Specialised exchange surfaces

​​In a nutshell

All organisms need to exchange substances with their environment to survive. The surface area to volume ratio of an organism affects how easy it is for the organism to exchange things. In particular, multicellular organisms need special exchange surfaces. Two examples of exchange surfaces in mammals are the alveoli in the lungs and the villi in the small intestine.

Exchange of substances

For all organisms to function properly and survive, they need to take in important substances from the environment and get rid of any waste products.

Examples

• Oxygen is needed by cells for aerobic respiration. 
• Carbon dioxide is a waste product of this process. Both gases move between the environment and the cell by diffusion.
  
• Cells take up water by a process called osmosis. In animals, dissolved food molecules and minerals diffuse into a cell with water.
• Urea is a waste product of protein breakdown. It diffuses from the cells into the bloodstream for removal by the kidneys.

Surface area to volume ratios

How easy it is for an organism to exchange substances with its environment varies and depends largely on the organism's surface area to volume ratio. Usually, the larger an organism is, the smaller its surface area is in comparison to its volume.

procedure

Example

A cat can be represented by a $$1\space cm \times 1\space cm \times 2\space cm$$​ block. What is the surface area to volume ratio of the cat?

Calculate the total surface area:

$$(1 \times 1) \times 2+ \space (1\times 2) \times 4= 10\space cm^2$$

Calculate the volume of the cat:

$$1 \times1\times2= 2\space cm^3$$​​

Calculate the surface area to volume ratio:

 $$10:2=5:1$$​

​$$\underline{Therefore,\ the\ surface\ area\ to\ volume\ ratio\ of\ the\ cat\ is\ 5:1}$$​​

Exchange surfaces

Unicellular vs multicellular organisms

The special exchange surfaces in multicellular organisms are adapted in different ways to maximise effectiveness.

Alveoli in the lungs

In mammals, gas exchange takes place in the alveoli (little air sacs) in the lungs. Blood that returns to the lungs from the rest of the body contains lots of carbon dioxide (a waste product of respiration) and a small amount of oxygen. This leads to a greater concentration gradient of these two gases, resulting in faster diffusion. 

The carbon dioxide diffuses out of the blood (where there is a high concentration) and into the air in the alveoli (where there is a lower concentration) to be breathed out. Oxygen diffuses in the opposite direction to be used by the cells in respiration.

Adaptations of alveoli

• The lining of the alveoli is moist for dissolving gases.
• The blood supply is good to maintain the concentration gradients.
• The walls of the alveoli are very thin, which reduces the travelling distance for the gases.
• They have a large surface area.

Villi in the small intestine

One part of the digestive tract is the small intestine. The small intestine is responsible for absorbing nutrients and water from digested food. The inside of the small intestine is covered with millions of small projections called villi. The villi allow digested food to be absorbed more quickly by increasing the surface area of the small intestine.

Adaptations of the villi 

• Villi have a single layer of surface cells (epithelium), reducing the travelling distance for substances.
• The blood supply is very good.