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Blood Vessels and Circulation - Anatomy & Physiology

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  • Three principal categories of blood vessels
    Arteries carry blood away from the heart, veins carry blood back to the heart, and capillaries connect smallest arteries to smallest veins to create a circuit.
  • Layers of blood vessel walls
    Blood vessel walls have three tunics: tunica interna (endothelium lining), tunica media (smooth muscle and elastic tissue), and tunica externa (loose connective tissue).
  • Function of tunica interna
    Lines the vessel, acts as a selectively permeable barrier, secretes chemicals for dilation/constriction, and normally repels blood cells to prevent clotting.
  • Function of tunica media
    Consists of smooth muscle and elastic tissue; controls vessel diameter by contraction and strengthens vessels to prevent rupture.
  • Function of tunica externa
    Outermost layer of loose connective tissue; anchors vessels and provides passage for nerves and lymphatic vessels; contains vasa vasorum in large vessels.
  • Classes of arteries by size
    Conducting (large) arteries like the aorta, distributing (medium) arteries to organs, and resistance (small) arteries including arterioles that control blood flow.
  • Aneurysm
    A weak point in an artery or heart wall forming a bulging sac that may rupture; caused by atherosclerosis, hypertension, trauma, or infection.
  • Arterial sense organs
    Sensory structures in vessel walls that monitor blood pressure and chemistry, including carotid sinuses (baroreceptors) and carotid/aortic bodies (chemoreceptors).
  • Types of capillaries
    Continuous (most tissues), fenestrated (rapid absorption organs), and sinusoids (liver, bone marrow) distinguished by permeability.
  • Capillary beds and precapillary sphincters
    Networks of capillaries controlled by sphincters that regulate blood flow; sphincters relax to perfuse capillaries and contract to bypass them.
  • Veins as capacitance vessels
    Veins have thin walls, collapse when empty, expand easily, and contain about 64% of blood at rest; they have valves to prevent backflow.
  • Types of veins
    Postcapillary venules, muscular venules, medium veins (with valves), large veins, and venous sinuses (thin walls, no smooth muscle).
  • Varicose veins
    Enlarged, twisted superficial veins caused by valve failure, blood pooling, and vessel wall weakening; risk factors include heredity, obesity, and pregnancy.
  • Simplest circulatory route
    Heart → arteries → arterioles → capillaries → venules → veins; blood passes through one capillary network per circuit.
  • Blood flow and perfusion
    Flow is blood volume per time (mL/min); perfusion is flow per tissue mass (mL/100g/min); cardiac output at rest is about 5.25 L/min.
  • Blood pressure (BP) measurement
    Measured at brachial artery; systolic pressure is peak during ventricular contraction; diastolic pressure is minimum during relaxation; normal ~120/75 mm Hg.
  • Pulse pressure and mean arterial pressure (MAP)
    Pulse pressure = systolic - diastolic; MAP = diastolic + 1/3 pulse pressure; MAP influences risk of edema, fainting, and vascular diseases.
  • Peripheral resistance factors
    Resistance depends on blood viscosity, vessel length, and vessel radius; radius has greatest effect, with flow proportional to radius to the fourth power.
  • Vasomotion
    Collective term for vasoconstriction and vasodilation; regulates blood pressure and flow by changing vessel radius.
  • Regulation of blood pressure and flow
    Controlled by local (autoregulation), neural (vasomotor center reflexes), and hormonal mechanisms (angiotensin II, aldosterone, ADH, catecholamines).
  • Capillary exchange mechanisms
    Substances cross capillary walls by diffusion, transcytosis, filtration, and reabsorption through endothelial cells, intercellular clefts, and fenestrations.
  • Filtration and reabsorption in capillaries
    Fluid filters out at arterial end due to hydrostatic pressure and is reabsorbed at venous end due to colloid osmotic pressure; about 85% reabsorbed.
  • Edema causes
    Excess fluid accumulation due to increased filtration, reduced reabsorption, or lymphatic obstruction; can impair oxygen delivery and cause tissue damage.
  • Mechanisms of venous return
    Venous return aided by pressure gradient, gravity, skeletal muscle pump, thoracic pump, and cardiac suction.
  • Skeletal muscle pump
    Contraction of limb muscles compresses veins, pushing blood toward the heart; valves prevent backflow.
  • Thoracic (respiratory) pump
    Inhalation lowers thoracic pressure and raises abdominal pressure, forcing blood upward in the inferior vena cava.
  • Effects of exercise on venous return
    Exercise increases heart rate, dilates vessels, and enhances skeletal muscle and thoracic pumps, improving venous return.
  • Arterial pressure points
    Superficial arteries close to body surface used for pulse taking and emergency pressure to reduce bleeding, e.g., femoral artery in femoral triangle.
  • Hypertension
    Chronic resting BP >130/80 mm Hg; damages heart and vessels, promotes atherosclerosis; primary (90%) and secondary forms exist.