Explain how high absorption of salt from the diet can lead to a build-up of tissue fluid

High absorption of salt from the diet results in a higher than normal concentration of salt (sodium chloride) in the blood plasma entering capillaries. This means that there is a higher concentration of sodium in the blood and therefore a higher concentration gradient for the diffusion of sodium ions from the blood stream to the interstitial fluid. As a result, more sodium ions will move from the blood to the interstitial or extracellular fluid. Importantly salt (sodium chloride) is dissolved in the blood which means that each sodium ion is surrounded by water molecules (hydrated) and therefore movement of sodium from blood to interstitial space will be accompanied by water. Furthermore, the increased concentration of sodium in the extracellular fluid will further cause water to move out of the surrounding cells, leading to extracellular volume expansion. This will lead to build-up of water/fluid in the tissues.