Corpus: Blood

Blood is a tissue that consists of blood cells and a liquid intercellular substance, the blood plasma. It fulfils a variety of transport and regulatory functions and reaches almost all parts of the body via the vascular system.

The total blood volume (total blood volume) of an adult is approx. 5 to 6 litres. The cellular components of the blood make up around 40 to 50 % of the blood volume, the other part is made up of blood plasma.

Histologically, three cell types can be distinguished in the blood:

• Erythrocytes (red blood cells)
• Leukocytes (white blood cells)
• Thrombocytes (blood platelets)

The cells together make up around 45% of the total blood volume (haematocrit). Erythrocytes make up the largest proportion with around 4 to 5 million cells per µl of blood. Platelets follow in second place with around 150,000 to 300,000 cells per µl of blood. However, the volume proportion of thrombocytes (thrombocrit) is only around 0.3 %. The numerically weakest fraction are the leukocytes with only 4,000 to 9,000 cells per μl of blood.

The term "cell" is only partially correct for erythrocytes and thrombocytes, as neither element has a nucleus.

Functions[Bearbeiten]

• Gas exchange: oxygen supply to the tissues, CO₂ removal
• Transport: nutrients, degradation products of cell metabolism, hormones
• Thermal regulation: distribution of body heat
• Immune defence: defence against foreign organisms and toxins by white blood cells and antibodies
• Securing integrity: wound closure through fibrin and coagulum formation.

Flow properties[Bearbeiten]

Blood is a non-Newtonian fluid with special flow properties that are described by haemodynamics and haemorheology. The blood viscosity depends on the cell count (haematocrit) and the plasma viscosity, among other things.

Coagulation[Bearbeiten]

Blood coagulation (haemostasis) creates the prerequisites for ensuring that blood does not leak uncontrollably in the event of injury, but that the bleeding stops again. The basis for this is formed by the thrombocytes and the coagulation factors, which trigger the formation of fibrin in a cascade-like reaction. The end result of haemostasis is a thrombus. If coagulation occurs post-mortem, this is referred to as cruor.

Transport[Bearbeiten]

The blood is pumped away from the heart via the arteries in the large circulation into the capillary system, which feeds the end organs. The blood returns to the heart via the veins. In the so-called small or pulmonary circulation, the blood is enriched with oxygen again and CO₂ (carbon dioxide) is released in exchange. After passing through the pulmonary vessels, the freshly oxygenated blood is ready for recirculation.

The erythrocytes carry certain surface antigens on their cell membrane in the form of glycolipids, which enable them to be categorised into blood groups. More than 20 different blood groups are known in humans. A blood group can be determined by one or more antigens. If blood of different blood groups is mixed, agglutination can occur. The most important blood group systems include the AB0 system and the Rhesus system, in particular the Rhesus factor.

Diseases[Bearbeiten]

Haematology deals with diseases of the blood. Important blood diseases include anaemia and leukaemia.

Laboratory tests[Bearbeiten]

The laboratory examination of the blood (blood test) is one of the most important routine diagnostic methods in medicine. It is usually based on blood taken by peripheral venipuncture. The complete blood count provides an overview of the rough cellular composition. The differential blood count provides more precise information on the composition of the leukocytes.

See also: Blood sample

Blood transfusion[Bearbeiten]

If the blood groups are compatible with each other, blood can be transferred from person to person. This is known as a blood transfusion. It takes place either in the form of whole blood, or more frequently as an erythrocyte concentrate. If you want to avoid using other people's blood, you can donate your own blood.

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