Complex sentence: Its composition varies but generally includes water and the dissolved solutes of the main circulatory fluid such as sap or blood, some or all plasma proteins, white blood cells, platelets and (in the case of local vascular damage) red blood cells.
Split into simpler sentences: Its composition varies but generally includes water and the dissolved solutes of the main circulatory fluid such as sap or blood. In the case of blood: it will contain some or all plasma proteins, white blood cells, platelets and (in the case of local vascular damage) red blood cells.