Avian respiration

 

The avian respiratory system has a two-way air-flow which ventilates the lungs twice in each respiratory cycle.

 The avian respiratory system has two main functions: first, gas exchange allowing oxygen in and carbon-dioxide out; second, thermal regulation by air cooling. The system comprises: The trachea subdivided into two bronchi, each supplying a lung together with a system of air-sacs which expand and contract with the movement of the ribcage and sternum. The lungs do not expand and contract during the respiratory cycle, instead the airflow through the lungs is caused by the expansion and contraction of the air-sacs. The lungs are dense and packed close up to the dorsal ribcage.

 To maximise the gas exchange between the air and the blood, the partial pressure difference needs to be maintained. If the air and blood are relatively stagnant, the oxygen partial pressure equalises quickly and the gas exchange slows.  In bird lungs that does not happen because the air in the lungs is kept moving and a counter-flow or cross-flow system has evolved in which the air moves in the opposite or crosswise direction to the blood flow so that oxygen-rich air constantly flows past relatively oxygen-depleted blood.

  The cooling function of the system has to deal with three main circumstances: first, resting in cool conditions; second resting in hot conditions and third, exercising ie flying in cool conditions (presuming that birds do not over-heat while flying?). Clearly, increased air-flow in the respiratory system, which can be achieved with a faster or deeper breathing rate, will promote oxygenation and cooling. However, when over-heating while resting, an increase in air-flow could lead to over-depletion of carbon-dioxide leading to an acid/base imbalance in the blood (alkalosis). Birds deal with over-heating by panting; rapid, short breaths move a small volume of air back and forth through the respiratory system causing heat to be ejected without a large amount of gas exchange. (Speculatively: the undesired hyper-ventilation could be dealt with by contraction or dilation of some of the air-ways in-order to divert more air direct to the air-sacs via the secondary bronchi and bypassing to some extent the bronchioles and air-capillaries).

 Within the lung, the bronchus subdivides into several secondary bronchi and these are inter-connected by tertiary bronchi or bronchioles. All of these air-vessels are open-ended so that air flows through them during the respiratory cycle. Branching off the bronchioles are short, blind-ended air-capillaries where the gas exchange takes place. The tertiary bronchi are gathered into secondary bronchi which emerge from the lungs into air-sacs. The primary bronchus becomes progressively narrower as it subdivides into secondary bronchi, while the secondary bronchi become progressively wider as they gather the tertiary bronchi proximal to the air-sacs.

avian respiratory cycle  There are normally up to nine air-sacs, one of which, the inter-clavicular air-sac connects to both lungs. The air sacs are thin-walled bladders which act as bellows to promote the flow of air through the lungs; they do not have a copious blood supply and do not take part in gas-exchange. They are tucked among the viscera and connected with some pneumatised bones to help with cooling the musculature and the internal organs.

  The path taken by the air through the lungs is a subject which is contentious. Although there is a direct and shortest path from the primary bronchus to the posterior air-sac, that does not mean that all of the air takes this path. Equally, just because there is a shortest path between air-sacs via secondary bronchi, does not mean that air flows from one air-sac to another. There is research which has shown that in some air-vessels the air flows more in one direction that another (Bretz and    Schmidt-Nielsen, 1970). Also, gas analysis suggests that the air in the posterior air-sacs is more oxygen-rich and that the air in the anterior air-sacs is more carbon-dioxide-rich. In some illustrations the primary bronchus is shown connected directly to the posterior air-sacs, by-passing the lungs which is nonsense.

  All of this has led to the explanation that air flows uni-directionally through the lungs and requires two respiratory cycles to achieve this. How can avian respiration be super-efficient if it takes two respiratory cycles to ventilate the lungs once in a one-way system? There is no known valve system in bird lungs which would support a uni-directional flow; also, if all the air-sacs inflate at the same time and deflate at the same time, and the lungs do not expand and contract, then the pressure gradient must be between the broncus and the all of the air-sacs acting collectively; there is no pressure gradient which would drive air from one air-sac to another.

  Birds do not have a diaphragm; the respiratory cycle is powered by expansion and contraction of the rib-cage, associated to some extent with the flapping cycle. As the rib-cage expands during inhalation, all of the air-sacs expand at the same time and air is drawn-in through the bronchus, through the lung and into the air-sacs. During exhalation the ribcage contracts, the air-sacs contract and air is forced out through the lung, through the bronchus and out of the trachea. This means that in each respiration cycle the lungs are twice flooded by moving air and no stagnant air is left in the lungs.

  Although the secondary bronchi connect the posterior and anterior air-sacs, as part of a network, that does not mean that air flows from one air-sac to another. That cannot happen if all air-sacs are at the same pressure at any particular time which is illustrated in Gill 2007 after Boggs et al 1997. The air-sacs are simply part of an interconnected network of secondary and tertiary bronchi that are thoroughly flooded and scoured during each respiratory cycle. It may well be that in some of the airways, the air flows more in one direction than another; however, that does not mean that there is an overall uni-directional flow.

 It makes more sense to say that one respiratory cycle ventilates the lungs twice in a two-way system and it is the double-ventilation which makes bird-respiration more efficient. Furthermore, a two-way, pressurise / de-pressurise cycle will promote air-flow in and out of the blind-ended air-capillaries.

 If this is the case, and in the absence of any valve system, then the primary bronchus should be found to be progressively narrower as it subdivides into secondary bronchi proximal to the air-sacs; whilst the secondary bronchi are progressively wider as they gather tertiary bronchi proximal to the air-sacs, which is indeed illustrated in Gill 2007 after Lasiewski 1972.

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