Cardiovascular System

• The heart


  The pump
• The vessels


  • Arteries
  • Veins
  • Capillaries
• Arteries


  • Carry blood from the heart to the tissues
  • High pressure
  • Contain a relatively small percentage of the blood volume

  
• Veins


  • Carry blood from the tissues back to the heart
  • Under low pressure
  • Contain the largest percentage of the blood volume

  
• Capillaries


  • Exchange of nutrients, wastes, and fluid across the capillary walls
• Primary function of the cardiovascular system

  • Participates in the regulation of arterial blood pressure
  • Delivers regulatory hormones from the endocrine glands to their sites of action in target tissues
  • Participates in the regulation of body temperature
  • Is involved in the homeostatic adjustments to altered physiologic states such as hemorrhage and exercise

  
• Heart


  Each side has two chambers, an atrium and a ventricle, connected by one-way valves, called atrioventricular (AV) valves
• Systemic circulation


  The left heart and the systemic arteries, capillaries, and veins

  
• Pulmonary circulation


  The right heart and the pulmonary arteries, capillaries, and veins

  
• Cardiac output


  The rate at which blood is pumped from either ventricle (ml/min)

  
• Venous return


  The rate at which blood is returned to the atria from the veins (ml/min)
• Blood vessels


  • Serve as a closed system of passive conduits, delivering blood to and from the tissues where nutrients and wastes are exchanged
  • Participate actively in the regulation of blood flow to the organs
• Steps of circulation


  1. 1

  2. 2

  3. 3

  4. 4

  5. 5

  6. 6
• Cardiac cycle


  Time of one contraction and relaxation (atrium or ventricle)

  
• Systole


  Ventricular contraction phase

  
• Diastole


  Ventricular relaxation phase

  
• Cardiac cycle overview


  1. Isovolumetric ventricular contraction

  2. Ventricular ejection (rapid, reduced)

  3. Isovolumetric ventricular relaxation

  4. Ventricular filling (rapid, reduced)

  
• Mid-diastole to late diastole

  1. Atrium and ventricle relaxed, atrial pressure slightly higher than ventricular

  2. AV valve opens, blood enters ventricle

  3. Aortic valve closed, aortic pressure decreasing

  4. Ventricular pressure increasing, volume expanding

  5. SA node discharges, atria depolarize

  6. Atrial contraction increases atrial pressure, forces blood into ventricle

  7. End-diastolic volume (EDV)

  
• Systole


  1. Ventricles depolarize, QRS complex

  2. Ventricles contract, pressure increases rapidly, exceeds atrial pressure

  3. AV valve closes, first heart sound

  4. Aortic valve opens, ventricular ejection begins

  5. Rapid ejection slows, most stroke volume ejected, atrial filling begins

  6. End-systolic volume (ESV)

  7. Aortic pressure increases with ventricular pressure

  8. Peak pressures reached before end of ejection, ejection rate reduces

  
• Early diastole


  1. T wave, ventricular repolarization

  2. Ventricular pressure decreases below aortic, aortic valve closes, second heart sound

  3. AV valve remains closed, isovolumetric ventricular relaxation

  4. Ventricular pressure decreases below atrial, AV valve opens, rapid venous filling

  
• Heart sounds


  First sound (lub) - AV valve closure, onset of systole

  Second sound (dup) - Aortic and pulmonary valve closure, onset of diastole
• Cardiac output


  Volume of blood each ventricle pumps per unit time (L/min)

  
• Heart rate control


  Inherent SA node discharge rate, negative chronotropic effect of parasympathetic, positive chronotropic effect of sympathetic, hormones, temperature

  
• Stroke volume control


  Changes in end-diastolic volume (preload)

  Changes in contractility (e.g. sympathetic stimulation)

  Changes in afterload (arterial pressure)
• Frank-Starling mechanism


  Stroke volume depends on end-diastolic volume, ensures cardiac output equals venous return

  
• Sympathetic regulation


  Increases ventricular contractility (positive inotropic effect), faster contraction and relaxation

  
• Afterload


  Increased arterial pressure tends to reduce stroke volume
• Cardiac reserve


  Maximum capacity cardiac output can increase above resting state
• Cardiac function measurements


  • Echocardiography

  Coronary angiography

  
• Conducting cells


  Specialized muscle cells including SA node (pacemaker), the atrial internodal tracts, the AV node, the bundle of His, and the Purkinje system
• Conducting cells


  • Spread action potentials
  • Have the capacity to generate action potentials spontaneously
• Contractile cells


  • Constitute the majority of atrial and ventricular myocardium
  • Are working cells that generate force or pressure
• Membrane potential


  Depends on conductance (or permeability) to ions and the concentration gradients
• Resting membrane potential of cardiac cells

  Close to the K+ equilibrium potential
• Role of Na+-K+ ATPase

  Maintain Na+ and K+ concentration gradients across the cell membrane
• Depolarization


  Inward current
• Hyperpolarization


  Outward current
• Threshold potential


  The membrane potential at which occurrence of the action potential is inevitable
• Ventricular muscle action potential

  • Long duration and Long refractory periods
  • Stable resting membrane potential
  • Plateau: a sustained period of depolarization

  
• Phases of ventricular muscle action potential

  1. Phase 0, upstroke: Rapid depolarization and inward Na+ current (positive feedback)

  2. Phase 1, initial repolarization: the inactivation gates on the Na+ channels close, an outward K+ current (Ito)

  3. Phase 2, plateau: inward Ca2+ current (L-type channels), outward K+ current (IK)

  4. Phase 3, rapid repolarization: inactivation of L-type Ca2+ channels, outward K+ current (IK, IK1)

  5. Phase 4, resting membrane potential: ≈ K+ equilibrium potential, IK1, Active ion transport