Circulatory pathways and the human heart

Circulation is of two kinds. In open circulation (arthropods, molluscs), the heart pumps blood into large open spaces (sinuses) around the organs. In closed circulation (annelids, chordates), blood always stays inside vessels, so its flow can be controlled precisely.

In vertebrates, the heart has evolved step by step:

• Fishes: a 2-chambered heart (one atrium, one ventricle). Blood passes through the heart only once per round: single circulation.
• Amphibians and most reptiles: a 3-chambered heart (two atria, one ventricle). Oxygen-rich and oxygen-poor blood mix in the single ventricle: incomplete double circulation.
• Crocodiles, birds and mammals: a 4-chambered heart (two atria, two ventricles). The two kinds of blood are kept fully separate: double circulation.

The human heart is about the size of your fist, made from mesoderm, and lies between the two lungs, protected by a double-layered sac, the pericardium. The heart does not make blood; it is a pump, like the water motor at home that lifts water to the roof tank. The two atria (upper chambers) are separated by a thin wall, the inter-atrial septum; the two ventricles (lower chambers) by a thick inter-ventricular septum. A wall called the atrio-ventricular septum separates each atrium from its ventricle.

Valves keep blood flowing one way only. The tricuspid valve (three flaps) lies between the right atrium and right ventricle; the bicuspid or mitral valve (two flaps) between the left atrium and left ventricle; and semilunar valves guard the exits into the pulmonary artery and the aorta. The ventricle walls are much thicker than the atrial walls, because the ventricles must pump blood much further.

The heart has its own special muscle, the nodal tissue: the SAN (sino-atrial node) in the upper right corner of the right atrium; the AVN (atrio-ventricular node) in the lower left corner of the right atrium; the AV bundle and its branches; and Purkinje fibres spreading through the ventricle walls. This tissue can fire impulses on its own (autoexcitable). The SAN fires the most, about 70-75 times a minute, so it sets the rhythm: it is the heart's pacemaker.