The Circulatory and Respiratory System of Birds

Circulatory Overview

• The circulatory system of birds is made up of a heart and blood vessels that function in a highly efficient manner. The circulatory system is responsible for transporting oxygen throughout the blood, clearing out waste materials, and helping to regulate body temperature. Birds have relatively large hearts, most likely due to the physically demanding act of flight. Generally speaking, the smaller the bird, the larger the heart. Hummingbirds have the largest heart relative to body size of all birds.

Circulatory Details

• Like mammals, birds have four-chambered hearts, which are more effective than the three-chambered hearts of reptiles and lizards. However, the hearts of birds are more efficient than those of mammals. In general, they pump more blood per beat and beat faster than mammalian hearts. This allows more blood to pass through the blood vessels of birds. Those vessels function similarly to mammalian vessels, with arteries transporting oxygenated blood away from the heart and veins returning the deoxygenated blood back to the heart.

Respiratory Overview

• Similar to that of mammals, the avian respiratory system circulates oxygen into the body, and carbon dioxide out. This is accomplished directly by their lungs, which are relatively smaller than in most other vertebrates. Unlike mammals, a bird's respiratory system is chiefly regulated by air sacs. Birds have a series of air sacs, usually nine, that regulate the circulation of air. This regulation is accomplished by changes in pressure, which are in turn caused by specific muscle contractions.

Respiratory Details

• Thanks to the regulation of air sacs, the respiratory system of birds allows for unidirectional air flow; that is, air moves in one direction throughout the respiratory system. In mammals, air flows in two directions (both in and out of the lungs). This unidirectional flow means that the air moving through a bird's respiratory is much richer in oxygen than the air that moves in both directions. As a result, the avian respiratory system is more effective than a mammalian -- more oxygen is transferred with each breath.