Vascular System – 2

Principal Arteries

• All systemic arteries branch directly or indirectly from the aorta.
• Thoracic aorta ➜ supplies chest wall & thoracic organs.
• Abdominal aorta major branches
  • Celiac trunk ➜ immediately divides into
  • Gastric artery: stomach
  • Splenic artery: spleen
  • Hepatic artery: liver
  • Renal arteries: kidneys
  • Superior mesenteric artery: most of small intestine + proximal large intestine
  • Inferior mesenteric artery: distal large intestine
• Terminal abdominal aorta ➜ right & left common iliac arteries
  • Internal iliac: pelvic organs
  • External iliac → Femoral → Popliteal → Anterior & Posterior tibial → Dorsalis pedis (foot)

Upper‐Limb Arterial Continuum

• Aortic arch branch ➜ Subclavian artery ➜ becomes:
  • Axillary artery (axilla)
  • Brachial artery (arm); primary site for cuff BP measurement
  • Radial artery (forearm); common pulse point

Arteries of Head, Neck & Brain

• Brain needs uninterrupted O₂; loss for seconds = loss of consciousness, $4\text{–}5 \;\text{min}$ = irreversible damage.
• Two major pathways supply cerebral blood:
  1. Common carotid arteries
  • Right: from brachiocephalic trunk
  • Left: directly off aortic arch
  • Each splits at Adam’s apple into
    • External carotid: superficial head structures
    • Internal carotid: enters cranium; supplies orbits + $\approx 80\%$ cerebrum
  1. Vertebral arteries
  • Arise from subclavians ➜ ascend transverse foramina ➜ join to form single basilar artery on brain-stem’s undersurface.
• Circle of Willis (arterial anastomosis safeguarding cerebral perfusion)
  • Components
  • Single anterior communicating artery
  • 2 anterior cerebral arteries
  • 2 posterior communicating arteries
  • 2 posterior cerebral arteries
  • Ensures alternate routes if any proximal artery occludes.

Principles of Circulation & Pressure Gradient

• Blood/any fluid flows from high ➜ low pressure (pressure gradient).
• Systolic pressure (left-ventricular ejection): $\approx 110\,\text{mm Hg}$ at aorta.
• Diastolic pressure: $\approx 70\,\text{mm Hg}$.
• By venae cavae pressure ≈ $1\,\text{mm Hg}$.
• Flow determined by
  • Cardiac output (CO)
  • Blood volume
  • Resistance (≡ peripheral resistance)
• Mathematical relationships
  • $\uparrow CO \Rightarrow \uparrow BP; \; \downarrow CO \Rightarrow \downarrow BP$
  • $\uparrow Volume \Rightarrow \uparrow BP; \; \downarrow Volume \Rightarrow \downarrow BP$
  • \uparrow Resistance \Rightarrow \uparrow BP \text{ & } \downarrow Flow

Peripheral Resistance

• Resistance stems from blood-vessel friction; governed mainly by blood viscosity + vessel diameter.

Blood Viscosity

• “Thickness/stickiness.”
• $\uparrow$ RBC count, plasma proteins (albumin) or dehydration ➜ $\uparrow$ viscosity ➜ $\downarrow$ flow.
• Analogy: milkshake vs water through a straw.

Vessel Diameter & Vasomotion

• Arteriolar smooth muscle changes lumen size (vasomotion) = body’s primary short-term BP regulator.
• Velocity relation: larger diameter ➜ faster flow.
  • Aorta ≈ $1200\,\text{mm/sec}$
  • Capillary ≈ $0.4\,\text{mm/sec}$

Vasoconstriction

• Lumen ↓ ➜ resistance ↑ ➜ BP ↑ ➜ downstream flow ↓.

Vasodilation

• Lumen ↑ ➜ resistance ↓ ➜ BP ↓ ➜ tissue flow ↑.

Elastic Recoil Insight

• Healthy arteries expand during systole, recoil during diastole
  • Propels blood & dampens pressure surges → protects smaller vessels.
• With age/atherosclerosis elasticity ↓, systolic force absorbed ↓ ➜ BP rises.

Flow Velocity Profile (Why blood slows then speeds)

• Capillary region: slowest due to
  • Distance from pump
  • Cumulative friction
  • Small diameters create resistance
  • Massive total cross-sectional area
• Venous side: velocity rises again because
  • Veins have larger diameters (less resistance)
  • Converging vessels ↓ total cross-sectional area.
• River metaphor: branching streams (capillaries) slow water; converging streams (veins) accelerate it.

Regulation of Blood Pressure & Flow

Autoregulation (Local Control)

• Most vigorous in heart, brain, kidneys.
• Hypoxia ➜ metabolic wastes (CO₂, H⁺, K⁺, adenosine, lactic acid) accumulate ➜ local vasodilation ➜ restores O₂ & removes wastes.
• Endothelial factors
  • Nitric oxide: potent vasodilator
  • Endothelins: strong vasoconstrictors
• Reactive hyperemia: marked ↑ flow after temporary occlusion.

Neural Regulation

• Baroreceptors (carotid sinus, aortic arch) monitor BP ➜ send signals via glossopharyngeal & vagus nerves to medullary centers.
• Medulla outputs
  • High BP: ↑ parasympathetic, induce vasodilation; ↓ HR & SV ➜ BP drops.
  • Low BP: ↑ sympathetic, induce vasoconstriction; ↑ HR & SV ➜ BP rises.
• Baroreceptors excellent for short-term adjustments (e.g., postural changes) but poor at chronic hypertension control.

Atherosclerosis (Life Lesson)

• Leading contributor to cardiovascular morbidity/mortality.
• Targets high-pressure arteries: coronary, carotid, renal.
• Pathogenesis
  1. Endothelial injury (HTN, hypercholesterolemia, smoking, hyperglycemia, gut-microbiota factors).
  2. Inflammation ➜ monocytes → macrophages ingest cholesterol ➜ fatty streak.
  3. Scar tissue forms fibrous cap; proliferating smooth muscle + lipid + macrophages constitutes atheroma.
  4. Lumen narrows; artery may stiffen (arteriosclerosis).
  5. Plaque rupture or clot on plaque can occlude vessel or embolize.
• $75\%$ coronary lumen reduction ➜ angina symptoms.
• Recent discovery: gut-derived bacteria DNA present in plaques; diet-induced intestinal permeability may permit translocation.

Blood Pressure Ranges & Clinical Relevance

• Normal: $<120/80\,\text{mm Hg}$
• Elevated: $120\text{–}129 / <80$
• Stage 1 HTN: $130\text{–}139$ systolic or $80\text{–}89$ diastolic
• Stage 2 HTN: $\ge 140$ systolic or $\ge 90$ diastolic
• Hypotension: < normal range; severe ↓ threatens perfusion ➜ shock.
• BP measured at brachial artery via sphygmomanometer; reflects systemic arterial pressure.
• Pulse pressure = systolic – diastolic (ex: $110/70 \Rightarrow 40\,\text{mm Hg}$) gauges arterial stress.

Principal Veins

• Two major trunks deliver systemic venous return to right atrium:
  • Superior vena cava (SVC): drains head, neck, upper limbs.
  • Inferior vena cava (IVC): drains lower body.
• Typical venous pathway may involve multiple merges (e.g., Axillary → Subclavian → Brachiocephalic → SVC).

Named Upper-Body Veins & Clinical Notes

• Internal jugular: drains brain; distends in right-sided heart failure.
• External jugular: drains scalp & superficial face.
• Cephalic vein (distal): frequent IV access.
• Median cubital vein: common phlebotomy site.

Lower Limb & Abdomen Veins

• Common iliac → Internal & External iliac → Femoral → Popliteal → Anterior/Posterior tibial → Fibular.
• Great saphenous: longest vein; harvested for coronary bypass grafts.

Hepatic Portal Circulation

• Digestive organs & spleen do NOT drain directly to IVC.
• Pathway
  1. Capillaries of stomach, intestines, pancreas, gallbladder, spleen → Splenic + Superior mesenteric veins.
  2. These converge forming Portal vein.
  3. Portal vein → liver sinusoids (microscopic liver capillaries).
  4. Hepatic veins exit liver → IVC → heart.
• Functional significance
  • Liver modulates nutrient levels (e.g., stores excess glucose as glycogen post-meal).
  • Detoxifies substances (bacteria, alcohol, drugs) before systemic release.

Velocity, Cross-Sectional Area & Clinical Implications

• Aorta (large diameter, near LV) = fastest flow.
• Capillaries (vast number, small diameter) = slowest ➜ allows exchange.
• Transition to venous side sees velocity rise as vessels converge & diameters enlarge.
• Injury patterns: artery spurts (pulsatile), vein oozes (steady) reflecting pressure differences.

Practical & Ethical/Health Implications

• Aging & lifestyle (high-fat, high-sugar, low-fiber diet) accelerate arterial stiffening & plaque formation ➜ underscores preventive medicine & dietary ethics.
• Understanding vasomotion informs pharmacology (vasodilators, vasoconstrictors) & critical care (after-load management).
• Blood pressure guidelines lowered in 2017; now ~50 % of US adults qualify for hypertension management—public health relevance.