The heart of a vertebrate is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ. The average human heart beating at 72 BPM, will beat approximately 2.5 billion times during a lifetime spanning 66 years.

Early development

main: Heart development The animal heart is derived from embryonic mesoderm germ-layer cells that differentiate after gastrulation into mesothelium, endothelium, and myocardium. Mesothelial pericardium forms the outer lining of the heart. The inner lining of the heart, lymphatic and blood vessels develop from endothelium. Myocardium develops into heart muscle

From splachnopleuric mesoderm tissue, the cardiogenic plate develops cranially and laterally to the neural plate. In the cardiogenic plate, two separate angiogenic cell clusters form on either side of the embryo. Each cell cluster coalesces to form an endocardial tube continuous with a dorsal aorta and a vitteloumbilical vein. As embryonic tissue continues to fold, the two endocardial tubes are pushed into the thoracic cavity and begin to fuse together and are completely fused at approximately 21 days.

After peaking at about 9.2 weeks after the LMP, it decelerates to about 152 BPM (+/-25 BPM) during the 15th week after the LMP. After the 15th week the deceleration slows reaching an average rate of about 145 (+/-25 BPM) BPM at term. The regression formula which describes this acceleration before the embryo reaches 25 mm in crown-rump length or 9.2 LMP weeks is Age in days = EHR(0.3)+6

There is no difference in male and female heart rates before birth.

Structure

The structure of the heart varies among the different branches of the animal kingdom. (See Circulatory system.) Cephalopods have two "gill hearts" and one "systemic heart". Fish have a two-chambered heart that pumps the blood to the gills and from there it goes on to the rest of the body. In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart.

Birds and mammals show complete separation of the heart into two pumps, for a total of four heart chambers; it is thought that the four-chambered heart of birds evolved independently from that of mammals.

The apex is the blunt point situated in an inferior (pointing down and left) direction. A stethoscope can be placed directly over the apex so that the beats can be counted. It is located posterior to the 5th intercostal space in the left mid-clavicular line. In normal adults, the mass of the heart is 250-350 g (9-12 oz), or about three quarters the size of a clenched fist, but extremely diseased hearts can be up to 1000 g (2 lb) in mass due to hypertrophy. It consists of four chambers, the two upper atria (singular: atrium ) and the two lower ventricles.