Gaseous Exchange

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Gas Exchange


Exchange in Organisms M.C Qu's 
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Gas Exchange
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Systems that increase the rate of exchange

As we saw earlier, Fick's law showed that for a fast rate of diffusion you must have a large surface area, a small distance between the source and the destination, and maintain a high concentration gradient. All large organisms have developed systems that are well-adapted to achieving these goals, as this table shows. For comparison, a tennis court has an area of about 260 m and a football pitch has an area of about 5000 m.


Large surface area

Small distance

High concentration gradient

human circulatory system

100m of capillaries with a surface area of 6000m

capillary walls are only one cell thick

constant blood flow replenishes the blood

human lungs

600 million  alveoli with a total area of 100m

each alveolus is only one cell thick

constant ventilation replaces the air

Fish gills

feathery filaments with lamellae

lamellae are two cells thick

water pumped over gills in countercurrent to blood

human small intestine

7m long, folds, villi and microvilli give surface area of 2000m

blood capillaries close to surface of villus

stirred by peristalsis and by microvilli


surface area of leaves of 1 tree is 200m, surface area of spongy cells inside leaves of 1 tree is 6000m.

gases diffuse straight into leaf cells

wind replaces air round leaves, and photosynthesis counteracts respiration

root hairs

1m area of lawn grass has 350m of root surface area due to root hairs

fairly short route through root to xylem

transpiration draws water and solutes away from roots.

Gas exchange takes place at a respiratory surface - a boundary between the external environment and the interior of the body. For unicellular organisms the respiratory surface is simply the cell membrane, but for large multicellular organisms it is part of specialised organs like lungs, gills or leaves. This leads to confusing terminology, for while the word "respiration" in biology usually refers to cellular respiration (ATP generation in cells), from time to time (such as here) it can also refer to breathing, which is what non-biologists mean by it anyway.

Gases cross the respiratory surface by diffusion, so from Fick's law we can predict that respiratory surfaces must have:

Many also have:

We shall examine how these requirements are met in the gas exchange systems of humans.


Gas Exchange in Humans   [back to top]

In humans the gas exchange organ system is the respiratory or breathing system. The main features are shown in this diagram.

The actual respiratory surface is on the alveoli inside the lungs. An average adult has about 600 million alveoli, giving a total surface area of about 100m, so the area is huge. The walls of the alveoli are composed of a single layer of flattened epithelial cells, as are the walls of the capillaries, so gases need to diffuse through just two thin cells. Water diffuses from the alveoli cells into the alveoli so that they are constantly moist. Oxygen dissolves in this water before diffusing through the cells into the blood, where it is taken up by haemoglobin in the red blood cells. The water also contains a soapy surfactant which reduces its surface tension and stops the alveoli collapsing. The alveoli also contain phagocyte cells to kill any bacteria that have not been trapped by the mucus.

The steep concentration gradient across the respiratory surface is maintained in two ways: by blood flow on one side and by air flow on the other side. This means oxygen can always diffuse down its concentration gradient from the air to the blood, while at the same time carbon dioxide can diffuse down its concentration gradient from the blood to the air. The flow of air in and out of the alveoli is called ventilation and has two stages: inspiration (or inhalation) and expiration (or exhalation). Lungs are not muscular and cannot ventilate themselves, but instead the whole thorax moves and changes size, due to the action of two sets of muscles: the intercostal muscles and the diaphragm.



  • The diaphragm contracts and flattens downwards

  • The external intercostal muscles contract, pulling the ribs up and out

  • this increases the volume of the thorax

  • this increases the lung and alveoli volume

  • this decreases the pressure of air in the alveoli below atmospheric (Boyle's law)

  • air flows in to equalise the pressure

Normal expiration


  • The diaphragm relaxes and curves upwards

  • The external intercostal muscles relax, allowing the ribs to fall

  • this decreases the volume of the thorax

  • this decreases the lung and alveoli volume

  • this increases the pressure of air in the alveoli above atmospheric (Boyle's law)

  • air flows out to equalise the pressure

Forced expiration

  • The abdominal muscles contract, pushing the diaphragm upwards

  • The internal intercostal muscles contract, pulling the ribs downward

  • This gives a larger and faster expiration, used in exercise

 These movements are transmitted to the lungs via the pleural sac surrounding each lung. The outer membrane is attached to the thorax and the inner membrane is attached to the lungs. Between the membranes is the pleural fluid, which is incompressible, so if the thorax moves, the lungs move too. The alveoli are elastic and collapse if not held stretched by the thorax (as happens in stab wounds or deliberately to rest a lung).



Controlling Breathing Rate  [back to top]

But what controls the breathing rate?  It is clearly an involuntary process (you dont have to think about it), and like many involuntary processes (such as heart rate, coughing and sneezing) it is controlled by a region of the brain called the medulla.  The medulla and its nerves are part of the autonomic nervous system (i.e. involuntary).  The region of the medulla that controls breathing is called the respiratory centre.  It receives inputs from various receptors around the body and sends output through two nerves to the muscles around the lungs.

The respiratory centre depends on information relayed via chemoreceptors that pick up changes in:

The chemoreceptors are stimulated by a rise in carbon dioxide levels and a fall in pH and oxygen in the blood.  The respiratory centre received the information as a nerve impulse from the chemoreceptors and uses this to regulate breathing.

Later in this module we will be look in more detail about the effects of exercise and how the breathing rate and heart rate is controlled via various changing conditions within the body.


How does the respiratory centre control ventilation?  [back to top]

Unlike the heart, the muscles that cause breathing cannot contract on their own, but need nerve impulses from the brain for each breath. The respiratory centre transmits regular nerve impulses to the diaphragm and intercostal muscles to cause inhalation. Stretch receptors in the alveoli and bronchioles detect inhalation and send inhibitory signals to the respiratory centre to cause exhalation. This negative feedback system in continuous and prevents damage to the lungs.

One difference between ventilation and heartbeat is that ventilation is also under voluntary control from the cortex, the voluntary part of the brain. This allows you to hold your breath or blow out candles, but it can be overruled by the autonomic system in the event of danger. For example if you hold your breath for a long time, the carbon dioxide concentration in the blood increases so much that the respiratory centre forces you to gasp and take a breath. Pearl divers hyperventilate before diving to lower the carbon dioxide concentration in their blood, so that it takes longer to build up.

During sleep there is so little cellular respiration taking place that it is possible to stop breathing for a while, but the respiratory centre starts it up again as the carbon dioxide concentration increases. It is possible that one cause of cot deaths may be an underdeveloped respiratory centre in young babies, which allows breathing to slow down or stop for too long.  

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  Last updated 20/06/2004