12.2-Monitoring the Circulatory
System
SBI 3U1
The Cardiac Cycle
The SA node sends out an
electrical stimulus that causes the
atria to contract.
systole = contraction
diastole = relaxation
Steps in the Cardiac Cycle:
1) The sinoatrial (SA) node (a.k.a. the pacemaker)
located in the R. atrium stimulates muscle cells to
contract and relax rhythmically.
2) It generates an electrical signal that spreads over the
atria so they contract simultaneously.
3) The signal reaches the atrioventricular (AV) node,
which transmits the signal through a bundle of fibres
called the bundle of His.
4) The signal is relayed to Purkinje fibres; these fibres
initiate the simultaneous contraction of the ventricles.
Cardiac muscles contract involuntarily…
Heartbeat= “lub-DUB”
The normal sound of the heartbeat, heard with a
stethoscope. The sound is described as “lub-DUB.”
These sounds are made as valves close:
• “lub” =closing of the atrioventricular valves
• “DUB” = closing of the semilunar valves
Electrocardiogram (ECG)
• Electrical impulses in your
heart create heartbeat
• electrodes placed on the
chest can measure the
voltage changes
• electrocardiogram = plot
of voltage measurements
over time
Electrocardiogram (ECG)
An ECG allows doctors to determine if the heart is generating signals
of normal strength, frequency, and duration.
Irregularities in the Heart:
Heart Murmur
Heart Murmur : an
abnormal swishing or
whistling sound in the
heart It is caused by blood
not flowing smoothly.
What causes it?
• valve stenosis (openings of
heart valves or arteries are
narrow)
• valve regurgitation
• septal defect
Blood pressure
• the pressure exerted by blood on
the walls of the blood vessels as it
travels through the body
• Measured using a sphygmomanometer
Systolic pressure- max. pressure in arteries when ventricles
contract and push blood from the heart
-Phase is called “systole”
Diastolic pressure- lowest pressure in arteries when the
ventricles are not contracting
-Phase is called “diastole”
Measuring Blood Pressure
• Pressure is recorded
in mmHg
(millimeters of
mercury)
• Average healthy
blood pressure is
120 mmHg (systolic)
80 mmHg (diastolic)
Factors that affect Blood Pressure:
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Genetics
Cardiac output/activity
Stress
Body temperature
Diet
Medications
*Note: It is normal for blood pressure to decrease
when you are sleeping and increase when you are
exercisng.
Frequent high blood pressure is called hypertension
Blood Pressure During The Cardiac Cycle
Ventricular Diastole:
▫ ventricles relax; pressure
drops and
SL valves close (“dub”)
▫ AV valves open
▫ blood flows from atria
into ventricles
Ventricular Systole:
▫ ventricles contract;
pressure rises
▫ AV valves close (“lub”),
and SL valves open
Cardiac Output & Stroke Volume
• Cardiac output : the amount of blood pumped
by the heart, in mL/min
• Stroke volume: the amount of blood pumped out of the
heart with each heartbeat
cardiac output
heart rate
stroke volume
(mL/min)
(beats/min)
(mL/beat)
Heart rate and stroke volume are used to calculate cardiac output
Cardiac Output & Stroke Volume cont’d…
• Average heart rate = 70 bpm
• Average stroke volume = 70 mL
• Average cardiac output [ adult at rest]:
▫ 4900 mL (70 beats/min × 70 mL/beat)
 almost the entire volume of blood in the body (5 L) passes
through the heart about once every minute
*Note: males are generally higher than females
Average Cardiac Output increases during periods of exercise
Cardiovascular Fitness
• Cardiovascular fitness is the capacity
of the lungs, heart, and blood vessels
to deliver oxygen to working
muscles.
• Another good indicator of
cardiovascular fitness is the length of
time it takes the heart to return to its
resting heart rate after strenuous
exercise.
Check Your Understanding…
Why would individual C be considered to be the
most fit? What factors determine this?
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Check Your Understanding…
Why do athletes tend to have higher
stroke volumes?
• stronger cardiac muscles
• resting heart rate is correspondingly
lower
• less work done by heart muscles but
they deliver the same amount of O2
to the muscles
Homework:
Read and make notes 12.1
Complete pg. 491 #13-18
Complete pg. 493 # 2, 5-7, 9, 11, 13, 14