If the cellular disruption has increased at any part of our body, meaning that tissue is defending itself against a pathological case, the cells of that tissue break down, releasing their components in blood. Hence, the increased amount of certain components show us that there is an abnormal functioning of those tissue cells.
For instance, the level of a metabolic component in our blood might be lower than expected compared to the normal references. For example, if iron deficiency in the blood might indicate low intake, low absorbtion in the intestines, a bleeding problem or decreased capacity of its transport in blood.
On the contrary, the level of a metabolic component in our blood might be higher than expected compared to the normal references. This may be due to an overproduction of a certain metabolite due to benign/malign tumor, or an issue with clearance from the kidneys.
Similarly, some of our cells might increase or decrease in our blood due to various reasons. The distribution and size of our red blood cells (erythrocytes), types of white blood cells (leukocytes) or the distribution of coagulation cells (thrombocytes) give us information about many blood diseases.