Transcript M19 - Lesson 11

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UNIT 4 Chapter 12: The Circulatory System
Section 12.2
12.2 Monitoring the Human Circulatory System
Within the heart, the sinoatrial (SA) node (the pacemaker)
stimulates the muscle cells to contract and relax rhythmically. The
SA node is in the right atrium. It generates an electrical signal that
spreads over the atria so they contract simultaneously. The signal
then reaches the atrioventricular (AV) node, which transmits the
signal through a bundle of fibres known as the bundle of His.
These relay the signal to Purkinje fibres; these fibres initiate the
almost simultaneous contraction of the ventricles.
UNIT 4 Chapter 12: The Circulatory System
Section 12.2
The Heartbeat
The SA node sends out an electrical stimulus that
causes the atria to contract.
Continued…
UNIT 4 Chapter 12: The Circulatory System
Section 12.2
The Heartbeat
The normal sound of the heartbeat, heard with a stethoscope, is
described as “lub-DUB.” These sounds are made as valves
close:
•“lub” is the closing of the atrioventricular valves
•“DUB” is the closing of the semilunar valves
Sound variations can indicate heart problems, such as a stenosis
murmur, or narrowing of a heart valve or artery.
UNIT 4 Chapter 12: The Circulatory System
Section 12.2
The Electrocardiogram (ECG)
Most heart function is analyzed by measuring the electrical
pulses generated by the nerve signals that stimulate
contractions. These create milliVolt charges that can be
detected with electrodes on the skin. An ECG allows doctors
to determine if the heart is generating signals of normal
strength, frequency, and duration.
UNIT 4 Chapter 12: The Circulatory System
Section 12.2
Blood Pressure
Blood pressure is the pressure that the blood exerts on the blood
vessel walls as it travels through the body. Changes in blood
pressure correspond to the phases of the heartbeat.
Systolic pressure: maximum pressure in arteries when the
ventricles contract and push blood through
Diastolic pressure: lowest pressure in arteries when the
ventricles are not contracting
(A) Small muscles surrounding the
veins contract and relax to squeeze
blood along the veins. (B) One-way
valves inside the veins prevent
blood from flowing backward due to
the pull of gravity.
UNIT 4 Chapter 12: The Circulatory System
Section 12.2
Measuring Blood Pressure
A sphygmomanometer (blood pressure cuff) placed on an
artery in the arm measures blood pressure. Pressure is recorded
in mmHg (millimeters of mercury) as systolic over diastolic.
The average blood pressure of a healthy young person is below
120 mmHg and over 80 mmHg.
Blood pressure is affected by genetics, activity, stress, body
temperature, diet, and medications. Continuous high blood
pressure (hypertension) can cause damage to the arteries and
increase the risk of heart attack, stroke,
and kidney failure.
Using a stethoscope, the doctor or nurse
can hear the flow of blood as the pressure
in the sphygmomanometer is released and
the blood starts to flow again.
UNIT 4 Chapter 12: The Circulatory System
Section 12.2
Cardiac Output
Cardiac output: the volume of blood pumped by the heart,
expressed as mL per minute. It can indicate:
• the total level of work the body’s muscles can perform
• how easily the heart fills with blood
• the distensibility (stretchiness) of the ventricles
Heart rate and stroke volume (the amount of blood pumped
out of the heart with each heartbeat) are used to calculate
cardiac output (heart rate X stroke volume).
Average heart rate = 70 bpm, stroke volume = 70 mL
Average cardiac output = 70 X 70 = 4900 mL/minute
UNIT 4 Chapter 12: The Circulatory System
Section 12.2
Cardiovascular Fitness
Cardiovascular fitness is the capacity of the lungs, heart, and
blood vessels to deliver oxygen to working muscles. In the
table below, individual C is considered to be the most fit.
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.
Why would individual C be considered to be the most
fit? What factors determine this?
UNIT 4 Chapter 12: The Circulatory System
Section 12.2 Review
Section 12.2