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Cardiac Cycle
Setting the Tempo
Cardiac Cycle – Overview
• Events of each heartbeat
– Highly coordinated so that both atria contract together
and then both ventricles contract together
• Systole
– contraction of heart muscle
• Diastole
– relaxation of heart muscle
Overview cont’d
• Time for each cycle is influenced by
autonomic nerves.
• Two regulatory nervous systems
– Sympathetic nervous system
• Forms a division of the autonomic nervous system
• Prepares the body for stress increases heart rate.
– Parasympathetic nervous systems
• Returns the body to normal resting state following stress
Cardiac Cycle
cont’d
• Cardiac muscles are able to contract
without being stimulated by external
nerves.
– Called myogenic muscle
– allows your heart to beat without a continual
reminder
– Also able to recover quickly, the only rest they get is
small time between beats.
• Normal heart rate at rest is about 60-80 beats per minute
Stages of the Cardiac Cycle
• Fig 12.5
12-5
Heart Beat - Sounds
• Caused by the closing of the heart valves
– Lubb – AV valves close
– Dubb – Semi lunar valves close
• Heart murmurs are caused by an
incomplete seal on a valve
– Blood leaks past the closed valve, or flows
backwards in the heart.
– causes a whoosing, or gurgling sound
Heart Beat - Intrinsic Control
• Heart has its own intrinsic conduction
system
• Autorhymicity-unlike skeletal muscle, cardiac muscle can
contract without neural stimulation
• The autonomic nervous system does has inputs to the heart and
normally regulates rate
• Nodal tissue-2 areas in the heart
• Has both muscular and nervous characteristics
• Can generate action potentials to cause contraction
• SA node and AV node
12-7
Heart Beat – The Pacemaker
• Sinoatrial Node (SA)
– A bundle of specialized
nerves and muscles
– located where the vena cava
enter the right atrium. (#1)
– Sends a signal over the two
atria
• Atrioventricular node
(AV).
– Located the lower part of the
right atrium close to the
tricuspid valve (#2)
– picks up electrical impulses
Heart Beat – The Pacemaker
• AV Node sends
nerve impulses via
the Purkinje Fibres
(#4)
– two large nerve fibres
– run through the septum,
• Each nerve impulse
triggers cardiac
contraction
– Atria contract first,
followed by ventricles.
Heart Beat
• Tachycardia
– Fast heart beat
– heart rate exceeds 100 beats per min
– can result from exercise or from the consumption of
such drugs as caffeine or nicotine.
• Bradycardia
– Slow heart beat
– Heart beat lower than 60 bpm
– Can result from degeneration of the muscle (age),
disease
Mapping the Heart Beat
• Electrocardiogram (ECG or EKG)
– test that measures the electrical activity of the heart.
– Traces how long the electrical wave takes to pass through your
heart
– Printed on paper covered with a grid of squares each
represents 0.04 seconds.
• 25 squares = 1 second
• ECG’s printouts are usually is 6 seconds; a "six second strip."
• Changes in electrical current reveal normal or
abnormal events of the cardiac cycle.
• Determines if
– activity is normal, fast or irregular.
– heart is enlarged or overworked.
Interpreting ECG
• P wave
– first little hump – atria
receives signal and
contracts
• QRS complex
– Ventricles receive signal
and contract
– May just be an RS… this
is still normal
• R wave is the first wave
ABOVE the midline
• T wave
– Recovery of the heart
Normal Rhythm
Bradycardia
Tachycardia
Abnormal Rhythym
V-Tach (NO QRS)
Blood Pressure
Blood Pressure
• A measure of the pressure or force of blood
against the walls of your arteries
• Systolic
– the pressure when your heart contracts and pushes blood out
– Highest pressure
• Diastolic
– the lowest pressure when the heart relaxes between beats
• Normal blood pressure is below 120/80 mm Hg.
– High blood pressure is consistently more than 140/90 mm Hg
Our Heart the Pump
The ventricles relax
• Backflow prevented by
closing of aortic and
pulmonary valves
Ventricles
contract
• Forces the AV valves
shut, pushes blood
into the arteries.
Atria relax
• causes them to
fill with blood
Atrial walls
contract
• Forces the AV valves
open, and blood flows
into the ventricles
filling them up
Sphygmomanometer
• Measures blood pressure indirectly
– Measures the pressure exerted by blood in the
brachial artery
• Blood flowing through the brachial
artery makes no noise
– A stethoscope is placed below the cuff to listen
for the blood
• The cuff is placed around your arm
and brachial artery.
– Air is pumped into the cuff until circulation is
restricted.
Sphygmomanometer
• Pressure is slowly lowered until the
blood is able to flow past, this is the
systolic pressure
– Doctors will hear this blood rushing through as a
beat
• Pressure continues to be lowered until
the noise disappears
– This is the diastolic pressure
– Blood no longer needs to force the artery open
to pass through.
Blood Pressure Clip
Factors affecting Blood Pressure
• Cardiac output
– The volume of blood pumped from the heart each minute
– Increased output will increase blood pressure.
• Raising your heart rate increases output!
Factors affecting Blood Pressure
• Arteriolar resistance
– Diameter of the arterioles is regulated by muscles in their walls
• Arteriolar Constriction reduces blood flow, causing higher blood
pressure.
• Arteriolar dilation, opens vessels increases blood flow and
decreases blood pressure
High Blood Pressure Factors
• High Blood Volume
– High Salt levels cause excess water in our blood increasing our
blood volume
• Increased Cardiac Output
– Tachycardia
– Overactive Sympathetic Nervous System
• Arteriolar Constrictors
– Caffeine and Nicotine
– Cold
– Blockages in the arteries
• Caused by artheriosclerosis
Low Blood Pressure Factors
• Low Blood Volume
– Dehydration
– Starvation (anorexia)
– Bleeding
• Decreased Cardiac Output
– Bradycardia
– Insufficient stroke volume
– Valve problems
• Arteriolar Dilation
– Heat
– Brain injury
• Arteriolar walls stop contracting