The composition of blood consists mainly of plasma and blood cells. Plasma makes up 55% of the volume and contains water, salts, electrolytes, hormones, and waste products. Blood cells make up 45% of the volume, divided into three types: erythrocytes, leukocytes, and thrombocytes. Erythrocytes carry oxygen and are the most abundant in blood. Leukocytes are responsible for immunity and vary between 4,000-11,000 per microliter. Thrombocytes help in blood clotting and range between 150,000-450,000 per microliter