What is the solvent in biological solutions such as cytoplasm and blood?

Cytoplasm and blood both have solvents of water. That's why the human body is mostly made up of water. Blood could also be said to have a solvent of plasma, which is 90% water, with other ingredients (carbon dioxide, fats, and glucose) totaling around 10%. Cytoplasm itself is a solution of mostly water with enzymes and salts dissolved in it, resulting in a gel-like substance.