Birds
 
 
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Avion Circulation

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Birds have very efficient cardiovascular systems that permit them to meet the  demands of flying, running, swimming, and/or diving. The cardiovascular system not only delivers oxygen to body cells  but also plays an important role in maintaining a bird's body temperature.    The avian circulatory system consists of a heart plus vessels that transport nutrients, oxygen and carbon dioxide, waste products, hormones, and heat.     
    Birds, just like mammals, have a 4-chambered heart with complete separation of oxygenated and de-oxygenated blood. The right ventricle pumps blood to the lungs, while the left ventricle pumps blood to the rest of the body. Because the left ventricle must generate greater pressure to pump blood throughout the body the walls of the left ventricular are much more thick and muscular than the other ventricles in the heart. 
    Birds tend to have larger hearts than mammals (relative to body size). The relatively large hearts of birds may be necessary to meet the high metabolic demands of flight. Among birds, smaller birds have relatively larger hearts (again relative to body mass) than larger birds. Hummingbirds have the largest hearts of all birds, because hovering takes so much energy.
    Avian hearts also pumps more blood than a mammals heart. In other words, cardiac output (amount of blood pumped per minute) for birds is typically greater than that for mammals of the same body mass. Cardiac output is influenced by both heart rate and stroke volume (blood pumped with each beat). 'Active' birds increase cardiac output primarily by increasing heart rate.  The only difference between the birds heart and the humans heart is the fact that the bird has a single aortic arch on the right side of its body while the mammalian heart has one on the left.


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Avion Respiration    

    Birds have an Avion Respiration.  The Avion Respiration System delivers oxygen from the air to the tissues and also gets rid of  carbon dioxide. The air sacs in a bird allow an unidirectional flow of air through the lungs. Unidirectional flow means that air moving through bird lungs has a higher oxygen content than normal air going though( for example) a human being. In contrast, air flow is 'bidirectional' in mammals, moving back and forth into and out of the lungs. As a result, air coming into a mammal's  lungs is combined with 'old' air (air that has been in the lungs for a while) & this 'combined air' has less oxygen. So, in bird lungs, more oxygen is available to spread out  into the birds blood stream.
    In the birds lung, oxygen spreads from the air capillaries into the blood & carbon dioxide from the blood into the air capillaries . This transfer is very efficient in birds for many reasons. First, the complex arrangement of blood and air capillaries in the avian lung creates a substantial surface area through which gases can disperse. The surface area available for exchange  varies with bird size.  
    Another reason why gas exchange in the Avion respiratory system is so efficient is that the blood-gas barrier through which gases spread out is extremely thin. This is important because the amount of gas diffusing across this barrier is inversely proportional to its thickness.  The blood-gas barrier is thinnest in birds. Thinner blood-gas barriers are better in birds and mammals because endotherms use oxygen at higher rates than ectotherms like amphibians and reptiles. Among birds, the thickness of the blood-gas barrier varies, with smaller birds generally having thinner blood-gas barriers than larger birds.

 
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