Cardiac Output/Blood Pressure

• Cardiac Output


  Cardiac output = Stroke volume (SV) x Heart rate (HR)
• Cardiac Output


  The total amount of blood pumped to the body in one minute
• Cardiac Output


  Regulated by extrinsic and intrinsic mechanisms
• Regulation of Cardiac (Heart) Rate


  1. Sympathetic innervation of the SA Node (norepinephrine) and epinephrine from the adrenal medulla

  2. Parasympathetic innervation (vagus nerve)
• Regulation of Cardiac (Heart) Rate

  Cardiac control center in the medulla oblongata dictates autonomic response
• Regulation of Stroke Volume

  1. Preload = End-Diastolic Volume (EDV)

  2. Afterload = diastolic blood pressure

  3. Contractility = the strength of contraction
• Preload


  Defined as the final volume in the ventricle before it contracts
• Preload


  Dictated by venous return = amount of blood returned to heart
• Preload


  The greater the preload the more forceful the contraction (Frank-Starling law), the greater the SV
• Afterload


  Defined as an impedance to the ejection of blood from ventricles
• Afterload


  Dictated by the Total Peripheral Resistance (TPR) of the blood vessels
• Afterload


  The greater the afterload, the less the SV
• Contractility


  The greater the stretch of the sarcomere the greater the strength of contraction
• Contractility


  The stretch is related to EDV
• Control of Contractility


  1. Intrinsic control (Frank-Starling law)

  2. Extrinsic control (Autonomic innervation (Ca2+))
• Venous Return


  Amount of blood that returns to the heart
• Venous Return


  Veins have higher compliance than arteries
• Venous Return


  Any degree of pressure will cause more distension in veins than in arteries, this means veins can hold more blood
• Variables that Affect Venous Return and End-diastolic Volume (Preload)


  1. Skeletal muscle pump = squeezes veins during muscle contraction

  2. Breathing = Δ P between thoracic and abdominal cavities

  3. Sympathetic vasoconstriction = smooth muscle contraction decreases compliance
• Fluid compartments of the body

  • Intracellular 67%
  • Extracellular 33%
  • Interstitial fluid 80%
  • Blood plasma 20%
• Capillary Exchange


  1. Determined by filtration, blood pressure, and osmotic pressure

  2. Combined pressure at the arteriole end of the capillary favor movement out of the capillary

  3. Combined pressure at the venular end of the capillary favor movement into the capillary
• Capillary Exchange


  1. Plasma (minus protein) leaves the capillary at the arteriole end carrying nutrients

  2. That fluid (tissue fluid) reenters the venous capillary carrying metabolic waste

  3. About 10 to 15% is left in the interstitial space and is returned by the lymphatic system
• Edema


  Excessive accumulation of tissue fluid
• Causes of Edema


  • BP
  • Venous obstruction
  • Leakage of plasma proteins into tissue fluid/ plasma protein concentration
  • Myxedema (hypothyroidism)
  • Lymphatic obstruction
• Regulation of Blood Volume

  1. Antidiuretic hormone (ADH) - Osmoreceptors in the hypothalamus detect osmotic pressure in the plasma, causes reabsorption of H2O by the collecting ducts of the renal tubules, also causes thirst and vasoconstriction

  2. Aldosterone - Released due to the activation of the renin-angiotensin-aldosterone system, causes reabsorption of Na+ and H2O in kidneys, and vasoconstriction

  3. Atrial Natriuretic Factor - Produced and released by the atria of the heart due to stretch, promotes Na+ and H2O excretion, possibly inhibits release of aldosterone
• Vascular Resistance to Blood Flow

  Blood flow in the circulatory system is dictated by change in pressure
• Vascular Resistance to Blood Flow

  Vasoconstriction and vasodilation of vessels determine blood flow to various organs of the body
• Variables that Regulate Blood Flow

  1. Pressure difference (ΔP) - Blood flow directional proportional to ΔP

  2. Resistance - Blood flow indirectly proportional to resistance, depends on length of vessel, viscosity of blood, radius of vessel
• Blood Pressure in Different Vessels

  • Arteries
  • Arterioles
  • Capillaries
  • Venules
  • Veins
• Total Peripheral Resistance


  Sum of all vascular resistances within systemic circulation
• Total Peripheral Resistance


  Changes in resistance in one organ generally affects blood flow in that organ only
• Regulation of Blood Flow

  1. Extrinsic Control - Autonomic, Hormonal, Paracrine control (Nitric oxide, bradykinin, prostacyclin, Endothelin-1)

  2. Intrinsic control - Myogenic, Metabolic (Decreased O2 concentration, Increased CO2 concentration, Decreased pH, Release of K+)
• Blood Pressure


  A measure of the force exerted on the wall of a blood vessel at the level of the heart
• Primary factors that effect blood pressure

  • Cardiac rate (Heart rate)
  • Stroke volume (determined by blood volume)
  • Total peripheral resistance (TPR)
• Cardiac output (CO) is amount of blood pumped by heart per minute


  CO = SV x HR
• Stroke Volume (SV)


  Blood pumped during each heart beat
• Heart Rate (HR)


  Number of times heart beats per minute
• Constriction increases blood pressure upstream (arterial pressure)


  Decreases pressure downstream (capillary and venous pressure)
• Resistance


  Highest in the arterioles due to their ability to vasoconstrict
• Blood pressure and blood flow

  Further reduced in the capillaries because of a higher cross-area in these capillaries