The blood in our bodies comprises three major components: blood plasma, formed elements, and the extracellular matrix. Blood plasma is a yellowish fluid that constitutes 55% of the total blood volume. It is primarily made up of water and essential substances such as electrolytes and proteins. Blood plasma serves as a medium for transporting blood cells and also contains nutrients, enzymes, hormones, antibodies, and gases.

Formed elements constitute the remaining 45% of the blood volume. These elements include red cells, white cells, and platelets. Red cells, or erythrocytes, account for about 99% of the formed elements. The proportion of erythrocytes in a blood sample's total volume is known as the hematocrit. Mature erythrocytes are anucleated and contain hemoglobin, a specialized protein that binds and transports oxygen and carbon dioxide between the lungs and tissues.

White cells, or leukocytes, are nucleated cells involved in immune responses and play a crucial role in defending the body against infections.

Lastly, platelets, small enucleated cell fragments, are essential for clotting and repairing damaged blood vessels.