The Heart Functions as a Pump. How is the pump action controlled by regulated waves of cell-cell depolarization within the pump? 2/12

Review: What are the steps to a cardiac cycle of contraction?How do the heart valves maintain one-way blood flow in two separate

circulatory loops?How does the heart work as a pump?What is the pathway taken by the wave of depolarization in the heart?What is the difference between nodal and contractile depolarization?What is a pacemaker potential?

How are cells initially become depolarized? (LEAKAGE=autorhythmicity)

How does the action potential pass between adjacent cells? (PASSAGE=electrical syncitium)

Your heart pumps blood using a five step Cardiac Cycle:

• Remember the two equal cycles: Pulmonary AND Systemic1) Diastolic Filling of Atria and Ventricles (V. Diastole)

– Semilunars are closed and AV valves are open!

2) Atrial Systole (VIP: occurs towards the end of V. Diastole)– Ventricles are primed with atrial blood (“topped off”)– Semilunar valves closed

3) Isovolumetric Ventricular Contraction (V. Systole)– Semilunar valves remain closed, AV valve flaps are

close by back flow of blood into atria when the ventricular pressure begins to increase.

– Pressure is generated until Ventricular mmHg > Arterial mmHg

• 4) Ventricular Ejection (V. Systole)– When Vent P > Arterial P, semilunars open and blood can

exit the ventricle– Volume of blood ejected from ventricle is dependent on

magnitude of pressure gradient– Semilunar valves must open before ejection can begin!

• 5) Isovolumetric Ventricular Relaxation (V. Diastole)– End of contraction, semilunars close when VentP< Arterial P– AV valves open and diastolic filling begins next cycle

• Remember the two ventricles BOTH do these activities at about same time with the same volumes at two different pressures!

• While “Atrial” Systole does occur, it is not as clinically important because the atria only do about 5% of the work done by the ventricles, so they just don’t use as much ATP or need as much oxygen.

Remember that the AV and semilular valves close to prevent flow of blood from high to low pressure back into

the atria or ventricles!

Your heart pumps blood using a five step Cardiac Cycle:

• Remember the two equal cycles: Pulmonary AND Systemic1) Diastolic Filling of Atria and Ventricles (V. Diastole)

2) Atrial Systole (VIP: occurs towards the end of V. Diastole)

3) Isovolumetric Ventricular Contraction (V. Systole)

4) Ventricular Ejection (V. Systole)

5) Isovolumetric Ventricular Relaxation (V. Diastole)

• Remember the two ventricles BOTH do these activities at about same time with the same volumes at two different pressures!

• While “Atrial” Systole does occur, it is not as clinically important because the atria only do about 5% of the work done by the ventricles, so they just don’t use as much ATP or need as much oxygen.

How do pressures change in the heart chambers and arteries during the cardiac cycle when you are healthy? Memorize List!Units of Pressure: measured in mmHg is the ability of fluid pressure to

maintain a column of mercury. Yes it’s archaic but it’s the unit used! Pressure gradients let blood move! ↑ gradient:↑movement point ABAtrial Pressures: Simply create enough P to load blood into ventricle

-Right Atrium: 0-10 mmHg and the Left Atrium: 0-10 mmHgVentricles: MUST generate enough pressure to load blood into arteries

Right Ventricle: 0 to ~15-25 mmHg Left Ventricle: 0 to ~120mmHgIf pressure in ventricle is not great enoughNo Blood Exits Ventricle

Arterial Pressures: Systolic pressure is from Vent Systole and Diastolic is pressure just before next loading from ventricle (Contraction).-Always expressed: Systolic mmHg/Diastolic mmHg-Pulmonary Artery: 15-25/8 tough to “feel” (a very short circuit) -Aorta: 120/80felt as a “Pulse” (a long-high resistance circuit)

Venous Pressures: always very very low (sometimes even negative)!-VenaCava and Pulmonary Veins: 5/0 or lower (WHEN HEALTHY!)

The volume of blood pumped by a ventricle per minute is the cardiac output of that ventricle! How is CO calculated? Memorize!

How do we get the ventricle ready to push blood out?Most ventricular filling passively occurs as blood drains from vena cava

and pulmonary veins through atria and into the apex (down hill) by simple gravity and draining…..water pours out of a glass onto the floor by the same effect.

End Diastolic Volume(EDV): ml of blood in ventricle after the “kick” Pre-systole (130 ml)– Mostly from passive filling during Vent and Atrial diastole, plus a

small bonus from atrial systole (about 100 ml + 30 ml from kick)Isovolumetric Contraction: no volume change (hopefully)

– AV Valves just closed and Semilunars are not yet able to open!– Volume in ventricle does not change but pressure on this 130 ml

goes up until pressure in ventricle is greater than in the artery in the blood on other side of the semilunar valve!

VentricularEjection/StrokeVolume(SV): blood leaving vent. (70ml)– “Ejection Fraction (EF)” – EF=SV/EDV X 100 = 70ml/130ml X 100 = 54%– SV can be larger and EF can be up to 90% during exercise!

End Systolic Volume (ESV): Amount left in vent. After systole– ESV=EDV-SV=130ml-70ml =60 ml (amt. remaining during

isovolumetric relaxation)– Heart disease= a weak ventricle so the EF is small and ESV large

Cardiac Output (CO): amount pumped by a ventricle/minute– CO= SV X HR = 70 ml/beat X 75 beats/min = 5,000 ml/min – CO=SV X HR = 100 ml/min X 50 beats/min = 5,000 ml/min– (X2 for “whole heart”: Rt/Lt ventricles pump equal volumes:

5X2=10 L/min)Range (cardiac reserve): CO can get up to 35 L/min/Ventricle for

Olympic Athletes! Your Blood volume (4-7 L) must be recycled many times/min at 35 L/min

Will I need to do the math for the calculation we just looked at on a lab or lecture test? Answer: Yes Will I get to use a calculator: No-Will math be “easy”:Yes

• Formulas should be understood for what they are….a logical prediction of what the body does.

• Know basic formulas we just looked at.• Know normal values we just looked at.• Know how to calculate missing information given

a set of known facts.

• KNOW HOW AND WHY WE MODIFY C.O.!

SA Node-->(Atrial depolarization/contraction)AV NodeBundle of HisLt/Rt Bundle BranchPukinjeMyocardium (Depol./Contract)

How do depolarizations pass between cardiac cells?• Intercalated discs hold adjacent cardiac cells together and the gap

junctions create tiny pores in the discs between the cells allowing passage of ions like Na+ and Ca++, this unites all cardiac cells into an electrical syncitium (all connected).

• Conduction Path: SA NodeAtriaAV NodeCommon bundle branchBundle of His R/L Bundle Branches Purkinje fibers Myocardium (contractile cells here).

Note: the Conduction rates and Consequence of rates in different locations in the pathway are not the same…this is very important!

• Then the wave of depolarization (excitation) spreads from endocardium to epicardium (Inside to Outside)

V.I.P. Atrioventricular septum is normally non-conductive to depolarization! Depol only passes through the AV-Node!

V.I.P. There is a bit of contraction in the interventricular septum prior to the walls of the ventricles (this is clinically significant!)

Heart Rhythm: is the heart conduction coordinated? Nodal vs. Arrhythmia vs. Ectopic Foci vs. Fibrillation

ACTION POTENTIAL IN INDIVIDUAL CELLS vs. DEPOLARIZATION OF ENTIRE HEART (MANY CELLS CONNECTED BY GAP JUNCTIONS) .HUGE Functional Difference: Pacemaker Cell vs. Contractile CellSA NodalCell(AV nodal or conduction)=(no contractile force) Cardiac Myocyte (force generation=contractile)Speed of Na+ leak determines rate of depolarization for SA/AV-Nodes Very

SteepDepolarizes FastRapid Heart Rate Not Steep: Cells depolarize slowly take more time to reach threshold for

voltage gated Na+ channels to open Slow Heart Rate Absolute refractory period determines rate of depolarization by determining

time the cell must wait until it can be depolarized againMyocytes and Timing: Why a delay in the onset of contraction?• Na+ channels must open and let Na+ into cell• Ca++ gets inside myocytes• Ca++ must find troponin and pull it off actin• Actin must find/bind myosin• Actin must ratchet across myosin and generate force• Ca++ Slow to exit myocytes• These all add to the x-axis (time) of contraction• This generates force in ventricle: Isovolumetic Contraction Ejection

SA Nodal cells are non-contractile and rhythmically depolarize themselves and adjacent cells. Nodal cells eventually depolarize adjacent contractile cells via gap junctions. As a result, the depolarizations of contractile and non-contractile cells are very different!

Remember: this is depolarization in single cells, not the whole heart!

Non-Contractile Cells!

Agents like epinephrine speed up the heart rate and the parasympathetic NS (ACH) slows the heart down. What happens to the rate of membrane leakage (pacemaker potential) to accomplish this change in heart rate at the SA Node?

Before and after Sympathetic Stimulation +pacemaker potentialLonger period between depol

Shorter period between depol

Longer time to nextdepolarization

Stimulation of the parasympathetic NS slows down the heart rate by hyperpolarizing the SA node and decreasing its leakiness (pacemaker potential)