topic 11: cardiovascular pt. 1

how does the cardiovascular system play an important role in homeostasis?

• ensures that oxygen, nutrients, and hormones reach every cell while at the same time is removing wastes

what does the cardiovascular system vital to?

• regulating blood pressure

• body temperature

• immune response to keep body in balance

why is understanding cardiovascular physiology important?

• crucial for recognizing how the heart and blood vessels function in health and disease, especially since heart-related conditions are among the leading causes of death worldwide

what are the parts in cardiac physiology?

1. heart - dual pump with valves

2. conduction system

what is the heart?

• dual pump with valves

• muscle cells connected by gap junctions connecting 2 areas of ISF

what is the conduction system?

• non-contractile cardiac muscle cells - modified to initiate and distribute impulses (APs) throughout the heart

• produce APs spontaneously (no stimulus) BUT at different rates (make faster and slower, space b/w AP)

what are the parts of the conduction system?

1. sinoatrial (SA) node - right atrium

2. atrioventricular (AV) node - right atrium

3. bundle of HIS (AV bundle)

4. Purkinje fibers

how does the AV node contribute to the conduction system?

• rate = 100 APs/min (modified by PSP to be 75 APs/min at rest)

• produced APs faster than other areas therefore is the pacemaker

how does the AV node contribute to the conduction system?

• rate = 50 APs/min

• on its own, so can have HR with no SA node but it would be a slower heart rate

how does the bundle of HIS contribute to the conduction system?

• originated at AV node

• ONLY route for electrical activity to go from atria to ventricles and Bundle Branches (right and left)

• 30 APs/min (too slow to keep you alive)

• CT is an insulator → cannot pass conduction through

how does the Purkinje fibers contribute to the conduction system?

• terminal fibers - stimulate contraction of the ventricular myocardium

• 30 APs/min

pathway of action potentials in the heart

what happens if the conduction system is damaged?

• next fastest part becomes the pacemaker

  • if SA node is damaged, AV node takes over (atria may not contract + ventricles contract at AV speed = 50 bpm) ok but not good

what are artificial pacemakers?

• can be used when the conduction system fails

• stimulus of SA or AV node damaged (sets up electrical current to reestablish HR)

action potentials of the SA and AV nodes

• cells = non-contractile autorhythmic cardiac muscle (self-excitable, don’t have conscious control)

• threshold = - 40mv

what are the phases of pacemaker activity?

1. pacemaker potential

2. AP depolarization

3. AP repolarization

4. Na+ channels open at -50mV

what occurs during pacemaker potential?

• low K+ permeability (K+ voltage gates closed)

• slow inward leak of Na+ (Na+ voltage gates open)

  • causes slow depolarization toward threshold (-40mV) (up to 0)

• autorhythmic

what occurs during AP depolarization?

• at threshold → AP

• Ca2+ voltage gates open - Ca2+ moves in → depolarization (Na+ voltage gates close at threshold so are NOT involved in AP)

• Ca2+ voltage gates close at peak (~0mv) (sodium closes at -40mV)

what occurs during AP repolarization?

• K+ voltage gated open at peak K+ out → repolarize

• K+ gates close below threshold

what occurs during the Na+ channels opening at -50mV?

• starts pacemaker potential (1) again (continuous cycle)

• self propagating APS

does the conduction system have a resting membrane potential?

• NO

• no relative refractory period because it is self regulating

phases of pacemaker activity

APs in ventricular myocardium

• cells = contractile

• Purkinje fibers AP → ventricular (contractile, they DO have RMP) myocardial AP (spread cell to cell by gap junctions)

• resting MP = -90mV

• heart needs to relax otherwise would remain contracted

where are pacemaker cells (autorhythmic) found?

• SA and AV node

• non-contractile

• generate AP automatically

where are contractile cells found?

• most of heart

• atria and ventricles

• contract and pump blood

what are the phases of ventricular myocardial

1. depolarization

2. plateau

3. repolarization

what happens during depolarization in ventricular myocardial APs?

• Na+ voltage gates open (fast) = same gates as neuron, skeletal muscle

• MP to +30mV

what happens during plateau in ventricular myocardial APs?

• Na+ channels close + inactivate (slight drop in MP)no longer letting pos. charges in

• Ca2+ slow voltage gates are open (Ca2+ influx maintains depolarization)

what happens during repolarization in ventricular myocardial APs?

• Ca2+ channels close

• K+ voltage gated channels open → increase K+ outflux → MP decrease to resting

• outflux of potassium

absolute refractory period of ventricular myocardium APs?

• long - Na+ channels inactivated until MP is close to -70mV

what is excitation-contraction coupling in myocardial cells

• how an electrical signal (AP) causes the heart muscle to contract

what are the steps to the excitation-contraction coupling in myocardial cells?

1. AP on sarcolemma of contractile cells triggers»>

2. voltage-gated Ca2+ channels open (plateau of AP) producing a small increase in cytosolic Ca2+ (from ECF). this is not enough to trigger contraction BUT…

3. Ca2+ binds to chemically-gated Ca2+ channels on sarcoplasmic reticulum (SR) and they open

4. Ca2+ is released from the SR into the cytosol producing a large increase in cytosolic Ca2+

5. Ca2+ binds to troponin → troponin-tropomyosin complex moves, exposing myosin binding sites on action; crossbridges form → leads to contraction

6. contraction

what happens during contraction phase of the excitation-contraction coupling myocardial cells?

• sliding filament mechanism

• begins a few msec after AP begins

• duration of AP = ~250 msec and duration of twitch = ~300 msec

• therefore contraction almost over when AP ends

• result = NO summation therefore NO tetanus - get alternation of contraction/relaxation

what are the components of cardiac cycle?

1. electrical activity (ECG)

2. mechanical activity

3. blood flow through heart

what is electrical activity (ECG)?

• small currents due to depolarization/repolarization of the heart move through salty body fluids

how is potential difference measured on the body surface?

• using electrode pairs: one pair = a lead

how is an ECG recording seen as?

• waves

  • = sum of electrical activity of ALL myocardial cells (NOT an AP)

what are the ECG waves?

• P wave

• QRS wave

• T wave

what does the P wave represent?

• atrial depolarization → followed by contraction

what does the QRS wave represent?

• ventricular depolarization → contraction

  • positive ions entering (Ca2+, Na+)

• atrial repolarization is also occurring (leads to relaxation of atria) - the wave created by atrial repolarization is masked by the larger ventricular electrical event (since the ventricles have a larger muscle mass compared to the atria, more cells depolarizing)

what does the T wave represent?

• ventricular repolarization → followed by relaxation

  • outflux of K+ as it repolarizes

what are the ECG intervals?

• P-Q

• S-T

• T-P

what is the P-Q interval?

• atria contracted, signals passing through AV node

what is the S-T interval?

• ventricles contracted, atria relaxed

what is the T-P interval?

• heart at rest (contractile muscle cells at rest)

what are abnormalities of heart beat?

• tachycardia

• bradycardia

• heart block

what is tachycardia?

• resting HR more than 100bpm (less distance between waves, don’t get proper relaxation)

what is bradycardia?

• resting HR less than 60 bpm

what is heart block?

• when conduction through AV node slowed, get an increased P→Q interval → ventricles may not contract after each atrial contraction

• not passing AP properly, still can contraction but slow (voltage gated Ca2+ are slow to close)

what is an example of heart block?

• 3rd degree heart block - no conduction through AV node - atria fire at SA node rate (~75 APs/min), ventricles at bundle/Purkinje rate (~30 APs/min)

what are the two main events of mechanical activity?

• systole

• diastole

what is systole?

• contraction, emptying (blood should be leaving)

what is diastole?

• relaxation, filling (allow to refill)

what are systole and diastole initiated by?

• electrical activity

what does 1 complete heart beat consist of?

• diastole + systole of atria AND diastole + systole of ventricles

what is the timing of mechanical events?

• average resting HR = 75 bpm

• therefore 0.8 sec/beat = 1 cardiac cycle (60 sec/min divide 75 beats/min)

• in 0.8 sec (start with atrial contraction at time 0)

  • atrial in systole for 0.1 sec, then diastole for 0.7 sec

  • ventricles enter systole after atria (0.1 sec delay at AV node) therefore the ventricles begin systole as atria begin diastole → in systole for 0.3 sec, then diastole for 0.5 sec