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The Heart

Chapter 18 – Day 4

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Wrap up PhysiologyMuscle contraction in cardiac tissuePacemaker cells (aka cells of the SA node)

♦Rhythmic depolarization and repolarization♦Triggers depolarization through internodal cells (aka – start

of action potential)

Cardiac tissue♦Ca2+ enters muscle cells via “slow calcium channels”♦This results in plateau (Fig. 18-15)♦Arrival of extracellular Ca2+ triggers the release of additional

Ca2+ from reserves in SR♦Sarcomeres shorten = muscle contraction♦Period of active muscle contraction continues until the

plateau ends

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Heart – Anatomy: Cardiac muscle

Fig. 18.5

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Fig. 18.13

Order of Rhythm in Heart

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Fig. 18.15

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Fig. 18.12

Journal of Action Potential in Heart

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Wrap up Physiology

Stimulates intercalated discs♦Moves action potential nearly instantaneously cell-to-cell♦Action potential simultaneously affects all tissue in the area

All muscle cells in ATRIA contract togetherAction potential is transferred to AV node (100msec

delay from SA node = Ventricles contract after atria)All muscle cells in VENTRICLES contract together

We’ve covered EKG/ECG, make sure you are comfortable with it for lecture AND lab exam

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Heart

Fig. 18.11

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Fig. 18.14

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Cardiac CycleCardiac cycle = 1 heart beat

Figure 18-16

2 important events♦Systole = contraction♦Diastole = relaxation

Each chamber has it’s own systole and diastole

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Cardiac CycleATRIAL SYSTOLE

♦Pushes blood into relaxed ventriclesVentricles build up pressureVENTRICULAR SYSTOLE

♦Early – there is enough pressure to close AV valves, but not enough to exceed pressure in blood vessels

♦Pressure exceeds that in blood vessels = 2nd phase of v. systole…

♦Ejects blood out of heart into circulation♦Meanwhile….ATRIAL DIASTOLE is in progress

At the beginning of VENTRICULAR DIASTOLE♦Atria fill up with blood (returning = venous return)

In late ventricular diastole all chambers are relaxed

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Fig. 18.16

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Fig. 18.17

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Fig. 18.18

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Heart HealthResting heart rate is efficient

Cardiac cycle can give information about heart efficiency

As the heart rate increases, less time is spent in each phase, thus…

…blood is ejected at a different volume

Heart efficiency can be measured as CARDIAC OUTPUT

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Cardiac Output = volume of blood ejected every minute

P. 523 and on board

Example

Cardiac Output = a major factor in determining blood pressure

How hard does the heart have to work for proper circulation

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Cardiac Output Depends on

♦Venous return♦Stretching of heart muscles♦Heart rate♦End diastolic volume (EDV)

•Volume remaining in ventricle after contraction b/c not all blood is ejected out

Figure 18-23 Amount of muscle stretching affects the force of contractions

(e.g. pulling a rubber band) Starlings Law of the Heart “Stretch” builds up resistance against the blood

♦More blood is pushed out

Why is C.O. important?♦Maintains proper BP levels to provide efficient blood supply

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Fig. 18.23

Factors Affecting Stroke Volume

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Cardiac OutputBlood Pressure

♦Pressure of blood within blood vessels♦Pressure exerted on ARTERY walls♦How would C.O. relate??

•Higher cardiac output◦ More blood = ↑ BP◦ Less blood = ↓ BP

♦Resistance of blood vessel walls•Small diameter = ↑ BP

•Blood flow is proportional to the radius of the “pipe”

♦Polycythemia (↑ in RBCs)•Thicker blood viscocity = ↑ BP (↑ RBCs or loss of water)

♦Length of blood vessel – longer length = ↑ BP

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Cardiac OutputCardiac output can be influenced by many factorsOne of these is HEART RATE

♦Heart conduction system♦Autonomic nervous system

Exercise can increase C.O.♦Heart muscles are working out♦Skeletal muscle cells need a fast blood supply♦The heart has to meet that demand – when exercising, it

meets increased demands♦The force of contraction increases ♦Blood moves faster, so venous return is faster♦More blood volume is pushed out♦Does not increase indefinitely, the Heart Rate can out-

compete C.O.

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Factors Affecting Heart Rate Hormones, ions, nervous system… ANS

♦Fibers connect to the SA node and the AV node

Sympathetic Control♦Medulla – remember the cardiovascular center♦Norepinephrine, epinephrine

•Affects the SA node, AV node & myocardium

Cardiac Nerve♦Dilates coronary vessels

Parasympathetic control♦Medulla – cardio-inhibitory center

Vagus Nerve (at S.A. node & A.V. node)♦Secretes ACh♦ACh changes ion distribution = decrease in heart rate

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Factors Affecting Heart RateIons affect heart rate

♦Ca2+ , Na+ - needed for depolarization♦K+ - needed for repolarization

Ion concentration in blood affects the action potential in the heart

Increase K+ outside♦Less K+ will leave the cell♦Partial repolarization – slower action potential HR

Increase Ca2+ outside cell♦ force of contraction – can lead to spasms

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Factors Affecting Cardiac Output

Fig. 18.24

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Heart ProblemsThe normal rhythm of the heart can be disrupted in

many ways – be ready to discuss on Friday those in book & those in handouts – you’ll also form your groups for the case study…