Systole heart beats—is cued by messages from the brain, reflecting the brain's responses to conditions of the body, such as pain, emotional stress, level of activity, and to ambient conditions including external temperature, time of day, etc. Electrical systole opens voltage-gated sodium, potassium and calcium channels in cells of myocardium tissue. Subsequently, a rise in intracellular calcium triggers the interaction of actin and myosin in the presence of ATP which generates mechanical force in the cells in the form of muscular contraction, or mechanical systole. The contractions generate intra-ventricular pressure, which is increased until it exceeds the external, residual pressures in
Composition of the human body g in men, ~2.3 g in women Of the 94 naturally occurring chemical elements, 60 are listed in the table above. Of the remaining 34, it is not known how many occur in the human body. Most of the elements needed for life are relatively common in the Earth's crust. Aluminium, the third most common element in the Earth's crust (after oxygen and silicon), serves no function in living cells, but is harmful in large amounts. Transferrins can bind aluminium. The composition of the human body is expressed in terms of chemicals: The composition of the human body can be