1. Option D.4: The Heart

2. Essential Idea: Internal and external factors influence heart function.

3. Understandings
U.D.4.1: structure of cardiac muscle cells allows propagation of stimuli through the heart wall.
U.D.4.2: signals from the sinoatrial node that cause contraction cannot pass directly from atria to ventricles.
U.D.4.3: there is a delay between the arrival and passing on of a stimulus at the AV node.
U.D.4.4: this delay allows time for atrial systole before the AV valves close.
U.D.4.5: conducting fibers ensure coordinated contraction of the entire ventricle wall.
U.D.4.6: normal heart sounds are caused by the AV valves and semilunar valves closing, causing changes in blood flow.

4. Applications and Skills
A.D.4.1: use of artificial pacemakers to regulate the heart rate.
A.D.4.2: use of defibrillation to treat life-threatening cardiac conditions.
A.D.4.3: causes and consequences of hypertension and thrombosis.
S.D.4.1: measure and interpretation of the heart rate under different conditions.
S.D.4.2: interpretation of systolic and diastolic blood pressure measurements.
S.D.4.3: mapping of the cardiac cycle to a normal ECG trace.
S.D.4.4: analysis of epidemiological data relating to the incidence of coronary heart disease.

5. The heart is composed of cardiac muscle cells
Skeletal muscle is the muscle responsible for moving your bones and is mostly voluntary. Many cells are fused together to create a muscle fiber. They are also multinucleate (they have many nuclei) which makes it easier to contract as a unit.
Cardiac muscle is similar to skeletal muscle in several ways.
The contracting unit of both cardiac and skeletal muscle is known as a sarcomere, and contains the proteins actin and myosin.

6. Cardiac muscle cont. (U.D.4.1)
Cardiac muscle contains interconnections called intercalated discs, which allows the cardiac muscle fibers to remain a “single” cell.
The intercalated discs contain openings called gap junctions, which allow the cytoplasm to flow freely between cardiac muscle cells.
This helps increase the speed of electrical signals, to ensure that the heart beat will happen as one unified event.

7. Cardiac muscle cont.
Additionally, cardiac muscle form fibrous units with many branches.
The muscle itself is dense with relatively large mitochondria, and is highly vascular. This prevents the fibers from becoming fatigued.
The evolutionary design of repeated branches and individual cells joined by intercalated discs is based on getting cells to work together as a unit. The signals synchronize contraction activity.

9. The cardiac cycle
The cardiac cycle is the same as one heartbeat, including all of the events that happen in between.
Your heart rate is the frequency of the cardiac cycle and is on average about 72 beats min-1.
When a chamber contracts, it is because the cardiac muscle has contracted, and the pressure on the blood in that chamber increases, forcing blood through any available opening.
When the cardiac muscle is relaxed, the chamber is relaxed, and the pressure on the blood is decreased.
Systole = heart is contracting
Diastole = heart is relaxing
Both atria undergo systole together, and both ventricles undergo systole together, but a little after the atria.

10. Valves
Recall the heart has four valves, and they are responsible for preventing the backflow of blood into the previous chamber.
The two atrioventricular valves are the tricuspid valve (between the right atrium and right ventricle) and the bicuspid or mitral valve (between the left atrium and left ventricle).
The semilunar valves are found between the ventricles and the pulmonary artery (right side) and aorta (left side).
To prevent the backflow of blood into the vena cava and pulmonary veins from the right and left atrium, respectively, there are valves found in the veins, and the pressure of atrial systole is very low, resulting in minimal pressure against these veins.

11. Valves

12. Heart sounds (U.D.4.6)
The sound that is heard when listening to the heart is the sound of one cardiac cycle.
The sounds are mostly made by the closing of the various cardiac valves.
The first sound, the “lub” of “lub, dub” is the sound of the AV valves closing as a unit.
The “dub” is the sound of the semilunar valves closing together.

13. Myogenic control of the heart
The ability of cardiac muscle to contract is intrinsic, that is, it is a property of the heart itself and does not depend on extrinsic nerve impulses (myogenic heart).
The independent but coordinated activity of the heart results from 2 factors:
A) the nerves of the involuntary nervous system (autonomic) which can act as brakes or accelerators and
B) the intrinsic conduction system or nodal system that is built into the heart tissue.

14. Myogenic control cont.
The heart consists of specialized nerve-like, but non-contractile, cardiac cells that initiate and distribute impulses throughout the heart, so that the myocardium contracts in an orderly sequence.
The area that initiates each heartbeat is the pacemaker or sinoatrial node. Nerves and hormones can transmit messages to the pacemaker to speed up beating while a second carries messages from the brain to slow down contractions. Damage to pacemaker slows heart beat.
Epinephrine (aka adrenaline), carried to the pacemaker in the blood stream tells the pacemaker to speed up the beating of the heart. This allows the person to prepare to fight or flee.

15. Myogenic control cont. (U.D.4.2 – U.D.4.5)
Let’s assume the heart is beating at 72 beats min-1.
This means that the SA node is sending an action potential every seconds. This action potential results in the atria undergoing systole.
The action potential also reaches the atrioventricular (AV) node, which delays the signal for .1 seconds.
From this point, the AV node sends its own action potential down conducting fibers in the interventricular septum, which branch off in Purkinje fibers. These fibers go into the muscles of the ventricles, and together with the intercalated discs, allow the ventricles to undergo systole at the same time.

16. The cardiac cycle

17. The cardiac cycle

19. ECG traces (S.D.4.3)
ECGs (electrocardiograms) are graphs that show the cardiac cycle in real time. The y-axis represents electrical activity, and the x-axis is time.
There are 4 portions to an ECG:
P wave: this is the electrical signal given off by the SA node, and represents atrial systole
Point Q: the point where the AV node sends its impulse
QRS complex: the point where the signal from the AV node spreads down the connecting fibers and out to the Purkinje fibers, marking the ventricular systole
T wave: the AV node repolarizes (going back to resting potential), and is ready for the next signal

20. ECG

21. ECG trace

22. Common heart problems and their treatments (A.D.4.1)
Artificial pacemakers: in some cases, a person’s heart rate may be too slow, too fast, or irregular in some other way. To combat this issue, an artificial pacemaker, a small battery powered device, is surgically inserted, with wires leading to the heart. The device will emit an electrical signal, causing the heart to contract.

23. Common heart problems and their treatments (A.D.4.2)
If a person suffers from a heart attack where the heart has stopped (cardiac arrest) or has been thrown out of rhythm (arrhythmia), a defibrillator may be used.
The defibrillator will provide an electric shock to the heart, and if successful, the heart will beat on its own once again.

24. Common heart problems and their treatments (A.D.4.3)
A thrombus is when a blood clot forms in the blood vessel.
This is a dangerous situation, as the clot can break loose and travel to another area of the body.
Should the clot become lodged in a smaller blood vessel, it could restrict the flow of blood to the tissue and cause major problems, especially in the heart, lungs, and brain.
To treat this, doctors will administer anticoagulants, or blood thinners, which slow the clotting process.

25. Common heart problems and their treatments (A.D.4.3)
Recall that blood pressure is a measurement of the force that blood exerts on the walls of the arteries.
For someone with hypertension, their “normal” blood pressure values are higher than the standard value of <120/80.
This can be caused by a variety of factors, including a loss of arterial elasticity (old age) and arterial plaque build-up.
To treat hypertension, a variety of medications are prescribed, as well as changes to activity level and diet.

27. Common heart problems and their treatments (S.D.4.4)
Coronary heart disease (CHD) is a term used for the slow build-up of plaque in the arteries and its corresponding problems.
There are a variety of factors that can contribute to CHD, including:
Excessive weight/obesity (higher b.p. and cholesterol result)
Sedentary lifestyle (contributes to weight gain/obesity)
Stress
Smoking
High blood pressure and/or cholesterol
CHD is difficult to study and treat, because many people may not experience symptoms for many years, and there are so many factors that contribute to the disease, as well as create a cascading series of problems.