Animal Circulatory Systems: Vertebrates & Invertebrates

Vertebrate Circulatory Systems

Simple Closed Circulation (Single Circuit)

Fishes

The heart in fishes has one atrium and one ventricle. From the ventricle, deoxygenated blood is pumped to the branchial artery, which branches into arteries supplying the gill lamellae. In the gills, blood becomes oxygenated. This oxygenated blood is then collected by efferent gill arteries that converge into the aortic root and then the dorsal aorta, which distributes it to the body tissues. This is a single-circuit system.

Closed Double Incomplete Circulation

Amphibians

The heart is composed of two atria and one ventricle. The two atria are separated by an interatrial septum. The right atrium receives deoxygenated blood from the body, and the left atrium receives oxygenated blood from the lungs (and often the skin). Both atria contract almost simultaneously, sending their blood into the single ventricle, where some mixing of oxygenated and deoxygenated blood can occur before it is pumped out.

Reptiles

The circulatory pattern in most reptiles is similar to that of amphibians, featuring two atria and a ventricle that is partially divided by a septum, which reduces the mixing of oxygenated and deoxygenated blood. However, crocodilians (crocodiles, alligators, caimans, and gharials) are an exception, as they have a heart with two atria and two completely separate ventricles, allowing for a more complete separation of pulmonary and systemic circuits.

Closed Double Complete Circulation (Four-Chambered Heart)

Birds and Mammals

The heart has two atria and two ventricles, ensuring a total separation of pulmonary (lung) and systemic (body) circulation. This is a highly efficient double-circuit system.

• In mammals: The right atrium receives deoxygenated blood from the body. This blood is then pumped by the right ventricle to the lungs for oxygenation. The left atrium receives oxygenated blood from the lungs, which is then pumped by the left ventricle to the rest of the body tissues.
• In birds: They also possess a four-chambered heart with complete separation of oxygenated and deoxygenated blood, functioning similarly to mammals. A key anatomical difference is that the main artery carrying oxygenated blood to the body (the systemic aortic arch) develops from the right side in birds, whereas in mammals, it develops from the left side.

Invertebrate Circulatory Systems

Open Circulation with Vessels

Arthropods

The heart is typically dorsal and tubular. Hemolymph (the circulatory fluid in open systems) is pumped by the heart into arteries and then released into open tissue spaces, collectively known as the hemocoel, where it bathes the organs directly. The hemolymph returns to the heart through small openings called ostia.

Non-Cephalopod Molluscs

The heart is generally compartmentalized and located in a dorsal position within the pericardial cavity. It typically consists of a single ventricle and one or two atria. Hemolymph is pumped from the ventricle into vessels, which then empty into tissue spaces (hemocoel). From these spaces, hemolymph is collected by other vessels that transport it to the gills (or mantle cavity in some) for oxygenation before it returns to the heart's atrium/atria.

Closed Circulation with Vessels

Cephalopods

Cephalopods (e.g., squid, octopus) have a closed circulatory system. Their system includes a main systemic heart and, in most, two accessory branchial hearts. The systemic heart usually has one ventricle and two (or four in Nautilus) atria. Blood is pumped by the systemic heart through vessels to the body. The branchial hearts are located at the base of each gill and boost blood pressure to drive blood through the gill capillaries for efficient oxygenation before it returns to the systemic heart.

Annelids

The circulatory system in annelids (e.g., earthworms, leeches) is typically closed. It consists of a main dorsal blood vessel and a main ventral blood vessel, which are connected by segmental vessels. Some of these connecting vessels, particularly in the anterior region (e.g., five pairs in earthworms, often referred to as aortic arches or "hearts"), are contractile and function to propel the blood. Blood generally flows forward (anteriorly) in the dorsal vessel and backward (posteriorly) in the ventral vessel.