An adult human has four to six litres of blood, depending on body size.
Blood is essential to human life. The main task of the blood and the circulatory system is to transport oxygen and nutrients to the cells, and carbon dioxide and waste products away from them. Blood also plays an important role in regulating body temperature.
Blood consists of
- Plasma – salts, water and proteins.
- Red blood cells (erythrocytes) – contain haemoglobin, which carries oxygen from the lungs and contains iron.
- White blood cells (leukocytes) – defend the body against infection and produce antibodies.
- Platelets (thrombocytes) – make the blood coagulate.
Plasma makes up the largest part of the blood and consists of about 92 % water.
Blood groups
The International Society of Blood Transfusion (ISBT) maintains the register of recognised blood group antigens. It currently lists 398 antigens across 48 blood group systems. The most important clinically are AB0 and Rh. The Rh factor, today called D, is present in about 85 % of the Swedish population (RhD positive); the remaining 15 % are RhD negative.