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Option D: Human Physiology
D.4 The Heart
D4
Essential idea: Internal and external factors influence heart function.
Nature of science: Developments in scientific research followed improvements in apparatus or
instrumentation – the invention of the stethoscope led to improved knowledge of the workings of the
heart. (1.8)
D.4 U1 Structure of cardiac muscle cells allows propagation of stimuli through the heart wall.
 Cardiac muscle tissue is unique to heart. It is striated in appearance and contractile proteins
actin and myosin is similar to that of skeletal muscles. Cardiac muscles are shorter and wider
than skeletal and have only one nucleus where skeletal has many.
 Contraction of cardiac is not voluntary.
 Cells are Y-shaped and join end to end in a network of interconnections. When two cell ends
meet they are connected by a specialized junction called an intercalated disc (only in
cardiac cells). The disc consist of a double membrane containing gap junctions that provide
channels of connected cytoplasm between the cells. This allows rapid movement of ions and
low electrical resistance.
 The Y shape and being electrically connected by gap junctions allows a wave of
depolarization to pass easily from one cell to a network of other cells and a synchronization of
contraction. So heart contracts as one large cell.
 Lots of mitochondria found in cells to supply energy.
D.4 U2 Signals from the sinoatrial node that cause contraction cannot pass directly from atria to
ventricles.
 Cardiac cycles is a repeating sequence of actions in the heart that result in pumping of blood
to lungs and body.
 Cycle starts with beginning of one heartbeat to the beginning of next. Contraction of heart’s
chambers is systole and relaxation is diastole.
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The Cardiac Cycle
During atrial systole:
the atria’s contract while the ventricles relax
so blood flows into ventricles
The AV valves are open and the aortic and
pulmonary valves are closed.
During ventricle systole:
The atria relaxes while the ventricles contract
pushing blood into the arteries. The semilunar
valves are open and the AV valves are closed.
During Cardiac diastole:
All chambers are relaxed and blood flows into
the heart.
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D.4 U3 There is a delay between the arrival and passing on of a stimulus at the atrioventricular node.
o The contractions of the atria and ventricles must be staggered so blood flows in the correct
direction.
o When action potential is delayed about 0.12 seconds.
o Action potential starts at SA node and travels to the AV node. The fivers in AV node take
longer to become excited. Reasons why: They have smaller diameter and don’t conduct as
quickly, reduced number of Na+ channels in membranes, fewer gap junctions between cells,
more non-conductive connective tissue in node.
D.4 U4 This delay allows time for atrial systole before the atrioventricular valves close.
o Delay allows atria to contract and empty blood into ventricles before the ventricles contract.
It this didn’t happen the ventricles would contract at the same time pushing blood back into
atrium and a small volume of blood being moved into ventricles.
D.4 U5 Conducting fibers ensure coordinated contraction of the entire ventricle wall.
o Once through the AV node and bundle, the signal is conducted rapidly to ensure the
coordinated contraction of the ventricles.
o The signal moves down fibers that split into the right and left bundle branches and these
branches conduct the impulse through the walls of the two ventricles.
o At the base or apex of the hear the bundles connect to Purkinje fibers which conduce the
signal even more rapidly to the ventricles. The Purkinje fibers have modifications that help
conduct at high speeds: have fewer myofibrils, bigger diameter, more voltage-gated sodium
channels, and lots of mitochondria.
o Contraction of ventricles begins at the apex of heart.
D.4 U6 Normal heart sounds are caused by the atrioventricular valves and semilunar valves closing
causing changes in blood flow.
o A normal heartbeat has two sounds, both caused by the closing of valves:
o Atrioventricular valves snap shut there is a “lub” sound
o Semilunar valve shut there is a “dub” sound
Application: Use of artificial pacemakers to regulate the heart rate.
o Pacemakers are medical devices that are surgically fitted in patients with malfunctioning
sinoatrial node. The devices maintains the rhythmic nature of the heartbeat.
o They can provide a regular impulse or discharge only when heartbeat is missed so that it beats
normally.
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o
Most common is the one that monitors heartbeat and initiates one if a beat is missed. The
ventricle is stimulated with a low voltage pulse.
Application: Use of defibrillation to treat life threatening cardiac conditions.
o Cardiac arrest occurs when blood supply to hear is reduced and heart tissues are deprived of
oxygen.
o First negative consequences of abnormalities in cardiac cycles is ventricular fibrillation or
twitching of ventricles due to rapid and chaotic contractions.
o Two paddles of a defibrillator can be placed on chest in diagonal line and will discharge
electrical impulse to restore a normal heart rhythm.
Application: Causes and consequences of hypertension and thrombosis.
o Atherosclerosis is hardening of arteries caused by formation of plaques on inner lining of
arteries. Plaques are swollen areas and accumulate debris. Can develop because of high
circulating levels of lipids and cholesterol. They reduce speed of which blood moves through
vessels. They can trigger a clot, or thrombosis, which can block blood flow in that artery and
deny the tissue access to oxygen. If this occurs on surface of heart it can cause a heart
attack (myocardial infarction)
o Hypertension: greater pressure on walls of arteries caused by slowed blood flow.
Consequences: damage to cells tat line arteries causing artery to become narrower and stiff,
constant high blood pressure can weaken artery causing wall to enlarge and form bulge
(aneurysm) that can bust and cause internal bleeding, chronic high blood pressure that can
lead to stroke by weakening blood vessels in brain, chronic high blood pressure causes kidney
failure.
o Read in book page 691
Skill: Measurement and interpretation of the heart rate under different conditions.
o Different variables can influence heart rate.
o Exercise, intensity of exercise, recovery from exercise, relaxation, body position including lying
down, breathing, and breath holding, exposure to a cold stimulus, and facial immersion in
water all affect heart rate.
Skill: Interpretation of systolic and diastolic blood pressure measurements.
o Blood pressure or arterial pressure is the pressure that circulating blood puts on the walls of the
arteries. Peak pressure occurs during ventricle systole to a minimum near beginning of the
cardiac cycle when ventricles are filled with blood and are in systole.
o Measurement in pressure units “mmHg” Example is 120 over 80. Higher number is the pressure
in the artery caused by ventricular systole and lower number refers to pressure in artery due to
ventricular diastole
o Read on page 691 on how to take blood pressure.
Skill: Mapping of the cardiac cycle to a normal ECG trace.
o Cardiac muscle contracts because it receives electrical signals. These signals can be
detected by electrocardiogram (EKG)
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P: P wave, caused by atrial systole
QRS: QRS wave, caused by ventricular
systole
T: T wave, coincides with ventricular
diastole.
Height of R-wave can be compared with
body changes position
Time interval analysis can be used.
Specialist can use changes to size of
peaks and lengths to detect heart
pathology
Skill: Analysis of epidemiological data relating to the incidence of coronary heart disease.
o Coronary heart disease refers to the damage to the heart as a consequence of reduced
blood supply to the tissues of the heart itself. Often caused by narrowing and hardening of
the coronary artery.
o Ethic groups can differ in their disposition of CHD because of differing diets and lifestyles.
Gender groups, age groups, groups that differ in level of physical activity can also differ.
o Epidemiology is the study of the patterns, causes and effects of diseases in groups of
individuals or populations.
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