External Heart Anatomy Quiz

Cardiac Cycle

• The cardiac cycle refers to all events associated with blood flow through the heart
• It consists of systole (contraction of heart muscle) and diastole (relaxation of heart muscle)
• Phases of the cardiac cycle:
  • Ventricular filling (mid-to-late diastole): heart blood pressure is low as blood enters atria and flows into ventricles
  • Atrial systole: AV valves are open, then atrial systole occurs
  • Ventricular systole (contraction): atria relax, rising ventricular pressure results in closing of AV valves, isovolumetric contraction phase, and ventricular ejection phase opens semilunar valves
  • Isovolumetric relaxation (early diastole): ventricles relax, backflow of blood in aorta and pulmonary trunk closes semilunar valves, and dicrotic notch – brief rise in aortic pressure caused by backflow of blood rebounding off semilunar valves

Heart Physiology

• Heart Physiology: Intrinsic Conduction System
  • Autorhythmic cells: initiate action potentials, have unstable resting potentials called pacemaker potentials, and use calcium influx (rather than sodium) for rising phase of the action potential
• Heart Physiology: Sequence of Excitation
  • Sinoatrial (SA) node generates impulses about 75 times/minute
  • Atrioventricular (AV) node delays the impulse approximately 0.1 second
  • Impulse passes from atria to ventricles via the atrioventricular bundle (bundle of His) and splits into two pathways in the interventricular septum (bundle branches)

Cardiac Output and Reserve

• Cardiac Output (CO) is the amount of blood pumped by each ventricle in one minute
• CO is the product of heart rate (HR) and stroke volume (SV)
• HR is the number of heart beats per minute
• SV is the amount of blood pumped out by a ventricle with each beat
• Cardiac reserve is the difference between resting and maximal CO

Regulation of Stroke Volume

• SV = end diastolic volume (EDV) minus end systolic volume (ESV)
• EDV = amount of blood collected in a ventricle during diastole
• ESV = amount of blood remaining in a ventricle after contraction
• Factors affecting stroke volume:
  • Preload – amount ventricles are stretched by contained blood
  • Contractility – cardiac cell contractile force due to factors other than EDV
  • Afterload – back pressure exerted by blood in the large arteries leaving the heart

Frank-Starling Law of the Heart

• Preload, or degree of stretch, of cardiac muscle cells before they contract is the critical factor controlling stroke volume
• Slow heartbeat and exercise increase venous return to the heart, increasing SV
• Blood loss and extremely rapid heartbeat decrease SV

Extrinsic Factors Influencing Stroke Volume

• Contractility is the increase in contractile strength, independent of stretch and EDV
• Increase in contractility comes from:
  • Increased sympathetic stimuli
  • Certain hormones
  • Ca2+ and some drugs
• Agents/factors that decrease contractility include:
  • Acidosis
  • Increased extracellular K+
  • Calcium channel blockers

Regulation of Heart Rate

• Positive chronotropic factors increase heart rate
  • Caffeine
• Negative chronotropic factors decrease heart rate
  • Sedatives
• Regulation of Heart Rate: Autonomic Nervous System
  • Sympathetic nervous system (SNS) stimulation is activated by stress, anxiety, excitement, or exercise
  • Parasympathetic nervous system (PNS) stimulation is mediated by acetylcholine and opposes the SNS
  • PNS dominates the autonomic stimulation, slowing heart rate and causing vagal tone
  • If the Vagus Nerve was cut, the heart would lose its tone