The biochemical processes we have been studying take place either in the aqueous solutions inside cells, in suspended particles, or on membranes. The water in cells makes up about two-thirds of the body's water. Of the remaining third, the most important part is the blood.
Blood has many functions.
Perhaps the most readily apparent is transport. The role of the blood in transporting oxygen from the lungs to cell mitochondria where it combines with electrons and hydrogen ions in oxidative phosphorylation is familiar. The corresponding transport of carbon dioxide from the citric acid cycle to the lungs for exhalation is perhaps less obvious. All of the carbohydrate, fat and protein products from digestion are transported by the blood or related systems to the sites of their further processing. We've noticed before that lactate produced under anaerobic conditions from pyruvate is transported by the blood to the liver for conversion back to pyruvate and then into gluconeogenesis.