The Conduction System of the Heart

AnatomyThe Circulatory System

Cardiac Physiology

• In a single cardiac contraction, or heartbeat, the entire heart contracts in series – first the atria and then the ventricles

• Specialized muscle cells of the conducting system control and coordinate the heartbeat

• Contractile cells produce the powerful contractions that propel blood

The Conduction System• The heart is controlled by the ANS

(autonomic nervous system) – it contracts on its own, without neural or hormonal stimulation

• The heart has its own regulating system = conduction system

• The conduction system is composed of specialized muscle cells/tissue that initiates and distributes electrical impulses (the action potential).

Components of the conducting system

• SA Node - Located in the right atrial wall below the opening to the superior vena cava

• AV Node - Located in the interatrial septum (floor of the right atrium)

• Conducting cells – interconnect the two nodes and distribute the stimulus throughout the myocardium

Components of the Conduction System

1. SA NODE • Contains pacemaker cells, which initiates and

establishes heartbeat• Known as the natural pacemaker• Average = 75 beats/minute2. AV NODE• Receives the action potential from the SA node• Action potential slows considerably allows time

for the atria to empty into the ventricles

Components of the Conduction System

3. AV BUNDLE• Receives impulse from the AV node• Located at the interventricular septum• Passes impulse onto the bundle branches4. BUNDLE BRANCHES• Located below the AV bundle• Passes impulse onto the conduction myofibers5. CONDUCTION MYOFIBERS (Purkinje Fibers)• Located in the ventricular myocardium• Receives action potential/impulse from bundle branches

and passes it onto ventricular myocardial cells to complete ventricle contraction

ECG or EKG• Impulses can be detected on the body surface and recorded with an

electrocardiograph.• The recording that is made, the electrocardiogram (ECG/EKG), traces the

flow of current through the heart. • Each time the heart beats, a wave of depolarization radiates through the

atria, pauses at the AV node, then travels down the interventricular septum to the apex, turns, and spreads through the ventricular myocardium toward the base

• An ECG integrates electrical information obtained by placing electrodes at different locations on the body surface

The typical ECG has three recognizable waves:1st Wave: P wave (depolarization of the atria)2nd Wave: QRS complex (ventricular depolarization)3rd Wave: T wave (ventricular repolarization)

Clinicians can use an ECG to access the performance of specific nodal, conducting, and contractile components. Abnormalities in the shape of the wave and changes in the timing could indicate something is wrong. Examples: An excessively large QRS complex often indicates that the heart has become enlarged. When a portion of the heart has been damaged by a heart attack the ECG reveals an abnormal pattern of impulse conduction.

The Cardiac Cycle

• Each heartbeat is followed by a brief resting phase, allowing time for the chambers to relax and prepare for the next heartbeat

• The period between the start of one heartbeat and the beginning of the next is a single cardiac cycle

• Includes alternating periods of contraction and relaxation

The Cardiac Cycle PhasesSystole: the phase of contraction of a chamber

(the chamber contracts and pushes blood into an adjacent chamber or into an arterial trunk)

Diastole: the phase of relaxation (the chamber fills with blood and prepares for the next cardiac cycle)

Blood Pressure• Pressure that blood exerts against the

inner walls of the blood vessels• Systolic Number – This is the TOP

number in your blood pressure…related to the pressure of contraction in the heart that is placed on the inside of the vessels

• Diastolic Number – This is the BOTTOM number in your blood pressure…related to the relaxed phase of your heart that is then transferred to your vessels

• Normal blood pressure is 120/80

Cardiac Output

• For the body to function properly, the heart needs to pump blood at a sufficient rate to maintain an adequate and continuous supply of oxygen and other nutrients to the brain and other vital organs

• Cardiac output = describes the amount of blood your heart pumps each minute

• A healthy heart pumps about 5-6 liters of blood every minute when a person is resting

Cardiac Output• Cardiac Output: It is determined by:

1. The amount of blood pumped by the left ventricle during each beat (stroke volume)2. The number of heart beats per minute

• The amount of blood ejected by a ventricle during each contraction (single beat) is called stroke volume

• Cardiac output is an indication of the blood flow through peripheral tissues; without adequate blood flow, homeostasis cannot be maintained; helps keep blood pressure at the levels needed to supply oxygen-rich blood to your organs

Cardiac Output• For an adult at rest…• Stroke volume = 80 mL (average)• Heart rate = 75 beats per minute (average)• Therefore, Cardiac Output (CO) = Stroke

Volume x Beats per Minute CO = 80 mL/beat x 75bpmCO = 6000mL/min (6L/min)

Arrhythmia• Abnormal heart rhythm (irregular

heartbeats and heart rates)• Bradyarrhythmia (Bradycardia) – heart

rates that are slow (less than 50bpm)• Tachyarrhythmia (Tachycardia) – rapid

heart rates (faster than 100 bpm)• About 5% of healthy individuals

experience a few abnormal heartbeats each day

Causes of Arrhythmia•Coronary artery disease (partial or complete blockage of coronary circulation)

•Electrolyte imbalances in your blood• Injury from a heart attack•Healing process after heart surgery

Example: Fibrillation (atrial or ventricular) – rapid, uncoordinated shuddering of the heart

High Blood Pressure/Hypertension(Silent Killer)

• Normal bp = 90/60 (birth) and 120/80 (adults)• Top number shows the pressure when the heart beats• Lower number measures pressure at rest between heart

beats, when the heart refills with blood• High blood pressure = 140/90 consistently• High blood pressure can threaten healthy arteries and

lead to life-threatening conditions (leading cause of stroke and major cause of heart attack)

• Significantly increases the workload on the heart, and the left ventricle gradually enlarges

• More muscle mass means a greater oxygen demand• Places physical stress on the walls of blood vessels

Hypertension

• Prehypertension – blood pressure is consistently just above normal (120-139 & 80-89)

• Smoking• Being overweight or obese• Lack of physical activity• Too much salt in the diet• Too much alcohol consumption• Stress• Genetics• Older age

What causes hypertension?

Atherosclerosis

• Usual cause of heart attacks & strokes• High blood pressure, smoking or high

cholesterol damage the endothelium (lines the arteries) & cholesterol plaque forms

• The arteries harden and narrow and block blood flow

• Plaque behaves in different ways: can stay within the artery wall, cause blockages, or rupture which allows blood to clot in the artery

• Part of the coronary circulation becomes blocked, and cardiac muscle cells die from lack of oxygen

• Coronary heart disease causes the arteries to become narrow and blood cannot flow as well

• Fatty matter, calcium, proteins, and inflammatory cells build up within the arteries to form plaque

Heart Attack/Myocardial Infarction

Heart attack continued

• When the plaque that forms becomes hard, the outer shell cracks, and blood clots form around the plaque

• If a blood clot blocks the artery, the heart muscle becomes “starved” for oxygen

• Within a short time, death of heart muscle cells occurs, causing permanent damage

1. Smoking2. High blood pressure3. High blood cholesterol levels4. Diabetes5. Obesity6. Sedentary lifestyle7. Genetics

Risk Factors of a Heart Attack

Pulmonary Edema• An abnormal buildup of fluid in the air

sacs of the lungs, which leads to shortness of breath

• In most cases heart problems cause pulmonary edema but fluid can accumulate for other reasons

• As pressure in the blood vessels increase, fluid is pushed into the air spaces (alveoli) in the lungs

• This fluid interrupts normal oxygen movement in the lungs

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