Introduction
Pulmonary Circuit
The heart pumping blood is a process
involving the coordinated contraction and
relaxation of the four heart chambers. The
two upper heart chambers are known as
atria, and the two lower heart chambers are
known as ventricles. Blood follows a specific
path through the heart with each beat. The
process of pumping blood is mediated by
electrical impulses and the autonomic
nervous system. These mechanisms work
together to keep the heart beating
continuously throughout the life of a human.
Deoxygenated blood enters the right atrium
from the superior and inferior vena cava. It
then travels into the right ventricle through
the atrioventricular valve (tricuspid). As the
ventricles contract, the AV valve closes and
blood is pumped through the pulmonary
semilunar valve and in to the pulmonary
artery. The pulmonary artery carries the
deoxygenated blood to the lungs where it
will become oxygenated. (1)
Systemic Circuit
The path blood follows:
Oxygenated blood enters the heart from
the left and right pulmonary veins and in to
the left atrium. The blood then moves into
the left ventricle through the AV valve
(bicuspid). Blood from the left ventricle
travels to the aorta through the aortic
semilunar valve. The aorta carries
oxygenated blood to all portions of the
body.
The blood travels through two circuits. The
pulmonary circuit carries deoxygenated
blood from the heart to the lungs, and the
systemic circuit carries oxygenated blood
from the heart to the rest of the body.
Aorta
The two circuits are operating concurrently
in the body to keep a consistent blood flow
throughout the body at all times. (1)
Superior Vena Cava
To Lungs
To Lungs
Pulmonary Veins
Pulmonary Veins
Bicuspid Valve
Atrial Septum
Tricuspid Valve
Aortic Valve
Ventricular Septum
Inferior Vena Cava
Image 1 (2)
How the heart pumps blood:
Electrical Impulse
Depolarization and repolarization work
The electrical impulse begins at the SA node
together to contract individual cardiac
(sinoatrial) located in the right atrium. This
cells. When cells contract, the electrical
node is known as the pacemaker of the
impulse travels throughout the heart muscle
heart. The electrical impulse travels through
like a wave. (4)
the atria walls and they contract together.
After atrial contraction, the impulse travels
to the AV node (atrioventricular). This node
slows down the electrical signal in the
center of the heart to allow for the atria to
fully contract and pump all the blood into
the ventricles. Then the signal spreads
throughout the ventricle walls through the
His-Purkinje network and causes ventricle
contraction. Bundle branches connect with
this network to aid with the transmission of
the electrical signal to the ventricles.
AV node
Bundle of His
Once ventricular contraction has been
completed, the SA node fires another
electrical impulse and the process starts
again. Each time the SA node fires an
impulse, a heartbeat is initiated. This is a
continuous process. (3)
SA node
Right and Left
Bundle Branches
Purkinje Fibers
Microscopic Level
The electrical impulse affects each cardiac
cell individually. A resting cardiac cell is
polarized, so there is no electrical activity
occurring at this time. Each cardiac cell has
a cell membrane which separates a
gradient of ions called the resting potential.
When the electrical impulse crosses a cell,
the ions cross the cell membrane. Now the
cell is depolarized or positive. This
movement of ions into the cardiac cell
leads to contraction. All of the stimulated
cardiac cells collectively are capable of
contracting the chambers of the heart in
sequence.
After the contraction phase, each cell
returns to its resting state and the ions cross
the membrane into the surrounding
environment. Now the cell is repolarized or
negative.
Image 2 (5)
Nervous System
The heartbeat is also controlled by the
autonomic nervous system. This is the
branch of the nervous system that controls
involuntary actions of the body. The
autonomic nervous system branches into
sympathetic and parasympathetic nervous
systems.
Sympathetic
The sympathetic nervous system releases
epinephrine and norepinephrine to control
the heart rate. These hormones control
heart rate by contracting arteries, which
pushes blood through at a faster rate. The
sympathetic nervous system is responsible
for increasing heart rate, heart contraction
strength, and conduction of the signals from
the AV node.
Essentially, the sympathetic nervous system is
responsible for accelerating heart rate and
function at times when the body is under
physical stress, and needs more blood flow.
(6)
Parasympathetic
The parasympathetic nervous system is
triggered by pressure receptors in the
carotid arteries and aorta. When the body is
cooling down it requires significantly less
cardiac output. This decrease in blood flow
triggers the pressure receptors to signal the
vagus nerves of the parasympathetic
nervous system. When stimulated, the vagus
nerve secretes acetylcholine, a hormone
that helps to slow the heart rate and the
conduction of the signals from the AV node.
The sympathetic and parasympathetic
nervous systems work together to aid the
heart rate, but they do not occur at the
same time. The conditions imposed on the
body dictate which branch of the
autonomic nervous system is triggered. (6)
Conclusion:
The beating of the heart is a complex
process that requires multiple systems to
perform correctly. The heart pumps blood
through its chambers in a specific manner to
send deoxygenated blood to the lungs to
receive oxygen, and oxygenated blood to
the body to supply muscles. Each heartbeat
is stimulated by an electrical impulse that
always starts at the SA node of the right
atrium. This electrical impulse travels through
the heart muscle causing contraction of the
chambers in a predetermined sequence.
Heart muscle contraction occurs at the
cardiac cell level, but each cell contracting
in concert spreads the signal through the
heart like a wave. The rate at which the
heart beats and electrical impulses are
transmitted depends on the two branches
of the autonomic nervous system. The
sympathetic increases heart rate and the
parasympathetic decreases heart rate. The
actions of the autonomic nervous system
are stimulated by actions of specific
hormones that are released upon
activation. The heart beats continuously due
to the combined actions of each of these
mechanisms. The beating of the heart is the
source of life in humans.
Decrease
Heart Rate
Sympathetic
Parasympathetic
Increase
Heart Rate
Footnotes:
1.
2.
3.
4.
5.
6.
7.
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Normal Heart Anatomy and Blood Flow.
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"Anatomy, Physiology, and
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The Conduction System. Digital image.
Monash Paramedic Nurse. N.p., n.d.
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Facey, Dorian. "How Does the Nervous
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Red Blood Cells. Digital image.
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