Here's why:
* Tissue is a group of similar cells that work together to perform a specific function. For example, muscle tissue, nervous tissue, and connective tissue.
* Organ systems are made up of different organs that work together to perform a major bodily function. For example, the digestive system or the circulatory system.
Blood cells, like red blood cells, white blood cells, and platelets, are individual cells that are suspended in a liquid called plasma. This combination of cells and plasma is what makes up blood, which is considered a connective tissue.
So while blood cells are not tissue or organ systems themselves, they are the building blocks of blood, which is a type of connective tissue.
