General Facts

General facts about blood

  1. A healthy adult has between 3 and 5l of blood in her or his body – that’s ca. 7% of its weight
  2. It moves due to contraction of the heart muscles, which pump it through vessels in the whole body
  3. Blood (namely the red cells in it) carries oxygen from our lungs to all cells, wastes like carbon dioxide the other way
  4. It consists of 60% fluid plasma and 40% solid parts –White and Red blood cells as well as platelets (they help clotting wounds etc.)
  5. The fluid part is made of water to 90% – so this plasma can be replaced rather easily
  6. Solid parts are build in bone marrow and take more time to be rebuild
  7. Nevertheless red blood cells (RBC) are destroyed and replaced every 120 days
  8. About a billion RBCs are in a few drops of blood
  9. As it reaches all parts, even the smallest ones of our body, nutrients providing power to the cells for work are transported through the blood
  10. The white blood cells are part of our immune system which fights disease and foreign particles
  11. Hormones are like couriers, bringing information from a ‘sender’ to a receiving ‘address’ within our system. They are carried by the blood.
  12. The body temperature is maintained by mechanisms in the blood
  13. Every person’s blood is different – the combination of blood groups and other anti body systems make it individual as a fingerprint
  14. That is why not everyone can receive any unit as transfusion – the anti gene systems would make his or her blood clot.

Blood Components

Red blood cells

Red Blood Cells
Red blood cells (RBCs), also called erythrocytes, contain the protein hemoglobin. This actually ‘carries’ the oxygen through the body by creating a chemical bond – it is fixed to the RBCs. The RBCs rush through small blood vessels called capillaries  in the lungs on their journey through the body. The capillaries are wound around alveoles, little  air sacks. As less oxygen is in the blood compared to the alveole inside it passes through the membrane – and is bound to the RBCs. When they pass body cells in need for oxygene it ‘jumps off’ and exits the blood stream again through the membrane.

The body produces carbon dioxide and needs oxygene. The RBCs ‘pick up’ that ‘waste’ gas when they pass the body’s cells and bring it to the lungs where it is emmitted.

Learn about transportation of oxygen and carbon dioxid through the body and travel with a red blood cell!

The percentage RBCs make in relation to other parts of the blood is called hematocrit. It should be between 40-47% (like the volume of blood that depends on gender, height and weight of an individual).


White blood cells

White Blood CellsThere are several types of WBCs, but all of them are involved in fighting foreign particles which could harm the body. Some are responsible for identifying those, some for disposing. Bacteria, viruses and other dangerous forms of being could just enter and have their destructive influence if they weren’t stopped by the granulocytes, leukocytes and monocytes.

Anti genes

As part of the defense system our body developed several antibody complexes. They detect cells which don’t belong to the body itself (like bacteria etc.) by fusing with them. Certain WBCs will notice and withdrawal the legated structure.

You can find a more detailed description about the immune system and its response here. Lots of lots of details you can find on this website.


If the tunnel like system of blood vessels is damaged through a wound etc without the platelets blood would just flood out and leave the body bleeding. These little cells help to build blood clots which block the new exit. They prevent that wounds make us bleed to death.

See how it works in this video or read more about it. Be informed what happens if the blood clotting becomes pathological


The plasma is the fluid part of our blood, a yellowish liquid consisting of water to 90%. The rest is made of salts, nutrients and hormones. The blood cells ‘swim’ in it.

A nice video about all the blood components explaining their role

Get more information about red and white blood cells, platelets and plasma . A very technical but informative description about blood components and their role you can find here, very nicely it is explained by the American Society of Hematology

Blood Groups

What blood groups exist in humans?

A + (positive), A – (negative), B+, B- , AB+, AB-, O+ (O positive) and O- .

These denotations consist of two parts – the letter and a factor which can be either positive or negative.

What does ABO mean?

The red blood cells (RBCs) have different types of protein complexes on their surface.

For one’s blood group one protein is important. It exists in two different versions – A and B. They may occur single or together, creating 3 of 4 existing major blood group types – A, B and AB. The fourth, 0 ( O or zero) is the condition when neither the  A or B protein is on one person’s cells.


Why is it important not to mix different blood groups when transfusing?

A person with group A has antibodies to the protein complexes of group B in the fluid part of blood, another person with group B has antibodies for group A. So everyone has the antibodies to the protein complexes that he doesn’t have on his own RBCs in the blood plasma.

Those antibodies don’t legate with one’s own red blood cells as they recognize them as part of the body. If group A and B are mixed the antibodies of group A in the blood of the person with group B will bond with the antigenes on the foreign group A RBCs. The blood will clot which can cause serious problems as oxygen supply of certain regions of the body is made impossible.

Blood group – protein on RBCs





Antibodies in Plasma



A and B

Why positive and negative? -The Rhesus factor

Being, for example,  A+ or A- refers to having or not having the other very important protein structure on our RBCs, it is called the rhesus factor. No different versions exist, one can either have it or not, be positive or negative. Just like the previous explained system Rh – persons have the antibodies in the plasma, Rh + ones don’t. This is independent from the ABO-system so 8 major blood groups emerge. Read more about the Rhesus factor.

Are there other things which must match when transfusing blood?

Like these antibody structures around 250 more exist in our blood having a more or less big influence when transfusing blood. It is strongly recommended to have a match which is as close as possible to one’s own blood in order to reduce the body’s immune reaction after transfusion.

What is a universal donor?

As Plasma from Group AB doesn’t contain any antibodies such as RBCs of Group O one can call someone with group AB a universal plasma donor, a person with group O is a universal RBC donor – if they are separated from other parts of the blood (which contains antibodies etc.) every blood group can receive those without risk if the status of the rhesus factor is the same.

Are some blood groups rarer than others?

Occurrence of blood groups

Occurrence of Blood Groups

A and O are the most common blood groups. Approximately 40% of people have group A, around 45% have group O. AB is the rarest blood group, specified AB -.

Who can give blood to who?

You heard about Bombay blood group? Find out what it is and why it is such an issue!

Read more interesting facts about the blood groups and how they are passed on from parents to children.

A great explanation about the role blood plays in our body and what all can go wrong your can find here.

If you really got into the topic you may like to read the wikibook on blood. There is also a quiz in the end where you can test yourself.

Forum Recent Topics

Skip to toolbar