Water as a Transport Medium

Transport systems in animals and plants move substances in solution, and water is the liquid that does the carrying. Several properties make it ideal for this.It is a good solvent. Because water is polar, it dissolves glucose, amino acids, ions, vitamins, hormones and urea. Blood plasma is about 90% water and carries all of these around the body. Most carbon dioxide travels in the plasma as dissolved hydrogencarbonate ions.It is liquid over a wide range of temperatures. Water is liquid from 0 °C to 100 °C, which includes the temperatures at which organisms live, so it can flow through vessels.It has cohesion and low viscosity. Hydrogen bonds hold water molecules together, so water flows as a continuous stream and can be pushed through narrow vessels and capillaries by the heart.It has a high specific heat capacity. Blood can carry heat from warm, active organs such as muscle and liver to cooler parts of the body without changing temperature much itself. This helps to keep body temperature stable.It removes waste. Urea and excess ions dissolve in water and are excreted in urine. The volume of water needed to dilute waste is one reason why mammals must drink regularly.Worked example: a person has 5 dm³ of blood, and plasma makes up 55% of blood. If plasma is 90% water, the volume of water in the plasma is5 × 0.55 × 0.90 = 2.5 dm³ (to 2 s.f.)Almost all of this water is available to dissolve and carry substances.