CIE A Level Biology复习笔记8.1.6 The Role of Water in Circulation

Solvent action

• Water is the main component of blood (where it constitutes 95% of plasma, a straw-coloured liquid) and tissue fluid
• Tissue fluid is formed when plasma passes through capillaries and some of it leaks into the spaces between the cells in the walls of the capillary. Tissue fluid is therefore mainly water, too
• Water’s properties as a solvent make it ideal for transport in mammals
• For example, glucose is transported in solution from the small intestine to every cell of the body for respiration. In addition, urea is transported in solution from the liver to the kidneys

Specific heat capacity

• Specific heat capacity is a measure of the energy required to raise the temperature of 1 kg of a substance by 1 oC
• Water has a high specific heat capacity of 4200 J / Kg oC – a relatively large amount of energy is required to raise its temperature
• This means that water is able to absorb a lot of heat without big temperature fluctuations
  • This is vital in maintaining temperatures that are optimal for enzyme activity
• Water in blood plasma is also vital in transferring heat around the body, helping to maintain a fairly constant temperature
  • As blood passes through more active (‘warmer’) regions of the body, heat energy is absorbed but the temperature remains fairly constant
  • Water in tissue fluid also plays an important regulatory role in maintaining a constant temperature

Worked example

Compare and contrast the similarities and differences in the composition of tissue fluid and blood plasma.

Blood plasma and tissue fluid are both mainly composed of water. This is because water is a small enough molecule to pass through the gaps in the capillary walls and into the tissue fluid. Blood plasma and tissue fluid differ because blood plasma contains proteins, while tissue fluid does not. This is because proteins, such as albumin, are too large to fit between the gaps in the capillary wall and so they remain in the blood.