Heart PhysiologyChapter 11

Microscopic AnatomyMicroscopic Anatomy Cardiac muscle fibers (cells)

Striated, short, and branched

One nucleus

Plasma membrane of adjacent cardiac cells interlock at intercalated discs

Large mitochondria account for 25% of the volume and give the cells high resistance to fatigue

Only operates on aerobic respiration

Contracts by the sliding filament mechanism

Self-excitable and can initiate their own depolarization

Spontaneous and rhythmic stimulation

The heart contracts as a unit or not at all because all cells are tied together at intercalated discs

Physiology of The HeartPhysiology of The Heart Cardiac muscle cells have

the ability to contract intrinsically (within) or with autonomic NS

Contractions occur in a regular continuous rhythm

Heart needs a control system to be an efficient pump

Main conduction system is the intrinsic conduction system Specialized non-

contractile cardiac cells that initiate and distribute impulses throughout the heart so that it contracts

Intrinsic Conduction SystemIntrinsic Conduction System Sinoatrial node (SA)

Located in the right atrial wall

Pacemaker (sets the pace for HR)

Atrioventricular (AV) node

Located between atria and ventricles

Atrioventricular bundle (Bundle of His)

Located in the septum

Right and left bundle branches

Located in the septum

Purkinje fibers

Spread in the ventricle walls

Intrinsic Conduction SystemIntrinsic Conduction System1. Sinoatrial (SA) node starts each

heartbeat and sets the pace – pacemaker

2. Impulse spreads through the atria to the AV node (causing the atria to contract)

3. Impulse pauses for a 0.1 sec to allow atria to finish contracting

4. Then the impulse goes through the AV bundle (bundle of His) and splits into the right and left bundle branches

5. The impulse ends in the Purkinje fibers causing the ventricles to contract

Nervous System Conduction SystemNervous System Conduction System

Basic heart rate is set by intrinsic conduction system

Autonomic nervous system can modify the heart rate

Sympathetic activation increases the heartbeat

Parasympathetic activation slows the heartbeat

Conduction System ImbalancesConduction System Imbalances Arrhythmias

Irregular heart rhythms due to defects in the intrinsic conduction system

Fibrillation

A condition of rapid and irregular contractions

Defibrillation is accomplished by electrically shocking the heart which interrupts the chaotic twitching

• Heart block

• Damage to the AV node, totally or partially releasing the ventricles from the control of the sinoatrial (SA) node

Electrical currents generated and transmitted through the heart spread throughout the body and can be detected with an electrocardiograph

A graphic record of heart activity is called an electrocardiogram (ECG or EKG)

Recording electrodes are positioned at various sites on the body surface

12 leads are typically used

Electrocardiography (ECG or EKG)

Typical ECG has 3 waves

P wave

Atrial depolarization before they contract

QRS complex

Depolarization of ventricles and contraction of ventricles

T wave

Ventricle repolarization

Electrocardiography (ECG or EKG)

P-R interval

From the atrial excitation to the beginning of ventricular excitement

S-T segment

Action potentials of the ventricular cells are in their plateau phases

Q-T interval

From the beginning of ventricular depolarization through ventricular repolarization

Electrocardiography (ECG or EKG)

In a healthy heart the size, duration and timing of the waves tend to be consistent

Changes in the pattern or timing may reveal a diseased or damaged heart or problems with the heart’s conduction system

Example: an enlarged R wave hints of enlarged ventricles

Electrocardiography (ECG or EKG)

Electrocardiography (ECG or EKG)

Heart SoundsHeart Sounds http://www.viddler.com/explore/humananatomy/videos/7/

Two sounds (lub-dup) can be heard when listening to the thorax with a stethoscope

Associated with the closing of the valves

First sound (lub) occurs when the AV valves close

Second sound (dup) occurs when the SL valves close

Heart SoundsHeart Sounds Heart murmurs

Abnormal heart sounds

Blood flows silently as long as the flow is smooth

If it strikes obstructions, its flow generates heart murmurs that can be heard with a stethoscope

The Cardiac CycleThe Cardiac Cycle

Cardiac cycle

All events associated with the blood flow through the heart during one complete heartbeat

Diastole

Relaxation period (atria and ventricles)

Systole

Contraction period (atria and ventricles)

The Cardiac CycleThe Cardiac Cycle

1. Ventricular filling: mid-to-late diastole (AV valves open)

2. Ventricular systole (SL valves open)

3. Early diastole (AV valves open)

Heart RateHeart Rate

• Heart rate can be affected by age, gender, exercise and body temperature

• Resting heart rate is fastest in the fetus • 140-160 beats/min

• Average heart rate for females • 72-80 beats/min

• Average heart rate for males • 64-72 beats/min

Heart RateHeart Rate

• Tachycardia• Abnormally fast heart rate more than 100 beats/min• Results from elevated body temperature, stress, certain drugs,

or heart disease

• Bradycardia• Heart rate slower than 60 beats/min• Results from low body temperature, certain drugs or

parasympathetic activation

The Vascular SystemThe Vascular System

Arteries Taking blood away from the heart to the tissues

Capillaries Gas and nutrient exchange

Walls of capillaries are only one cell layer thick to allow for exchanges between blood and tissue

Veins Taking blood back to (visit) the heart

Skeletal muscle “milks” blood in veins toward the heart

PulsePulse

Pressure wave of blood

Monitored at “pressure points” where pulse is easily palpated

Blood PressureBlood Pressure

Pressure the blood exerts against the inner blood vessels walls

Because the heart contracts and relaxes, the rhythmic flow of blood causes the pressure to rise and fall each beat.

Systolic – pressure at the peak of ventricular contraction

Diastolic – pressure when ventricles relax

Variations in Blood PressureVariations in Blood Pressure Human normal blood pressure range is variable Pressure in blood vessels decreases as the

distance away from the heart increases Normal

140–110 mm Hg systolic 80–75 mm Hg diastolic

Hypotension Low systolic (below 110 mm HG) Often associated with illness

Hypertension High systolic (above 140 mm HG) Can be dangerous if it is chronic

Measuring Arterial Blood PressureMeasuring Arterial Blood Pressure

Figure 11.18