1 Chemically, ATP is an adenine nucleotide bound to three phosphates. 2  There is a lot of energy stored in the bond between the second and third phosphate groups that can be used to fuel chemical reactions. 3  When a cell needs energy, it breaks this bond to form adenosine diphosphate (ADP) and a free phosphate molecule.
calf muscle pump. calf muscle pump. an action of the calf (soleus) muscles in which the muscles contract and squeeze the popliteal and tibial veins, forcing the blood in those veins to move upward toward the heart. Also called soleus pump.
Why is the heart considered a double pump? A: The heart is a single organ, but it functions as a double pump sending oxygen-depleted blood to the lungs to pick-up oxygen and unload carbon dioxide, while simultaneously sending oxygen-loaded blood coming from the lungs to cells throughout the body.