In mammals the gas exchange surfaces are the lungs, which are derived as a ventral outgrowth from the digestive tract in the early embryo, a fact which relates us to a group of lobe-finned fish from the Carboniferous period! The larynx is formed from the cartilages which made up the gill arches of our distant ancestors. It is connected to the trachea which is essentially a flexible tube held open at all times by incomplete rings of cartilage. The trachea divides into the left and right bronchi which then enter the lungs and continue to divide forming narrower and narrower tubules called bronchioles. these are surrounded by circular smooth muscle fibres. At the ends of the bronchioles we find groups of alveoli or air-sacs. It is in the alveoli that gas exchange actually occurs.

The alveoli have a one cell thick wall and on their outer surfaces run a network of blood capillaries which are also one cell thick. Since only two layers of cells separate the air from the lungs it is easy for gases to diffuse from the alveolar air into the blood and vice-versa. To maintain the concentration gradient between the blood and the alveolar air it is necessary to ventilate the lungs. This is accomplished by the intercostal muscles (between the ribs) and the diaphragm muscles. The lungs are enclosed by the 12 pairs of ribs and the intercostal muscles (for the keen of eye the 13 pairs of spots on the diagram include the clavicles!). Around the lungs we have the inner and outer pleural membranes which secrete mucus to lubricate the movement of the lungs as they slide against the rib-cage wall, friction here would be disastrous. The pleural membranes also prevent the leakage of gases into the space between the lungs and the rib-cage wall....in fact, their is no space between the pleural membranes, lungs and rib-cage in a living animal, the pleural cavity is a potential cavity. When the ribs are raised and the diaphragm lowered, the thorax increases in volume and this immediately causes a drop in pressure on the outer surface of the lungs. The pressure on the inner surface of the lungs is at atmospheric pressure. We have a greater force pressing outwards and a lesser force pressing inwards, as soon as the difference in these two forces is great enough to overcome the elastic tension of the lung tissues then the lungs expand and air will flow into them from the atmosphere following the pressure gradients. When we wish to breath out, we merely have to lower the ribs and relax the diaphragm. When the diaphragm muscle relaxes the gut which has been compressed by the downward movement of the diaphragm as it contracted will push the diaphragm into a dome shape. The movement downwards and inwards of the ribs will also cause a reduction in volume of the thorax. As the reduction in volume begins the pressure on the outer surface of the lungs begins to rise and as soon as it gets close to the internal air pressure of the lungs the elastic tension of the lung tissue will cause the lungs to deflate and air will flow out of the lungs back into the atmosphere.

You should be aware that you do not breath air into from the atmosphere directly into your alveoli. The inspired air passes into the bronchi and bronchioles but does not reach the alveoli. Oxygen must diffuse from the bronchioles into the alveoli and Carbon dioxide must diffuse from the alveoli into the bronchioles. The reason for this is to maintain a slightly higher CO₂ content in the alveolar air to slow the loss of CO₂ from the blood. If you hyperventilate, breathing very fast and very deeply you will soon feel the effects of losing too much CO₂ from your system and you will very likely faint!

The lungs have the typical features required by an efficient gas exchange system: a large surface area (about the area of a doubles tennis court); a short diffusion pathway (only two layers of cells from the alveolar air to the blood); high concentration gradients (maintained by ventilation and flow of blood in the capillaries); a gas permeable surface (the moist surface of the alveoli allow gases to dissolve and then diffuse through the cells. The lungs are protected by the secretion of mucus containing antibacterial enzymes, and by ther action of the cilia which sweep the mucus out of the lungs. The action of a single cilium is shown in the animation below

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In fish, the gas exchange surfaces are the gills. Water is a more dense medium than air and contains only a small quantity of dissolved oxygen per litre. The fish must do relatively more work to push water over the gills and will obtain less oxygen per litre, it follows from this that the design of fish gills must give maximum efficiency for exchange. There are two major groups of fish, the Chondrichthyes, sharks and rays, and the Osteichthyes, or bony fish. Their ventilation structures are different. The Chondrichthyes have what might be called a push system of ventilation while the Osteichthyes have a push-pull system.

In the Chondrichthyes the gills are all separated from each other in chambers which allow water from the pharynx to pass over the gill lamellae and then out through separate gill slits. Each gill slit has a thin flap over it. When the fish opens it’s mouth it sucks in water, the low pressure in the gill chambers will pull the gill flaps closed to prevent water being drawn back over the gill lamellae. The fish then closes it’s mouth and compresses the water in it's pharynx by contracting its buccal muscles. The increased pressure forces the water over the gills, opens the gill flaps and the water flows out in a backwards direction away from the fishes mouth. In the Osteichthyes the same sort of thing occurs to create a flow of water from the pharynx and over the gills, but as the fish opens its mouth to take in more water, the opercular bones are pulled outwards which sucks water from the pharynx over the gills and into the opercular space. The opercular valve prevents water from entering the opercular space while this is happening. thus the Chondrichthyes have a push-pause-push intermittent water flow over the gills, while the Osteichthyes have a push-pull-push constant flow over the gills. This explains why many sharks will drown if held still in the water for a long period.

The gill lamellae are arranged as a series of flat plates sprouting from the gill arch. On their upper and lower surfaces there are many thin vertical flaps which contain blood capillaries. The blood flows through these capillaries in the opposite direction to the flow of water over the gills. This is called a counter-current flow system and gives a highly efficient diffusion pathway since as the blood flows along and picks up oxygen it meets water which always has a higher oxygen content than itself and the diffusion of oxygen into the blood will be maintained.

Consider the two diagrams above right. The first shows parallel flow and the second counter-current flow. The number refer to the concentration of a material dissolved in the flowing solutions and the small arrows indicate diffusion. Notice that with parallel flow the diffusion process stops once the concentration gradient is equalised whereas with countercurrent flow this does not occur and more material is absorbed from the water

Counter-current flow systems are found in many biological structures, for example the kidney, liver and the blood vessels in the legs of penguins and polar bears. They are also an important feature in many engineering systems, for example heat exchangers.

In Insects the tough impermeable exoskeleton makes gas exchange impossible through the skin and therefore a system of tracheoles has evolved which gives every cell of the insect a continuous airline to the atmosphere. Each section of the insect apart from the head has a pair of lateral openings called spiracles, from these run tubes called tracheoles which branch and divide and finally their numerous microscopic ends penetrate into the individual body cells. The spiracles frequently have valves to allow them to be closed to prevent excessive loss of water by evaporation from the tracheae. The tracheae in many insects are connected to a longitudinal trachea which runs from the head to the abdomen of the insect. By contracting the abdominal muscles between the body segments the insect can make itself shorter and this can be used to compress the longitudinal tracheae to effectively pump gases in and out of the tracheal system. In a few insects there are soft sac like swellings in the longitudinal tracheae and these make effective pumps forcing the air along the longitudinal tracheae. By opening and closing the spiracle valves in a particular order it is possible for the insect to suck air into the tracheal system at one end of the body and to circulate the air through the system and pass it out at the other end of the body. It must be remembered that insects generally have a small volume and therefore are in great danger of desiccation, so control of gas intake and release from the tracheal system is most important.