Airflow in birds is unidirectional through a pair of smallyet rigid lungs and nine thin-walled, expandable air sacs. Unlike in mammals,the volume of the lungs doesn’t change to effect airflow; the air sacs worklike bellows to create a continuous flow of air through the lungs – the systemis more efficient than a bidirectional air flow and allows for the highmetabolic rate found in birds.Physiology1. The cycle begins when inspiratory muscles contract,moving the sternum outwards, increasing the volume of the air sacs anddecreasing the pressure in the cavity below atmospheric pressure.2. Oxygenated air enters the mouth or nares. The air passesthe pharynx and into the trachea, the trachea divides into two primary bronchiat the syrinx. 3.

The air sacs exist in two functional groups, the cranialair sacs and the caudal air sacs, they extend from the main bronchus at cranialand caudal ends respectively. Air enters the caudal air sacs first, due toaerodynamic valving,  air previously inthose sacs being drawn through the lungs into the cranial air sacs as theyexpand.4. Primary bronchi enter the lungs and are then known asintrapulmonary bronchi, these branch into smaller dorsobronchi, which branchinto even smaller parabronchi (Figure 1). 5. The parabronchi contain hundreds of tiny inlets calledair capillaries. They are surrounded by a network of blood capillaries, this iswhere gas exchange takes place via diffusion.

After passing through theparabronchi, the now deoxygenated air moves into the ventrobronchi, which thenconnects back to the main bronchus.6. Contraction of the expiratory muscles pulls the sternumtowards to spine, reducing the volume of the system and increasing pressurewithin the air sacs.

Air from the caudal air sacs is pushed through the lungsinto the cranial air sacs, with the air in the cranial air sacs being expelledthrough the trachea and then out of the mouth or nares. (Sjaastad, 2010)Cross-current Gas ExchangeThe parabronchi’s air capillaries and blood capillaries crosseach other perpendicularly (Figure 2), this is to sustain a high concentrationgradient and increase the amount of O2 diffusion into the blood. Capillariestowards the start of the parabronchi contain air with a higher pO2 than thoseat the end. The overall pooled capillary blood returning to the heart has ahigher pO2 than the exhaled air. In mammals, the partial pressures remainrelatively constant, this means they can never achieve blood with a higher pO2than exhaled air. This means birds can adequately oxygenate their blood at muchhigher altitudes than mammals can.

(Sjaastad, 2010)How does it differ from a typical mammal?• Birds have a higher tidal volume, and therefore lowerrespiratory rate, than mammals. This is beneficial as there is a large amountof dead space within the bird, the increased tidal volume allows more of theair in the system to be used for gas exchange. • They also have pneumatic bones which contain some of theair sacs (Figure 3).

They are beneficial for flying as they decrease theoverall weight of the animal, as there is no marrow, but do not compromisestrength, as avian bones are denser than those of mammals. (Dumont, 2010)

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