physio cardiovascular

• Blood vessels


  Carry blood throughout the body
• 5 main types of blood vessels

  • Arteries
  • Arterioles
  • Capillaries
  • Venules
  • Veins
• Arteries


  • Carry blood AWAY from the heart
• Veins


  • Carry blood TO the heart
• Tunica interna (intima)


  Inner lining in direct contact with blood
• Tunica media


  Muscular and connective tissue layer
• Tunica externa


  Elastic and collagen fibers
• Arteries

  • Thick muscular-to-elastic tunica media
  • High compliance - walls stretch and expand in response to pressure without tearing
  • Vasoconstriction - decrease in lumen diameter
  • Vasodilation - increase in lumen diameter
• Elastic arteries


  • Largest arteries
  • Largest diameter but walls relatively thin
  • Function as pressure reservoir
  • Help propel blood forward while ventricles relaxing
  • Also known as conducting arteries - conduct blood to medium-sized arteries
• Muscular arteries


  • Tunica media contains more smooth muscle and fewer elastic fibers than elastic arteries
  • Walls relatively thick
  • Capable of great vasoconstriction/vasodilation to adjust rate of blood flow
  • Also called distributing arteries
• Anastomoses


  • Union of the branches of 2 or more arteries supplying the same body region
  • Provide alternate routes - collateral circulation
• Arterioles


  • Abundant microscopic vessels
  • Metarteriole has precapillary sphincter which monitors blood flow into capillary
  • Sympathetic innervation and local chemical mediators can alter diameter and thus blood flow and resistance
  • Resistance vessels - resistance is opposition to blood flow
  • Vasoconstriction can raise blood pressure
• Capillaries


  • Smallest blood vessels connect arterial outflow and venous return
  • Exchange vessels - primary function is exchange between blood and interstitial fluid
  • Lack tunica media and tunica externa
  • Substances pass through just one layer of endothelial cells and basement membrane
  • Capillary beds - arise from single metarteriole
  • Vasomotion - intermittent contraction and relaxation
  • Throughfare channel - bypasses capillary bed
• Types of capillaries

  • Continuous
  • Fenestrated
  • Sinusoids
• Venules


  • Thinner walls than arterial counterparts
  • Postcapillary venule - smallest venule
  • Form part of microcirculatory exchange unit with capillaries
  • Muscular venules have thicker walls with 1 or 2 layers of smooth muscle
• Veins


  • Very thin walls in relation to total diameter
  • Tunica interna thinner with little smooth muscle
  • Tunica externa thickest layer
  • Not designed to withstand high pressure
  • Valves - folds on tunica interna forming cusps
  • Aid in venous return by preventing backflow
• Blood distribution


  • Largest portion of blood at rest is in systemic veins and venules
  • Blood reservoir
  • Venoconstriction reduces volume of blood in reservoirs and allows greater blood volume to flow where needed
• Capillary exchange

  1. Diffusion

  2. Transcytosis

  3. Bulk flow
• Diffusion


  • Substances move down their concentration gradient
  • O2 and nutrients from blood to interstitial fluid to body cells
  • CO2 and wastes move from body cells to interstitial fluid to blood
  • Can cross capillary wall through intracellular clefts, fenestrations or through endothelial cells
  • Most plasma proteins cannot cross
  • Except in sinusoids - proteins and even blood cells leave
  • Blood-brain barrier - tight junctions limit diffusion
• Transcytosis


  • Small quantity of material
  • Substances in blood plasma become enclosed within pinocytotic vessicles that enter endothelial cells by endocytosis and leave by exocytosis
  • Important mainly for large, lipid-insoluble molecules that cannot cross capillary walls any other way
• Bulk flow


  • Passive process in which large numbers of ions, molecules, or particles in a fluid move together in the same direction
  • Based on pressure gradient
  • Diffusion is more important for solute exchange
  • Bulk flow more important for regulation of relative volumes of blood and interstitial fluid
  • Filtration - from capillaries into interstitial fluid
  • Reabsorption - from interstitial fluid into capillaries
• Blood hydrostatic pressure (BHP)


  • Generated by pumping action of heart
  • Falls over capillary bed from 35 to 16 mmHg
• Interstitial fluid osmotic pressure (IFOP)

  1 mmHg
• Blood colloid osmotic pressure (BCOP)


  • Promotes reabsorption
  • Due to presence of blood plasma proteins to large to cross walls
  • Averages 36 mmHg
• Interstitial fluid hydrostatic pressure (IFHP)


  Close to zero mmHg
• Starling's Law

  • Nearly as much reabsorbed as filtered
  • At the arterial end, net outward pressure of 10 mmHg and fluid leaves capillary (filtration)
  • At the venous end, fluid moves in (reabsorption) due to -9 mmHg
  • On average, about 85% of fluid filtered in reabsorbed
  • Excess enters lymphatic capillaries (about 3L/ day) to be eventually returned to blood
• Blood flow


  • Volume of blood that flows through any tissue in a given period of time (in mL/min)
  • Total blood flow is cardiac output (CO)
  • CO = heart rate (HR) x stroke volume (SV)
  • Distribution of CO depends on pressure differences that drive blood through tissue and resistance to blood flow in specific blood vessels
• Blood pressure


  • Contraction of ventricles generates blood pressure
  • Systolic BP - highest pressure attained in arteries during systole
  • Diastolic BP - lowest arterial pressure during diastole
  • Pressure falls progressively with distance from left ventricle
  • Blood pressure also depends on total volume of blood
• Vascular resistance

  • Opposition to blood flow due to
  • 1. Viscosity of blood
  • 2. Diameter of blood vessels
  • 3. Length of blood vessels
• Hemodynamics


  Factors affecting blood flow
• Blood flow


  Volume of blood that flows through any tissue in a given period of time (in mL/min)
• Cardiac output (CO)


  Total blood flow, volume of blood that circulates through systemic (or pulmonary) blood vessels each minute
• Calculating cardiac output


  CO = heart rate (HR) x stroke volume (SV)
• Distribution of cardiac output


  • Depends on pressure differences that drive blood through tissue, flows from higher to lower pressure, resistance to blood flow in specific blood vessels
• Blood pressure


  Contraction of ventricles generates blood pressure, systolic BP is highest pressure attained in arteries during systole, diastolic BP is lowest arterial pressure during diastole, pressure falls progressively with distance from left ventricle, also depends on total volume of blood
• Vascular resistance


  Opposition to blood flow due to friction between blood and walls of blood vessels, depends on size of lumen, blood viscosity, and total blood vessel length
• Venous return


  Volume of blood flowing back to heart through systemic veins, occurs due to pressure generated by constriction of left ventricle, small pressure difference from venule to right ventricle is sufficient
• Skeletal muscle pump


  Milks blood in 1 direction due to valves
• Respiratory pump


  Due to pressure changes in thoracic and abdominal cavities
• Velocity of blood flow


  Speed in cm/sec is inversely related to cross-sectional area, velocity is slowest where total cross sectional area is greatest, blood flow becomes slower farther from the heart, slowest in capillaries, aids in exchange, circulation time is normally 1 minute at rest