Exchange Surfaces

Gaseous exchange is the process by which oxygen is taken in by the body for respiration and carbon dioxide, the waste product of respiration, is released. In humans and most other air-breathing animals the lungs are the organ used for gaseous exchange.

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Breathing

When we breathe, air moves in and out of the body via our lungs. In the lungs, the air moves into the alveoli and oxygen diffuses into the blood stream. Carbon dioxide diffuses in the opposite direction, from the bloodstream and into the alveoli, where it’s then breathed out into the air.

The action of breathing involves muscles contracting and relaxing, depending on whether we want to breath in (inhale) or out (exhale). Movement of air in and out of the lungs is called ventilation.

The main muscles involved are:

• the intercostal muscles which are located between the ribs
• the diaphragm which is located between the abdomen and the thorax

When you inhale:

• The intercostal muscles contract which causes the ribcage to expand.
• The diaphragm also contract and pulls downwards making it flatter so that the volume of the chest increases.
• When pressure in the thorax decreases, air is pulled into the lungs.

When you exhale:

• The intercostal muscles relax which causes the ribcage to drop in and down.
• The diaphragm also relaxes and moves upwards which decreases the chest volume.
• The increase in pressure in the thorax pushes air out of the lungs.

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The lungs

The lungs are very well adapted for gaseous exchange. Instead of being two large sacs, inside each lung are a multitude of little sacs called alveoli. Each alveoli is smaller than a grain of salt yet, because there are about 300 million of them in an adult, they massively increase the surface area of the lungs. This means that diffusion can happen at a much faster rate.

The alveoli themselves have a very thin and moist surface. This helps gases to move in and out of them quicker.

They’re also covered with capillaries which are very thin blood vessels. This means that oxygen can quickly pass into the bloodstream and carbon dioxide can quickly pass back into them to be expelled by the lungs.

Lung structure

The lungs located in the upper half of the body in the thorax and separated by the lower part of the body, the abdomen, by the diaphragm. The ribcage surrounds the lungs and provides them with protection.

NOTE: You should be able to recognise these structures on a diagram.

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The small intestine

The small intestine absorbs food molecules which are then passes onto the bloodstream and taken to where required in the body.

The small intestine is well adapted to ensuring the efficiency of this transfer. Firstly, the walls are very thin, only one cell thick, thereby decreasing the distance of diffusion.

Secondly, it has a large surface area. This is because it’s lined with tiny villi which stick out like minute fingers. These increase the rate of absorption by about 600 times.

Lastly, each villi is surrounded by a network of capillaries which connect to the main blood supply so that the molecules can quickly be transported to where they’re needed. The supply of blood around the small intestine also has a lower concentration of food molecules so that diffusion can occur more rapidly.

Digestion also relies on active transport. When carbohydrates are broken down they form simple sugars like glucose. These are absorbed by the villi using active transport.