3.2 - Electrical conduction, ECG, pacemakers, blood vessels

ELECTRICAL CONDUCTION OF THE HEART

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What is unusual about cardiac muscle compared with skeletal muscle?

Cardiac muscle cells can contract without input from the nervous system.

What happens if individual cardiac myocytes are isolated?

They continue to beat rhythmically on their own.

Why can cardiac muscle contract in unison?

Because cardiac myocytes are electrically coupled by intercalated discs.

SINOATRIAL (SA) NODE

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What is the sinoatrial node?

The heart’s natural pacemaker.

Where is the SA node located?

In the wall of the right atrium near the entry of the superior vena cava.

What is the function of the SA node?

To generate electrical impulses that set the heart’s rhythm.

ATRIOVENTRICULAR (AV) NODE

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What is the atrioventricular node?

A relay point of specialised muscle tissue between atria and ventricles.

What is the role of the AV node?

To delay electrical impulses before they pass to the ventricles.

How long is the AV node delay?

Approximately 0.1 seconds.

Why is AV node delay important?

It ensures the atria contract and empty before ventricular contraction begins.

PURKINJE FIBRES

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What are Purkinje fibres?

Specialised conducting muscle fibres in the ventricles.

What is the role of Purkinje fibres?

To rapidly spread electrical impulses throughout the ventricles for coordinated contraction.

ELECTRICAL CONDUCTION PATHWAY

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Describe the electrical conduction pathway through the heart.

SA node → atrial walls → AV node → Purkinje fibres → ventricular muscle.

What happens after the SA node fires?

Electrical impulses spread across the atria causing them to contract.

What happens after the AV node delay?

The impulse travels rapidly through Purkinje fibres to trigger ventricular contraction.

REGULATION OF HEART RATE

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What determines the basic heart rhythm?

The SA node.

Can heart rate be modified despite SA node control?

Yes, by nerves, hormones, and physiological factors.

CHRONOTROPIC FACTORS

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What are positive chronotropic factors?

Factors that increase heart rate.

Example of a positive chronotropic factor?

Adrenaline (epinephrine).

Why does adrenaline increase heart rate?

It is part of the fight-or-flight response.

What are negative chronotropic factors?

Factors that decrease heart rate.

Example of a negative chronotropic factor?

Beta-adrenergic receptor antagonists (beta blockers).

TEMPERATURE EFFECTS

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How does temperature affect heart rate?

An increase of 1°C raises heart rate by approximately 10 beats per minute.

Why does fever increase pulse rate?

Because increased temperature stimulates pacemaker activity.

ELECTROCARDIOGRAM (ECG)

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What is an ECG?

A recording of the electrical activity of the heart during the cardiac cycle.

How are ECG signals detected?

Electrical currents generated by the heart travel through body fluids and are detected by skin electrodes.

What does an ECG measure?

Changing electrical potential differences over time.

ECG WAVES

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What does the P wave represent?

Atrial depolarisation from impulses originating in the SA node and spreading through both atria.

What does the QRS complex represent?

Ventricular depolarisation as impulses spread from the AV node through Purkinje fibres.

What does the T wave represent?

Ventricular repolarisation before the next impulse.

ECG IMPULSE SEQUENCE

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What happens at the beginning of the ECG sequence?

The SA node generates an impulse and atrial excitation begins.

What happens when the impulse reaches the AV node?

It is delayed while the P wave completes.

What happens after AV node delay?

The impulse is transmitted toward the apex and ventricular excitation begins.

What happens during the QRS complex?

Ventricular depolarisation completes.

What happens during the T wave?

The ventricles repolarise in preparation for the next beat.

ARTIFICIAL PACEMAKERS

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When are artificial pacemakers needed?

When the SA node fails or when there are conduction abnormalities.

What can artificial pacemakers correct?

Slow heart rate, irregular heart rhythm, or conduction defects after heart damage.

Where are pacemakers implanted?

Under the skin.

How do pacemakers connect to the heart?

Through electrodes attached to the heart.

What is the function of an artificial pacemaker?

To regulate heart rhythm artificially.

BLOOD VESSELS

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What is the circulatory system vessel arrangement?

A closed system beginning and ending at the heart.

What are the three major blood vessel types?

Arteries, veins, and capillaries.

ARTERIES

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What is the function of arteries?

To carry blood away from the heart.

Why do arteries have thick walls?

To withstand high pressure generated by the heart.

VEINS

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What is the function of veins?

To carry blood toward the heart.

Why do veins have thinner walls than arteries?

Because blood pressure is much lower in veins.

CAPILLARIES

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What is the function of capillaries?

To directly exchange substances with tissues.

Why are capillary walls thin?

To allow efficient exchange of oxygen, nutrients, and waste products.

VESSEL STRUCTURE

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Why do different blood vessels have different structures?

Because they manage different blood pressures and transport functions.

Which vessels experience the highest pressure?

Arteries.

Which vessels are specialised for exchange?

Capillaries.

Which vessels act as blood reservoirs?

Veins.

IMPORTANT NOTES

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Are arteries defined by oxygen content?

No.

How are arteries defined?

By carrying blood away from the heart.

Are veins defined by oxygen content?

No.

How are veins defined?

By carrying blood toward the heart.

Do all arteries carry oxygen-rich blood?

No.

Do all veins carry oxygen-poor blood?

No.

What happens to blood volume after blood loss?

Venous blood volume decreases to help maintain arterial pressure and capillary blood flow.