AB

Week 3: Types of Blood Vessels

Vessel Overview

  • Arteries: carry blood away from heart; high, pulsatile pressure; “resistance vessels”.

  • Veins: return blood to heart; low, steady pressure; thin, distensible walls; “capacitance vessels” store ≈ 64\% of blood.

  • Capillaries: microscopic links between arteries & veins; site of exchange.

Wall Structure (Tunics)

  • Tunica interna: endothelium + thin areolar CT; permeable; secretes vaso-active chemicals; initiates clotting when damaged.

  • Tunica media: smooth muscle + elastic tissue + collagen; regulates diameter (vasoconstriction / dilation).

  • Tunica externa: CT that anchors vessel; passage for vasa vasorum, nerves, lymphatics.

  • Capillaries possess only tunica interna.

Arteries

  • Elastic (conducting): largest; tunica media 40–70 elastic layers; stretch & recoil (e.g. aorta, pulmonary trunk).

  • Muscular (distributing): 1.0–0.3\,\text{cm}; up to 40 smooth-muscle layers; direct blood to specific organs (renal, femoral, etc.).

  • Resistance: 300–10\,\mu\text{m}; include arterioles that set tissue perfusion.

  • Metarterioles: short vessels linking arterioles → venules, bypassing capillary beds.

Capillaries

  • Continuous: tight endothelium; small clefts; most tissues (skin, muscle, CNS).

  • Fenestrated: pores for rapid filtration/absorption; kidneys, small intestine.

  • Sinusoidal: wide gaps for proteins & cells; liver, spleen, bone marrow.

  • Capillary bed: web of 10–100 capillaries fed by one arteriole; flow controlled by upstream arteriole tone.

Veins

Order (small → large):

  • Post-capillary venules 10–20\,\mu\text{m}; porous, still exchange.

  • Muscular venules: 1–2 smooth-muscle layers.

  • Medium veins (≤10\,\text{mm}): named; contain valves.

  • Venous sinuses: thin wall, no muscle (dural, coronary).

  • Large veins (≥10\,\text{mm}): SVC, IVC, pulmonary, renal, internal jugular.

Special Vascular Features

  • Vasa vasorum: tiny vessels nourishing outer half of large-vessel walls.

  • Arterial sense organs:
    • Carotid sinuses – baroreceptors (BP).
    • Carotid bodies – chemoreceptors (O2, CO2, pH).
    • Aortic bodies – chemoreceptors in aortic arch.

  • Anastomoses:
    • Arterial (e.g., Circle of Willis) – collateral supply.
    • Venous – one vein into another.
    • Arteriovenous (shunt) – artery → vein, bypass capillary (ears, toes for thermoregulation).

Clinical Correlations

  • Aneurysm: arterial wall weakness → bulging sac; risks rupture & compression; causes include genetics, arteriosclerosis, hypertension, trauma; treated by monitoring, graft, stent.

  • Varicose veins: valve failure → pooling, dilation (legs; hemorrhoids in anal canal); risk factors: prolonged standing, age, pregnancy, obesity; managed by exercise, compression, ablation.

Circulatory Route & Sequence

Standard flow: heart → artery → arteriole → capillary → postcapillary venule → vein → heart.
Detailed arterial–venous sequence:
1 Heart → 2 Elastic artery → 3 Muscular artery → 4 Arteriole → 5 Capillary → 6 Postcapillary venule → 7 Muscular venule → 8 Medium vein → 9 Large vein → 10 Heart.

Key Mnemonics:
“Resistance = Arteries; Capacitance = Veins.”
“Intima – Media – Externa: Inside → Middle → External.”