measured. Differences in these measures can affect what a comparison shows.

Association and causation

If higher exposure tends to occur alongside higher lung disease incidence, the variables are correlated or associated. This pattern alone does not establish that the exposure caused the disease.

Another factor may influence both exposure and disease incidence. Such a factor is a confounder. For example, if populations with different pollution exposures also differ in their smoking habits, smoking could influence the difference in lung disease incidence. The observed difference cannot automatically be attributed to pollution.

A causal interpretation therefore depends on more than finding a correlation. Researchers consider the study design, including how exposure and disease were measured and whether possible confounders were accounted for. They also consider the strength of the association and whether the findings are consistent across studies. Evidence from well-designed studies that address confounding and produce consistent findings provides a stronger basis for judging causation.

Evidence for lung disease risk-factor restrictions

Experimental data can help decision-makers judge whether a source of exposure is likely to cause lung disease and whether restricting it could reduce that risk. A statutory restriction is a measure imposed by law, such as a legal limit on exposure to a harmful substance.

Evaluating the evidence

First, identify what the data actually show: who or what was exposed, how much exposure occurred, and what signs of harm were measured. A comparison between exposed and less-exposed groups can show an association: a difference in exposure occurs alongside a difference in harm.

An association alone does not prove that exposure caused the harm. The groups may also differ in other ways that affect lung disease risk. Evidence for a causal link is stronger when the study controls these other differences, exposure occurs before the harm, and the findings are consistent across well-conducted studies.

The quality and limitations of the data also matter. For example, a small sample, inaccurate measurements or an unsuitable comparison group can make a result less reliable. Findings from an experiment may not apply exactly to people exposed at different levels or for different lengths of time.

Assessing a restriction

The proposed or adopted restriction should target a source of exposure linked to harm. Assessing it means asking how much exposure the measure is expected to prevent and how strongly the evidence supports the expectation that this reduction will lower disease risk.

For example, if evidence links inhaled workplace dust to lung damage, a legal limit on that dust could reduce workers’ exposure. Its expected effect depends on how much exposure the limit prevents and on the strength of the evidence that the dust causes the damage. Risk may not disappear if exposure continues from other sources or is not eliminated by the restriction.