Blood is composed of cells and fluid, contain many inorganic and organic molecules. Functions of blood could be falls into three categories: transport, defense and regulation, they all have different function. Blood transport oxygen and nutrients and removes waste from the body. It also transports hormones, which control the body's organ. Blood defense pathogens such as bacteria and viruses and it clots, which prevents the loss of blood. In regulatory functions, blood helps maintain body temperature and the pH body fluids.

A molecule cells use in cellular respiration, transport oxygen, is called red blood cells. Red blood cells also known as erythrocytes, they are very small. There are about 4 to 6 millions of red blood cells in our whole blood. It's containing a respiratory pigment called hemoglobin. It's carries oxygen and it is red in color. In red blood cell, it's contains about 200 million hemoglobin molecules.

If we losing a big amount of hemoglobin in our blood, our blood will become thicker, so then is hard to pumping to our heart. All blood cells are produced from stem cells with in the red bone marrow. Stem cell, which can divide, and producing new cells into specific type of cells. Red blood cells can only live for 120 days, after that, they will destroyed in the liver and spleen and there are about 2 million cells are destroyed per second. Therefore, an equal number much be produced to keep the cell count in balance. People who are lack of hemoglobin will suffer from anemia.

Blood that playing the role of defends is the white blood cell. White blood cells also known as leukocytes are differ from red blood cells. It's fight infection, they are more largely and being lack hemoglobin. White blood cells are came from the stem cells in the red bone marrow. They defend us against pathogens that have invaded into our body. White blood cells are classified into two leukocytes, which are granular and agranular.

Both are surrounding the nucleus. The granules contain enzymes and proteins, which helps the white blood cells to defend. There are types of cells in granular are neutrophils, eosinophils and basophils. Neutrophils respond to an infection. Eosinophils known to increase the number of cells while infection. Basophil is dilates blood vessel caused contraction of smooth muscle.

Inside Agranular, there are two types of cell, which are Monocytes and lymphocytes. Monocytes are responsible for defense to particular pathogens and their poison substance. Lymphocytes are in B and T type. B is target pathogens or destruction. T is destroying any cell that has antigens. People, who have large amounts of immature white blood cells, will suffer from leukemia.

When a blood vessel in the body is breaks into two parts, platelets formed element to put them back together. Produced at a rate of 200 billion a day. Their job is involved in the process of blood clotting. Functions of plasma proteins are to maintain homeostasis. Plasma contains mostly water and small amount of proteins, which produced by the liver which, occurs in 3 categories: album is, globulin's, and fibrinogen. The plasma proteins helps maintain pressure, regulate pH and transport molecules.

The most common system of the blood is ABO system, which are A, B, AB, and O. It is important to determine their own blood, because whenever that person have an anti-A with and type A blood, the red blood cell will clumping and it will cause blood to stop circulating in small blood vessel which may cause that person to death and when we do a blood transfusions, the donor match with the recipient's blood. This is how the ABO system works, to determine each own blood.