The circulatory system, a network of tubes and pumps, is fundamental to life, with the heart propelling blood. Animals have evolved diverse circulatory solutions reflecting their lifestyles and environments.
The Animal with the Most Hearts
Cephalopods, marine mollusks like octopuses, squids, and cuttlefish, possess the most hearts in the animal kingdom, each having three. These three hearts play distinct roles. Two smaller branchial hearts pump blood through the gills for oxygenation. The oxygenated blood then flows to a larger, single systemic heart, which circulates it throughout the body, delivering nutrients and oxygen.
This three-heart system supports their active, predatory lifestyle and high metabolic rate. Unlike vertebrates that use iron-rich hemoglobin, cephalopods utilize copper-rich hemocyanin to transport oxygen, giving their blood a blue color. Hemocyanin is less efficient than hemoglobin, necessitating higher blood pressure and robust circulation to meet their needs. The branchial hearts overcome resistance in the gills, allowing the systemic heart to focus on distributing oxygenated blood throughout the body.
Diverse Approaches to Circulation in the Animal Kingdom
Beyond cephalopods, the animal kingdom has diverse circulatory strategies adapted to different body plans and metabolic needs. These systems fall into two categories: open and closed. In an open circulatory system, found in many invertebrates like insects and most mollusks, blood (hemolymph) flows freely within body cavities, directly bathing organs. This fluid returns to the heart through openings, aided by muscular movements.
Insects, for example, have an open circulatory system where hemolymph is propelled by a dorsal vessel, acting as a heart and aorta. This tube runs along their back, collecting hemolymph in the abdomen and pumping it forward to the head. This system is less efficient in regulating blood flow to specific areas compared to closed systems.
A closed circulatory system, found in vertebrates and some invertebrates like earthworms and cephalopods, confines blood within a continuous network of vessels. This design allows for more efficient and precise delivery of oxygen and nutrients. Fish possess a two-chambered heart: one atrium and one ventricle. Deoxygenated blood enters the atrium, moves to the ventricle, and is pumped to the gills for oxygenation before circulating throughout the body in a single loop.
Amphibians, such as frogs, have a three-chambered heart with two atria and one ventricle. This arrangement allows some mixing of oxygenated and deoxygenated blood in the single ventricle, supporting their ability to absorb oxygen through both lungs and skin. Earthworms have multiple pairs of aortic arches that function like hearts. These five pairs of muscular arches pump blood into dorsal and ventral blood vessels, circulating blood throughout their segmented bodies.