The Composition of Blood

Blood is a fluid connective tissue. It circulates around the body through the blood vessels by the pumping action of the heart.
Blood accounts for about 7% of the human body weight. The average adult has a blood volume of roughly 5 liters, composed of:

1. A fluid called plasma where are the blood cells and other substances.
2. Several kinds of blood cells. Within the blood plasma, are:

• Erythrocytes (red blood cells)
  
• Leukocytes (white blood cells)
  
• Thrombocytes (platelets)

The cells that make up the blood can be seen in Figure

Plasma
Plasma is the golden-yellow liquid part of the blood. Plasma is 90% water and 10% dissolved materials including proteins, glucose, ions, hormones, and gases.
Red Blood Cells
Red blood cells, also known as erythrocytes, are flattened, doubly concave cells that carry oxygen. There are about 4 to 6 million cells per cubic millimeter of blood. Red blood cells are continuously made in the red bone marrow of long bones, ribs, skull, and vertebrae. Each red blood cell lives for only 120 days, after which they are destroyed in liver and spleen.
Mature red blood cells do not have a nucleus or other organelles. They contain the protein hemoglobin which iron and this protein gives blood its red color
White Blood Cells
White blood cells, also known as leukocytes, are generally larger than red blood cells,. They have a nucleus. White blood cells make up less than one percent of the blood's volume. They are made from stem cells in bone marrow. They take part in the immune response. There are five types of white blood cells but the most important are:
Monocytes enter the tissue fluid by squeezing through capillary walls and transform into Macrophages that phagocytose (swallow) cell debris and. bacteria or viruses
Neutrophils also swallow foreign bodies.
Lymphocytes fight infection. They produce antibodies, proteins that travel in blood to identify and neutralize foreign objects, such as bacteria and viruses
Macrophage showing cytoplasmic extensions that allow it to swallow particles or pathogens. In the image here, a mouse macrophage stretches its arms to engulf two particles at once.
Platelets
Platelets, also known as thrombocytes, are small, regularly-shaped clear cell fragments. They are important in blood clotting. Platelets carry chemicals essential to blood clotting. This clot stops more blood from leaving the body when damage to wall of blood vessels occurs

Cells of the blood. From left to right: Red blood cell, platelet, white blood cell.

The Components of Blood