Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells, platelets). Medical terms related to blood often begin in hemo- or hemato- (BE: haemo- and haemato-) from the Greek word "haima" for "blood".
The main function of blood is to supply nutrients (oxygen, glucose) and constitutional elements to tissues and to remove waste products (such as carbon dioxide and lactic acid). Blood also enables cells (leukocytes, abnormal tumor cells) and different substances (amino acids, lipids, hormones) to be transported between tissues and organs. Problems with blood composition or circulation can lead to downstream tissue dysfunction.
The blood is circulated around the lungs and body by the pumping action of the heart..
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 normal pH of human arterial blood is approximately 7.40 (normal range is 7.35-7.45). Blood that has a pH below 7.35 is acidic, while blood pH above 7.45 is alkaline.
Blood pH along with paCO2 and HCO3 readings are helpful in determining the acid-base balance of the body. Blood is about 7% of the human body weight, so the average adult has a blood volume of about 5 liters, of which 2.7-3 liters is plasma. The combined surface area of all the erythrocytes in the human anatomy would be roughly 2,000 times as great as the body's exterior surface. 
The corpuscles are:
- Red blood cells or erythrocytes (96%). In mammals, mature red blood cells lack a nucleus and organelles. They contain the blood's hemoglobin and distribute oxygen. The red blood cells (together with endothelial vessel cells and some other cells) are also marked by proteins that define different blood types.
- White blood cells or leukocytes (3.0%), are part of the immune system; they destroy infectious agents.
- Platelets or thrombocytes (1.0%) are responsible for blood clotting (coagulation)
Blood plasma is essentially an aqueous solution containing 96% water, 4% blood plasma proteins, and trace amounts of other materials. Some components are:
Together, plasma and corpuscles form a non-Newtonian fluid whose flow properties are uniquely adapted to the architecture of the blood vessels.
What is Blood