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Recommended video: Blood [15:56]
Microscopic appearance of the blood.

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% of adults (hematocrit). The rest consists of liquid plasma (e.g. water, plasma proteins, electrolytes etc.).

The blood is composed of:

  • Cells.
  • Cell fragments.
  • Aqueous solution (plasma).
Key facts about blood
Functions Transports gases (oxygen, carbon dioxide, nitrogen), nutrients, and hormones
Helps with the maintenance of acid-base homeostasis
Maintains a constant body temperature
Thrombogenesis and thrombolysis
Erythrocytes They are round and bioconcave cells without a nucleus, transporting oxygen bound to their heme groups
Leukocytes Neutrophils, eosinophils, basophils, lymphocytes (B,T), and monocytes
Platelets They derive from megakaryocytes and are responsible for homeostasis
Clinical Anemia, leukemia
  1. Function
    1. Messenger and waste removal
    2. Acid-Base Balance
    3. Oxygen supply and carbon dioxide removal
    4. Coagulation
  2. Blood cellular components
    1. Erythrocytes
    2. Leukocytes
    3. Platelets
  3. Anemia
    1. Definition
    2. Therapy
  4. Leukemia
    1. Definition of leukemia
    2. Therapy
  5. Sources
+ Show all


Messenger and waste removal

Blood 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.

Acid-Base Balance

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.

Start with the structure and function of blood with our study unit.

Oxygen supply and carbon dioxide removal

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 a 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.

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 constant body temperature.

Want some practice identifying blood cells? Try our free histology slide quizzes!


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 cellular components


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

Erythrocytes (histological slide)


Unlike mature erythrocytes, leucocytes have a nucleus. Different types of leucocytes can be found in the blood:

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 hematopoietic 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.

Try our free histology slide quizzes to see how well you've learned the different types of blood cells! 


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.

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