Circulatory Pathways

Circulation in animals follows one of two broad patterns — open or closed — and among vertebrates the heart itself varies in the number of chambers it has, which in turn determines how oxygenated and deoxygenated blood are kept apart.

Open versus closed circulation

• Open circulatory system: Found in arthropods and molluscs. Here the heart pumps blood into large vessels that empty into open spaces or body cavities called sinuses, so the blood bathes the tissues directly.
• Closed circulatory system: Found in annelids and chordates. The blood pumped by the heart stays within a closed network of blood vessels throughout its journey. This arrangement is regarded as more advantageous, because when the fluid is confined to vessels its flow can be regulated much more precisely.

Chambered hearts in vertebrates

Every vertebrate has a muscular, chambered heart, but the number of chambers differs across groups:

• Fishes: a 2-chambered heart — one atrium and one ventricle.
• Amphibians and reptiles (except crocodiles): a 3-chambered heart — two atria but only a single ventricle.
• Crocodiles, birds and mammals: a 4-chambered heart — two atria and two ventricles.

How blood flows in each type

The heart's structure decides the route the blood takes:

• In fishes, the heart pumps out deoxygenated blood. This blood is oxygenated at the gills, supplied to the body parts, and the deoxygenated blood then returns to the heart. Because the blood passes through the heart only once in each complete circuit, this is called single circulation.
• In amphibians and reptiles, the left atrium receives oxygenated blood (from the gills, lungs or skin) while the right atrium receives deoxygenated blood from the rest of the body. However, both streams empty into the single ventricle, where they mix, and the ventricle then pumps out this mixed blood. This is described as incomplete double circulation. …