Plasma

Plasma is approximately 90% water and is the liquid part of blood. The composition of plasma varies continuously as cells remove or add substances to the blood. If the body is receiving a healthy balanced diet, plasma is kept relatively constant.
There are over 100 different substances which dissolve in plasma, these being nutrients, metal ions (salts), respiratory gases, hormones, plasma protiens and various wastes and products of cell metabolism.
Plasma proteins serve a variety of functions. For example, albumin (the most abundent plasma protein) contributes to the osmotic pressure of the blood, which helps to keep water in the blood stream.