The cardiovascular system

Birds have evolved a tightly controlled cardiovascular system that meets the rigorous energetic demands of flight. The bird heart has a functional anatomy similar to that of the mammalian heart with four chambers and an extensive coronary circulation. The electrical conducing system of the avian heart includes a sinoatrial node, atrioventricular (AV) node, AV Purkinje ring, and the bundle branches. The vascular system is composed of arteries, capillaries, and veins and includes a pair of ductus arteriosi as embryos. Control of cardiovascular function includes both cardiac and the vasculature limbs. Peripheral vasculature reactivity is regulated by local mechanical autoregulation, humoral factors, and the autonomic nervous system altering vascular resistance. Humoral factors that regulate vascular resistance include pH, CO2, nitric oxide, angiotensin II, and circulating catecholamines. Both sympathetic and parasympathetic inputs have important roles in regulating the vasculature and the heart function. Sympathetic control of the cardiac function increases both contractile force and heart rate. Parasympathetic control affects cardiac contractile force and heart rate at the pacemaker and conducting tissues. Cardiac output is regulated by chemoreflexes and baroreflexes. All of these regulatory pathways mature late in embryonic life, providing fine control by the time of hatch. Finally, the avian cardiovascular system has evolved to function effectively during long migration flights, some at high altitude and during diving.