In insects, other arthropods, and most molluscs, blood bathes the organs directly in an open circulatory system. There is no distinction between blood and interstitial fluid, and this general body fluid is more correctly termed haemolymph. One or more hearts pump the haemolymph into an interconnected system of sinuses, which are spaces surrounding the organs. Here, chemical exchange occurs between the haemolymph and body cells. In insects and other arthropods, the heart is an elongated tube located dorsally. When the heart contracts, it pumps haemolymph through vessels out into sinuses. When the heart relaxes, it draws haemolymph into the circulatory system through pores called ostia. Body movements that squeeze the sinuses help circulate the haemolymph.
The mammalian cardiovascular system : Note that the dual circuits operate simultaneously, not in the serial fashion that the numbering in the diagram suggests. The two ventricles pump almost in unison; while some blood is traveling in the pulmonary circuit, the rest of the blood is flowing in the systemic circuit.
Located beneath the breastbone (sternum), the human heart is about the size of a clenched fist and consists mostly of cardiac muscle. The two atria have relatively thin walls and serve as collection chambers for blood returning to the heart, most of which flows into the ventricles as they relax. Contraction of the atria completes filling of the ventricles. The ventricles have thicker walls and contract much more strongly than the atria—especially the left ventricle, which must pump blood to all body organs through the systemic circuit.
The cardiac cycle. For an adult human at rest with a pulse of about 75 beats per minute, one complete cardiac cycle takes about 0.8 second.
1 During a relaxation phase (atria and ventricles in diastole), blood returning from the large veins flows into the atria and ventricles.
2 A brief period of atrial systole then forces all remaining blood out of the atria into the ventricles.
3 During the remainder of the cycle, ventricular systole pumps blood into the large arteries.
Note that seven–eighths of the time—all but 0.1 second of the cardiac cycle—the atria are relaxed and are filling with blood returning via the veins.
A region of the heart called the sinoatrial (SA) node, or pacemaker, sets the rate and timing at which all cardiac muscle cells contract. Composed of specialised muscle tissue, the SA node is located in the wall of the right atrium, near the point where the superior vena cava enters the heart. Because the pacemaker of the human heart (and of other vertebrates) is made up of specialised muscle tissues and located within the heart itself, the vertebrate heart is referred to as a myogenic heart.
The SA node generates electrical impulses much like those produced by nerve cells. Because cardiac muscle cells are electrically coupled (by the intercalated disks between adjacent cells), impulses from the SA node spread rapidly through the walls of the atria, causing both atria to contract in unison.
The impulses also pass to another region of specialised cardiac muscle tissue, a relay point called the atrioventricular (AV) node, located in the wall between the right atrium and right ventricle. Here the impulses are delayed for about 0.1 second before spreading to the walls of the ventricles. The delay ensures that the atria empty completely before the ventricles contract. Specialised muscle fibres called bundle branches/his and Purkinje fibres then conduct the signals to the apex of the heart and throughout the ventricular walls.
The impulses that travel through cardiac muscle during the heart cycle produce electrical currents that are conducted through body fluids to the skin, where the currents can be detected by electrodes and recorded as an electrocardiogram (ECG or EKG).