Microcirculation – Intra-vascular, Extra-vascular, Vascular Wall

Microcirculation is the circulation of the blood in the smallest blood vessels, present in the vasculature embedded within organ tissues.  This contrasts with macro-circulation, which is the circulation of blood to and from the organs.  The micro-circulation is composed of terminal arterioles, capillaries, and venules which drain capillary blood. The vessels on the arterial side of the micro-circulation are called the arterioles, which are well innervated, are surrounded by smooth muscle cells and 10 – 100 µm in diameter.  Arterioles carry the blood to the capillaries, which are not innervated, have no smooth muscle, and are about 5-8 µm in diameter.  Blood flows the capillaries into the venules, which have little smooth muscle and are 10-200 µm. The blood flows from venules into the veins.  In addition to these blood vessels, the micro-circulation also includes lymphatic capillaries and collecting ducts.  The main functions of microcirculation are the delivery of oxygen and nutrients and the removal of CO2.  It also serves to regulate blood flow and tissue perfusion thereby affecting blood pressure and responses to inflammation which can include edema (swelling).  Most vessels of the micro-circulation are lined by flattened cells, the endothelium, and many of them are surrounded by contractile cells the smooth muscle or pericytes.  The endothelium provides a smooth surface for the flow of blood and regulates the movement of water and dissolved materials in the plasma between the blood and the tissues.  The endothelium also produces molecules that discourage the blood from clotting unless there is a leak.  The smooth muscle cells can contract and decrease the size of the arterioles and thereby regulate blood flow and blood pressure.

Main disorders of microcirculation are divided into 3 main groups:-

1.    Intra-vascular

2.    Extra-vascular

3.     Changes in the components of the vascular wall

Intra-vascular: – These disorders maybe because of some reasons i.e. Can develop because of aggregation of blood cells (predominantly erythrocytes) and agglutination of erythrocytes.  In order to differentiate agglutinations and aggregation, the agglutinations are an irreversible process.  In the process of erythrocyte aggregation may develop separation of blood.  Aggregate generally consists of erythrocytes which close the entry in pre-capillary regions and therefore these capillaries contain only plasma. The next intravascular disorder of microcirculation is increased coagulation of blood (dis-coagulation of blood and thromboembolism).  The reaction of thrombocytes and factors of blood coagulation on the damaged tissues results in the formation of thrombus, difficult in circulation.  Disorders of micro-circulation are also related to decreased flow of blood.  Decrease blood flow may be because of: –

      1.     Change in rheological properties of blood.
2.     Disturbed outflow of blood from organs.
3.     Decreased arterial blood flow.
4.     Disorders of blood in the arteriovenous anastomosis.  Decrease blood flow results in thrombus formation. 

Extra-vascular: – these disorders develop as the result of damage of stroma cells and parenchyma of tissues surrounding the microcirculatory unit.  These disorders involve different etiological factors: disorders of perivascular structure, the action of mast cells which contain biologically active substances in their granules, disorders of the lymphatic system, neuro-dystrophic tissue process. 

Changes in the components of the vascular wall: – these disorders maybe because of the pathology of the endothelium of micro-vessels and attachment of blood cells to them, change in vessels permeability, emigration, and diapedesis of blood cells;