Over accumulation of fluid in the interstitial space is generally bad and it affects the proper function of the tissue. This is because the formation of oedema increases the diffusion distance of oxygen and other nutrients. And for the same reason it reduces the diffusional removal of potentially toxic substances of cellular metabolism. This is especially important in the lungs where pulmonary oedema can greatly affect exchange of gasses. Also, certain anatomical structures limit tissue expansion in response to oedema. For example, the kidneys, brain, and the skeletal muscles (Scallan et al., 2010a). This means that small incremental changes in transcapillary fluid filtration will induce a large increase in interstitial fluid pressure.
Congestion is closely related to oedema, poor venous drainage, distension of the veins, venules and capillaries. It may be localised or systemic