Cardiovascular Vessels & Conduction System – Detailed Exam Notes

Blood Vessel Fundamentals

• Circulatory direction
  • Arteries: carry blood away from heart (to lungs via pulmonary circuit or to body via systemic circuit).
  • Veins: return blood to heart.

Classes of Arteries

• Elastic (conducting) arteries

  • Examples: aorta, pulmonary trunk, initial segments of carotid & subclavian.
  • Function: accept the high-pressure/large-volume bolus ejected by ventricular systole.
  • Structure highlights
  • Thick tunica media rich in elastic fibers; smooth muscle mainly provides tensile strength.
  • Terminology
    • "Elastic" ≠ stretchy; compliance/extensibility = ability to stretch; elasticity = tendency to recoil.
  • Pressure reservoir concept
  • Ventricular ejection stretches wall (stores energy).
  • Recoil during diastole maintains forward flow (cannot back-flow because semilunar valves are closed).
  • Engineering analogy: PVC pipe vs flexible hose—the latter prevents wasted cardiac work.

• Muscular (distributing) arteries

  • Vast majority of named arteries (brachial, radial, femoral, etc.).
  • Tunica media dominated by smooth muscle → active vasoconstriction/vasodilation.
  • Principal role: regulate systemic blood pressure (BP) rather than fine-tune flow to one capillary bed.

• Arterioles

  • Smallest true arteries; unnamed, highly numerous.
  • Key site of resistance; their diameter determines how much blood enters each capillary bed.
  • Two broad reasons to change diameter
  1. Local distribution (metabolic needs of a tissue).
  2. Systemic BP regulation (via sympathetic tone).

Capillaries

• General traits
  • Microscopic (≈ diameter of a single RBC); wall = only tunica intima (endothelium + thin basement membrane).
  • Ubiquitous—form vast networks; exceptions: epithelia, cartilage, cornea/lens, anterior eye, some tendons/ligaments (poorly vascularized → slow healing).
  • Bleeding from any dermal cut underscores density of capillary beds.

Types of Capillaries

Capillary Bed Architecture & Microcirculation

• Terminal arteriole → capillary network → post-capillary venule (= microcirculation).
• Local flow control mechanisms
  1. Arteriolar tone: smooth-muscle constriction ↑ resistance ↓ flow.
  2. Pre-capillary sphincters: rings of smooth muscle guarding true capillaries; when closed, blood takes metarteriole–thoroughfare (vascular shunt) path.
• Physiological examples
  • Rest-and-digest: sphincters in gut open, skeletal muscle arterioles constrict.
  • Exercise/fight-or-flight: reverse pattern.

Venous System

• Post-capillary venules → venules → veins → venae cavae.
• Features
  • Thin tunica media, minimal muscle; large lumen → capacitance vessels (≈60 % of blood volume).
  • Venous valves (infoldings of tunica intima) prevent retrograde flow, especially in limbs working against gravity.
  • Skeletal-muscle pump, respiratory pump, and sympathetic venoconstriction aid return.
• Venous & Arterial Anastomoses
  • Interconnections that provide collateral routes; e.g., cerebral circle of Willis (lab), palmar arches.

Hemodynamics: Flow, Pressure, Resistance

• Definitions
  • Blood flow (F): volume/time (mL·min⁻¹) through tissue, vessel, or entire system.
  • Blood pressure (P): force exerted by blood on vessel wall; expressed in mm Hg.
  • Resistance (R): opposition to flow due to friction.
• Core relationship F = \frac{\Delta P}{R}
  • \Delta P = pressure gradient between two points.
• Determinants of R (Poiseuille’s law simplified) R \propto \frac{L\,\eta}{r^{4}}
  • L length (constant after growth)
  • \eta viscosity (↑ with hematocrit; changes slowly)
  • r radius (dynamic; most powerful because of 4th-power effect)
• Clinical/lecture applications
  • Vasoconstriction ↑ R drastically (coffee-stirrer vs boba-straw analogy) →
    • In arterioles: ↓ local flow.
    • In muscular arteries: ↑ systemic BP by raising upstream \Delta P.

Distinct Roles of Vasomotor Activity

1. Muscular arteries
   • Sympathetic vasoconstriction raises mean arterial pressure (MAP), ensuring adequate perfusion during blood loss or exercise.
2. Arterioles / Pre-capillary sphincters
   • Match regional blood flow to metabolic demand; redistribute limited cardiac output.

Action Potentials in Cardiac Tissue

• Skeletal muscle/neurons: rapid AP, brief refractory.
• Cardiac contractile cells (≈95 % myocardium)
  • Phases: rapid Na⁺ influx → plateau via L-type Ca²⁺ channels → K⁺ repolarization.
  • Plateau yields prolonged refractory period, preventing tetany.
• Cardiac pacemaker cells (SA, AV nodes, etc.)
  • Pacemaker (autorhythmic) potential
  1. Slow Na⁺ “funny” channels leak in (If) → drift to threshold.
  2. At threshold, Ca²⁺ influx (T- then L-type) → depolarization.
  3. K⁺ efflux → repolarization; cycle repeats ≈ 0.8 s at rest.

Intrinsic Conduction Pathway

1. Sino-atrial (SA) node – master pacemaker (~100 bpm intrinsic).
2. Atrioventricular (AV) node – 0.1 s delay (allows ventricular filling).
3. AV bundle (Bundle of His) – only electrical bridge through fibrous skeleton.
4. Right & Left bundle branches – course down interventricular septum.
5. Purkinje fibers (a.k.a. sub-endocardial conducting network) – spread through ventricular myocardium; depolarization begins at apex, driving upward squeeze.

Effects

• Sequential depolarization → atrial contraction (top-down) then ventricular contraction (apex-up).
• Heart sounds
  • S_1\ ("lub"): AV valves snap shut at onset of ventricular systole.
  • S_2\ ("dub"): semilunar valves close at beginning of diastole.

Gap-junction concept

• Conduction tissue and working myocardium interlinked by gap junctions in intercalated discs → ions (Na⁺/Ca²⁺/K⁺) flow cell-to-cell.
• Conduction fibers are modified muscle cells, not neurons (no axons, no contraction).

Illustrative & Clinical Examples Mentioned

• Body Worlds “vascular hand”: removing all tissue except vessels still preserves precise anatomy → capillaries everywhere.
• Paper-cut bleeding shows dermal capillary density; superficial epithelium itself is avascular.
• Drug crossing BBB
  • Caffeine & alcohol (lipid-soluble) vs Claritin (loratadine, non-sedating) & Benadryl (diphenhydramine, crosses BBB → drowsy).
• Milk-shake straw analogy for radius–resistance relationship.
• Water-bottle squeeze demonstrates fluid moves from higher to lower pressure.
• Rest-digest vs fight-flight scenarios relating vasomotor changes to nutrient delivery.

Key Equations & Numeric Facts to Retain

• Flow-pressure-resistance: F = \frac{\Delta P}{R}
• Poiseuille emphasis on radius: R \propto \frac{1}{r^{4}} (small radius change → large resistance change).
• Systemic venous system holds ≈ 60 % of total blood volume (no rote percentages needed for exam per instructor but concept important).

What to Study for Exam/Lab (per instructor notes)

• Lecture focuses
  • Functions & distinguishing features of elastic vs muscular arteries vs arterioles.
  • Capillary types and blood-brain barrier.
  • Concepts of pressure reservoir, systemic BP regulation, local flow control.
  • Hemodynamic relationships and role of vessel radius.
  • Dual purpose of vasoconstriction (systemic vs local).
  • Intrinsic conduction pathway and differences in AP types.
• Lab focuses
  • Identify components of the intrinsic conduction system on models/diagrams.
  • Recognize vessel histology, valves, continuous-fenestrated-sinusoidal slides.
  • Circle of Willis and other named anastomoses.
  • EKG basics (P wave, QRS, T wave) discussed in lab, not required for lecture.

Ethical / Philosophical / Practical Touches

• Engineering parallels (pressure reservoirs, compliance) illustrate bio-physics synergy.
• Slow healing of cartilage & ligaments sparks discussion on vascular privilege and tissue repair.
• BBB selectivity influences pharmacology and neuro-ethics (drug design, toxin protection).

End of consolidated study notes – replicate lecture video without re-watching!