The heart is literally the center of circulation. Which is why “right to the heart of the matter” is a common expression. All types of multi-cellular animals have hearts as the pump that provides the primary motive force for the circulation of their blood. From the higher mammals including people, to the reptiles and amphibians, birds and fish, all the way down to the lowest insects and worms, all have circulatory systems driven by hearts. Whether they are the closed cardiovascular systems of vertebrates or the open circulatory systems of invertebrates.
The cardiovascular system of all vertebrates (mammals, reptiles, birds, etc.) except for fish has two closed circuits, both start and finish at the heart. The circuits are made up of a network of blood vessels; arteries carry the blood from the heart into capillary webs that feed into veins that carry the blood back to the heart. The heart has four chambers, two atria that receive blood and two ventricles that pump blood out; a pair for each circuit, diagonally opposite each other.
The pulmonary circuit takes deoxygenated blood from the right ventricle of the heart through the lungs to offload carbon dioxide and re-oxygenate the blood, then back to the left atrium of the heart. The systemic circuit takes oxygenated blood from the left ventricle throughout the body, then back to the right atrium, with the oxygen depleted and a load of carbon dioxide waste picked up. The heart is the start, finish and main driving force of both circuits. Smooth muscle within and the actions of nearby skeletal muscles aid the movement of blood returning to the heart through the veins.
Fish have but a single closed circuit and therefore a two-chambered heart. The circuit goes from the heart to the gills where it is oxygenated and slowed, it then moves throughout the body including the cardiac muscle of the heart, before arriving back inside the heart to get another push.
Invertebrates, such as insects and arachnids, have an open circulatory system that transports nutrients and waste products but not oxygen. It has a dorsal vessel where the spine of vertebrates is located. The rear half of this, located above the abdomen, is called the heart. It is a series of chambers with one-way valves between. Insect blood, a green fluid called hemolymph, can only move towards the head of the organism. Each chamber has a pair of muscles to contract it, forcing the hemolymph forwards. The forward portion of the dorsal vessel, above the thoracic cavity, is a simple tube called the aorta which empties out into the head. From there the hemolymph floods the organism’s body cavities, with the motion of the invertebrate causing it to seep towards its rear. It is because of this rearward motion that the dorsal vessel is necessary, to insure the head is amply supplied with nutrients.
The human heart is primarily composed of specialized muscle tissue called the myocardium. This is made up of cardiac muscle cells, found nowhere else in the human body. The cardiac muscle cells are striated or lined in appearance, the same as skeletal muscle, due to the structure of the myofibrils within them that enable the contraction and relaxation to occur. They have fewer nuclei than skeletal muscle cells, usually just one or two, but an even higher concentration of mitochondria to supply them with energy. They are comparatively small and fat, and unlike skeletal muscle cells, they branch. This allows them to connect to multiple other cardiac muscle cells at junctions called intercalated discs. These junctions allow the passage of electrical signals that trigger contraction as well as holding the muscle cells together throughout the cardiac cycle, the contraction called systole and the relaxation called diastole.
The hearts of all multi-cellular animals are pumps that provide the primary motive force for circulation. The circulatory system, whether open or closed, moves most of the materials that need to travel from one part to another in the organism’s body, to enable its life to continue.