Download Applied Exercise Physiology: The Cardiovascular System

Applied exercise physiology
heart, called the right and left ventricles. In doing
so, it passes the atrioventricular (AV) valves. The
right AV valve is the tricuspid valve and the left AV
valve is the bicuspid valve. The purpose of these
valves is not merely to separate the atria from the
ventricles but also to ensure that the blood can only
flow in one direction through the heart. When the
ventricles contract, blood on the right side of the
heart is forced through the semi-lunar pulmonary
valve into the pulmonary artery from where it
travels towards the lungs. Meanwhile, blood from
the left ventricle enters the aorta via the semi-lunar
aortic valve. The aorta branches into many different
arteries, which then transport the blood around
the whole body. Once again, the semi-lunar valves
ensure the unidirectional flow of blood, preventing
backflow of blood into the heart.
As the left side of the heart is responsible for
pumping blood around the whole body, the wall of
cardiac tissue (myocardium) surrounding the left
ventricle is much thicker than that on the right side
of the heart.
HOT LINKS
Visit a website such as www.zerobio.com/videos/dissect_
sheepheart1.html or heartlab.robarts.ca/dissect/dissection.
html to see the dissection of a heart
Task 4.01
Copy and complete Table 4.01, identifying the
relevant function for each anatomical structure of
the heart in the list.
Structure of the heart
Aorta
Av bicuspid valve
Right ventricle
Pulmonary vein
Septum
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Function
The cardiovascular system
The conduction system of
the heart
yourself with the components of
this system.
Cardiac tissue is extremely specialised. To start
with, it is ‘myogenic’, which means that it can
generate its own electrical impulses and does not
require stimulation by the brain. It also possesses
an intricate network of nerves. Together, these two
factors ensure an efficient flow of blood through the
heart and around the body.
The cardiac cycle
Key terms
Myocardium:
cardiac muscle that makes up the heart
Myogenic:
the ability of the heart to produce its own impulses
The cardiac impulse originates from the
sinoatrial node (SA node). This is a specialised area
of cardiac muscle fibres located in the muscular wall
of the right atrium and acts as the heart’s intrinsic
pacemaker. Once the SA node has emitted the
electrical impulse, it rapidly spreads throughout both
atria, creating a wave of excitation and causing them
both to contract. The impulse then arrives at and
activates another specialised area of cardiac tissue
known as the atrioventricular node (AV node).
The AV node initially delays the transmission of the
cardiac impulse from spreading to the ventricles (for
approx 0.1 seconds). This enables the atria to contract
fully before ventricular contraction begins. After this
short delay, the impulse is sent down the septum of
the heart via the bundle of His and throughout the
muscular walls of the ventricles via Purkinje fibres.
Both ventricles now contract, forcing the blood out
of the heart and around the body. Fig 4.03 traces
the journey of an impulse through the heart’s
conduction system. Take a few minutes to familiarise
Left atrium
Sinoatrial node
(SAN)
The cardiac cycle refers to the
HIS bundle
electrical and mechanical events
Left bundle
Right atrium
branch(LBB)
that take place in the heart during
Left posterior
one complete heartbeat. Typically, Atrioventricular node
fascicle(LPS)
(AVN)
at rest, one complete heartbeat
Right bundle
Left ventricle
will occur every 0.8 seconds and
branch(RBB)
occurs approximately 72 times
Left anterior
per minute. During this time, the
fascicle (LPS)
Right ventricle
heart will at first relax and fill with
Purkinje fibres
blood – known as the diastolic
(PF)
phase – and then contract,
Fig 4.03 Control of the heart rate is myogenic
forcing blood from one part of the
heart to another or forcing blood
out of the heart altogether – this is referred to as the
systolic phase.
The control of the heart rate
It is possible to summarise the cardiac cycle into
It was stated earlier that the heart is myogenic – it
four stages:
generates its own impulses from its own intrinsic
Stage 1
Atrial diastole
pacemaker, the sinoatrial node (SA node). However,
0.5 seconds
the rate at which cardiac impulses are fired can
Stage 2
Ventricular diastole
be altered and controlled by mechanisms external
Stage 3
Atrial systole
0.3 seconds
to the heart. During exercise, for example, the
Stage 4
Ventricular systole
heart rate must increase and the SA node must fire
impulses more rapidly in order to meet the body’s
demands for oxygen. It is able to do this through two
REMEMBER!
main regulatory mechanisms:
The cardiac cycle and conduction system of the heart are
inextricably linked.
• neural control mechanism
• hormonal control mechanism.
Table 4.02 A summary of the events of the cardiac cycle
Stage of cardiac cycle
Description
Action of valves
Atrial diastole (relaxation)
Atria fill with blood
Atrioventricular valves are closed
Semi-lunar valves are open
Ventricular diastole (relaxation)
Rising pressure in the atria causes the AV
valves to open and the ventricles to fill with
blood
Atrioventricular valves open
Semi-lunar valves are closed
HOT LINKS
Atrial systole (contraction)
Atria contract, forcing blood into the ventricles
Atrioventricular valves open
Semi-lunar valves are closed
View an animation of the conduction system of the heart
at www.bostonscientific.com/templatedata/imports/
HTML/CRM/heart/interact_8.html
Ventricular systole (contraction)
Ventricles contract, increasing pressure in the Atrioventricular valves are forced to close
ventricles, and forcing blood into the aorta and
pulmonary artery
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