Cardiovascular | Microcirculation

Summary of Scientific Concepts and Phenomena in Microcirculation

Microcirculation Anatomy and Structures:

• Terminal arterial: The small artery feeding into the capillary bed.
• Meta-arterial: Branches off the Terminal arterial and gives rise to True capillaries.
• True capillaries: Smallest blood vessels (10-100 per capillary bed) where gas and nutrient exchange occurs.
• Thoroughfare channel: A vessel that connects the Meta-arterial to the post-capillary venule, allowing blood to bypass True capillaries.
• Vascular shunt: Combination of Meta-arterial and thoroughfare channel; an example of an arteriovenous anastomosis.
• Post-capillary venule: Drains blood from the capillary bed.

Precapillary Sphincters:

• Rings of smooth muscle around True capillaries that regulate blood flow into capillaries.
• Controlled by sympathetic nervous system and local chemical signals.
• Constriction blocks blood flow; dilation allows blood flow to tissues.

Pressures Governing Fluid Exchange in Capillaries (Bulk Flow):

• Capillary hydrostatic pressure (HPc): Pushes fluid out of capillaries (~35 mmHg arterial side, ~17 mmHg venous side), dependent on systolic blood pressure.
• Capillary osmotic pressure (OPc): Pulls fluid into capillaries (~25 mmHg), mainly due to plasma proteins like albumin.
• Interstitial fluid hydrostatic pressure (HPif): Pushes fluid into capillaries (~0 mmHg, usually negligible).
• Interstitial fluid osmotic pressure (OPif): Pulls fluid out of capillaries (~1 mmHg), due to small solutes in interstitial fluid.

Net Filtration Pressure (NFP):

• On arterial side: NFP ≈ (HPc + OPif) - (OPc + HPif) = (35 + 1) - (25 + 0) = +11 mmHg → net fluid out (filtration).
• On venous side: NFP ≈ (17 + 1) - (25 + 0) = -7 mmHg → net fluid in (reabsorption).

Clinical Relevance of Pressures:

• Loss of plasma proteins (e.g., albumin) in conditions like Nephrotic syndrome reduces OPc → increased filtration → edema.
• Lymphatic vessel blockage (e.g., cancer, lymphoma) impairs drainage → increased interstitial hydrostatic pressure → edema.

Types of Anastomoses (Collateral Circulation):

• Arterial anastomosis: Connections between arteries (e.g., Circle of Willis in the brain, coronary circulation, joints).
• Venous anastomosis: Connections between veins (most common, e.g., median cubital vein connecting basilic and cephalic veins in the arm).
• Arteriovenous anastomosis: Direct connections between arteries and veins bypassing capillaries (vascular shunts).

Circle of Willis:

• A key arterial anastomosis in the brain formed by internal carotid arteries, anterior and posterior communicating arteries, vertebral arteries, basilar artery, and cerebral arteries.
• Provides collateral blood flow to protect brain tissue in case of arterial blockage.
• Clots in some arteries (e.g., middle cerebral artery) can still cause stroke despite anastomosis.

Localized Blood Flow Regulation:

• Skeletal muscle during exercise:
  • Increased CO2, H+ (from lactic acid), and low pH cause vasodilation of arterioles.
  • Relaxation of smooth muscle in tunica media increases blood flow (active hyperemia).
• Brain:
  • Regulated by mean arterial pressure (MAP).
  • High MAP → vasoconstriction (myogenic mechanism) to protect fragile cerebral vessels.
  • Low MAP → vasodilation to maintain perfusion and prevent syncope.
• Lungs:
  • Unique regulation: low oxygen partial pressure causes vasoconstriction of pulmonary arterioles to divert blood to better-ventilated areas.
• Gastrointestinal tract (GIT), skin, kidneys:
  • Under sympathetic stimulation (e.g., stress, fight or flight), vasoconstriction reduces blood flow to these less vital organs.
  • Blood is shunted to vital organs like brain, heart, and skeletal muscles.

Arteriovenous Malformation (AVM):

• A pathological condition where arteries connect directly to veins without capillaries.
• Causes high-pressure arterial blood to flow directly into veins, risking vessel rupture and hemorrhage.
• Commonly found in the brain.
• Treated by Embolization therapy to prevent rupture.

Key Methodologies and Concepts Outlined

• Microcirculation anatomy identification: Terminal arterial → Meta-arterial →