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Blood

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Composition of blood

The blood is composed of cells, cell fragments and an aqueous solution (plasma).

Blood makes up about 8% of the human body weight. It contains erythrocytes, leucocytes, thrombocytes (platelets) and plasma. The volume percentage of all blood cells in the whole blood is about 45% in adults (hematocrit). The rest consists of liquid plasma (e.g. water, plasma proteins, electrolytes etc.).

Circulatory system
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Anatomy and function of the circulatory system.

Function

bloodBlood is the most important transport medium in the human body. It transports gases (oxygen, carbon dioxide, nitrogen etc.) as well as nutrients (metabolism) and end products of cell metabolism. Hence the blood has the task of assuring the exchange of substances. It provides the tissues with blood gases and nutrients and in exchange transports end products (e.g. carbon dioxide, urea, uric acid, creatinine etc.) to the eliminating organs (lung, liver, kidney). Furthermore it carries chemical messengers (hormones) to their target organs.

The acid-base homeostasis is regulated in the blood: Through the diffusion of gases between alveoli and blood in the lung (alveolar diffusion) oxygen diffuses from the alveoli into the blood due to the concentration gradient. It is taken up by the carrying protein hemoglobin (hem = iron-containing, globin = protein). Contrariwise carbon dioxide diffuses from the blood into the alveoli due to its higher blood concentration where it is breathed out.

The blood transports the oxygen from the alveoli to the remotest cells of the body. Because of the higher gas pressure in the plasma (relative to the cells) it diffuses to the tissues.

Carbon dioxide diffuses from the cells into the blood due to the higher gas pressure in the tissue. Here it undergoes a chemical reaction and forms carbonic acid (CO2 + H2O → H2CO3) which dissociates into hydrogen ion (H+) and bicarbonate (HCO3-). Thus the metabolism end product carbon dioxide is transported in the form of carbonic acid (or rather hydrogen ion and bicarbonate). In the lung the above mentioned chemical reaction reverses and carbon dioxide is exhaled.

(For detailed information about the complex biochemical processes further studies of physiology and biochemistry textbooks is recommended.)

To sum it up the blood regulates the acid-base homeostasis by the gas exchange. The blood is also responsible for the homeostasis, e.g. balancing the water between the blood capillaries on the one hand and intracellular and extracellular space on the other hand. It also maintains a constant body temperature.

Coagulation factors (proteins) are solved in the blood and stop bleeding after a complex (cascade-like) activation of coagulation factors through damage to blood vessels finally leading to the building of thrombus (thrombogenesis). Simultaneously, fibrinogen/fibrin prevents the pathological development of blood clots in the blood vessels. Blood coagulation and fibrinolysis influence each other and maintain a sensitive equilibrium.

Blood contains erythrocytes, leucocytes, monocytes and platelets. The function of the erythrocytes is the transport of oxygen from the lung to the tissue by bonding oxygen to the iron-containing heme group of the hemoglobin. Erythrocytes are round and have a biconcave shape as they have no nucleus. An erythrocyte has a diameter of 8 to 10 µm. A healthy adult has about 5 million/µl erythrocytes.

The blood group antigens are expressed on the surface membrane of the erythrocytes.

Unlike mature erythrocytes leucocytes have a nucleus. Different types of leucocytes can be found in the blood: neutrophil granulocytes (banded and segmented), eosinophil granulocytes, basophil granulocytes, lymphocytes and monocytes. The normal concentration of leucocytes ranges from 4,000 to 10,000 per µl, depending on age and health status.

Both leucocytes and erythrocytes are descendants of pluripotent hematopoetic stem cells from the bone marrow.

The primary function of leucocytes is the immune defense. Especially lymphocytes (25 to 40% of leucocytes) are responsible for the adaptive immune response, the specific defense from pathogenic germs. The B lymphocytes produce antibodies, whereas T lymphocytes mediate the antibody production and the direct cellular immune response.

Monocytes (4 to 8% of leucocytes) have the task of phagocytosis (e.g. removing foreign materials, bacteria etc.) by producing extremely reactive free oxygen radicals which are capable of penetrating and destroying bacteria wall. Monocytes may differentiate into fixed macrophages (histiocytes) in connective tissue or into free macrophages.

Platelets (thrombocytes) are another type of blood cells. They derive from megakaryocytes (bone barrow giant cells). Their task is the hemostasis when damage to blood vessels occurs (wound closure). The platelets adhere to the vascular wall of the damaged blood vessel and react with fibrin building a solid clot within 1 to 3 minutes (bleeding time). The physiological range for platelets is 150,000 – 400,000/µl.

Pathology of the blood

As a liquid tissue blood may be – as all other tissues - subject to malignant processes. These are disorders of the hematopoetic system. From the wide range of hematologic diseases anemia and leukemia are exemplarily mentioned and discussed in the following.

Definition of anemia

Decrease of the hemoglobin concentration in the blood below the normal level.

The normal range of hemoglobin in male is 14 to 18 g/dl, in women 12 to 16 g/dl. An anemia goes along with a reduction in erythrocytes in the blood (erythropenia). Main symptoms are light tiredness, breathlessness (dyspnea) and headache. Anemia is either acquired or hereditary. Acquired anemia may be due to loss of blood (hemorrhagic anemia), hemolysis, disorders of the blood-forming system, kidney diseases, tumors etc. Hereditary anemia can be caused by abnormal forms of hemoglobin (hemoglobinopathies).

Therapy:

Iron substitution, vitamin B12/B6 supplementation, erythrocyte transfusion in severe blood loss.

Definition of leukemia

Leukemia (known as „blood cancer“) is a disorder of the blood-forming system. It is characterized by an overproduction of immature leucocytes (myelocytes, myeloblasts). These cells spread in the bone marrow suppressing the production of normal blood cells. The blood shows a typical „left shift“ which means that leucocyte precursor cells are detectable in the peripheral blood (differential blood cell count). These cells are usually not found in the peripheral blood of healthy people.

Due to disorders of the hematopoiesis the blood shows in addition to the increase of leucocytes a decrease in erythrocytes (anemia).

Symptoms of leukemia are fatigue, feeling unwell (malaise), increased susceptibility to infection, enlargement of lymph nodes etc.

Therapy:

Cytostatic drugs, allogeneic stem cell transplantation (donator of bone marrow), radiation therapy

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Show references

References:

  • Benninghoff/Drenckhahn: Anatomie, Band 2, 16. Auflage (2004)
  • Silbernagl/Despopoulos: Taschenatlas Physiologie, 7. Auflage, Thieme Verlag (2007)
  • Hick: Intensivkurs Physiologie, 6. Auflage, Elsevier Verlag (2009)
  • Koolmann/Röhm: Taschenatlas der Biochemie, 3. Auflage, Thieme Verlag (2003)
  • Löffler/Petrides: Biochemie und Pathobiochemie, 8. Auflage, Springer Verlag (2007)
  • Herold: Innere Medizin, Auflage 2010

Foto: Flickr / rosmary

Author & Layout:

  • Christopher A. Becker
  • Achudhan Karunaharamoorthy
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