An exothermic reaction<span> is a chemical </span>reaction<span> that releases energy by light or heat. It is the opposite of an </span>endothermic reaction<span>. Expressed in a chemical equation: reactants → products + energy.</span>
If it has valves then it is a vein.
Evaporate.
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The wall of the heart<span> consists of three </span>layers<span>: the epicardium (external </span>layer<span>), the myocardium (middle </span>layer<span>) and the endocardium (inner </span>layer<span>). The epicardium is the thin, transparent </span>outer layer<span> of the wall and is composed of delicate connective tissue.
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