8.1.8—Water as main blood component

Water is the main liquid component of plasma and tissue fluid, so its properties make the circulation a useful transport medium. The key applications here are solvent action and heat distribution.

• Solvent → transport of dissolved substances: water dissolves solutes in plasma → they can be carried in solution through blood and tissue fluid → glucose can move from the small intestine to cells for respiration, while urea can move from the liver to the kidneys for excretion.
• High specific heat capacity → temperature distribution: water can absorb substantial heat with relatively little temperature change → plasma and tissue fluid can take up heat from warmer, active regions and redistribute it in the circulation → body temperature is kept more stable, supporting conditions suitable for enzyme activity.

Boundary: these are circulation applications of water’s solvent and thermal properties; they do not require a catalogue of every property of water or invented plasma chemistry.

Do not describe water as merely filling the vessels: connect each property to the transported material or heat. The carrier is plasma/tissue fluid, whose water-rich phase supports these functions. No image generated or bound.