Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells, platelets).

Human blood is a liquid tissue; its major function is to transport oxygen necessary to life throughout the body. It also supplies the tissues with nutrients, removes waste products, and contains various components of the immune system defending the body against infection. Endocrine hormones also travel in the blood. There are about 6 quarts (or 5.6 liters) of blood in an average human body, accounting for ~8% of body mass. Adult humans have ~60 millilitres of blood per kilogram of body weight.

Human blood is red, ranging from bright red when oxygenated to dark red when not. It owes its colour to hemoglobin, a respiratory protein containing iron in the form of heme, to which oxygen binds. Because veins are blue, there exists a popular misconception that deoxygenated blood is blue and that blood only becomes red when it comes into contact with oxygen.

Anatomy of blood :

Blood is composed of several kinds of corpuscles; these formed elements of the blood constitute about 45% of whole blood. The other 55% is blood plasma, a yellowish fluid that is the blood's liquid medium.

The corpuscles are:

• Red blood cells or erythrocytes (about 99%). These corpuscles lack a nucleus and organelles, so are not cells strictly speaking. They contain the blood's hemoglobin and distribute oxygen. The red blood cells also give rise to the system of blood types.
• Platelets or thrombocytes (0.6 - 1.0%) are responsible for blood clotting or coagulation.


• White blood cells or leukocytes (0.2%), are part of the immune system; they destroy infectious agents.

• albumin
• blood clotting factors
• immunoglobulins (antibodies)
• hormones
• various other proteins
• various salts

The normal pH of arterial blood is approximately 7.40.