Know the three types of blood vessels, how they differ, and their functions



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Know the three types of blood vessels, how they differ, and their functions

  • Know the three types of blood vessels, how they differ, and their functions

  • Know the mechanisms of venous return

  • Understand the principle of blood flowand pressure

  • Know what peripheral resistance is and what causes it

  • Understand how materials are exchanged in the capillaries





Resistance vessels because they have relatively strong, resilient tissue structure that resists high blood pressure

  • Resistance vessels because they have relatively strong, resilient tissue structure that resists high blood pressure

    • conducting (elastic or large) arteries
      • aorta, common carotid, subclavian, pulmonary trunk, and common iliac arteries
      • have layer of elastic tissue, internal and external elastic lamina, at the border between interna and media and between media and externa
    • distributing (muscular or medium) arteries
      • distributes to specific organs
      • brachial, femoral, renal, and splenic arteries
      • smooth muscle layers constitute three-fourths of wall thickness


resistance (small) arteries

  • resistance (small) arteries

    • arterioles – smallest arteries
      • control amount of blood to various organs
    • thicker tunica media in proportion to their lumen than large arteries and very little tunica externa
  • metarterioles

    • short vessels link arterioles to capillaries
    • muscle cells form a precapillary sphincter
      • constriction of sphincters reduces or shuts off blood flow


capillaries - site where nutrients, wastes, and hormones pass between the blood and tissue fluid through the walls of the vessels (exchange vessels)

  • capillaries - site where nutrients, wastes, and hormones pass between the blood and tissue fluid through the walls of the vessels (exchange vessels)

    • composed of endothelium and basal lamina
    • absent or scarce in tendons, ligaments, epithelia, cornea and lens of the eye


capillaries organized into networks called capillary beds

  • capillaries organized into networks called capillary beds

    • usually supplied by a single metarteriole
  • thoroughfare channel - metarteriole that continues through capillary bed to venule

  • precapillary sphincters control which beds are well perfused

    • when sphincters open
      • capillaries are well perfused with blood and engage in exchanges with the tissue fluid
    • when sphincters closed
      • blood bypasses the capillaries
      • flows through thoroughfare channel to venule
  • three-fourths of the bodies capillaries are shut down at a given time





greater capacity for blood containment than arteries

  • greater capacity for blood containment than arteries

  • thinner walls, flaccid, less muscular and elastic tissue

  • collapse when empty, expand easily

  • have steady blood flow

  • merge to form larger veins

  • subjected to relatively low blood pressure

    • remains 10 mm Hg with little fluctuation


venous return – the flow of blood back to the heart

  • venous return – the flow of blood back to the heart

    • pressure gradient
      • blood pressure is the most important force in venous return
      • 7-13 mm Hg venous pressure towards heart
      • venules (12-18 mm Hg) to central venous pressure – point where the venae cavae enter the heart (~5 mm Hg)
    • gravity drains blood from head and neck
    • skeletal muscle pump in the limbs
      • contracting muscle squeezed out of the compressed part of the vein
    • thoracic (respiratory) pump
      • inhalation - thoracic cavity expands and thoracic pressure decreases, abdominal pressure increases forcing blood upward
        • central venous pressure fluctuates
      • 2mm Hg- inhalation, 6mm Hg-exhalation
      • blood flows faster with inhalation
    • cardiac suction of expanding atrial space




exercise increases venous return in many ways:

  • exercise increases venous return in many ways:

    • heart beats faster, harder increasing CO and BP
    • vessels of skeletal muscles, lungs, and heart dilate and increase flow
    • increased respiratory rate, increased action of thoracic pump
    • increased skeletal muscle pump
  • venous pooling occurs with inactivity

    • venous pressure not enough force blood upward
    • with prolonged standing, CO may be low enough to cause dizziness
      • prevented by tensing leg muscles, activate skeletal muscle pump
    • jet pilots wear pressure suits


postcapillary venules – smallest veins

  • postcapillary venules – smallest veins

    • even more porous than capillaries so also exchange fluid with surrounding tissues
    • tunica interna with a few fibroblasts and no muscle fibers
    • most leukocytes emigrate from the bloodstream through venule walls
  • muscular venules – up to 1 mm in diameter

    • 1 or 2 layers of smooth muscle in tunica media
    • have a thin tunica externa
  • medium veins – up to 10 mm in diameter

    • thin tunica media and thick tunica externa
    • tunica interna forms venous valves
    • varicose veins result in part from the failure of these valves
    • skeletal muscle pump propels venous blood back toward the heart


venous sinuses

  • venous sinuses

    • veins with especially thin walls, large lumens, and no smooth muscle
    • dural venous sinus and coronary sinus of the heart
    • not capable of vasomotion
  • large veins – larger than 10 mm

    • some smooth muscle in all three tunics
    • thin tunica media with moderate amount of smooth muscle
    • tunica externa is thickest layer
      • contains longitudinal bundles of smooth muscle
    • venae cavae, pulmonary veins, internal jugular veins, and renal veins


blood pools in the lower legs in people who stand for long periods stretching the veins

  • blood pools in the lower legs in people who stand for long periods stretching the veins

    • cusps of the valves pull apart in enlarged superficial veins further weakening vessels
    • blood backflows and further distends the vessels, their walls grow weak and develop into varicose veins
  • hereditary weakness, obesity, and pregnancy also promote problems

  • hemorrhoids are varicose veins of the anal canal



simplest and most common route

  • simplest and most common route

    • heart  arteries  arterioles  capillaries  venules  veins
    • passes through only one network of capillaries from the time it leaves the heart until the time it returns
  • portal system

    • blood flows through two consecutive capillary networks before returning to heart
      • between hypothalamus and anterior pituitary
      • in kidneys
      • between intestines to liver
  • anastomosis – the point where two blood vessels merge



blood supply to a tissue can be expressed in terms of flow and perfusion

  • blood supply to a tissue can be expressed in terms of flow and perfusion

    • blood flow – the amount of blood flowing through an organ, tissue, or blood vessel in a given time (ml/min)
    • perfusion – the flow per given volume or mass of tissue in a given time (ml/min/g)
  • at rest, total flow is quite constant, and is equal to the cardiac output (5.25 L/min)

  • important for delivery of nutrients and oxygen, and removal of metabolic wastes

  • hemodynamics

    • physical principles of blood flow based on pressure and resistance
      • F is proportional to P/R, (F = flow, P = difference in pressure, R = resistance to flow)
      • the greater the pressure difference between two points, the greater the flow; the greater the resistance the less the flow


blood pressure (bp) – the force that blood exerts against a vessel wall

  • blood pressure (bp) – the force that blood exerts against a vessel wall

  • measured at brachial artery of arm using sphygmomanometer

  • two pressures are recorded:

    • systolic pressure: peak arterial BP taken during ventricular contraction (ventricular systole)
    • diastolic pressure: minimum arterial BP taken during ventricular relaxation (diastole) between heart beats
  • normal value, young adult: 120/75 mm Hg

  • pulse pressure – difference between systolic and diastolic pressure

    • important measure of stress exerted on small arteries by pressure surges generated by the heart
  • mean arterial pressure (MAP) – the mean pressure one would obtain by taking measurements at several intervals throughout the cardiac cycle

    • diastolic pressure + (1/3 of pulse pressure)
    • average blood pressure that most influences risk level for edema, fainting (syncope), atherosclerosis, kidney failure, and aneurysm


hypertension – high blood pressure

  • hypertension – high blood pressure

    • chronic is resting BP > 140/90
    • consequences
      • can weaken small arteries and cause aneurysms
  • hypotension – chronic low resting BP

    • caused by blood loss, dehydration, anemia


one of the body’s chief mechanisms in preventing excessive blood pressure is the ability of the arteries to stretch and recoil during the cardiac cycle

  • one of the body’s chief mechanisms in preventing excessive blood pressure is the ability of the arteries to stretch and recoil during the cardiac cycle

  • importance of arterial elasticity

    • expansion and recoil maintains steady flow of blood throughout cardiac cycle, smoothes out pressure fluctuations and decreases stress on small arteries
  • BP rises with age

    • arteries less distensible and absorb less systolic force
  • BP determined by cardiac output, blood volume and peripheral resistance





peripheral resistance – the opposition to flow that blood encounters in vessels away from the heart

  • peripheral resistance – the opposition to flow that blood encounters in vessels away from the heart

  • resistance hinges on three variables

    • blood viscosity “thickness”
      • RBC count and albumin concentration elevate viscosity the most
      • decreased viscosity with anemia and hypoproteinemia speed flow
      • increased viscosity with polycythemia and dehydration slow flow
    • vessel length
      • the farther liquid travels through a tube, the more cumulative friction it encounters
      • pressure and flow decline with distance
    • vessel radius - most powerful influence over flow
      • only significant way of controlling peripheral resistance.
      • vasomotion - change in vessel radius
        • vasoconstriction - by muscular effort that results in smooth muscle contraction
        • vasodilation - by relaxation of the smooth muscle


vessel radius (cont.)

  • vessel radius (cont.)

    • vessel radius markedly affects blood velocity
    • laminar flow - flows in layers, faster in center
    • blood flow (F) proportional to the fourth power of radius (r), F  r4
      • arterioles can constrict to 1/3 of fully relaxed radius
        • if r = 3 mm, F = (34) = 81 mm/sec; if r = 1 mm, F = 1mm/sec
        • an increase of three times in the radius of a vessel results in eighty one times the flow




from aorta to capillaries, blood velocity (speed) decreases for three reasons:

  • from aorta to capillaries, blood velocity (speed) decreases for three reasons:

    • greater distance, more friction to reduce speed
    • smaller radii of arterioles and capillaries offers more resistance
    • farther from heart, the number of vessels and their total cross-sectional area becomes greater and greater
  • from capillaries to vena cava, flow increases again

    • decreased resistance going from capillaries to veins
    • large amount of blood forced into smaller channels
    • never regains velocity of large arteries


arterioles are most significant point of control over peripheral resistance and flow

  • arterioles are most significant point of control over peripheral resistance and flow

    • on proximal side of capillary beds and best positioned to regulate flow into the capillaries
    • outnumber any other type of artery, providing the most numerous control points
    • more muscular in proportion to their diameter
      • highly capable of vasomotion
  • arterioles produce half of the total peripheral resistance



vasomotion is a quick and powerful way of altering blood pressure and flow

  • vasomotion is a quick and powerful way of altering blood pressure and flow

  • three ways of controlling vasomotion:

    • local control
    • neural control
    • hormonal control


the most important blood in the body is in the capillaries

  • the most important blood in the body is in the capillaries

  • only through capillary walls are exchanges made between the blood and surrounding tissues

  • capillary exchange – two way movement of fluid across capillary walls

    • water, oxygen, glucose, amino acids, lipids, minerals, antibodies, hormones, wastes, carbon dioxide, ammonia
  • chemicals pass through the capillary wall by three routes

    • through endothelial cell cytoplasm
    • intercellular clefts between endothelial cells
    • filtration pores (fenestrations) of the fenestrated capillaries
  • mechanisms involved

    • diffusion, transcytosis, filtration ,and reabsorption


diffusion is the most important form of capillary exchange

  • diffusion is the most important form of capillary exchange

    • glucose and oxygen being more concentrated in blood diffuse out of the blood
    • carbon dioxide and other waste being more concentrated in tissue fluid diffuse into the blood
  • capillary diffusion can only occur if:

    • the solute can permeate the plasma membranes of the endothelial cell, or
    • find passages large enough to pass through
      • filtration pores and intracellular clefts
  • lipid soluble substances

    • steroid hormones, O2 and CO2 diffuse easily through plasma membranes
  • water soluble substances

    • glucose and electrolytes must pass through filtration pores and intercellular clefts
  • large particles - proteins, held back



endothelial cells pick up material on one side of the plasma membrane by pinocytosis or receptor-mediated endocytosis, transport vesicles across cell, and discharge material on other side by exocytosis

  • endothelial cells pick up material on one side of the plasma membrane by pinocytosis or receptor-mediated endocytosis, transport vesicles across cell, and discharge material on other side by exocytosis

  • important for fatty acids, albumin and some hormones (insulin)



fluid filters out of the arterial end of the capillary and osmotically reenters at the venous end

  • fluid filters out of the arterial end of the capillary and osmotically reenters at the venous end

    • delivers materials to the cell and removes metabolic wastes
  • opposing forces

    • blood hydrostatic pressure drives fluid out of capillary
      • high on arterial end of capillary, low on venous end
    • colloid osmotic pressure (COP) draws fluid into capillary
      • results from plasma proteins (albumin)- more in blood
      • oncotic pressure = net COP (blood COP - tissue COP)
  • hydrostatic pressure

    • physical force exerted against a surface by a liquid
      • blood pressure is an example
  • capillaries reabsorb about 85% of the fluid they filter

  • other 15% is absorbed by the lymphatic system and returned to the blood



capillary filtration at arterial end

  • capillary filtration at arterial end

  • capillary reabsorption at venous end

  • variations

    • location
      • glomeruli- devoted to filtration
      • alveolar capillary - devoted to absorption
    • activity or trauma
      • increases filtration




capillaries usually reabsorb most of the fluid they filter – exception:

  • capillaries usually reabsorb most of the fluid they filter – exception:

    • kidney capillaries in glomeruli do not reabsorb
    • alveolar capillaries in lung absorb completely to keep fluid out of air spaces
  • capillary activity varies from moment to moment

    • collapsed in resting tissue, reabsorption predominates since BP is low
    • metabolically active tissue has increase in capillary flow and BP
      • increase in muscular bulk by 25% due to accumulation of fluid


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