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34 Cards in this Set

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What is an electrocardiogram?
The electrical events occurring in the heart are powerful enough to be detected by electrodes on the surface of the body. A recording of these events is an electrocardiogram, also called an ECG or EKG. No contraction.
What happens each time the heart beats?
Each time the heart beats, a wave of depolarization radiates throught the atria, reaches the AV node, travels down the interventricular septum to the apex, turns, and spreads through the ventricular myocardium toward the base.
ECG features:
1. The small P wave accompanies the depolarization of the atria. The atria begin contracting about 100 msec after the start of the P wave.
2. The QRS complex appears as the ventricles depolarize. This is a relatively stong electrial signal, because the ventricular muscle is much more massive than that of the atria. It is also a complex signal, because it incorporates atrial repolarization as well as ventricular depolarization. The ventricles begin contracting shortly afer the peak of the R wave.
3. The smaller T wave indicates ventricualar repolarization. You do not see a deflection corresponging to atrial repolarization, because it occurs while the venticles are depolarizing and the electrial events are masked by the QRS complex.
What do you do to analyze an ECG?
To analyze an ECG, you must measure the size of the voltage changes and determine the durations and temporla relationships of the various components. Attention is usuallu is focuse on the amount of depolarization occurring during the P wave and the QRS complex. For example, an excessively large QRS comples often indicates that the heart has become enlarged. Also starvation and low cardiac energy reserves, and coronary ischemia, or abnormal ion concentrations will reduce the size of the T wave.
You must also measure the time between the waves. Name the two intervals?
1. P-R interval extends from the start of atrial depolarization to the start of the QRS comples (ventricular depolarization) rather than to R, becasue in abnormal ECGs the peak can be difficult to determine.
2. Q-T interval indicates the time required for the ventricles to undergo a single cycle of depolarization and repolarization. It is usually measured from the end of the P-R interval rather than from the bottom of the Q wave.
Contractile cells
The purkinje fibers distribute the stimulus to the contractile cells, which form the bulk of the atrial and ventricular walls.
How are skeletal and cardiac muscle fibers similar?
1. an action potential leads to the apperance of calcium among the myofibrils
2. the binding of calcium to troponin on the thin filaments initiates the contraction
How are skeletal and cardiac muscle fibers different?
In the terms of the nature of the action potential, the source of the calcium, and the duration of the resulting contraction.
When does the action potential begin?
An aciton potential begins when the memebrane of the ventricular muscle cell is brought to threshold, usually at -75mv. Threshold is normally reached in a portion of the membrane next to an intercalated disc. The typical stimulus is the excitation of an adjacent muscle cell.
Once threshold has been reached, the action potential proceeds in three basic steps:
1. RAPID DEPOLARIZATION. Resembles that in a skeletal muscle fiber. At threshold, voltage-regulated sodium channels open and the membrane suddendly become more preamable to sodium. The result is a massive influx of sodium ions and rapid depolariztion of the sarcolemma. The channels involved are called fast channels, because they open quickly and remain open for only a few milliseconds.
Once threshold has been reached, the action potential proceeds in three basic steps:
2. THE PLATEAU. As the transmembrane potential approaches +30mv, the voltage-regulated sodium channels close. They remain closed and inactivated until the membran potential reaches -60mv. The cell now begins actively pumping Na out of the cell. However, a net loss of positive charges does not continue, becasue as the sodium channels are closing, voltage-regulated calcium channels are opening. These channels are called slow calcium channels, because they open slowly and remain open for a relatively long period. While the slow calcium channels are open, clacium ions enter the sarcoplasm. The entry of positive charges through the calcium channels, in the form of calcium, roughlu balances the loss of positive ions through the active transport of sodium and the transmembrane potential remains near 0mv for an extended period. This portion of the action potential curve is called a plateau. MAJOR difference because skeletal muscle fibers have rapid depolarization followed immediately by rapid repolarization.
Once threshold has been reached, the action potential proceeds in three basic steps:
3. REPOLARIZATION. As the plateau continues, slow calcium channels begin closing and the slow potassium channesl begin opening. As the channels open, potassium ion rush out of the cell, and the net result is a period of rapid repolarization that restores the resting potential.
Absolute Refractory Period
In a ventricular muscle cell, the absolute refractory period spans the duration of the plateau and the initial period of repolarization.
Relative Refractory Period
Voltage-regulated sodium channels are closed, but can open. The membrane will respond to a stronger than normal stimulus by initiating another aciton potential.
The apperance of an action potential in the cardiac muscle cell membrane produces a contraction by causing an increase in the concentration of calcium around the myofibrils. This process occurs in two steps:
1. Calcium ions entering the cell membrane during the plateau phase of the action potential provid roughly 20% fo the calcium required for a contraction.
2. The arrival of extracellular calcium trigger for the release of additional calcium from reserves in the sarcoplasmic reticulum.
Cardiac Cycle
Each heartbeat is followed by a brief resting phase, allowing time for the chambers to relax and prepare for the next heartbeat. The period between the start of one heartbeat and the beginning of the next is a single caridac cycle. The cardiac cycle, therefore, includes alternating periods of contraction and relaxation.
For any one chamber in the heart, the cardiac cycle can be divided into two phases:
1. Systole
2. Diastole
or contraction, the chamber contracts and pushes blood into an adjacent chamber or into an arterial trunk.
Or relaxation, the chamber fills with blood and prepares for the next cardiac cycle.
How do fluids move through the heart relating to pressure?
Fluids move from an area of high pressure to one of lower pressure. The pressure within each chamber rises during systole and falls during diastole. Valves between adjacent chambers help ensure that blood flows in the desired direction, but blood will flow from one chamber to another only if the pressure in the first chamber exceeds that in the second. This basic principle governs the movement of blood between atria and ventricles, between ventricles and arterial trunks, and betwwen major veins and atria.
Phases of the Cardiac Cycle
a. ATRAIL SYSTOLE BEGINS. Atrial contraction forces a small amount of additional blood into relaxed ventricles.
c. VENTRICULAR SYSTOLE-FIRST PHASE. Ventricular contaction pushes AV valves closed but does not create enough pressure to open the semilunar valves.
d. VENTRICULAR SYSTOLE-SECOND PHASE. As ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected.
e. VENTRICUALR DIASTOLE-EARLY. As ventricles relax, pressure in the ventricles drops; blood flows back against cusps of semilunar valves and forces them closed. Blood flows into the relaxed atria.
f. VENTRICULAR DIASTOLE-LATE. All chambers are relaxed. Ventricles fill passivly.
Pressures are lower in the right artium and right ventricle.
Although pressures are lower in the right artium and right ventricle, both sides of the heart contract at the same time, and they eject equal volumes of blood.
Atrial systole
1. As the atria contract, rising atrial pressures push blood into the ventricles through the open right and left AV valves.
2. At the end of atrial systole, each ventricle contains the maximum amount of blood that it will hold in this cardiac cycle. The quantiy is called end-diastolic volume (EDV). -130ml normal.
Ventricular Systole
As the pressure in the ventricles rise above those in the atria, the AV valves swing shut.
3. Ventricles are contracting. Blood flow has yet to occur, however, because ventricular pressures are not high enough to force open the semilunar valves and push blood into the pulmonary or aortic trunk. Volumes of the ventricles remain constant and the ventricular pressure rise.
4. Once pressure in the ventricles exceeds that in the arterial trunks, the semilunar valves open and blood flows into the pulmonary and aortic trunks. This point marks the beginning of the period of ventricular ejection. The muscle cells shorten, and tension production remain constant.
During ventricular ejection, each ventricle will eject 70-80 ml of blood, the stroke volume(SV) of the heart.
5. As the end of ventricular systole approaches, ventricular pressures fall rapidily. Blood in the aorta and the pulmonary trunk now starts to flow back toward the ventricles, and this movement closes the semilunar valves. As the backflow begins, pressure decreases in the aorta. When the semilunar valves close, pressure rises again as the elastic arterial walls recoil. The amount of blood remaining in the ventricles when the semilunar valves closes is the end-systolic volume(ESV). 50 ml.
Ventricular diastole
6. All the heart valves are now closed, and the ventricular myocardium is relaxing. Because ventricular pressures are still higher than atrial pressures, blood cannot flow into the ventricles. Ventricular pressures drop rapidly over this period because the elasticity of the connective tissues of the heart and fibrous skeleton helps re-expand the ventricles toward their resting dimensions
7. When ventricular pressures fall below those of the atria, the atrial pressures force the AV valves open. Blood now flows from the atria into the ventricles. Both the atria and the ventricles are in diastole, but the ventricular pressures continue to fall as the ventricualr chambers expand. Throughout this period, pressures in blood pours through the relxed atria and on through the open AV valves into the ventricles. This passive mechanism is the primary method of ventricualr filling.
Heart Sound
When you listen to your own heart, you usually hear the first and second heart sounds. These sounds accompany the closing of your hear valves. The first hear sound, known as "lubb"(S1) last a little longer than the second, call "dupp"(S2). S1 which marks the start of ventricular contraction, is produced as the AV valves close; S2 occurs at the beginning of ventricular filling, when the semilunar valves are close.
What is Cardiac Output?
The amount of blood pumped by each ventricle in one minute.
What is stroke volume?
The amount of blood pumped out of each ventricle during a single beat.
What is End-Diastolic Volume?
The amount of blood in each ventricle at the end of ventricular diastole.
What is End-sytolic Volume?
The amount of blood remaing in each ventricle at the end of ventricular systole.
Cardiac output can be adjusted by?
heart rate or stroke volume
Control of cardiac output:
The heart rate can be adjusted by the activities of the autonomic nervous system or by the circulating hormones. The stroke volume can be adjusted by changing the end-diastolic volume (how full the ventricles are when they start to contract), the end-systolic volume (how much blood remains in the ventricle after it contracts), or both.
What hormones effect heartrate?
Epinephrine and norepinephrine increase the heartrate by their effect on the SA node.
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