Cardiovascular Microcirculation & Vessel Anatomy – Comprehensive Study Notes

Blood Vessel Types & Flow Dynamics

• Arteries
  • Carry blood away from the heart toward systemic tissues.
  • Gradually branch into smaller vessels (arterioles ➜ metarterioles ➜ capillaries).
• Capillaries
  • Microscopic vessels where gas and nutrient exchange occurs.
  • Connect the arterial side (metarteriole end) to the venous side (post-capillary venule end).
• Venules and Veins
  • Capillaries converge to form venules.
  • Venules enlarge into veins, regaining structural layers (tunics) lost at the capillary level.
  • Veins return blood to the heart.

Capillary Structure & Epithelium

• Lined by simple squamous epithelium ("squashed" flat cells with intercellular gaps).
  • Thinness + gaps = minimal diffusion distance ⇒ efficient exchange of O₂, CO₂, nutrients, and wastes.
• Types (briefly referenced)
  • Continuous: tight junctions, least permeable (e.g., blood–brain barrier).
  • Fenestrated: have "fenestrae" (holes); greatly increased permeability (intestine, endocrine organs, glomerulus).
  • Sinusoidal: large gaps & discontinuous basement membrane (liver, spleen, marrow).
• Transcript emphasis: “fenestrated means holes – not on the current test, but know the term.”

Vessel Wall Layers (Tunics)

• Tunica externa (adventitia)
  • Outermost; connective tissue that anchors vessel.
  • Easy mnemonic: externa is external.
• Tunica media
  • Middle, thickest in arteries.
  • Smooth muscle + elastic fibers generate the arterial pulse and regulate diameter/pressure.
• Tunica intima
  • Innermost; endothelium + thin connective tissue.
• Functional Sequence as Vessels Branch:
  • Large arteries possess all three tunics robustly.
  • As diameter decreases, outer layers thin; at true capillaries the outer tunics are essentially absent to maximize exchange.
  • Returning venules/veins gradually regain externa and media as they enlarge.

Pre-capillary Sphincters & Perfusion Regulation

• Rings of smooth muscle located at the capillary entrance (between metarteriole & true capillary).
• Function
  • Open when local tissue O₂/nutrient levels are low.
  • Close when tissue is saturated, preventing unnecessary flow and allowing blood to bypass via the thoroughfare (metarteriole) to distal tissues.
• Sprinkler-system analogy: Without these “valves,” proximal regions would receive all the flow/pressure, leaving distal tissues (e.g., fingertips, toes) under-perfused.
• Clinical tie-in: Peripheral tissues are often first to suffer in vascular disease because they are farthest from the heart and rely heavily on proper sphincter control.

Fluid Exchange: Filtration vs Reabsorption

• Filtration
  • Occurs primarily at the arterial end of capillaries.
  • Hydrostatic pressure (blood pressure) pushes water, O₂, and solutes out into interstitial fluid.
• Reabsorption
  • Dominant at the venous end.
  • Colloid osmotic pressure (from plasma proteins) pulls most fluid back into the capillary.
• Only ≈85 % of filtered fluid is reabsorbed; the remaining ≈15 % is collected by the lymphatic system.
  • Failure to collect this excess leads to edema (tissue swelling).

Net Filtration Pressure (NFP) Equation

• \text{NFP} = \Delta HP - \Delta COP
  • \Delta HP = change (difference) in hydrostatic pressure between blood & interstitium.
  • \Delta COP = change in colloid osmotic pressure.
• Interpretation
  • \text{NFP} > 0 ⇒ net filtration (fluid exits capillary).
  • \text{NFP} < 0 ⇒ net reabsorption (fluid enters capillary).

Lymphatic Role & Edema Prevention

• Lymphatic capillaries pick up the 15 % excess interstitial fluid plus escaped proteins.
• Return fluid to venous circulation via thoracic duct/right lymphatic duct.
• Blockage of lymphatics (e.g., parasitic infection, tumor removal) ➜ lymphedema.

Vascularization & Angiogenesis

• Vascularization: the degree to which a tissue is supplied with blood vessels.
  • Highly metabolically active tissues (muscle, liver) are richly vascularized.
• Angiogenesis: growth of new vessels/capillaries.
  • Driven by protein growth factors (e.g., VEGF, FGF) mentioned as “Proteins” in transcript.
  • Important in development, wound healing, tumors.

Aorta & Branches (Overview for Labeling)

• After the aortic arch, the descending aorta travels the thoracic cavity then through the diaphragm into the abdominal cavity, supplying:
  1. Celiac trunk
  2. Superior mesenteric artery
  3. Inferior mesenteric artery
  4. Renal arteries (left & right)
  5. Gonadal arteries (testicular/ovarian)
  6. Lumbar arteries
  7. Middle suprarenal arteries
  8. Inferior phrenic arteries
  9. Median sacral artery
  10. Common iliac arteries (branching into internal & external iliacs)
  11. Internal iliac artery (pelvis)
  12. External iliac artery (leg via femoral)
  13. (Any additional regional branch specified by instructor)
• Students are expected to label ~13 branches on the provided diagram (blueprint will supply exact list & slide numbers).

Surgical & Clinical Considerations

• Aortic repair
  • Requires opening chest (breaking ribs), suturing aneurysm/tear.
  • Post-op risk: new tears elsewhere (“see if it’s gonna crack anywhere else”).
• Peripheral vascular disease & aging
  • Distal digits (fingers, toes) often first to show ischemia due to long distance + sphincter dysfunction.

Exam / Course Logistics (Mentioned for Context)

• Upcoming exam: Friday; will include ~30 new questions (≈15 from new lecture, 15 from earlier material).
• Instructor will provide blueprint + slide numbers and conduct a review on Wednesday.
• Students should be prepared for a difficult/commonly missed diagram question and labeling of aortic branches.