vascular system 1

Overview of Circulatory Routes

  • Blood normally follows the sequence heart → arteries → capillaries → veins → heart.
  • Exceptions to the single-capillary-bed rule:
    • Portal system – blood passes through two capillary networks before returning to the heart (main example: hepatic portal system in the liver).
    • Anastomosis – two vessels join directly, creating alternate routes if one path is obstructed.
    – Types: arterio-arterial, arteriovenous, venous (e.g., venous anastomosis between superficial veins).
  • Two major circuits:
    • Pulmonary circulation – begins at the right ventricle; exchanges \text{CO}2 for \text{O}2 in the lungs.
    • Systemic circulation – begins at the left ventricle; supplies every organ/tissue with oxygenated blood and removes wastes.

Pulmonary Circulation

  • Step-by-step flow:
    1. Right ventricle ejects blood into the pulmonary trunk.
    2. Pulmonary trunk splits into right & left pulmonary arteries.
    3. Each pulmonary artery branches into lobar arteries (one per lung lobe), then into smaller arteries → arterioles → capillary beds.
    4. Capillaries surround alveoli; gas exchange occurs ((\text{O}2\uparrow, \text{CO}2\downarrow)).
    5. Capillaries coalesce into venules → veins → four pulmonary veins (two per lung) that return oxygenated blood to the left atrium.
  • Key concept: In pulmonary routes, arteries carry O₂-poor blood and veins carry O₂-rich blood—the opposite of systemic routes.
  • Pulmonary circuit does not supply lung tissue itself; bronchial arteries (systemic) do that.

Systemic Circulation

  • All systemic arteries arise, directly or indirectly, from the aorta (origin: left ventricle).
  • Regions of the aorta:
    • Ascending aorta – a few cm long; gives off right & left coronary arteries to the myocardium.
    • Aortic arch – inverted U over the heart; branches:
    – Brachiocephalic artery → right common carotid & right subclavian (supplies head & right arm).
    – Left common carotid artery (neck/head).
    – Left subclavian artery (left shoulder & upper arm).
    • Descending aorta – travels through thoracic then abdominal cavities.
    – Above diaphragm: thoracic aorta.
    – Below diaphragm: abdominal aorta → branches into right & left common iliac arteries (pelvis & legs).
  • Venous return mirrors arteries but converges into progressively larger veins, terminating in the superior & inferior venae cavae.

Specialized Circulatory Systems

  • Hepatic portal circulation – drains digestive organs → hepatic portal vein → liver sinusoids (2nd capillary bed) for nutrient processing & detoxification.
  • Cerebral circulation (Circle of Willis) – extensive anastomoses safeguard brain perfusion.
  • Fetal circulation – unique shunts (ductus arteriosus, foramen ovale, ductus venosus); present only before birth (see Chapter 24).

Comparative Oxygen Content in Vessels

  • Systemic circuit: Arteries = O₂-rich, Veins = O₂-poor.
  • Pulmonary circuit: Arteries = O₂-poor, Veins = O₂-rich.
    (Mnemonic: Arteries – Away from heart, regardless of oxygen content.)

Pulse Points (Arterial Pressure Surges)

  • Common palpation sites where arteries run close to surface:
    • Radial artery (wrist – most common for pulse check).
    • Carotid artery (neck).
    • Brachial artery (antecubital region – BP measurement).
    • Femoral artery (groin), Popliteal artery (behind knee).
    • Posterior tibial & dorsalis pedis arteries (ankle/foot).

Vessel Classes and Structure

  • Wall layers (tunics) common to arteries & veins:
    1. Tunica intima – endothelium + basement membrane; smooth, chemical mediator of dilation/constriction.
    2. Tunica media – smooth muscle & elastic tissue; thickness varies; sympathetic innervation.
    3. Tunica externa – fibrous connective tissue; protection & anchoring; thickest layer in veins.

Arterial Classes

  • Conducting (elastic) arteries – largest; expand with ventricular systole, recoil with diastole ⇒ pressure smoothing.
    Examples: aorta, common carotid, subclavian.
  • Distributing (muscular) arteries – direct blood to specific organs/regions.
    Examples: brachial, femoral, renal.
  • Arterioles (resistance vessels) – smallest; major role in regulating BP & blood flow via smooth-muscle contraction.
    • Connect to capillaries through short metarterioles.

Arterial Clinical Insight: Aneurysm

  • Aneurysm = localized arterial wall dilation due to wall weakness + high pressure.
  • Common causes: atherosclerosis + hypertension, congenital defects, trauma, infection.
  • Frequent sites: aorta, renal arteries, cerebral arterial circle.
  • Risks: rupture → hemorrhage; compression of adjacent structures.

Venous Classes and Features

  • General vein traits:
    • Thinner walls, lower pressure, ability to distend (capacitance vessels).
    • Lie superficial; contain majority of circulating volume ((>60\%)).
    • Can constrict markedly (sympathetic stimulation) to redistribute blood.

  • Venules – smallest; porous; exchange fluid with tissues; thin tunica media.

  • Medium-sized veins – have one-way valves (infoldings of tunica intima) to prevent backflow, especially in legs (e.g., radial, ulnar, great saphenous).

  • Large veins – thick tunica externa; form by convergence of mediumsized veins (e.g., venae cavae, pulmonary veins, internal jugulars).

Capillaries: Structure & Distribution

  • Microscopic (just wide enough for single RBCs).
  • Wall = only endothelium + basement membrane ⇒ ideal for exchange.
  • Distribution varies with metabolic demand:
    • High density: liver, kidneys, myocardium.
    • Low density: tendons, ligaments.
    • None: epidermis, cartilage, lens/cornea.
  • Sinusoids – large, irregular, highly permeable capillaries in liver, bone marrow, spleen; allow proteins & blood cells to enter/exit.

Capillary Beds & Microcirculation Regulation

  • Arterioles → capillary beds → venules = microcirculation.
  • Entrance to each capillary bed guarded by a pre-capillary sphincter (smooth muscle ring):
    • Open during activity (e.g., exercise) → full perfusion.
    • Closed during rest → blood bypasses via thoroughfare channel into venule.
  • Body lacks enough blood to fill entire network simultaneously; ~90\% of skeletal-muscle capillaries are closed at rest.

Mechanisms of Capillary Exchange

  1. Diffusion – movement down concentration gradient; primary mechanism.
    – \text{O}2 diffuses out; \text{CO}2 diffuses in.
  2. Filtration – driven by blood (hydrostatic) pressure; dominant at arterial end.
    – Capillary pressure: \approx 30–35\ \text{mm Hg} vs. tissue fluid \approx 2\ \text{mm Hg}.
    – Plasma + dissolved nutrients pushed into interstitial fluid.
  3. Colloid osmotic pressure (reabsorption) – dominant at venous end as BP falls to \approx 10\ \text{mm Hg}.
    – Albumin remains in blood → pulls fluid & wastes back (oncotic pressure).
    – About 85\% of filtered fluid re-enters; remaining 15\% is picked up by lymphatic vessels and returned to circulation.

Fluid Balance: Edema

  • Edema = excess interstitial fluid when filtration > reabsorption.
  • Three primary causes:
    1. Increased capillary filtration – high BP (right heart failure), kidney failure, prolonged standing/inactivity.
    2. Reduced capillary reabsorption – low plasma albumin from liver disease, severe burns, nephrotic syndrome.
    3. Obstructed lymphatic drainage – tumors, parasites, surgical lymph-node removal.
  • Clinical signs: swelling of ankles, fingers, abdomen, face; possible organ dysfunction internally.

Quantitative & Fast Facts

  • Total vascular length ≈ 60{,}000 miles (twice Earth’s circumference).
  • Blood volume distribution at rest:
    • Heart: 4\%.
    • Brain circulation: 13\%.
    • Veins contain >60\% of total blood; arteries ≈11\%.
  • Estimated capillary count: >1\times10^9.
  • No body cell is >4–6 cell-widths away from a capillary.

Learning Outcomes Checklist (for self-review)

  • Trace blood flow leaving & returning to heart (✓).
  • Distinguish wall structure of arteries vs. veins (✓).
  • Explain conducting, distributing arteries & arterioles (✓).
  • List traits that make veins distinct; describe venules, medium & large veins (✓).
  • Describe capillary structure, organization, & exchange mechanisms (✓).
  • Explain diffusion, filtration, colloid osmotic pressure (✓).
  • Identify three causes of edema (✓).
  • Follow pulmonary circulation path (✓).
  • Name aortic regions & major branches (✓).
  • Outline head/neck blood supply (via common carotids & vertebral arteries) (review further if needed).
  • Identify principal systemic veins (e.g., jugulars, brachiocephalic, hepatic, renal, iliac) (review diagrams).
  • Discuss hepatic portal circulation (✓).
  • Relate pressure gradient to BP & circulation (review hemodynamics formulas (\Delta P = Q \times R)).
  • Summarize how cardiac output, blood volume, resistance affect BP ((BP \propto CO \times TPR)).
  • Explain why capillary flow is slowest (large total cross-section) (✓).
  • Describe skeletal-muscle & respiratory pumps aiding venous return (review concepts).