Download The Heart

SICM Tuition
Biology AS
The Heart
So, we have covered a lot of material so far and there’s not thaaaaat much left. 2 pages worth of
syllabus and we are done…..FOREVER! Well…until next year. Anyhoo, one of the things
we’ve looked at is the way in which humans need a circulatory system to ensure that all cells
receive the oxygen they need for respiration. So let’s talk about the heart! ☺
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The heart lies between the lungs – behind the sternum (the sternum is in the centre of
the chest. You can feel it very easily by lightly pressing the middle of your chest)
the sternum protects the heart from damage in the thoracic cavity
Pericardium
consists of two membranes which surround the heart:
a) the inner one – attached to the heart
b) the outer one – attached to the surrounding tissue (e.g. diaphragm)
-
the pericardium holds the heart in position
it reduces friction between the heard and the surrounding tissue
it is non-elastic and so prevents the heart from over stretching
pericarditis – inflamation of
the membrane: the heart no
longer functions properly
Structure of the heart
What is the heart?? (apart from the thing that we give away to those we love: awwww…)
complex pump
two pumps side by side
the right side pumps to lungs via the pulmonary artery
left side pumps to the head/body via the aorta
-
four chambered structure
made of cardiac muscle
o cardiac just means related to the heart
Atria
thin walled
receive blood from:
a) vena cava (coming back from the head and body: full of CO2)
b) pulmonary vein (coming back from the lungs: full of yummy oxygen!)
Ventricles
thick walled
pumping chambers
left ventricle has a thicker muscular wall to create higher pressure to pump the
blood all the way round the head/body and back.
Page 1
SICM Tuition
Biology AS
As the right side contains blood that has come back from the head and the body, it is
deoxygenated. The blood on the left side has just come back from the lungs. So it is
oxygenated. Therefore, mixing the two would be silly…and inefficient. There is therefore a
septum in between the two sides separating them.
One way flow needs to be ensured:
a) semi-lunar valves: valves in pulmonary artery and aorta
to stop the backflow of blood into the ventricles when they relax
b) atrio-ventricular valves – between the atria and the ventricle (tricuspid / bicuspid)
to prevent the backflow of blood into the atria when the ventricle contracts
valve do not turn inside out as they are attached by non-elastic tendons to
muscle “bumps” on the inside wall of the ventricles.
c) Ventricles – muscular chambers
contract to create a “force” to pump blood to the lungs or head and body
Differences
-
the differences in the thickness between the atria and the ventricle walls
relate to their function.
The walls of the atria are thinner than the walls of the ventricles
The left ventricle is thicker than the right ventricle as the left ventricle
pumps blood to the head and the body whereas the right ventricle only
pumps blood to the lungs.
Coronary artery:
Immediately above the semi-lunar valve in the aorta is the entrance of the coronary artery
this supplies blood to the heart muscle itself
this branches over the surface of the heart muscle
deoxygenated blood is “collected” in the coronary vein which empties
directly into the right atrium (along with the rest of the blood from the head
and body)
Take a blank piece of paper and draw a reasonably big picture of the heart. Show the vessels
going to and away from it and label each part of the heart and each vessel. Try using colours
(blue and red) to show the oxygenated (red) and deoxygenated (blue) blood. Write a summary
of the chambers of the heart.
Good…that should keep you occupied for a while! Muhahahaha.
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SICM Tuition
Biology AS
Lungs
Head
Pulmonary vein
Pulmonary artery
Right Atrium
Head
Left Atrium
aorta
Body
vena cava
Coronary artery going
back to the heart
Body
Tricuspid atrioventricular valve
Right ventricle
Left ventricle
Bicuspid atrioventricular valve
The heart has four chambers of equal volume.
2 Atria (left and right)
receiving chambers
thin walled
Right Atrium:
receiving vena cava (from
head and body)
blood rich in CO2, low in O2
Left Atrium:
receiving pulmonary vein
(from lungs)
rich in O2
2 Ventricles
pumping chambers
thick muscular walls to create high pressure
Right ventricle:
pumps blood to lungs via pulmonary vein
blood rich in CO2, low in O2
Left Ventricle:
pumps blood to the head and the body via the aorta
blood rich in O2
muscular wall much thicker than right ventricle as it has to pump the blood
around the whole body.
Page 3
SICM Tuition
Biology AS
Cardiac Cycle – 1 heart beat
a) deoxygenated blood enters the right atrium (from the vena cava)
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oxygenated blood enters the left atrium (from the pulmonary artery)
b) the resulting pressure forces open the tricuspid and bicuspid (mitral) valves and blood
flows from the atria into the ventricles.
these stages represent the DIASTOLIC phase
this is passive filling of the ventricles: no contraction of atria
c) when the diastolic phase ends, the two atria contract completely filling the ventricles
with blood.
this is the ATRIAL SYSTOLE (A.S.)
d) the ventricles then contract – ventricular systole (V.S.)
the tricuspid valve and mitral valves close to stop backflow into the atria
e) the blood is then forced simultaneously into the pulmonary artery and aorta
the semi-lunar valves prevent the backflow from the aorta and pulmonary
artery into the ventricles – unidirectional flow (valves closed)
f) the atria fill with blood again
the cycles continues
N.B. all the contraction in the cycle STARTS in the right atrium and spreads across the heart
muscle from he Sino-Atrial Node (SAN)
Thus the heart operates in two ways:
a) contraction phase – systole
b) relaxation phase – diastole
The heart muscle is myogenic:
it contracts without nerve stimulation
nerve impulses to the SAN merely modify the speed and the
strength of the contraction
Page 4
SICM Tuition
Biology AS
Initiation and propagation of contractions
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the sino atrial node is a natural pacemaker, which provides the basic rhythm of
the heart contractions / initiates / sends out the heart beat
-
the heart muscle is myogenic / beats spontaneously / does not require nerve
impulse
-
the rate of the beating is influenced / modified by nerve impulses to the SAN
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a wave of nerve impulses / electrical activity / excitation passes over the atrium
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this triggers the contraction of the atria
-
the electrical activity cannot pass to the ventricles because of fibrous tissue
between the atria and the ventricles
-
when the electrical activity reaches the AVN (atrio ventricular node) at the
base of the atria, the AVN passes the nerve impulse along “the bundle of His”
to the base of the ventricles
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there is a (time) delay at the AVN (this allows the atria to fully contract and
empty)
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once the impulses have reached the base of the ventricles, the ventricles
contract from the base upwards via “Purkinje fibres”
SAN
AVN
fibrous tissue
bundle of His
Purkinje
fibres
Heartbeat
base
The pattern of the spread of excitory nerves through the heart ensures this.
(a)
the atria contract to force the blood down into the ventricles
(b)
the ventricles contract top force the blood up into the pulmonary artery and
aorta
Detection of excitation through the heart by electrodes attached to the skin of the chest are
displayed as an “electrocardiograph” (ECG trace)
Page 5
SICM Tuition
Biology AS
Role of nervous and hormonal systems
SAN connects to the central nervous system directly by two nerves:
(a)
vagus nerve
slows down the heart beat
(b)
accelerator / sympathetic nerve
speeds up the heart beat
Hormones (e.g. adrenaline) act directly on the SAN and increase the rate.
pH and temperature:
low pH (caused by a high CO2 concentration) and high temperature
- causes accelerator nerve to act
other drugs (e.g. cyanide)
Beta blockers:
Caffeine:
-
blocks electron transfer in cytochrome
immediate effect on the heart rate as a respiratory
inhibitor
decrease the heart rate
has an indirect effect:
it stimulates “dopamine” which stimulates the heart beat
The heart rate is mainly affected by:
-
blood pressure
oxygen and carbon dioxide concentration in the blood
The heart beats without fatigue. This is a property of cardiac muscle.
Effects of exercise
Cardiac Output is the product of the heart rate and the stroke volume
(i.e. heart rate × stroke volume = cardiac output)
Stroke volume
= the amount of blood that the left ventricle pumps each time it contracts
Heart rate
= the number of times the heart “beats” in one minute
Take exercise
-
immediate increase in heart rate
both the rate and the force with which the heart beats are
continuously adjusted
the cardiac output is matched to the needs of the body
i.e. that enough oxygen can be supplied and CO2 taken away
Adjustment of cardiac output
(a)
nerve impulses via vagus nerve – slows the heart rate to SAN
(b)
nerve impulses via accelerator nerve – increases the heart rate to the SAN
(c)
As more blood enters the right atrium, the wall stretches as a result, the heart beats
faster and with greater strength
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