Blood Basics

What is Blood?


Blood is a liquid that circulates throughout the body, carrying oxygen and nutrients to every cell, and carrying away waster products. Blood plays a central role in the body’s defense against intruders.

Blood is pumped by the heart, through a network of miles of blood vessels, to every part of the body. It accounts for 7% of the human body weight, with an average density of approximately 1060 kg/m³. (pure water's density is 1000 kg/m³). The average adult has a blood volume of roughly 5 litres, composed of plasma and several kinds of cells (occasionally called corpuscles); these formed elements of the blood are erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets). By volume the red blood cells constitute about 45% of whole blood, the plasma constitutes about 55%, and white cells constitute a minute volume.

Blood performs many important functions within the body including:

• Supply of oxygen to tissues (bound to hemoglobin which is carried in red cells)
• Supply of nutrients such as amino acids, gluscose and fatty acids(dissolved in the blood or bound to plasma proteins)
• Removal of waste such as carbon dioxide, urea and lactic acid
• Immunological functions, including circulation of white cells, and detection of foreign material by antibodies
• Coagulation, which is one part of the body's self-repair mechanism
• Messenger functions, including the transport of hormones and the signaling of tissue damage
• Regulation of body pH (the normal pH of blood is in the range of 7.35 - 7.45)
• Regulation of core body temperature
• Hydraullic functions

The Origins of Blood

Blood is produced in the bone marrow, a jellylike substance inside the bones. In adults, the spine, ribs, and pelvis are the primary bones that make blood. As the blood cells develop from the stem cells in the marrow, they seep into the blood that passes through the bones and on into the bloodstream.
The different blood cells have different life spans — red blood cells last about 120 days in the bloodstream; platelets about 10 days; and the various kinds of white blood cells can last from days to years.
The body has feedback systems that tell it when to make new blood cells. For example, if bodily oxygen levels are low, the kidneys produce a hormone called erythropoietin, which stimulates the stem cells in the marrow to produce more red blood cells.

Can you accumulate and store blood?

Blood has a limited shelf life.

• Whole blood may be stored for 35 days.
• Red blood cells may be stored under refrigeration for a maximum of 42 days.
• Platelets must be used within 5 days of donation.
• Plasma is generally frozen and must be used within one year.

Because blood is perishable, new donations are needed every day.