The Cardiovascular System

Heart Beat

• Electrical impulse carried from parts of the brain to the heart from the vagus nerve

• The impulse passes from one part of the heart to another which causes the contraction of muscles to start the pumping action

The cardiac cycle

1. SA node is stimulated2. SA node stimulates the AV

node causing AV valves to open and atrium to contract to push blood into ventricle

3. AV node passes impulse to Bundle of His and Purkinje fibers causing ventricle to contract to push blood through the semi-lunar valves into aorta or pulmonary artery

4. The cycle starts over again

Cardiac efficiency

• Cardiac output (amount of blood pumped by the heart in one minute) is determined by multiplying the heart rate (beats per minute) by the stroke volume ( the amount of blood pumped out of the left ventricle in one contraction

• CO=HRXSV• Used to determine the

efficiency of the heart.

• What is the CO if a person had a HR of 60 BPM and a SV of 70 mL per beat?

Blood Pressure• Pumping action of the heart forces

the blood through the vessels under pressure

• The pressure that is exerted is measured with a blood pressure cuff in millimeters of mercury or mmHg.

• BP is measured when ventricles contract (systolic pressure) and when ventricle relaxes (diastolic pressure)

• Example: 120/80 means that the pressure of blood caused by a pumping ventricle is measured at 120 mmHg while the pressure of the blood caused by a relaxing ventricle is 80 mmHg

Taking a blood pressure• Put BP cuff on arm and place

stethoscope over Brachial artery• Inflate the BP cuff until about 160

mmHg. This will completely close the artery, stopping all blood flow.

• Slowly let air out of the cuff and listen to when you hear the first sound. This sound is the systolic pressure (first number)

• Continue to slowly let air out until you hear the sound disappear completely. This is the diastolic pressure (second number)

• Record the first number over the second number to give the correct blood pressure.

Systole vs Diastole

• Systole is when the heart chambers are contracting (pumping).

• Diastole is when the heart chambers are relaxed (filling).

Figure 20–13

Impulse Conduction through the Heart

The Sinoatrial (SA) Node

• In posterior wall of right atrium

• Contains pacemaker cells

• Connected to AV node by internodal pathways

• Begins atrial activation (Step 1)

The Atrioventricular (AV) Node

• In floor of right atrium

• Receives impulse from SA node (Step 2)

• Delays impulse (Step 3)

• Atrial contraction begins

The AV Bundle

• In the septum

• Carries impulse to left and right bundle branches:– which conduct to Purkinje fibers (Step 4)

• And to the moderator band:– which conducts to papillary muscles

4. The Purkinje Fibers

• Distribute impulse through ventricles (Step 5)

• Atrial contraction is completed

• Ventricular contraction begins

Ectopic Pacemaker

• Abnormal cells

• Generate high rate of action potentials

• Bypass conducting system

• Disrupt ventricular contractions

What electrical events are associated with a

normal electrocardiogram?

The Electrocardiogram

Figure 20–14b

Electrocardiogram (ECG or EKG)

• A recording of electrical events in the heart

• Obtained by electrodes at specific body locations

• Abnormal patterns diagnose damage to heart cells/tissue

Features of an ECG

• P wave:– atria depolarize

• QRS complex:– ventricles depolarize

• T wave:– ventricles repolarize

Time Intervals

• P–R interval:– from start of atrial

depolarization– to start of QRS

complex

• Q–T interval:– from ventricular

depolarization– to ventricular

repolarization

Cardiac Arrhythmias

• Abnormal patterns of cardiac electrical activity

• Bradycardia:– abnormally slow heart rate

• Tachycardia:– abnormally fast heart rate

KEY CONCEPT (1 of 3)

• Heart rate is normally established by cells of SA node

• Rate can be modified by autonomic activity, hormones, and other factors

KEY CONCEPT (2 of 3)

• From the SA node, stimulus is conducted to AV node, AV bundle, bundle branches, and Purkinje fibers before reaching ventricular muscle cells

KEY CONCEPT (3 of 3)

• Electrical events associated with the heartbeat can be monitored in an electrocardiogram (ECG)

How is the heart supplied with blood?

Blood Supply to the Heart

• Coronary circulation

Figure 20–9

Coronary Circulation

• Coronary arteries supply blood to muscle tissue of heart

• cardiac veins return the blood from the muscle and tissue of the heart to the systemic circulation.

• Blocka

Heart Attacks

• A blockage of these coronary arteries causes heart attacks

• Blockages can be held open with a stent

• When blockages are too large for a stent, cardiac bypass surgery is required