Section Three

• Arteries carry blood away from the heart whereas veins carry blood to the heart
• Deoxygenated blood flows from the heart to the lungs. Oxygenated blood flows from the lungs back to the heart
• Systemic circulation is blood flow from the heart (oxygenated) to the body (except the lungs) and from the body (deoxygenated) back to the heart
• Only systemic arteries carry oxygenated blood
• The left ventricle must generate more pressure than the right ventricle
• Atrioventricular valves are between the atria and ventricles. The right valve is the tricuspid and the left valve is the mitral (bicuspid) valve
• Tricuspid and mitral valves are attached to chordinae tendinae and tethered to papillary muscle
• Semilunar valves include the pulmonary valve and the aortic valve
• The pulmonary valve is between the right ventricle and the pulmonary artery
• The aortic valve is between the left ventricle and the aorta
• Cardiac muscle cells are striated, mononucleated, have desmosomes (shock absorbers), and have more blood vessels and mitochondria than skeletal muscles
• The heart can contract without a connection to the nervous system, and the signal for contraction is myogenic
• Autorhythmic and pacemaker cells initiate action potentials and establish the heartbeat rate and generates electrical activity to contractile cells
• Pacemaker cells in the SA node set the heart rate (primary pacemaker)
• The SA node is on top of the right atria
• Action potentials spread from the SA node to the AV node through internodal pathways
• The AV node is on the bottom of the right atria
• The interatrial pathway spreads electrical activity to the other atrium
• When a defibrillator is used when there is a failure of electrical conductance and shocks the SA node
• Pacemaker cells have an unstable resting membrane potential, generate no contractile force, and account for 1% of the heart
• Contractile cells make up the bulk of the heart. They have a stable resting membrane potential, long lasting action potentials, and are responsible for force generation needed for pumping blood
• The only electrical connection between the atria and ventricles is the Bundle of His
• The path of electrical conductance is:

  1. SA Node
  2. AV Node
  3. Bundle of His
  4. Bundle branches
  5. Purkinje fibers
• Action potentials are generated spontaneously in pacemaker cells because of their unstable membrane potential (pacemaker potential)
• Pacemaker cells are innervated by the sympathetic and parasympathetic nerve fibers
• Sympathetic nerves increase heart rate
• Parasympathetic nerve fibers decrease heart rate
• Sympathetic postganglionic fibers release norepinephrine. They act mainly on beta1 adrenergic receptors in the pacemaker cells
• Parasympathetic nerve fibers: acetylcholine activates muscarinic receptors that can lead to two outcomes:

  1. Increases K+ permeability (hyperpolarizing the cell)
  2. Decreases Ca2+ permeability
• The vagas nerve has a large influence on the heart. Cutting off its access will increase heart rate
• Under resting conditions, the heart is dual innervated, but the dominant influence is parasympathetic (rest and digest)
• The electrocardiogram measures summed electrical activity in the heart
• The PQRST complex:

  • P: atrial depolarization
  • QRS: progressive ventricular depolarization/atrial repolarization
  • T: Repolarization of the ventricles
• Atrial contraction on an ECG is the P to R segment
• Ventricular contraction on a ECG is just after the Q wave through the T wave (QT interval)
• On a ECG, the heart rate is shown by the R to R interval
• Diastole is relaxation whereas systole is contraction
• The volume of blood in ventricle at the end of diastole is called end-diastolic volume (EDV)
• The volume of blood in ventricle at the end of systole is called end-systolic volume (ESV)
• The amount of blood ejected from ventricle during a contraction is called the stroke volume