Lecture on Arteries and Arterioles

Macroscopic Arteries – Overview

• Two macroscopic (gross-anatomy) sub-types

  • Elastic (Conducting) arteries

  • Muscular (Distributing) arteries

• Diagram in video shows one quarter of each vessel’s cross-section, exposing all three tunics and facilitating side-by-side layer comparison.

• Microscopic counterpart: arterioles (discussed later).

Elastic (Conducting) Arteries

• Alternate name: Conducting arteries – main task is simply to conduct blood from the heart to downstream vessels, not to regulate lumen size.

• Examples

  • Aorta

  • Immediate branches of the aorta (brachiocephalic, common carotid, subclavian, etc.)

  • Pulmonary trunk & its immediate branches

• General dimensions

  • Largest individual diameter and greatest total cross-sectional area of any artery type.

• Tunica media

  • Thickest of the three tunics in elastic arteries.

  • Contains abundant elastic protein fibers (wavy blue lines in histology slide) ⇒ gives the name “elastic.”

• Compliance

  • Definition: degree to which a vessel stretches (distends) for a given internal pressure.

  • Elastic arteries exhibit low compliance relative to veins: hard to stretch, small ΔV for a given ΔP.

  • Graph shown: compliance curve sits below venous curve – at any pressure, veins distend far more.

• Analogy

  • Thick, tough rubber band (elastic artery) vs thin flimsy rubber band (vein).

• Elastic recoil

  • Because they resist stretch, release of tension produces a forceful snap-back.

  • Functional consequence during the cardiac cycle:

  • Systole: LV ejection ↑ aortic pressure → wall distends (stores elastic potential energy).

  • Diastole: Aortic valve closes, blood flows downstream, wall recoils quickly → lumen volume ↓, pressure inside ↑ (Boyle’s law P \propto \frac{1}{V}).

  • Creates dicrotic notch/bump on aortic pressure curve right after semilunar valve closure.

  • Physiological significance: prevents diastolic arterial pressure from falling too low (helps keep MAP high enough to perfuse tissues between heartbeats).

Muscular (Distributing) Arteries

• Alternate name: Distributing arteries – deliver blood from elastic arteries to organs/tissues.

• Size

  • Smaller diameter & lumen than elastic arteries (farther from heart).

• Tunica media

  • Contains fewer elastic fibers, leaving space for more densely packed smooth muscle cells.

  • Total thickness still less than in elastic arteries but muscle density higher.

• Functional capacity

  • Can perform modest vasoconstriction/vasodilation (more than elastic arteries but far less than arterioles).

  • Therefore, minor role in peripheral resistance.

• Histology slide comparison shows scant wavy elastic fibers and tight smooth-muscle bundles.

Microscopic Arterioles

• Primary resistance vessels of the circulation.

• Dimensions

  • Microscopic lumen → small absolute radius; small Δr changes translate to large ΔR (Poiseuille’s law R \propto \frac{1}{r^4}).

• Tunica media

  • 1–3 concentric layers of smooth muscle.

  • Cells arranged in discrete circular “ring” units ⇒ act like tiny sphincters pinching lumen diameter.

• Tunica adventitia

  • Very thin outer connective layer.

• Functional advantages

  • Rings allow large percent change in lumen size → powerful control over resistance and therefore blood flow.

Metarterioles & Pre-capillary Sphincters

• Arterioles branch into metarterioles that thread through the center of a capillary bed.

• Pre-capillary sphincters

  • Rings of smooth muscle (same architecture as arteriolar rings) located where metarteriole channels meet individual capillaries.

  • Regulate entry of blood into capillaries on a moment-to-moment basis.

• Routing consequence of constriction

  • When sphincters close, blood bypasses true capillaries via metarteriole → venule pathway, reducing capillary perfusion.

Extrinsic vs Intrinsic Control of Arterioles

• Extrinsic (neural & hormonal)

  • Goal: maintain Mean Arterial Pressure (MAP) via adjusting Total Peripheral Resistance (TPR).

  • Sympathetic tone, epinephrine, vasopressin, angiotensin II, etc.

• Intrinsic (local) controls – two key mechanisms

  • Metabolite-mediated (active hyperemia) – dominates in most tissues.

  • Myogenic response – present in select vascular beds (kidney, brain, etc.).

Intrinsic Control 1: Metabolite-Driven (Active Hyperemia)

• Trigger: ↑ metabolic activity of a tissue (e.g., contracting biceps).

• Local extracellular chemistry changes

  • ↓ O_2 (purple dots in diagram)

  • ↑ CO_2 + ↑ H^+ (green dots) – metabolic by-products

• Vascular smooth muscle senses these metabolites (act as chemical messengers)

  • ↓ O2, ↑ CO2, ↑ H^+ ➔ smooth-muscle relaxation ➔ arteriolar dilation

• Consequences

  • ↓ Resistance (R)

  • ↑ Blood flow (Q) through arteriole and downstream capillaries

  • Matches delivery of O_2 / nutrients & removal of wastes to heightened demand.

• Operates continuously, making fine-tuned adjustments “in the background.”

Intrinsic Control 2: Myogenic Response

• “Myo” (muscle) + “genic” (generated) – response generated by muscle’s stretch-sensitive ion channels.

• Purpose: Maintain relatively constant capillary blood flow despite changes in perfusion pressure.

  • Especially important in kidney (constant filtration) & brain (constant perfusion).

• Mechanism (flowchart)

  1. ↑ MAP → ↑ Perfusion pressure (P) within arteriole.

  2. ↑ Distending pressure → arterial wall stretches.

  3. Stretch-activated smooth muscle channels open → smooth muscle depolarizes → contracts.

  4. Arteriole constricts → ↑ Resistance (R).

  5. Goal: ΔR equals ΔP so that Q = \frac{\Delta P}{R} returns to baseline.

  6. Response latency: 1–2 s; quickly restores steady flow.

• Works in reverse: ↓ MAP → ↓ stretch → smooth muscle relaxes → dilation → keeps flow constant.

Comparative Compliance: Arteries vs Veins

• Compliance curves presented side-by-side.

• For any given intraluminal pressure:

  • Vein undergoes large volume change (high compliance).

  • Artery undergoes small volume change (low compliance).

• Functional corollaries

  • Arteries: pressure reservoir (elastic recoil sustains pressure).

  • Veins: volume reservoir (store extra blood without large pressure rise).

Boyle’s & Poiseuille’s Laws – Connections

• Boyle’s law underpinning elastic recoil effect: P \propto \frac{1}{V}

  • As aortic volume ↓ during recoil, pressure ↑ creating the diastolic “bump.”

• Poiseuille’s law explaining resistance role of arterioles: Q = \frac{\Delta P}{R} \quad \text{with} \quad R \propto \frac{1}{r^4}

  • Tiny radius changes (via sphincter rings) produce huge resistance shifts.

Ethical & Clinical Relevance

• Aortic stiffening (loss of elastic fibers with age or disease) ↓ compliance even further → widened pulse pressure & higher systolic load on LV.

• Dysfunction of arteriolar control (e.g., sepsis → loss of tone; diabetes → microvascular disease) leads to tissue ischemia or edema.

• Pharmacologic targets

  • Vasodilators (e.g., nitric-oxide donors) exploit metabolite pathways.

  • Calcium-channel blockers dampen myogenic constriction (used in hypertension, cerebral vasospasm).

Key Takeaways / Study Checklist

• Recognize structural differences among elastic, muscular, and arteriolar walls (tunica content).

• Explain low compliance ➔ elastic recoil ➔ diastolic pressure maintenance.

• Differentiate extrinsic vs intrinsic controls of arteriole tone (MAP vs tissue needs).

• Outline metabolite vs myogenic mechanisms and identify tissues where each predominates.

• Relate Poiseuille’s law to why arterioles are chief resistance vessels.

• Apply concepts to clinical scenarios (aging arteries, kidney autoregulation, exercise hyperemia).