Transcript Cardiovascular system - The Grange School Blogs

Structure of the
heart and Cardiac
Cycle
Structure of the heart
DEFINITIONS/EXPLANAT
IONS
SYSTOLE
• Period of contraction of heart
DIASTOLE
• Period of relaxation of heart
SINO-ATRIAL NODE (SAN)
• Pace maker of the heart, initiates cardiac
impulse
• Found in region of right atrium
• A specialised area of cardiac muscle fibres.
DEFINITIONS/EXPLANAT
IONS
ATRIO-VENTRICULAR NODE (AVN)
• Second node in right atrium
responsible for passing on cardiac
impulse
BUNDLE OF HIS
• Found in the septum and conducts
the cardiac impulse to the apex (tip)
of the heart
PURKINJE FIBRES
• Send impulses to ventricles to
contract
SEQUENCE OF EVENTS
1. Originates in the SA Node
(sinoatrial node).
2. Sends a wave of excitation through
the atria causing them to contract.
3. This then stimulates the AV node
(atrioventricular node).
4. Sends a wave through the bundle of
his and the purkingjie fibres
causing the ventricles to contract.
SEQUENCE OF EVENTS
5. This is called VENTRICULAR
SYSTOLE.
6. Once systole has finished
DIASTOLE follows
• All this takes place in a
fraction of a second
Cardiac Cycle
• Refers to the electrical and
mechanical movements of the
heart.
• Rest one complete heartbeat
takes 0.8 seconds.
CONTROL OF HEART
RATE
• The timing of the cardiac
contractions is altered by two
extrinsic factors:
1. Neural control
2. Hormonal control
• One intrinsic factor can also alter
HR
• Neural control is the most
important control mechanism
NEURAL CONTROL
•
The SA node is controlled by the
autonomic nervous system
• Two nerves stimulate this node;
1. Sympathetic cardiac accelerator
nerve (speeds up HR)
2. Parasympathetic vagus nerve (slows
down HR)
• Overall control of both nerves is with
the cardiac control centre in the
medulla of the brain.
STIMULATION OF
CARDIAC CONTROL
CENTRE
Stimulated by:
1. Muscle receptors in muscle and
joints at the start of exercise
2. Chemoreceptors in the muscle
that sense chemical changes in
muscle and blood (changes in O2,
CO2 and pH levels)
3. Emotional excitement
4. Changes in blood pressure
detected by baroreceptors in
aortic and carotid arteries
HORMONAL CONTROL
•
•
•
•
Adrenaline (secreted from adrenal
glands in the kidney) stimulates the
SA node
This causes an increase in HR
Prepares body for the ‘flight or
fight’ scenario
Adrenaline also increases the
strength of contraction produced
by the myocardium (heart muscle)
INTRINSIC CONTROL
• Warm muscle conducts nerve
impulses faster
• Thus HR of a warm heart increases
• A drop in temperature will reduce HR
• Increased venous return stretches
heart which stimulates SA node and
thus increases HR and SV
• This is called ‘Starling’s Law of the
heart’
Changes in Heart Rate, Stroke
Volume, and Cardiac Output
Activity
Heart rate Stroke volume Cardiac output
(beats/min)
(ml/beat)
(L/min)
Resting (supine)
55
95
5.2
Resting (standing
and sitting)
60
70
4.2
Running
190
130
24.7
Cycling
185
120
22.2
Swimming
170
135
22.9
Changes of Cardiac
output
• Increases why?
• What does this enable to
happen?
• What happens to heart rate
why?
• What happens to stroke volume
why?
Did You Know…?
Arteries always carry blood away from the heart; veins
always carry blood back to the heart with the help of
breathing, the muscle pump, and valves.
THE CARDIOVASCULAR
RESPONSE TO
EXERCISE
BLOOD
DISTRIBUTION
AT REST
THE MUSCLE
PUMP
Blood Distribution
w Matched to overall metabolic demands
w Autoregulation—arterioles within organs or tissues dilate
or constrict
w Extrinsic neural control—sympathetic nerves within walls
of vessels are stimulated
w Determined by the balance between mean arterial
pressure and total peripheral resistance
Key Points
The Vascular System
w
Blood returns to the heart with the help of
breathing, the muscle pump, and valves in
the veins.
w
Blood is distributed throughout the body
based on the needs of tissues; the most
active tissues receive the most blood.
w
Autoregulation controls blood flow by
vasodilation in response to local chemical
changes in an area.
(continued)
Key Points
The Vascular System
w
Extrinsic neural factors control blood flow
Key Points
primarily by vasoconstriction.
The Vascular System
w
flow
factors control blood
neural pressure
w Extrinsic
Systolic
blood
(SBP)
is the
primarily by vasoconstriction.
highest
pressure
within
the vascular
(SBP) is the
blood pressure
w Systolic
pressure within the vascular
systemhighest
while
diastolic
blood pressure
blood pressure
while diastolic
system
is the lowest.
(DBP) (DBP)
is the
lowest.
w
w
Mean arterial pressure (MAP) is the
average pressure on the arterial walls.
Mean arterial pressure (MAP) is the
average pressure on the arterial walls.