Energy is well-defined as the capacity to do work. Heat is the kind of energy. It is transmitted from the high-temperature object to the low-temperature object. When electricity runs over a wire, energy is transferred from one end of the wire towards another end. Electricity flow in the wire by electrons. When electricity flows, the electrons collide with one another and give off heat due to collision. Less collision of electrons produce less heat, and more collision of electrons produce more heat. When the toaster wire plugged into the electricity supply, electric energy began to flow through the wire—the electric current run over a series of filaments. The filaments are thin and connected. When electric energy flows through the filaments, they glow red hot and transfer the bread. The filaments are present on both sides of the toaster. So the bread cook from both sides at a similar time.
It is necessary to separate oxygenated and deoxygenated blood separate in mammals and birds to maintain their body temperature. Also, if they both are mixed, the blood will get impure and it would be difficult for the heart to supply impure blood. We also can't live with impure blood so we most probably will die.
