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David Beeson, May 2022
While nearly everyone you meet on a nature exploration can identify most of the birds they encounter, few know much about how they work. This article is an introduction to some aspects of their physiology.
For those who require more detail, you should obtain: Handbook of Bird Biology (Cornell Lab of Ornithology) and see the chapter on Physiology.
Supplying oxygen to flight muscles
Flight is a high energy-demanding activity. Birds use the energy in wind to reduce this requirement, yet even getting off the ground can be difficult … as I found out attempting the high jump at school!
Human ventilation works by drawing oxygen-rich air into the lungs using the diaphragm and inter-rib muscles. This air goes into the small sac-like alveoli of the lungs, where oxygen is exchanged for carbon dioxide. Expiration expells that air back into the environment. Of the 20% oxygen in the incoming air only around 4% is taken up by the lungs; an efficiency of around 20%. Now, when I achieved that percentage in examinations I was not proud! This method is insufficient for a high-energy demand such as flight.
The human gas exchange system is in Diagram B below. (Blue is air, red blood.)
In A, the bird lung, there is no mixing of input and output gases, and that is much more efficient, so much more suited to an aerial life-style.
Birds avoid the mixing problem by moving air through their lungs in one direction via a series of 7 to 9 air sacs, connected by loopy tubes. Birds take oxygen into their body tissues when they breathe in and when they breathe out. So, for every bird breath, humans would need to take two. Effectively, air flows continuously through a bird’s lungs, while in the human it pulsates.
Birds do not have a diaphragm and the lungs do not flex as in humans. Instead, the air sacs change volume and act as bellows, and these sacs are spread around the body. The whole body cavity, not just the pleural (lung) cavity, changes in volume during breathing.
Bird lungs and air sacs occupy twice the comparative body volume in birds to humans, while their lung is smaller. The reasons are clear: flying is very oxygen demanding, they have a higher body temperature (42 compared to 37 Celsius) and need to fly above ground level where the percentage of oxygen is reduced.
To cope with the high oxygen demands of flight the heart rate needs also to be enhanced, so birds typically have larger hearts than mammals of similar sizes, and they also have much higher heart rates with resting heart rates generally sitting between 150-350 beats per minutes for a medium-sized bird. (Humans 60 – 70bpm). The capacity for lung O2 diffusion is also greater in birds because of the exceptional thinness and large surface area of the gas exchange tissue. Nevertheless, the diffusion barrier appears to be mechanically stronger in birds than in mammals, so pulmonary blood flow and pressure can increase without causing stress failure.
So, with a one-way airflow, high volume of blood arriving at the thin gas exchange surfaces and high heart rate (8 times higher during flight) the natural flow of oxygen from the incoming air by diffusion is high. Coupled with this, avian blood has a greater capacity to take up and deliver oxygen to the body tissues.
Interestingly, birds with long necks have to take deeper breathers than those with shorter necks, as there will be more ‘dead space’ of air that does not reach the lung’s gas exchange surfaces.
Yet there is even more efficiency in the system. Avian muscle fibres are smaller than those of mammals, ensuring the easier movement of gases into and out of those highly metabolic tissues. And their nervous system is less vulnerable to high carbon dioxide levels. All of this allows the system to cope with the demands of their lifestyle.
Surface area to volume
Organisms gain or lose heat from their outer surfaces. They can, if it is kept moist, use their outer surface for the exchange of gases.
Imagine cubical animals:
|size||1 x 1 x 1 Small organism||2 x 2 x 2||10 x 10 x 10 Larger organism|
|Surface area to volume ratio||6 : 1||3 : 1||0.6 : 1|
|comment||Easy exchange of gases over their comparatively large surface, BUT easy heat gain or loss.||Gas exchange through the surface will be difficult BUT heat loss or gain is less significant. Needs a gas exchange organ – lung.|
As organisms increase in size they need specialised organs (internal and moist) to take up and release waste gases, but heat control is easier.
Smaller organisms have a comparatively large surface area to volume, so gas exchange is easier. Yet, small warm-blooded organisms are in danger of losing too much heat as there is little body volume to generate heat through their metabolism.
Consider a minute aquatic organism such as Amoeba. From the data above one can suspect that diffusion through its surface could supply sufficient exchange of gases (O2 and CO2). It is unconcerned, mostly, by gain and loss of heat. However, a small bird living in a cool or cold environment, not exchanging gases through its outer layer, is in trouble because of potential heat loss with its comparatively large surface area when compared to a much bigger bird. Hence why small hummingbirds must hibernate at night. (But not larger birds). With such losses of heat, the energy intake of a small bird needs to be much greater (for each unit of weight) than a larger animal. Little birds seldom ‘enjoy’ cold climates and may force some to migrate.
Feathers are vital not only to aid flight but also for thermoregulation. They may need to trap insulatory air to reduce heat loss or gain. So, not all feathers are flight feathers, some are ‘down’ feathers, and their percentage and orientation will vary with environmental conditions. Maintaining feathers in optimal condition is a vital activity.
The figures usually quoted for the energy content of fats (lipids) and carbohydrates is 38KJ to 17KJ a gramme. A gramme of fat contains about twice the energy than carbohydrate. If birds need to have high energy stores, e.g. for migration, it is better to use fats. Migratory geese have higher fat stores than chickens (non-migratory) and will need to use it in energy release during migration.
Fats may be great energy stores, yet they metabolically need more oxygen than carbohydrates, and with high-flying migratory birds, this introduces a new problem. At high altitudes air pressure is lower, so lift is less. Hence greater flapping is needed at a time when oxygen levels are lower than at ground level. Under such conditions, birds risk going anaerobic with carbon dioxide levels causing body stress or death. Flying high may reduce air resistance and allows the chance to use back winds to aid you, but other negative factors come into play.
Birds, such as the high-flying, migrating Bar-headed Geese, take much deeper breaths and have larger lungs to cope with the low oxygen levels. Additionally, like high-living mammals, they possess blood haemoglobin that takes up oxygen more readily than a non-migratory species. Also, the goose heart’s left ventricle is more enriched with blood vessels to reduce the chance of it being oxygen-deficient. There are cellular modifications too to make oxygen usage more efficient.
Of course, we all know that the muscles of birds use one of two systems. Explosive muscles work mainly anaerobically (white meat), while slowly working muscles use oxygen (red meat). Red meat contains the fixed oxygen-holding pigment myoglobin which is absent in white meat.
The explosive-functioning muscles are to fly from predators, yet they soon become oxygen-deficient and exhausted. This is ideal for wild chickens and turkeys that are ground-dwelling and burst upwards but coast down to a safer location. Their leg muscles are functioning much of the time so will be red meat. However, those explosive muscles are not the correct design for migratory birds and they need slower but longer-lasting red muscle.
Birds are wonderful examples of how evolution adapts a basic animal design to many different niches – environmental options.