Bruising that happens easily (by easy I mean from even the smallest knocks), is due to capillary fragility in which the smallest branches of your blood vessels are so thin and weak that they fail to contain the blood within them. A slight bang causes these weakened blood vessels to break and the blood gets into the surrounding tissues. This bleeding causes the black and blue appearance that I particularly see in women.
Ideally our capillaries should withstand a certain degree of pressure upon them. Research shows us that most cases of capillary degeneration and fragility is due to a deficiency of vitamin C and its complex, the bioflavanoids.
The main cause in most people is refined foods in the diet particularly grains. Removal of these often brings a great improvement in tissue integrity. However I see many people with pretty good diets (compared to the usual junk food uk diets), with fresh vegetables, fruit and even supplements of vitamin C and bioflavanoids. The reason behind the continued deficiency? Underfeeding yet again, see my previous posts on semi-starvation.