Blood is life. "For the life of the flesh is in the blood…" says the word of the Lord. It is primarily liquid, with numerous cells and proteins suspended in it. And this makes the blood thicker than water. An adult body has about five liters (1.2-1.5 gallons) of blood. Blood is approximately 10% of an adult's weight.  

Blood constantly circulates in the human body, providing nutrition and oxygen and facilitating waste removal. It flows through arteries and veins. The arteries carry blood to the other body parts from the heart, and the veins carry it back to the heart.

About half of blood consists of a liquid called plasma. Plasma contains proteins that help clotting, transport substances through the blood, and perform other functions. It also contains glucose and other dissolved nutrients. The other half of the blood volume consists of red blood cells, white blood cells, and platelets. 

• Red Blood Cells (RBC) or Erythrocytes carry oxygen to the tissues.  
• White Blood Cells (WBC) or Leukocytes fight infections and other diseases. Types of white blood cells include Lymphocytes, Monocytes, Eosinophils, Basophils, and Neutrophils.
• Platelets are tiny blood cells that help the body form clots and stop bleeding. 

Bone marrow -the soft, spongy material in the center of the bones - produces blood cells. About 95% of the body's blood cells are produced in the bone marrow. In an adult's body, bone marrow is mainly in the pelvic bones, breast bones, and spine bones. 

Blood cells formed in the bone marrow start out as stem cells from which all other cells with specialized functions are generated. A stem cell is the first phase of all blood cells, and as the stem cell matures, several distinct cells evolve. These include red blood cells, white blood cells, and platelets. Lymph nodes, spleen, and liver help regulate the cells' production, destruction, and differentiation (developing a specific function). 

On average, adult men have more blood than most adult women. However, children have much less blood than adults. Generally, men can lose a little more blood than women before experiencing adverse effects. In the case of children, even slight blood loss could affect a child negatively.