Exchange Surfaces

What are exchange surfaces in biology?

Exchange surfaces are essential for all organisms, as without them life would not be possible: organisms would die from lack of nutrients or excess of waste products, and an organism would not be able to communicate with its environment fully.

Characteristics of Gaseous Exchange Surfaces

Exchange surfaces exchange gases, water, nutrients from food and many other substances, and they need to be able to do this as efficiently as possible. This means that they must share a set of features that allow the rapid movement of substances into and out of the cell or the body. These shared characteristics of gaseous exchange surfaces include:

• Moisture
• Good blood supply
• Large surface area
• Short transport distance for substances to move across
• Refreshing the material substances are being exchanged with (air, food, etc.).

We will look over these in more detail in the following paragraphs.

Moisture in Exchange Surfaces

By keeping the exchange surfaces moist, the exchange rate is significantly increased, especially when the substance being exchanged is a gas, such as in the lungs. By dissolving substances in water, they may more readily diffuse across the barrier of an exchange surface than when they are in their gaseous form. This applies to most exchange surfaces, however, the source of moisture might be different in each exchange surface: in exchange surfaces such as the GI tract, the substance (i.e. food) generally brings its own moisture, but the lungs have their own moisture production.

Heavy Blood Supply in Exchange Surfaces

As you may already know, the passive transport of substances relies on the presence of a diffusion gradient, or, in the case of water, an osmotic gradient or a difference in water potential. This is achieved using two methods: a heavy blood supply and constantly refreshing the material substances are being exchanged with. For optimal exchange, both methods are combined.

By making sure exchange surfaces have a heavy blood supply, substances absorbed are readily removed, and fresh blood with no absorbed substances present is supplied in its place, maintaining a higher concentration in the material substances are being absorbed from.

This also serves to function the other way. When waste substances such as carbon dioxide (CO₂) are being excreted from the body through the exchange surface, the blood supply brings fresh waste materials to the exchange surface, ensuring that a concentration gradient which guarantees the flow of waste out of the body is maintained.

Large Surface Areas in Exchange Surfaces

Exchange surfaces generally have a large surface area related to the volume of the organism. This is generally achieved either by the organism being small, such as in single-celled organisms, where the surface area of the cell membrane is sufficient to allow enough exchange, or through specialised structures in exchange systems, such as the alveoli of the lungs and villi of the GI tract.

By increasing the surface area, there are more opportunities for things to be exchanged. You can think of it as every square centimetre (cm²) of exchange surface being able to exchange a limited amount per minute. Let's say that each square centimetre of lung tissue can exchange 20 molecules of oxygen per minute. If the lung had a surface of 10 cm², it could exchange 200 molecules of oxygen per minute. But if it were 20 cm² it would be able to exchange 400 molecules of oxygen per minute.

By increasing the surface area, you increase the number of square centimetres able to exchange a limited amount of molecules every minute, speeding the exchange. This exchange can occur using either passive or active transport.

Short Transport Distance in Exchange Surfaces

By minimising the distance substances have to travel during an exchange, the rate of exchange is increased, as the particles have to travel less far. In other words, they would pass to the opposite side of the exchange, for example the blood, sooner, and thus be transported away from the exchange spot faster. This helps keep the exchange going at a fast rate.

Refreshing the Exchange Substance

Refreshing the exchange substance refers to constantly renewing the substance from which an organism is obtaining nutrients, or to which an organism is excreting waste. This aids in keeping the concentration gradients different and steep enough to ensure fast exchange at the exchange surfaces. In the lungs for example you constantly breathe, refreshing the gases in the lungs, removing CO₂ and maintaining a concentration gradient.

When we are excreting waste into a substance through an exchange surface, we obviously want to maintain the gradients as steep as possible, to remove the substances from the body as fast as possible. It's also extremely important to keep absorbing nutrients at a high enough rate to ensure every part of an organism gets enough nutrition to survive.

This also impacts substances being taken in by the body. If you simply leave material next to an exchange surface, it will eventually begin to run out of the substances being absorbed. By refreshing the material, you bring a constant supply of new nutrients ready to be absorbed. This is shown with breathing constantly bringing fresh oxygen into the lungs, or peristalsis moving food through the GI tract.

Exchange Systems

As described above, some single or very simple multicellular organisms may rely purely on exposure to the environment for some or all of their exchange needs. Once organisms get larger, this stops being the case, and specific exchange surfaces become necessary to ensure organisms can take in sufficient nutrition and excrete enough waste.

The square-cube law describes the relationship between an organism's size and its surface area. It is this law that creates an upper limit on the size of a single cell, unless it is adapted to have a large surface area relative to its total size, such as by being very long and thin, with multiple nuclei or by being surrounded by support cells like nerve cells are.

The largest single cell is an ostrich egg, which runs about 18x18 cm. Single-celled organisms such as Caulperla taxifolia, a type of algae, can get bigger in some dimensions; however, they have several adaptations to allow them to reach this size, which create exchange surfaces and also possess multiple nuclei.

The results of the square-cube law can be shown as the surface area to volume ratio. If the surface area to volume ratio gets too low, the organism must develop exchange systems to allow for efficient substance exchange or risk running out of nutrients or being poisoned by its own waste.

Exchange Surface Adaptations

Efficient exchange surfaces are of extreme importance for the survival of each organism. Thus, exchange surfaces have evolved and adapted to optimise exchanges in the different conditions and situations where they are needed. We will see two examples of this: the gills and the stomata.

Fish Gill Adaptations as Exchange Surfaces

Absorbing oxygen from water is harder than absorbing it from the air, as there is a lower percentage of molecular oxygen dissolved in water than in the air that land animals breathe. However, fish (and other aquatic animals) have developed gills to optimise the process. Gills are especially suited for gas exchange in water, as you can see in the table below.

Table 1. Fish gill adaptations to increase the rate and efficiency of gas exchange.

Fig. 2. Fish gill structure. Notice how there are many filaments per gill, and many lamellae per filament, increasing the exchange surface compared to a single, straight exchange surface.

Stomata Adaptations as Exchange Surfaces

Plants also have gas exchange surfaces that have evolved to optimise the release or absorption of oxygen and carbon dioxide: stomata.

Fig. 3. Plant stomata are the big red structures you can observe in the image. Source: Flickr.

What are the adaptations that stomata have to make gas exchange in the leaves work? The major issue for plants is that by exchanging gases with the environment they also increase the loss of water. Think about it: the leaves are the parts of the cell the sun beats down on the most. Therefore, stomata have two adaptations to reduce water loss:

• Stomata are located on the lower surface of the leaves so that the sun is not directly hitting on the pores through which gases and water will be lost.
• Stomata can be opened and closed. The aperture of the stomata is regulated by the amount of water the plant has. If it has plenty of water, the stomata can remain open for a longer period of time, but when the plant loses a lot of water, the stomata close to reduce water loss.

Exchange Surfaces in the Human Body

An example of an organism whose surface area to volume ratio is too low to rely simply on exchange with the environment is you and me (remember the square-cube law!). Humans have several systems designed to maximise the exchange of substances into and out of the body, including the lungs and gastrointestinal (GI) tract.

The Lungs as Exchange Surfaces in the Human Body

The lungs are adapted to support gas exchange in a number of ways, each of which adheres to the principles described above:

• Moist surfaces are provided by the release of a small amount of liquid into the interior of the lungs, which contains a substance known as surfactant. This is made up of proteins and lipids, which reduce surface tension in the lungs. This aids the lungs in expanding, limits the pull of liquids from the capillaries into the lungs, moistens the air we breathe and contributes to innate immunity. This allows gases to dissolve, meaning they can more effectively diffuse into the body.
• A good blood supply is provided by many capillaries surrounding each alveolus, which rapidly exchange oxygen and carbon dioxide with the air in the lungs, before carrying them elsewhere around the body, bringing fresh deoxygenated blood loaded with carbon dioxide ready to exchange more.
• The large surface area is provided by the structure of the lungs. The airways gradually branch into sets of smaller and smaller tubes, going from the trachea, into the descending tiers of bronchi and then bronchioles where the air eventually flows into sacs known as alveoli. These small, expandable sacs are wrapped by capillaries, and provide a surface area of ~50-70 square metres.
• The short diffusion distance is provided by the thin nature of the epithelial cells lining the alveoli, along with the single-celled wall of the capillary. This means that oxygen and carbon dioxide must only move across two cells before they enter the bloodstream and are carried away.
• Continuous refreshing of the material being exchanged is provided by constantly breathing, removing air filled with carbon dioxide and replacing it with oxygen-filled fresh air.

Fig. 2. The alveoli, the main exchange surface of the lungs

The Gastrointestinal Tract as Exchange Surface in the Human Body

The GI tract absorbs nutrients and water from food and liquids we consume. Like the lungs, it has several adaptations to do this:

• Moist surfaces are provided by the liquid we consume being mixed with food, along with the release of mucus and other moisture-filled substances into the GI tract.
• Like in the lungs, a good blood supply is provided by capillaries running up into each villus within the intestine, along with the regular circular folds within the intestines.
• A large surface area is provided by the villi, small finger-like projections into the lumen of the small intestine, along with the circular folds that the walls of the intestine possess. The intestines are also incredibly long, meaning the transit time of material through them is extended, increasing the surface area material is exposed to.
• Short diffusion distances are provided by the capillaries and lacteals, which extend into the length of each villus, meaning that, like in the alveoli, substances must only cross two cells to enter the bloodstream.
• Food moves along the GI tract, meaning that fresh material is constantly being brought in with fresh supplies of nutrients and water.

Fig. 3 - The villi of the small intestine, which provides the bulk of the surface area of the GI tract

The GI tract, unlike the lungs, also uses active transport to ensure it can absorb substances regardless of concentration gradients, maximising the efficiency of nutrient extraction from resources we consume.