Cellular elements and large molecules that function within the vascular system can be kept within it All vertebrates have closed circulatory systems and chambered hearts with valves that prevent the backflow of blood. From fishes to amphibians to reptiles to birds and mammals, the complexity and number of chambers of the heart increase. Pulmonary circuit 2. Systemic circuit Pulmonary circuit - blood is pumped from the heart to the lungs and back to the heart. A closed vascular system includes arteries that carry blood away from the heart, and veins that carry blood back to the heart.
Arterioles are small arteries, and venules are small veins.
Arterioles and venules are connected by very small thin-walled vessels called capillaries . Fishes The atrium receives blood from the body and pumps it to the muscular ventricle which pumps the blood to the gills. After leaving the gills, blood collects in a large dorsal artery called the aorta which distributes blood to smaller arteries and arterioles leading to the organs and tissues. In the tissues, blood flows through capillary beds, collects in venules and veins, and eventually returns to the heart. Amphibians Separate atria receive the oxygenated blood from the lungs and the deoxygenated blood from the body.