Blood is often referred to as a connective tissue. Its chief function is to connect blood cells with other tissues and body organs. It helps maintain homeostasis in a number of areas including water, nutrients, gases, hormones, and wastes. Blood is composed of a formed or cellular portion and a fluid portion.

The formed portion of the blood contains the cellular elements of erythrocytes (red blood cells), leucocytes (white blood cells) and platelets.

These elements make up the formed portion of blood and are called this because these blood components are enclosed by a membrane and have a definite structure. They are formed in the bone marrow from generic, undifferentiated cells more commonly known as stem cells. These stem cells are capable of specializing and becoming any type of cell.

The fluid portion of the blood is the plasma that consists primarily of water (92%). The other 8% of the plasma contains dissolved carbon dioxide, proteins (fibrinogen, albumin, globulin), carbohydrates, hormones, vitamins and minerals, and waste products like urea.

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