Blood is the only fluid tissue with a complex composition (plasma, red blood cells, white blood cells and platelets and other substances in solution) and non-Newtonian behavior. This tissue is enclosed in a system of channels communicating with each other (arterial vessels and venous vessels), in which it can circulate because it is pushed mainly by the heartbeat, but also by venous suction, muscle movements, pulmonary excursions, the force of gravity: so the blood reaches all the districts of the body where it performs multiple functions.

Blood is a particular type of connective tissue, consisting of a corpuscular component (figured elements) suspended in a liquid component, comparable to the extracellular matrix. For the histological analysis of the figured elements, sections are not used but smears are made on glass slides, which are then generally fixed and stained with the May Grünwald-Giemsa method. From a physical point of view, blood possesses the properties of a fluid, which moves in a closed system (circulatory system) by means of the energy provided by the contraction of the heart. The fluid part of the blood is the plasma, an aqueous solution containing about 90% water, proteins, organic components of a different nature than proteins, and inorganic components. Plasma is in continuous exchange with the fluids in the tissues. The figured elements of blood are:

  • the red blood cells or erythrocytes or haematics
  • white blood cells or leukocytes
  • platelets (in Mammals) and thrombocytes (in non-Mammals).

Erythrocytes are the most abundant cells in the blood, specialized in the transport of respiratory gases; they are small (diameter 7.5-8 µm), eosinophilic due to the presence of cytoplasmic hemoglobin (basic protein). In Mammals, erythrocytes are anucleated, discoidal, biconcave (except in Camelids where the shape is ellipsoidal). In non-Mammals they are nucleated, ellipsoidal, biconvex. On the membrane of erythrocytes there are antigens, consisting of glucidic residues bound to proteins and membrane lipids, which determine the different blood groups.

Leukocytes include different cell types: granulocytes (or polymorphonuclear) that are divided into neutrophils, eosinophils or acidophils, basophils; lymphocytes; monocytes. Leukocytes perform their function outside the bloodstream. The following description of the characteristics of leukocytes refers to the human species.

  • Neutrophil granulocytes: they are the most abundant cellular component of leukocytes (50-70%), they present phagocytic activity (polymorphonuclear phagocytes). They have a diameter of 10-12 µm. In the cytoplasm numerous small granules are observed, with low affinity for dyes, both acidic and basic. The nucleus is multilobed and well visible: in the neutrophils of female individuals there is a masserelle of heterochromatin (Barr body or drumstick) joined to the nucleus by a thin and short peduncle; this particular heterochromatin corresponds to the X chromosome inactivated during embryonic development.
  • Eosinophilic (or acidophilic) granulocytes: they constitute 2-4% of leukocytes, of which less than 1% circulate in the blood, the remaining part localized in the red bone marrow and tissues. Their activity is mainly against parasitic infestations. They present a diameter of 12 µm. The cytoplasm is rich in large acidophilic granules, which therefore show up well with eosin. The nucleus is generally bilobed (with the lobes connected by a thin segment of chromatin).
  • Basophilic granulocytes: they represent 0.5-1% of leukocytes, they are responsible for immediate hypersensitivity (anaphylaxis, asthma, urticaria, etc.). They present a diameter of about 10 µm. In the cytoplasm are voluminous granules (larger, but less numerous than the granules of eosinophils), containing sulfur acid glycosaminoglycans (including heparin), responsible for the intense basophilic and metachromatic coloration. The nucleus is bilobed or kidney-shaped, often only partially visible because it is masked by large granules.
  • Lymphocytes: they represent 20-30% of leukocytes and are the ones with the smallest size, although variable: small lymphocytes are the most numerous (diameter 6-9 µm) and large lymphocytes (diameter 9-15 µm). They are the fundamental component of the immune system as they recognize foreign antigens and perform the main effector functions (B, T and NK lymphocytes). They have amoeboid movement and the ability to cross blood and lymphatic vessels (diapedesis). The nucleus is spherical well and occupies most of the cell volume. The cytoplasm is rather small, surrounding the nucleus in a thin, slightly basophilic halo with rare bluish granulations.
  • Monocytes: represent 3-8% of leukocytes and those circulating in peripheral blood are the precursors of tissue macrophages (mononuclear phagocytes). Among leukocytes, they are the largest cells: 12-18 µm in diameter. The nucleus, in eccentric position, is voluminous and generally kidney-shaped; in the cytoplasm some small azzurrophilic granules are visible.

The platelets (present in Mammals) are corpuscular elements that derive from the cytoplasmic fragmentation of megakaryocytes. They are not cells, but cellular fragments, without nucleus but with plasma membrane, with dimensions 1,5÷3,5 µm. They have a spherical or ellipsoidal shape and have a central colored area and a peripheral area less colored. They participate in the phenomena of hemostasis (direct mechanism to close the lesion formed in the vessel wall) and coagulation (formation of the clot or platelet plug).

In non-Mammalian vertebrates there are thrombocytes, real cells with a large central nucleus. Thrombocytes have a function similar to that of platelets but are not derived from the fragmentation of megakaryocytes.

Blood morphology

Cellular Components

  • 95% red blood cells (5 · 106 mm3);
  • 0.13% white blood cells (5 · 103 ÷ 8 · 103 mm3);
  • 4÷5% platelets (250 · 106 ÷ 300 · 103 mm3).

Non-cellular components

  • Water: represents 90% of the plasma;
  • Proteins:
    • fibrinogen: responsible for blood coagulation;
    • albumin: responsible for osmotic pressure at the capillary level;
    • globulins: carriers of lipids and antiviral antibodies;
  • Inorganic salts and ions resulting from their dissociation;
  • Glucose;
  • Urea.

Rheological properties of blood

If the blood was composed only of plasma could be considered, with good approximation, a Newtonian fluid; in fact the plasma is a Newtonian fluid, with a viscosity 0.016÷0.0135 P (or in centipoise 1.16÷1.35 cP) at 37 °C. At the same temperature, the viscosity of water is 0.59 cP. In contrast to plasma, blood presents a non-Newtonian behavior, mainly due to the presence of red blood cells. In fact, blood exhibits a “yield stress”, i.e., a shear stress threshold value (non-Newtonian behavior), below which the fluid moves only with a piston-like flow.

Red blood cells possess extreme flexibility and a tendency to aggregate, demonstrated by the ability to move easily within capillaries up to 3 μm in diameter, or through the pores of microfilters of around 5 μm in size. The behavior of red blood cells varies as the strain rate varies:

  • for values less than 0.1 s-1 red blood cells tend to form rouleaux of 3D structures (sequences of erythrocytes that adhere to each other by dilating along the maximum dimension and reducing thickness;
  • for values equal to about 1 s-1 they rotate as rigid disks around their minor axis, the rouleaux begin to disintegrate due to the normal and tangential actions to which they are subjected in the course of the movement;
  • for values greater than 20 s-1 the cells deform, cease to rotate and translate with their major axis parallel to the axis of the duct.


Literally means “blood shift” (or blood slide): indicates a reaction that always occurs, albeit with different intensity, in all conditions of stress body, when there is a need to increase the flow of blood to the internal organs key to survival (such as the lungs and heart), at the expense of peripheral tissues that have a greater resistance to hypoxia. As a result, the muscles of the peripheral limbs are inevitably deprived of oxygen, which can be partially compensated by effective training aimed at adequate toning of muscle tissue.

Stress and blood redistribution

The orthosympathetic activation of the autonomic nervous system, through the action of catecholamines on the arteriovenous anastomoses present at the level of the deep arterial plexuses of the dermis, of the muscular or visceral districts, involves a displacement of blood from the dermal-cutaneous microcirculation and of the gastro-intestinal apparatus to the cephalic (mainly encephalic), thoracic (heart/lungs) and renal areas: Simultaneously, peripheral vasoconstriction occurs, which, by increasing resistance to blood flow in the tissues, increases blood pressure.

A part of the blood is also directed to the muscular structures; the perfusion area varies according to the type of behavioral response, determined by the “fight & flight reaction”: in case of prevalence of an aggressive fight type attitude, we observe the activation of the cervical-brachial muscles, with consequent blood perfusion to optimize the defensive or offensive activity towards the stressor.

In case of dominance of the flight attitude, typical in case of prevalence of anxiety or fear, the blood will tend to vascularize the musculature of the legs, to facilitate the eventual escape of the subject; in case of dominance fright, that is the feeling of the person to be paralyzed, there may be a paradoxical displacement of blood in depth, with manifestations similar to syncope: the vascular response is predetermined by the instinctual reaction resulting from distress, although it can be influenced by conditioned reflexes, training or education of the person.

This mechanism ensures greater perfusion in the vital organs, which could undergo hypoxic manifestations and put at risk the survival of the individual, and allows a greater availability of oxygen in the functional cells of the “noble” organs, i.e. brain, heart-lungs, kidney. Even within the brain, phenomena of blood redistribution are observed, with a reduction in cortical flow in favor of a selective increase in vascularization at the level of the basal nuclei.

Blood redistribution during immersions

A particular type of blood-shift occurs as a result of diving reflex; the term, used in hyperbaric medicine, identifies the “blood shift” that occurs, albeit with different intensity, in all diving conditions: due to the effect of water pressure, the blood mass is pushed from the periphery of the body towards its center. As a result, the muscles of the peripheral limbs undergo an inevitable subtraction of oxygen, which can be partially compensated by an effective training aimed at an adequate toning of muscle tissue.

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