Heart Physiology Chapter 11

Microscopic 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 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 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 Purkinje fibers Spread in the ventricle walls

Intrinsic Conduction System Sinoatrial (SA) node starts each heartbeat and sets the pace – pacemaker Impulse spreads through the atria to the AV node (causing the atria to contract) Impulse pauses for a 0.1 sec to allow atria to finish contracting Then the impulse goes through the AV bundle (bundle of His) and splits into the right and left bundle branches The impulse ends in the Purkinje fibers causing the ventricles to contract

Nervous 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 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

Electrocardiography (ECG or EKG) 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)

Heart 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 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 Cycle Cardiac cycle Diastole Systole 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 Cycle Ventricular filling: mid-to-late diastole (AV valves open) Ventricular systole (SL valves open) Early diastole (AV valves open)

Heart 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 Rate Tachycardia Bradycardia 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 System Arteries Capillaries Veins 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

Pulse Pressure wave of blood Monitored at “pressure points” where pulse is easily palpated

Blood 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 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 Pressure Figure 11.18