2. The
human heart and its pumping action - diagrams and explanation
The heart is an organ that pumps blood around
the body
You need to refer to the diagram of the
structure of the heart above.
Much of the wall of the heart is made
from muscle tissue which continually contracts and relaxes.
The heart can act as a pump
because of the physical action of a muscular wall to move the tissue of the
heart chambers and one-way valves are need so that there is a
continuous flow of blood in the same direction.
An involuntary, striated muscle constitutes the
main tissue of the wall of the heart.
This cardiac muscle
contains lots of mitochondria to provide the cells with
lots of ATP
for respiration - lots of energy is needed to work to
contract the heart muscles.
Heart muscles need their own
blood supply to continually access a source of nutrients and
oxygen so they can work non-stop - a continuously beating heart
to keep all of us mammals alive!
The cardiac muscles are
supplied with blood from two coronary arteries which branch from
the base of the aorta. They ensure a constant supply of glucose
and oxygen for respiration, that can diffuse through the thick
walls of the heart into the heart muscle cells .
The heart has valve systems
that ensure the blood only flows in the right direction - one way
only, no reverse flow allowed.
The heart has two pumps to
work the double circulation system that beat together, typically 60 -
100 times per minute.
Each pump has an upper chamber (atrium)
that receives blood and a lower chamber (ventricle) that
pumps blood out.
Both atria and ventricles fill
and pump blood at the same time.
The natural resting heart rate is
controlled by a group of cells in the right atrium which act as a
pacemaker and act as part of the nervous system (* see advanced
note below).
Heart rate is usually measured as the number of
times the pump action (sensed as a heart beat) occurs per
minute.
For a healthy heart, it should normally be
around 60-90 beats per minute at rest.
It will increase with more strenuous aerobic
exercise OR potentially a cardiovascular problem.
(* Heart rate is controlled by the two branches
of the autonomic (involuntary) nervous system. The sympathetic
nervous system (SNS) and the parasympathetic nervous system (PNS).
The sympathetic nervous system (SNS) releases the hormones like
epinephrine to accelerate the heart rate if more oxygen is
needed for respiration.
Know that there are four main chambers of the heart
(left and right atria and ventricles) to pump the blood around and the associated control valves which only allow flow
in one direction.
See the diagram of the heart above/below where the important
parts of the organ are labelled and the white arrows show the direction of
flow of the two circuits for:
(i) the oxygenated blood from the
lungs to the rest of the
body (the organs etc.)
and (ii) the deoxygenated blood to the lungs.
The septum of the heart is a
wall of tissue that separates the left and right sides of the heart and
enables blood to flow as it should. The septum consists of the atrial
septum and the ventricular septum.
The atrial septum separates the
left and right atria, while the ventricular septum separates the
left and right ventricles. The heart’s septum is critical for
ensuring that blood flows properly through the heart.
Note: For visual simplification, note
that oxygenated blood is shown as bright red and deoxygenated blood as blue (even
though it is a dark red colour in reality!).
Repeated diagram for visual convenience
The mechanism by which the heart works
The blood flows from the heart to the organs,
including the lungs,
through arteries and returns through veins.
Blood is supplied to the heart by two
coronary arteries which branch from the base of the aorta - the largest
artery in the body.
The heart itself needs a good supply of
oxygen and glucose for lots of energy to keep the heart muscles continuously
working.
The four major blood vessels associated with the heart
are the pulmonary artery,
pulmonary vein, aorta, vena cava.
Blood is pumped away from the
heart into arteries and returns to the heart in veins.
The aorta (main artery) -
transports oxygenated blood
from the left ventricle to the body.
The vena cava (main vein) -
transports the deoxygenated
blood flow from the body into the right atrium.
The pulmonary artery -
transports the
deoxygenated blood from the right ventricle to the lungs
The pulmonary vein - brings the
oxygenated blood to the heart from the lungs into the left atrium.
The heart valves
The heart needs a set of valves to keep the fluid
flowing in one direction.
The heart has two types of valves that keep the blood
flowing in the correct direction.
The valves between the atria and ventricles are called
atrioventricular valves those at the bases of the large vessels leaving
the ventricles are called semilunar valves.
When the ventricles contract, atrioventricular valves
close to prevent blood from flowing back into the atria.
When the ventricles relax, semilunar valves close to
prevent blood from flowing back into the ventricles.
Flow and valve action (simple
summary)
The heart has valves to ensure the
blood only flows in the one correct direction.
When the ventricles
contract, the valves to the atria close and the valves to the blood
vessels open.
This prevents any reverse blood flow.
Ventricles have thicker walls than
atria because they operate at a higher pressure to pump the blood further.
The left ventricle has a thicker wall than the right
ventricle because the muscle tissue must be stronger because it operates at
a higher blood pressure to pump blood around all of the body.
The right
ventricle muscle tissue is thinner because it only has to pump the blood to
the lungs.
The blood enters the two atria
(right atrium and left atrium, atria) of the heart from the vena carva and the
pulmonary vein.
The left atrium and ventricle pump oxygenated blood
from the lungs around the body (see diagram above).
The left atrium receives the oxygenated blood from the
lungs via the pulmonary vein and is pumped through the aorta to the rest of
the body. It passes into the left ventricle to be pumped around the body.
The right atrium and ventricle pump deoxygenated
blood
The right atrium of the heart receives the
deoxygenated blood from all of the body, entering the heart by the vena
cava.
The deoxygenated blood flows through the right ventricle which pumps
it to the lungs via the pulmonary artery to be oxygenated.
The basic sequence of heart function
is:
The heart relaxes and the blood
enters both atria.
The atria contract
at the same time forcing blood into the ventricles.
The ventricles
contract from the bottom upwards which forces blood out of the heart through the pulmonary artery and aorta.
The atria fill up again and the whole
cycle is repeated
The valves in
the heart ensure that blood flows in the correct
direction.
How is it, that normally the heart
can beat at a regular rate?
In the right atrium are a group of cells
that act as a pacemaker.
They control the heart beat by producing
tiny electrical impulses (signals) that spread into the surrounding muscle
cells causing them to contract.
If this group of cells are
malfunctioning leading to an irregular, and dangerous heartbeat, an
artificial pacemaker can be surgically implanted under the skin
and wired up to the heart.
This device produce tiny electrical
signals at the right frequency to generate a correct and regular
heartbeat.
The pumping rate of the heart
The cardiac output is the total
volume of blood pumped by a ventricle every minute.
It is calculated using the following
equation:
cardiac output (cm3/min)
= heart rate (beats/min) x stroke volume
(cm3)
Abbreviated rearrangements:
heart
rate = cardiac output / stroke volume and stroke volume =
cardiac output / heart rate
Your heart rate is the number of
beats per minute e.g. when you measure your pulse rate.
The stroke volume is the volume of
blood pumped by one ventricle every time it contracts.
Example questions
Q1 A person has a
heart rate of 60 beats/min and average stroke volume of 70 cm3.
Calculate the cardiac output.
cardiac output = heart
rate x stroke volume = 60 x 70 =
4200 cm3/min
Q2 An athlete
sprinter heart rate is 120 beats per minute and a cardiac output of
6000 cm3/min.
Calculate the athlete's
stroke volume.
stroke volume =
cardiac output / heart rate = 6000 / 120 =
50 cm3
Q3 The cardiac
output from a person is 3600 cm3/min.
If the person's stroke volume
is 60 cm3, what is the pulse rate?
heart rate beat = cardiac
output / stroke volume = 3600 / 60 =
60 beats/min
Summary of learning objectives and key words or phrases
Be able to interpret diagrams explaining how the heart works
including the valves, cardiac
muscles, aorta, pulmonary artery, pulmonary vein, vena cava, left atrium,
left atrium, right ventricle and left ventricle.
Knowledge of the names of the blood vessels
associated with the heart ...
-
arteries are blood vessels that
carry deoxygenated blood away from the heart to the lungs,
-
veins are blood vessels that
carry oxygenated blood to the heart from the lungs and exit the heart via
the aorta to the rest of the body
-
capillaries are the smallest
blood vessels
-
aorta - the oxygenated blood
flow exit to the organs
-
vena cava - the deoxygenated
blood flow entry from the organs
-
pulmonary artery - carries the
deoxygenated blood to the lungs
-
pulmonary vein - brings the
oxygenated blood to the heart from the lungs
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