Blood (Latin: sanguis, Greek: haima) is a suspension of different cells in a saline solution containing proteins. It makes up approximately 6-8 % of the body weight and has a pH value of 7.4. The blood is responsible for numerous essential functions and is a crucial point of the whole organism. This article provides an overview of the important facts, in order to better understand the whole concept of the "organ" blood.
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System of hematopoiesis in bone marrow

Image: “Hematopoietic System of Bone Marrow” von Phil Schatz. Licence: CC BY 4.0

Blood components

If a centrifuge is used for a blood sample, a more or less cloudy liquid (depending on the blood’s fat content) can be seen from which red sediment has settled. The liquid component represents the blood plasma and the sediment the cruor of the blood cells. The ratio of plasma to blood cells is indicated by the hematocrit. Physiologically, it is at 42 % in women and at 45 % in men.

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Blood plasma

The plasma makes up approximately 55 % of the blood. It contains water, proteins, ions, hormones and physically dissolved gases. Furthermore, serum can be extracted from the blood plasma, which is free of clotting factors.

Composition of Blood

Image: “Composition of Blood” by Phil Schatz. License: CC BY 4.0

Normal values of the plasma electrolytes:

  • Na+  140 mmol/L
  • K4.5 mmol/L
  • Ca2+ 2.4 mmol/L
  • Cl– 105 mmol/L
  • HCO3–   24 mmol/L

Therefore, plasma has osmotic pressure of 5.600 mmHg and osmolarity of 290 mOsm/L. Plasma proteins represent the largest part of the plasma, around 75 g/L. They barely contribute to the osmolarity but play an important role for the colloid osmotic (also: oncotic) pressure, meaning the osmotic pressure between plasma and interstitium. With 25 mmHg, it is significantly higher than that of the interstitium (5 mmHg). It prevents a diffusion of too much plasma water from the blood vessels into the tissue.

Albumin is the most common plasma protein (54-60 %). Therefore, it is essential for the oncotic pressure. Albumin is produced in the liver and is one of the smallest plasma proteins. It serves as carrier for cations, bilirubin, fatty acids and cholate.

Other important plasma proteins are the globulins, which are classified as α, β and γ. Just like albumin, α and β globulins act as carriers. Thus, for example, LDLs (low density lipoproteins) that carry fat and HDLs (high density lipoproteins) are important representatives of globulins.

γ globulins contain immunoglobulins (Ig) and have immunological functions. They are divided into five chemically different Ig classes, whereby the immunoglobulins IgG, IgA and IgM are particularly present in the plasma.

Blood components in % Blood subcomponents in % Type in % (if any) Place of production Major functions
Plasma 43 – 63 %  Water 92 % Liquid Resorbed by the intestinal tract or produced during metabolic processes Transport medium
Plasma proteins 7 % Albumin 54 – 60 % Liver Maintains the osmotic balance; transports lipid molecules.
Globulins 35 – 38 % Alpha – Globulins – Liver Transport functions; maintains the osmotic concentration.
Beta – Globulins – Liver Transport functions; maintains the osmotic balance.
Gamma – Globulins (Immunoglobulins) – Plasma cells Immune response
Fibrinogen 4 -7 % Liver Blood clotting during hemostasis
Regulatory Proteins < 1 % Hormones and Enzymes Various places Regulate various bodily functions
Further dissolved substances 1 % Nutrients, gases and waste substances Resorbed by the intestinal tract, exchanged into the respiratory tracts or produced by cells Numerous different functions
Form elements 37-54 % Erythrocytes 99 % Erythrocytes Red bone marrow Transport gases, mostly oxygen and some carbon dioxide
Leukocytes <1 %, platelets <1 % Granular Leukocytes: Neutrophils, Eosinophils and Basophils Red bone marrow Unspecific immunity
Agranular Leukocytes: Lymphocytes, Monocytes Lymphocytes: Bone marrow and lymphatic tissue Lymphocytes: Specific immunity
Monocytes: red bone marrow Monocytes: Unspecific immunity
 Platelets <1 % Megakaryocytes red bone marrow Hemostasis

The blood cells


In a blood smear specimen, the red blood cells are easily recognizable by their biconcave shape. Their diameter is about 7.5 µm, and they are the most common blood cells with 4.5 x 106 /µL in women and 5.2 x 106 / µL in men. Erythrocytes contain hemoglobin. They have neither a cell nucleus nor cell organelles and, therefore, only have a lifespan of about 120 days before being broken down in the spleen.

Erythrocytes are generated in the red bone marrow and are stimulated by erythropoietin, which is dumped by the kidneys. The last preliminary step to erythrocytes is the reticulocyte, which also has no cell nucleus. It can be detected in blood smear specimen in cases of increased new synthesis (i.e., after heavy blood loss).

Erythrocytes have a larger diameter than some capillaries. Therefore, the malleability of red blood cells is extremely important. This is secured by their special red cell membrane cytoskeleton made of ankyrin, spectrin and actin.


With a diameter of only 2.5 µm, platelets are significantly smaller than erythrocytes and biconvex in their free floating form. The blood of a healthy human contains 150,000-400,000 thrombocytes/µL blood. These cell fragments without cell nuclei are produced by megakaryocytes of the bone marrow and circulate within the blood for approximately ten days if they are not activated for blood clotting.


Leukocytes are the blood’s immune cells. Their number varies from between 4,000 to 10,000/µL blood. They are divided into monocytes, granulocytes and lymphocytes.

  1. Monocytes stand out in blood smear specimen because of their size (15-20 µm). They usually have a kidney-shaped nucleus, and their cytoplasm appears pale gray under the optical microscope. Monocytes are very rare (6 % of the leukocytes) and circulate only between one and three days in the blood. During this time they mature to macrophages, which then migrate into the tissue. Therefore, they can barely be detected in blood smear specimen.
  2. Granulocytes have a size of 10 – 12 µm, are produced in the red bone marrow like all blood cells and are divided into three groups:
    1. Neutrophil granulocytes represent the largest part of leucocytes and mostly have a segmented cell nucleus. Therefore, they are called segmented neutrophils. Only 8% of the neutrophil granulocytes are banded (= banded neutrophils). The granules of the neutrophils appear pale pink in the Pappenheim stain.
    2. Eosinophil granulocytes constitute only 3% of leucocytes and have a cell nucleus consisting of two segments. Their granules stand out in blood smear specimen because of their brick-red coloring.
    3. Basophil granulocytes are the least common in blood smear specimen. The granules mostly cover the cell nucleus and are dark blue in color.
  3. Lymphocytes exhibit a round, purple-dyed nucleus that is surrounded by a thin, pale-gray cytoplasm layer. They are classified as natural killer cells as well as B- (bone marrow) and T-lymphocytes (thymus). However, the three types are not distinguishable using the light microscope.

Functions of the blood

Erythrocytes are responsible for transporting gases

The oxygen content of a healthy person’s arterial blood is about 200 mL O2/L of blood. Hemoglobin (Hb) binds 85% of the oxygen. The remainder is physically dissolved in the plasma.

Hemoglobin is located in erythrocytes, and the adult Hb consists of 2α and 2β subunits. Each subunit has a Fe2+ ion that binds O2. Thus, one Hb can transport four oxygen molecules. The Hb concentration in men is 16 g/dL and in women is 14 g/dL.

In the periphery, Hb delivers the oxygen to the tissue and accepts a part of the carbon dioxide. The majority of CO2, however, is transported by erythrocytes as bicarbonate and exhaled via the lungs.

Thrombocytes ensure the clotting of the blood

The clotting of the blood is called hemostasis and is divided into primary and secondary hemostasis.

Primary hemostasis

Primary hemostasis is triggered by damage to the vascular endothelium when thrombocytes come into contact with collagen located outside the vessel. Thereby, the von-Willebrand factor in collagen binds to its receptor, the glycoprotein Ib of the thrombocytes, which leads to the adhesion of the thrombocytes to the damaged endothelium.

Simultaneously, Ca2+ activates the platelets. This means that the tubulin ring within the thrombocytes contracts, thus creating pseudopodia. Together with adenosine diphosphate (ADP), which is released from damaged cells, a thrombocyte aggregation takes place. A blood clot is formed. In addition, the activated thrombocytes secrete serotonin, fibrinogen and thromboxane A2. The latter also causes vasoconstriction and hereby promotes the closure of the vessel opening.

In order to prevent the clot from clogging the entire vessel, the endothelium releases prostacyclin which inhibits the aggregation of thrombocytes. Thus, a targeted clot formation at the damaged endothelium is assured.

Secondary hemostasis

Secondary hemostasis is divided into extrinsic and intrinsic system.

The extrinsic system intervenes in cases of tissue destruction and is very fast, with a speed of about 14 seconds thromboplastin time (quick test). The tissue factor from vascular muscle cells forms a complex with phospholipids. This is called tissue thromboplastin. It binds the coagulation factor VII which activates the factor X in the presence of Ca2+.

In the intrinsic system, factor XII comes in contact with negatively charged surfaces, such as collagen (or glass), and is thereby activated. Thereafter, factor XI and factor IX are activated. Factor IX forms an enzymatic complex with phospholipids and Ca2+ ions, which, in turn, activates factor X (thrombokinase) via limited proteolysis. This process can be accelerated rapidly by factor VIII activated by thrombine. In this case, the partial thromboplastin time is 40-50 seconds.

After activation of factor X, the intrinsic and extrinsic systems converge. Factor X, together with factor V, phospholipids and Ca2+ ions, form the prothrombin activator which splits factor II (prothrombin) into thrombin. Thrombin, in turn, splits factor I (fibrinogen) into soluble fibrin, which turns firm via factor XIII and Ca2+ ions and forms a red thrombus.

The process of preventing blood clots is called fibrinolysis. Hereby, plasminogen is activated by blood factors and becomes plasmin. It splits fibrin into individual peptides. Furthermore, it splits the clotting factors I, II, V, VIII, IX, XI and XII and thus decreases the clotting ability of the blood.

Leukocytes ensure the defense of foreign bodies

Neutrophil granulocytes function as fast defense mechanisms in cases of infections and thus belong to the unspecific defense of the immune system. In an infection, the number of neutrophils increases quickly by pulling the reserves from the marginal pool (at the endothelium of large veins) or the bone marrow. They fight bacteria and can break down debris of the body’s own cells. The suspension of dead neutrophils and tissue debris in the lymph is called pus.

Eosinophil granulocytes serve the defense of worm parasites.

Lymphocytes, except for natural killer cells, belong to the specific immune system. In the thymus, T-lymphocytes are trained to differentiate between foreign antigens and endogenous antigens and react accordingly. In the process, helper T-cells as well as cytotoxic lymphocytes are developed, which support the mechanisms of the specific and unspecific immune defense, and which recognize virus-infected or degenerated cells and kill them.

B-lymphocytes can be activated as plasma cells that produce free antibodies which bind the foreign antigens and render them harmless. Thus, B-lymphocytes belong to the humoral component of the specific defense. During the activation process, one part of the B-lymphocytes differentiate themselves as memory cells which remain in the body for many years. In case of renewed contact with the antigen, they already know which antibodies are needed.

Natural killer cells are part of the unspecific immune system and can cause apoptosis in degenerated cells because of their cytolytic granules.

Formula elements Main sub-types Amount per microliter and importance (scale) Appearance in the standard blood smear specimen Summary of functions Comment
Erythrocytes (red blood cells)Rote-Blutzellen  5.2 million (4.4 – 6.0 million) Flattened biconcave shape; no nucleus; light-red color Transport oxygen and carbon dioxide between tissue and lungs. Lifespan of about 120 days.
Leukocytes (white blood cells) 7000 (5000 – 10,000) Significantly dark-colored nucleus All functions of the immune system. Exiting the capillaries and migrating into the tissue, lifespan: usually a few hours or days.
Leukocytes Granulocytes 4360 (1800 – 9950) Plenty of granules in the cytoplasm; usually capped nucleus Unspecific disease resistance Classified by membrane-bound granules in the cytoplasm
NeutrophilsNeutrophile-Granulozyten 4150 (1800 – 7300) Amount of nuclear lobes increases with age Phagocyte; particularly effective against bacteria, releasing cytotoxic chemical form granules. Most common leukocyte, lifespan: minutes up to days.
EosinophilsEosinophile-Granulozyten- 165 (0 – 700) Nucleus generally two-lobed; bright, red-orange granulate Phagocytic cells; particularly effective with antigen-antibody complex; releasing histamines. Lifespan: minutes up to days.
BasophilsBasophile-Granulozyten 44 (0 – 150) Nucleus generally two-lobed but due to heavy, dense and dark purple granulate difficult to detect. Promote infections Rarest leukocyte, lifespan: unknown
Agranulocytes 2640 (1700 – 4950) Lack of granulate in the cytoplasm; have a simply shaped nucleus that can be indented. Immune defenses Group consists of two major cell types of different origin.
Lymphocytes480px-Blausen_0625_Lymphocyte_T_cell 2185 (1500 – 4000) Spherocytes with a single, often large nucleus, making up a large part of the cell volume; dark spots; appears as a large (natural killer cells) and a small (B- and T-cells) variant Primary specific (adaptive) immunity; T- cells directly attack other cells (cellular immunity); B-cells release antibodies (humoral immunity); natural killer cells are similar to T cells, but unspecific. Initial cells originate in the bone marrow; secondary production takes place in the lymphoid tissue; several different subtypes; Memory cells form after contact with a pathogen and ensure a faster reaction. Lifespan: many years.
MonocytesMonozyten 455 (200 – 950) Largest leukocyte with indented or hoof-shaped nucleus  Particularly effective phagocytes, engulf pathogens or damaged cells. They also serve as antigen-presenting cells for other components of the immune system. Produced in the red bone marrow. Designated as macrophages after leaving the circulation system
PlateletsBlutplättchen 350,000 (150,000 – 500,000) Cell fragments which are surrounded by the plasma membrane and contain granules; violet spot. Hemostasis and release of growth factors in order to repair and heal tissues. Derived from megacaryocytes which remain in the red bone marrow and release platelets in the circulation system.

Blood diseases

Inflammatory values in the blood

Inflammatory values play an important role in diagnosis. They are obtained by examination of the blood. Here, the erythrocyte sedimentation rate, the C-reactive protein and the number of leukocytes is mostly determined. In addition, a differential blood count can be made as well.

To determine the erythrocytes sedimentation rate, the blood is stored vertically in a sedivette for two hours. After every hour the level of the erythrocyte column is defined. Normal values for men after one hour are below 15 mm and for women below 20 mm. A change of these values may indicate anemia, inflammations and even cancer.

The C-reactive protein belongs to the immune system and is deemed to be a marker for acute inflammations. In a healthy person, its concentration in the blood is below 10 mg/L.

The number of leukocytes is determined by a machine. If the numberis too high, this is called leukocytosis. If the value is lowered, however, it is called leukopenia.

To produce a differential blood count the leukocytes are identified in subtypes in the blood smear. In 100 leucocytes, there are normally four monocytes, 58 neutrophils (of which only 3 are segmented), one eosinophil, up to one basophil and 35 lymphocytes. If the number of banded neutrophils is significantly increased, then it is called a left shift. It indicates a bacterial infection. If there are too many hypersegmented granulocytes, there is a shift to the right. This may indicates a vitamin B12 deficiency.


  • Iron deficiency anemia: erythrocytes are microcytic and hypochromic, cause: chronic blood loss, insufficient iron intake.
  • Megaloblastic anemia: erythrocytes are megacytic and normochromic, cause: vitamin B12 deficiency, folic acid deficiency, alcohol.
  • Renal anemia: erythropoietin, cause: renal failure.
  • Aplastic anemia: erythrocyte deficiency, cause: bone marrow damage.
  • Hemolytic anemia: erythrocytes are degraded too strong, cause: incorrect cytoskeleton (spherocytosis, sickle cell anemia, thalassemia), malaria, sepsis, poisoning)


Peripheral blood with chronic lymphocytic leukemia

Image: “Peripheral blood with chronic lymphocytic leukemia,” by Gabriel Caponetti. License: CC BY-SA 3.0

Leukemia is characterized by a large number of leukocytes, in particular of their precursors. These precursors have no function in the immune defense yet. They spread in the bone marrow, the blood and the lymphoid organs and thus reduce their function. This interferes with the formation of other blood cells. The result is aplastic anemia, decreased blood clotting and a dramatically weakened immune defense.

Coagulation disorders

Coagulation disorders are either innate or acquired. An increased bleeding tendency is mostly based on malfunction or lack of a coagulation factor:

  • Hemophilia A: Factor VIII deficiency;
  • Hemophilia B: Factor IX deficiency;
  • Von Willebrand disease: Von Willebrand factor deficiency;
  • Vitamin K deficiency: decreased synthesis of factors II, VII, IX and X.

Popular exam questions regarding blood

The solutions are below the sources.

1. The blood plasma is designated as:

  1. The remaining liquid after the coagulation of the blood.
  2. The ionized components of the blood.
  3. An isotonic saline solution.
  4. The supernatant after centrifugation of the blood.
  5. The cellular amount of the blood.

2. What substance serves the platelet aggregation best?

  1. Factor IX
  2. Factor XI
  3. Fibrinogen
  4. Factor X
  5. Plasmin

3. Iron deficiency anemia is most commonly caused by:

  1. Chronic blood loss.
  2. Vitamin B12 deficiency.
  3. Increased iron excretion by renal failure.
  4. Increased alcohol consumption.
  5. Damage of the blood-forming bone marrow.
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