36- Physiology of microcirculation. Functional organization of the microcirculatory unit. Organ related specific of the capillaries. Control of the microcirculation

microcirculation

Microcirculation- part of the circulatory system,, The microcirculation is made of the arterioles, capillaries, and venules, terminal lymphatic cells.

flow of blood within the smallest vessels.

• Occurs in vessels typically <100 micrometres in diameter

• Controls tissue perfusion, oxygen delivery, and waste removal

• Regulated by local metabolic demand, neural, and hormonal signals

• Maintains fluid balance via Starling forces (hydrostatic and oncotic pressures)

These vessels are embedded in the organ tissues.

1. Arterioles- Arterioles are small blood vessels that branch off from arteries and lead into capillary beds. They are primarily responsible for regulating blood flow and blood pressure through vasoconstriction and vasodilation, controlled mainly by sympathetic nerve fibers and local metabolic signals.

2. capillaries one cell thick for diffusion

• There are 3 main structural classifications:

• The capillaries are known for their high surface area and high permeability, to fluid and

macromolecules,

• primary site of the exchange for fluid, electrolytes gases and macromolecules.

3. venules

• The sympathetic innervation of the large venules alters the venular tone, which regulates the

hydrostatic pressure.

4. Terminal lymphatic vessels

• The spontaneous and stretch activated vasomotion acts as a “pump” to the lymph.

• The sympathetic nerves cause contraction.

• have one-way valves direct lymph from the tissue" Terminal lymphatic capillaries contain one-way valves that direct lymph away from tissues toward the larger lymphatic ducts — including the right lymphatic duct and the thoracic duct — which ultimately drain into the right and left subclavian veins, returning lymph to the systemic circulation."

Organ related specification of the capillaries

Continuous capillaries – Found in muscle, skin, lungs, and CNS. They have an uninterrupted endothelial lining, limiting movement mostly to small molecules through tight junctions and intercellular clefts.

Fenestrated capillaries – Seen in areas like endocrine glands, kidneys, and the intestinal mucosa. These have small pores (fenestrations) covered by thin diaphragms, allowing selective passage of small proteins and solutes.

Discontinuous (sinusoidal) capillaries – Located in the liver, spleen, and bone marrow. These have large gaps between endothelial cells and a discontinuous basement membrane, allowing free movement of large molecules and even cells.

Control of microcirculation

• Blood flow is controlled by the capillary perfusion by and among the terminal arterioles,

• the distribution of blood flow in the arteriole networks is according to the metabolic and the

hemodynamic feedback from the active muscles,

• balance between then muscle blood flow and the arterial blood pressure is controlled by the

sympathetic nerve activity.

• Osmotic pressure

Osmotic (oncotic) pressure, driven by plasma proteins, pulls water into capillaries and opposes hydrostatic pressure, helping regulate fluid balance in microcirculation

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