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.
All veins have valves. These valves have two functions: 1  When leg muscles contract and squeeze the veins, the vein valves work to keep venous blood flowing one way, upwards, towards the heart.  When leg muscles relax, the vein valves hold up the weight of the column of blood returning to the heart.