ASPA on the case of the stenosed giraffe
The law of unforeseen consequences in science is fascinating, sometimes painful. Medical consultants are always on the lookout for this principle applying to the patients they treat. This law applies also to the way nature “designs” her animals. The giraffe is a perfect example where nature has solved one problem only to create several others. All she wanted was an animal with a hugely long neck to beat the competition to food sources in the tree tops.
Nature has fashioned the giraffe to have a much narrower trachea than would be expected for an animal of its size. The reason for this structural feature is a little involved but let me try to explain. And while you read on, note the side effects this causes and the adaptations developed to cope with these problems.
You can see from the graphic how long the trachea is (in red) – at least 3 metres in a large animal. When the giraffe breathes in, waste air has to be flushed out from the space inside the trachea (the dead space) before fresh air gets to the lungs. The smaller this volume, the less the dead space problem interferes with getting fresh air. So, the more nature can narrow the trachea, without starving the lungs of air, the smaller the dead space becomes.
Like so many physiological mechanisms in animals, the giraffe respiratory system is full of compromises and “fixes”.
The graphic on the right is a tracheal ring dissected from the neck of a giraffe. The internal diameter is about 4.0 cm and with a wind pipe 3m in length, this represents a dead space of 1.2 litres. There is a resting tidal volume of about 3.3 litres (man 0.5 L) with a dead-space/tidal-volume ratio cited by various authors of about 0.34 ; a ratio actually very close to the expected value.
Up to this point nature is doing quite well but that narrow trachea causes another problem – impeded air flow. A stenosis by design! Nature has had to adapt the lungs to cope with this. The giraffe’s lungs are larger than expected for an animal this size in order to draw through this 1.0 L of “bad” air and still allow its respiratory and circulatory systems to get oxygen to all parts of its body. The breathing rate is very low, again less than expected, to address the problem of resistance to air flow.
Body temperature in a giraffe is not so closely controlled as in man, with a range of at least 3.3 degrees C. Oxygen consumption, respiratory frequency and minute volume all drop at lower temperatures. The point is that while it is relaxing, the animal the animal cools down and has to breathe less often (less energy burned off) and can build up reserves of oxygen in the muscles in case a burst of speed is required later on. A human would pant under exercise load but the narrow trachea makes panting hard impossible for a giraffe. Instead they raid stored oxygen from their muscles when the demand is high and have lungs designed to get that little bit more extra oxygen out of the air that enters.
Article written by Dr Michael Buck 18/02/2014