Microcirculation

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Microcirculation
Microcirculation in the capillary
Details
SystemCirculatory system
ArteryArteriole
VeinVenule
Identifiers
MeSHD008833
Anatomical terminology

The microcirculation is the

veins.[citation needed
]

In addition to these blood vessels, the microcirculation also includes

lymphatic capillaries and collecting ducts. The main functions of the microcirculation are the delivery of oxygen and nutrients and the removal of carbon dioxide (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 microcirculation are lined by flattened cells of the

pericytes
. The endothelium provides a smooth surface for the flow of blood and regulates the movement of water and dissolved materials in the interstitial plasma between the blood and the tissues.

The microcirculation contrasts with macrocirculation, which is the circulation of blood to and from the organs.

Structure

Microvessels

Blood flows away from the heart to arteries, which follow into arterioles, and then narrow further into capillaries. After the tissue has been perfused, capillaries branch and widen to become venules and then widen more and connect to become veins, which return blood to the heart.
Transmission electron microscope image of a capillary with a red blood cell within the pancreas. The capillary lining consists of long, thin endothelial cells, connected by tight junctions.

The vessels on the arterial side of the microcirculation are called the

veins. Metarterioles connect arterioles and capillaries. A tributary to the venules is known as a thoroughfare channel.[citation needed
]

The microcirculation has three major components: pre-capillary, capillary, and post-capillary. In the pre-capillary sector, arterioles, and

endothelial cells that allow free movement of some substances.[3]

Microanatomy

Most vessels of the microcirculation are lined by flattened cells of the

pericytes. The endothelium provides a smooth surface for the flow of blood and regulates the movement of water and dissolved materials in the interstitial plasma between the blood and the tissues. The endothelium also produces molecules that discourage the blood from clotting unless there is a leak. Pericyte cells can contract and decrease the size of the arterioles and thereby regulate blood flow and blood pressure.[citation needed
]

Function

In addition to these blood vessels, the microcirculation also includes

lymphatic capillaries and collecting ducts. The main functions of the microcirculation are the delivery of oxygen and nutrients and the removal of carbon dioxide (CO2). It also serves to regulate blood flow and tissue perfusion thereby affecting blood pressure and responses to inflammation which can include edema (swelling).[citation needed
]

Regulation

The regulation of tissue

Noradrenaline and adrenaline have effects on alpha and beta adrenergic receptors. Other hormones (catecholamine, renin-angiotensin, vasopressin, and atrial natriuretic peptide) circulate in the bloodstream and can have an effect on the microcirculation causing vasodilation or vasoconstriction. Many hormones and neuropeptides are released together with classical neurotransmitters.[1]

Arterioles respond to metabolic stimuli that are generated in the tissues. When tissue metabolism increases,

catabolic products accumulate leading to vasodilation. The endothelium begins to control muscle tone and arteriolar blood flow tissue. Endothelial function in the circulation includes the activation and inactivation of circulating hormones and other plasma constituents. There are also synthesis and secretion of vasodilator and vasoconstrictor substances for modifying the width as necessary. Variations in the flow of blood that circulates by arterioles are capable of responses in endothelium.[1]

Capillary exchange

The term capillary exchange refers to all exchanges at microcirculatory level, most of which occurs in the capillaries. Sites where material exchange occurs between the blood and tissues are the capillaries, which branch out to increase the swap area, minimize the diffusion distance as well as maximize the surface area and the exchange time.[4]

Approximately, seven percent of the body's blood is in the capillaries which continuously exchange substances with the liquid outside these blood vessels, called interstitial fluid. This dynamic displacement of materials between the interstitial fluid and the blood is named capillary exchange.[5] These substances pass through capillaries through three different systems or mechanisms: diffusion, bulk flow, and transcytosis or vesicular transport.[3] The liquid and solid exchanges that take place in the microvasculature particularly involve capillaries and post-capillary venules and collecting venules.[citation needed]

Capillary walls allow the free flow of almost every substance in plasma.[6] The plasma proteins are the only exception, as they are too big to pass through.[5] The minimum number of un-absorbable plasma proteins that exit capillaries enter lymphatic circulation for returning later on to those blood vessels. Those proteins which leave capillaries use the first capillary exchange mechanism and the process of diffusion, which is caused by kinetic motion of molecules.[6]

Regulation

These exchanges of substances are regulated by different mechanisms.[7] These mechanisms work together and promote capillary exchange in the following way. First, molecules that diffuse are going to travel a short distance thanks to the capillary wall, the small diameter and the close proximity to each cell having a capillary. The short distance is important because the capillary diffusion rate decreases when the diffusion distance increases. Then, because of its large number (10-14 million capillaries), there is an incredible amount of surface area for exchange. However, this only has 5% of the total blood volume (250 ml 5000 ml). Finally, blood flows more slowly in the capillaries, given the extensive branching.[4]

Diffusion

hydrostatic and osmotic pressures (the so-called Starling forces) in the movement of fluid across capillary endothelium. Lipids, which are transported by proteins, are too large to cross the capillary walls by diffusion, and have to rely on the other two methods.[9][10]

Bulk flow

The second mechanism of capillary exchange is

Starling forces. If the NFP is positive then there will be filtration, but if it is negative then reabsorption will occur.[12]

Transcytosis

The third capillary exchange mechanism is transcytosis, also called vesicular transport.[13] By this process, blood substances move across the endothelial cells that compose the capillary structure. Finally, these materials exit by exocytosis, the process by which vesicles go out from a cell to the interstitial space. Few substances cross by transcytosis: it is mainly used by large, lipid-insoluble molecules such as the insulin hormone.[14] Once vesicles exit the capillaries, they go to the interstitium.[14] Vesicles can go directly to a specific tissue or they can merge with other vesicles, so their contents are mixed. This intermixed material increases the functional capability of the vesicle.[5]

See also

References

  1. ^
    ISBN 978-970-10-7341-4.[page needed
    ]
  2. ISBN 8847011515
    . Retrieved 1 March 2023.
  3. ^ a b c Drucker, René. Medical physiology (1st ed.). Modern Manual. p. 137.
  4. ^
    ISBN 970-729-069-2
    .
  5. ^
    ISBN 978-0470565100
    .
  6. ^
    ISBN 978-84-8086-819-8
    .
  7. ^
    ISBN 9781451113846
    .
  8. ISBN 978-0123875846
    .
  9. PMID 6995154
    .
  10. ^ "Fluid Physiology: 4.1 Microcirculation".
  11. ISBN 9781615040667
    .
  12. ISBN 978-3-13-144061-7
    .
  13. ISBN 978-0071780032
    .
  14. ^
    ISBN 978-0-7234-3388-0
    .
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