Blood is a body fluid which carries oxygen and other nutrients to the cells of human and other animals. It also transports carbon dioxide and other metabolic waste products from the same cells. It is the liquid which pumped by the heart to all parts of the body. Blood also fights infections by its constituents.

Human blood color is red due to protein hemoglobin. Healthy adult produces about 400 to 2000 milliliters in a day.

Main Constituents of blood

 The main constituents of blood are as given below –

1. Blood Plasma – It is a yellowish liquid component of blood holds the blood cells of whole blood in suspension that carries proteins and cells throughout the body. It is the largest component of human blood being about 55 percent of total blood volume in body. Most of the water in blood is about 95 percent by volume that contains proteins (6 to 8 percent) , clotting factors, minerals, hormones, oxygen and glucose.
2. Red Blood Cells – These are the most abundant cells in human blood and contains hemoglobin (iron containing protein) which carries oxygen throughout the body. These cells are about 4.7 to 6.1 million cells per microliter in men and 4.2 to 5.4 million cells per microliter in women (not pregnant). Red Blood Cells reduces than normal count maybe caused due to anemia, bone marrow failure, internal or external bleeding, thyroid disorder and many more.
3. White Blood Cells – These cells protect the human body against infectious diseases and foreign invaders which plays an important role in making strong immune system. White Blood Cells is also known as leucocytes or leukocytes are produced and derived from multipotent cells in the bone marrow. Bone marrow is also called as hematopoietic stem cells. White Blood Cells are present throughout the body including the lymphatic system and blood. The normal count of white blood cells in the blood is about 4500 to 1100 per microliter. White Blood Cells are found in the blood is about only 1 percent.
4. Platelets – These are small blood cells which helps in clot to prevent bleeding from any part of the body and made in the bone marrow along with red and white blood cells. The normal count of platelets is about 1,50,000 to 450,000 per microliter of blood. There is an average of 10 ¹¹ platelets are produced daily in a healthy adult.

Important Note – An healthy individual  has about 4.5 to 5.5 liters of blood circulating inside the body.