The Cardiovascular Systemo The circulatory system is composed of the heart, blood vessels, and blood.o The cardiovascular system consists of the heart, veins, arteries, and capillaries and is divided

into two major circuits: the pulmonary circuit (right side of the heart) carries blood to the lungs for gas exchange, while the systemic circuit (left side of the heart) supplies blood to all organs of the body. Blood is carried away from the heart by arteries, or efferent vessels, and to the heart by veins, or afferent vessels. Capillaries interconnect the smallest of the veins and arteries and are known as exchange vessels, as their thin walls permit the exchange of nutrients, gasses, and waste products between the blood and surrounding tissue.

Position of the Hearto The heart is located in the mediastinum, the region between the lungs which also contains the

great vessels, thymus, esophagus and trachea, and directly posterior to the sternum. o The base of the heart is the broad superior portion where the great vessels are attached, and

the apex is the inferior ends, which tilts to the left and tapers into a point.Pericardium

o The pericardium encloses the heart and allows it to beat without friction. It also allows enough room for the heart to expand without allowing excessive expansion. The pericardial sac consists of a network of dense collagen fibers and stabilizes the position of the heart and great vessels in the mediastinum.

o The parietal pericardium is the tough outer layer of fibrous connective tissue. It encloses the pericardial cavity, which is filled with pericardial fluid. The epicardium of the heart wall, also known as the visceral pericardium, is a thin, smooth serous layer that covers the heart surface. The small space in between the parietal and visceral surfaces is the pericardial cavity. The pericardial cavity contains pericardial fluid, which acts as a lubricant and prevents friction between the surfaces as the heart beats.

o Pathogens can infect the pericardium, producing the condition pericarditis, in which the inflamed surfaces rub against one another producing a scratching sound that can be heard through a stethoscope. This often results in an increased production of pericardial fluid by the pericardial membranes, which can restrict the movement of the heart. This condition is called cardiac tamponade and can also be caused by bleeding into the pericardial cavity due to traumatic injury.

Superficial Anatomy o The heart consists of four chambers: right and left atria- receive blood returning to the heart,

and right and left ventricles: pump blood into arteries o The atria have thin muscular walls that are highly expandable, when not filled with blood, the

outer portion of each atrium deflates and becomes a wrinkled flap. This expandable appendage of an atrium is called an auricle.

o The corony suclus is a deep groove the marks the border between the atria and ventricles.o The anterior and posterior interventricular sucli are more shallow depressions that mark the

boundary between the left and right ventricles. The sucli contain the arteries and veins that supply blood to the heart tissue, as well as a substantial amount of fat.

Layers of the Heart Wallo Epicardium (or visceral pericardium) - serous membrane that covers the outside of the heart

(inside the pericardial cavity). Composed of an exposed mesothelium and an underlying layer of loose areolar connective tissue that is attached to the myocardium.

o Myocardium- thick , muscular layer that has a fibrous skeleton made up of collagen and elastic fibers which forms both the atria and the ventricles. The layer contains of cardiac muscle tissue,

blood vessels, and nerves, and is made up of concentric layers of cardiac muscle tissue. The atrial myocardium contains muscle bundles that wrap around the atria and form figure eights to encircle the great vessels. Superficial ventricular muscles wrap around both ventricles, while deeper ventricular muscles spiral around and between the ventricles toward the apex. This layer is an electrical nonconductor, which helps to coordinate contractile activity of the heart.

o Endocardium- smooth inner lining of the heart and valves that consists of simple squamous epithelium.

Cardiac Muscle Tissueo Structure of cardiac muscle- short, branched cells with one central nucleus, decreased

sarcoplasmic reticulum with large T tubules which enables cells to admit for Ca2+ from ECFo Myocytes are joined end to end with intercalated discs. At each discs, the interlocking

membranes of adjacent cells are held together by desmosomes and linked by gap junctions, which allow ions to flow between cells. Mechanical junctions tightly join myocytes (fascia adherens- actin anchored to plasma membrane, transmembrane proteins link cells).

o Intercalated discs convey the force of contraction from cell to cell and propagate action potentials.

o Interdigitating folds increase surface area.o Metabolism of cardiac cells: aerobic respriration, rich in myoglobin and glycogen, large

mitochondria, fatigue resistant, uses organic fuels such as fatty acids, glucose, and keytones.Internal Anatomy

o Interatrial septum- separates atria, interventricular septum- separates ventricles, much thickero Pectinate muscles- internal ridges of myocardium in right atrium and both auricles, trabeculae

carnae- internal ridges in both ventriclesThe Right Atrium

o Receives blood from the systemic circuit from the two greater veins, the superior vena cava (opens into the posterior and superior portion of the atrium, delivers blood from the head, neck, arms, and chest) and the inferior vena cava (opens into the posterior and inferior portion of the atrium, delivers blood from the rest of the trunk, viscera, and legs).

o The cardiac veins return blood from the heart tissue to the coronary sinus, a large, thin-walled vein that opens into the right atrium inferior to the connection with the superior vena cava. The opening lies near the posterior edge of the interatrial septum.

o From five week gestation until birth, a hole in the interatrial septum known as the foramen ovale connects the two atria. The enables the blood flow to bypass the lungs while they are still developing. At birth, the foramen ovale closes, and becomes permanently sealed off within three months. In adults, a small, shallow depression remains, known as the fossa ovalis.

o The posterior wall and the interatrial septum have smooth surfaces. The anterior wall and the inner surface of the auricle contain muscular ridges called the pectinate muscles.

The Right Ventricleo Blood flows from the right atrium to the right ventricle through the right AV valve, or tricuspid

valve. The valve is made up of three fibrous flaps, or cusps. The free edge of each cusp is attached to tendinous connective tissue fibers called the chordae tendineae, which originate at the paillary muscles,conical muscular projections that arise fram the inner surface of the right ventricle. The right AV valve close when the right entricle contracts, precenting the backflow of blood into the right atrium. The chordae tendineae enable the valve to hold tight and prevent the backflow of blood. When the ventricle is relaxed, the chordae tendineae are loose. When the ventricles contract, the contraction of the papillary muscles tense the chordeae tendineae, which stops the cusps before they swing into the atria.

o The internal surface of the ventricle contains a series of muscular ridges known as the trabeculae carneae. The moderator band is a muscular ridge that extends from the interventricular septum and connects to the papillary muscle. This ridge also contains a portion of the conducting system, and delivers the stimulus for contraction to the papillary muscles which enables them to begin tensing the chordate tendineae before the rest of the ventricle contracts.

o The superior end of the right ventricle tapers to the comus arteriosus, a conical pouch that ends in the pulmonary semilunar valve. Blood flowing from the right ventricle passes through this valve to enter the pulmonary trunk, the start of the pulmonary circuit. The arrangement of the cusps prevent backflow as the right ventricle relaxes.

o Once in the pulmonary trunk, blood flows into the left and right pulmonary arteries, which branch repeatedly in the lungs before supplying the capillaries where gas exchange occurs.

The Left Atriumo The posterior wall of the left atrium receives blood from the two left and two right pulmonary

veins.o The left AV valve, or bicuspid/mitral valve, guards the entrance to the left ventricle and prevents

backflow during ventricular contraction.The Left Ventricle

o The left ventricle is much larger than the right due to its thicker walls, though it holds the same amount of blood. The thick muscular wall gives the left ventricle the power to push blood through the entire systemic circuit, while the right ventricle only needs to push blood a short distance into the lungs.

o The left ventricle does not have a moderator band. o Blood leaves the left ventricle by passing through the aortic valve, or aortic semilunar valve, into

the ascending aorta. Blood then flows through the aortic arch and into the descending aorta.o The pulmonary trunk is attached to the aortic arch by the ligamentum arteriosum, a fibrous

band that was once an fetal blood vessel linking the pulmonary and systemic circuits.The Heart Valves

o The semilunar valves do not rquire muscular braces because the atrial walls do not contract and the position of the cusps are stable. When the valves close, the three symmetrical cusps support one another like the legs of a tripod.

o Saclike dilations at the base of the ascending aorta adjacent to each cusp, aortic sinuses, prevent the cusps from sticking to the aorta when the valve opens. The right and left coronary arteries originate at the aortic sinuses.

o Valvular heart disease- valve function deteriorates to the point where the heart cannot maintain adequate circulatory flow. Generally this develops after carditis, inflammation of the heart (which can be caused by rheumatic fever), occurs.

o Antroventricular (AV) valves- right AV valve has three cusps (tricuspid valve) while left AV valve has two cusps (mitral/bicuspid valve). Chordae tendinae are cords hat connect the AV vavles to the papillary muscles on the floor of the ventricles.

o Semilunar valves control the flow of blood into the arteries- pulmonary semilunar valve (right ventricle into pulmonary trunk), and aortic semilunar valve (left ventricle into aorta)

o When the ventricles are relaxed: pressure in the heart drops, semilunar valves are closed, AV valves are open, and blood flows from atria to ventricles.

o When the ventricles contract: pressure in the heart rises, semilunar valves are open, AV valves are closed, blood is pumped from ventricles into great vessels

Connective Tissue of the Heart

o Each cardiac muscle cell is wrapped in a strong, but elastic, sheath and adjacent cells ares tied together by fibrous cross-links, or struts. These fibers are then interwoven into sheets that separate the superficial and deep muscle layers.

o The connective tissue fibers: provide physical support for the cardiac muscle fibers, blood vessels, and nerves of the myocardium, help distribute forces of concentration, add strength and preven overexpansion of the heart, and provide elasticity that helps return the heart to its original size and shape after a contraction.

o The fibrous skeleton of the heart consists of four dense bands of elastic tissue that encircle the heart valves and he bases of the pulmonary trunk and aorta. These bands stabilize the positions of the heart valves and ventricular muscles cells and electrically insulate the ventricular cells from the atrial cells.

The Blood Supply to the Heart- Arterieso Left coronary artery (LCA) consists of two branches: anterior interventricular branch- supplies

blood to interventricular septum and anterior walls of ventricles, and circumflex branch- passes around left side of heart in coronary suclus, supplies blood to left atrium and posterior wall of left ventricle.

o Right coronary artery (RCA) also consists of two branches: right marginal branch- supplies lateral right atrium and ventricle, and posterior interventricular branch- supplies posterior walls of ventricles

o The left and right coronary arteries originate at the base of the ascending aorta, at the aortic sinuses. When the left ventricle relaxes and blood is no longer flowing into the aorta, pressure decreases and the walls of the aorta recoil, called elastic rebound. This recoil pushes the blood forward into the systemic circuit and backward into the aortic sinuses and then the coronary arteries. Because of this myocardial blood flow is not steady, it peaks when the heart muscle relaxes and almost ceases when it contracts.

o The right coronary artery follows the coronary suclus around the heart and supplies blood to the: right atrium, portions of both ventricles, and portions of the conducting system of the heart including the SA and AV nodes.

o Inferior to the right atrium, the right coronary artery gives rise to the marginal arteries, which extend across the surface of the right ventricle.

o The right coronary artery continues to the posterior surface of the heart, supplying the posterior interventricular artery, or posterior descending artery, which runs towards the apex within the posterior interventricular suclus, and supplies the interventricular septum and adjacent portions of the ventricles

o The left coronary artery supplies blood to the left ventricle, left atrium, and the interventricular septum.

o On the anterior surface of the heart it gives rise to the circumflex branch and an anterior interventricular branch. The circumflex artery curves around the coronary suclus, meeting and fusing with small branches of the right coronary artery. The anterior interventricular artery goes around the pulmonary trunk and runs within the anterior interventricular suclus.

o Arterial anastomoses, or interconnections between arteries, ensure that the blood supply remains constant despite pressure fluctuations in the left and right coronary arteries as the heart contracts.

The Blood Supply to the Heart- Veinso The great cardiac vein begins on the anterior surface of the ventricles along the interventricular

suclus. It drains blood supplied by the anterior interventricular artery. The great cardiac vein comes up to where the atria are then curves around the heart within the coronary suclus. It

empties into the coronary sinus, which lies in the posterior portion of the coronary suclus, and opens into the right atrium near the base of the inferior vena cava.

o The posterior cardiac vein drains the area supplied by the circumflex artery, the middle cardiac vein drains the area supplied by the posterior interventricular artery, the small cardiac vein drains the posterior surfaces of the right atrium and ventricle, and all of these empty into the great cardiac vein or coronary sinus.

o The anterior cardiac vein drains the anterior surface of the right ventricle and empties directly into the right atrium.

o Venous drainage of the heart: 20% of blood pumped to the heart tissue (via he coronary arteries) drains directly into the right atrium via the thebesian veins, while 80% returns to the right atrium via: the great cardiac vein (blood from anterior interventricular suclus), the middle cardiac vein (from posterior suclus), the left marginal vein, or the coronary sinus (collects blood and empties into the right atrium)

The Conducting Systemo Automaticity- the heart muscle contracts on its own, without the presence of neural or

hormonal stimulation.o Conducting system- cells responsible for initiating and distributing the electrical

impulse/stimulus. The conducting system consists of the SA node, the AV node, and the conducting cells.

o The conducting cells interconnect the two nodes and distribute the contractile stimulus throughout the myocardium. In the atria, conducting cells are found in intermodal pathways, which distribute the contractile stimulus to atrial muscle cells as the impulse travels from the SA node to the AV node.

o Most of the cells in the conducting system are smaller than the contractile cells in the myocardium but have a larger diameter, which enables them to conduct action potentials more quickly.

o Membranes of conducting cells in the SA and AV nodes cannot maintain a stable resting potential. After each repolarization, the membrane drifts toward threshold, and this gradual depolarization is called prepotential or pacemaker potential.

o The rate of depolarization is fastest in the SA node, and because of this it establishes the heart rate. The impulse generated by the SA node brings the AV nodal cells to threshold faster.

o Though the SA node depolarizes 80-100 per minute, the resting heart rate is somewhat slower due to parasympathetic innervation.

The Sinoatrial Nodeo Embedded in the pposterior wall of the right atriumo Contain pacemaker cells which establish the heart rateo Connected to the AV node by intermodal pathways in the atrial walls, as the signal travels down

the pathway, conducting cells pass the stimulus to contractile cells of both atria. The action potentional then spreads across the atrial surfaces by cell-to-cell contact.

o This stimulus affects only the atria because the heart’s fibrous skeleton insulates the ventricles from the atria.

The Atrioventricular Nodeo Sits within the floor of the right atrium near the opening of the coronary sinuso The rate of propagation of the impulse slows as it leaves the intermodal pathways and enters

the AV node because the nodal cells are smaller in diameter than the conducting cells and the connections between the nodal cells are less efficient than those between the conducting cells.

o After this brief delay, the impulse is conducted along the purkinje fibers and the papillary muscles. The purkinje fibers then distribute the impulse to the ventricular myocardium and ventricular contraction begins.

o The AV node can conduct impulses at a maximum of 230 bpm, which means even if the SA node is generating impulses faster, the ventricles cannot contract more than 230 bpm. The pumping efficiency of the heart begins to decrease at rates above 180bpm, and rates above 230 bpm can only be achiever if there is damage to the heart or by drugs. 300-400 bpm= dangerously/fatally reduced pumping efficiency.

o Bradycardia= slower than normal HR, tachycardia= faster than normal HRThe AV Bundle, Bundle Branches, and Pukinje Fibers

o The connection between the AV node and the AV bundle is the only electrical connection between the atria and ventricles.

o Once an impulse is in the AV bundle, it travels to the interventricular septum and enters the right and left bundle branches.

o The left budle brach, which supplies the left ventricle, is much larger than the right bundle branch. Both branches extend toward the apex of the heart, turn and fan out deep to the endocardial surface. As they diverge the branches conduct the impulse to the papillary muscles of the right ventricle through the moderator band, and to the purkinje fibers.

o The purkinje fibers contract very quicly, and within 75 milliseconds the signal has traveled to the ventricular muscle cells. They radiate from the apex to the base of the heart, and thus contraction moves in a wave from the apex to the base, pushing blood towards the base of the heart, where the pulmonary trunk and aorta are.

o Because the signal goes straight to the papillary muscles of the right ventricle through the moderator band, the papillary muscles begin contracting before the rest of the ventricular muscle cells. This contraction applies tension to the chordae tendineae, bracing the AV valves.

o Conduction deficits- resulting problems from the conducting pathways being damaged.o Ectopic pacemaker- abnormal conducting cell or ventricular contractile muscle begins producing

action potentials faster than those of the SA or AV node and thus overrides them. Disrupts the timing of ventricular contraction and can dangerously reduce pumping efficiency.

Electrocardiogramo A recording of the electrical events occurring in the heart detected by electrodes on the surface

of the body. The results can be used to access the performance of specific nodal, conducting, and contractile components.

o The small P wave accompanies depolarization of the atria, which begin contracting 25milliseconds after the start of the wave. The QRS complex appears as the ventricles depolarize, and this is a stronger signal because the ventricles are much larger than the atria. The ventricles begin contracting shortly after the peak of the R wave. The smaller T wave indicates ventricular repolarization. Atrial repolarization occurs during the QRS complex but isn’t represented on the EKG.

o The EKG is analyzed by measuring the size of the voltage changes and determining the durations and temporal relationships of the various components.

o Excessively large QRS complex indicates the heart has become enlarged, smaller than normal electrical signal may indicate the mass of the heart has decreased.

o The size and shape of the T wave will be affected by anything that slow repolarization, such as low cardiac energy reserves, coronary ischemia, or abnormal ion concentrations that will reduce the size of the T wave.

o The time between the waves is known as segments or intervals. Segments extend from one end of a wave to the other, intervals are more variable but always include at least one entire wave.

o The P-R interval extends from the start or atrial depolarization to the start of the QRS complex, because the peak at R can be difficult to determine in abnormal EKGs. (A P-R interval of more than 200 milliseconds can indicate damage to the conducting pathways or the AV node).

o The Q-T interval indicates the time required for the ventricles to undergo a single cycle of depolarization and repolarization. Can be lengthened by electrolyte disturbances, some medications, conduction problems, coronary ischemia, or myocardial damage.

o EKG analysis is especially useful in detecting and diagnosing cardiac arrhythmias, or abnormal patterns of cardiac electrical activity.

Contractile Cellso Purkinje fibers distribute the stimulus to the contractile cells, which form the bulk of the atrial

and ventricular walls. o In both cardiac muscle cells and skeletal muscle fibers action potential leads to the appearance

of Ca2+ among the myofibrils and the binding of Ca2+ to troponin on the thin filaments initiates the contraction.

o The resting potential of a ventricular contractile cell is -90mV. An action potential begins when the membrane of the ventricular muscle is brought to threshold, about -75mV. Threshold is normally reached in a portion of the membrane near an intercalated disc.

o Once threshold has been reached, rapid depolarization begins. Voltage-regulated sodium channels open, and the mebrane becomes permeable to Na+, which results in an influx of sodium and depolarization of the sarcolemma. The channels involved are called fast channels because they open quickly and stay open for a few milliseconds.

o As the transmembrane potential approaches -30mV, the sodium channels close, and remain close until the potential reaches -60mV. The cell now begins pumping Na+ out of the cell, but as the sodium channels are close, calcium channels open so there is no net loss of positive charges. The calcium channels are called slow channels because they open slowly and stay open for about 175 milliseconds. The entry of Ca2+ ions balances the loss of Na+ ions and the potential remains at 0mV for an extended period. This portion is called plateau, not present in skeletal muscle cells.

o As plateau continues, slow calcium channels close as slow potassium channels open. As potassium ions rush out of the cell, the net result is a period of rapid repolarization that restores the resting potential of -90mV.

o Refractory period- for some time after the action potential begins, the membrane will not respond normally to a second stimulus. During the absolute refractory period, the cell cannot respond at all, which lasts about 200 milliseconds, spanning the time of the plateau and initial period of rapid repolarization. During this period, the sodium channels are closed but can open. The membrane will respond to stronger-than-normal stimulus by initiating another action potential.

o A total action potential last 250-300 milliseconds.o The appearance of an action potential in the cardiac muscle cell membrane produces a

contration by causing an increase in the concentration of Ca2+ around the myofribils. Calcium ions entering the cell membrane during the plateau phase provide 20% of the Ca2+ required for concentration, and the arrival of extracellular Ca2+ is the trigger for the release of additional Ca2+ from reserves in the sarcoplamic reticulum.

The Cardiac Cycle

The period between the start of one heartbeat and the beginning of the next is a single cardiac cycle. The cycle is divided into two phases, systole and diastole.

During systole, or contraction, the chamber contracts and pushes blood into an adjacent chamber or the aorta.

During diastole, or relaxation, the chamber fills with blood and prepares for the next cardiac cycle.

The pressure in each chamber rises during systole and falls during diastole. Blood will flow from one chamber to another only if pressure in the first chamber exceeds that in the second, as fluid always flows from an area of higher pressure to an area of lower pressure.

When the cardiac cycle begins, all four chambers are relaxed and the ventricles are partially filled with blood. During atrial systole, the atria contract filling the ventricles completely with blood. Then the atria enter diastole at the same time the ventricles enter systole. During this period, blood is pushed through the systemic and pulmonary circuits and towards the atria. Then, the ventricles enter diastole and filling begins passively as the whole heart is relaxed, before the next cycle begins.

When the heart rate is increased, the time spent in each part of the cycle decreases, but the greatest decrease is the time spent in diastole.

At the end of atrial systole, the ventricles contain the maximum amount of blood they will hold in this cardiac cycle, know as end-diastolic volume.

Isovolumetric contraction- the ventricles are contracting, but the pressure has yet to become enough to force open the semi lunar valves. During this period the valves are all closed, the volumes in the ventricles remain constant, and the pressure is increasing.

Once the pressure in the ventricles exceeds that of the arterial trunks, the semilunar valves are forced open, which is known as ventricular ejection.

Stroke volume is the amount of blood each ventricle ejects during ventricular ejection. The stoke volume at rest is roughly 60% of the end-diastolic volume. (SV=EDV-ESV)

At the end of ventricular systole, pressure in the ventricles falls and the blood begins to flow back towards them (and the lower pressure). The semilunar valves close to prevent backflow, and thus increase the pressure in the arterial trunks as the elastic arterial walls recoil. This small, temporary rise in pressure is called the dicrotic notch.

The amount of blood left in the ventricles after contraction is know as end-systolic volume, and is about 40% of end-diastolic volume.

During ventricular diastole, all the heart valves are closed, and pressure in the ventricles is still higher than that in the atria so blood cannot flow, this is called isovolumetric relaxation. When the ventricle pressure falls below that of the atria, the AV valves are forced open and blood begins to flow into the ventricles. This passive method is the primary way the ventricles fill, and they will be 75% full at the end of the cardiac cycle, before the atria contract.

Ejection fraction- percentage of the EDV represented by the SV.Heart Sounds

S1- sound of the AV valves closing, marks the start of ventricular contraction S2- sound of the semilunar valves closing, beginning of ventricular filling S3- blood flowing into the ventricles S4- atrial contraction

Energy for Cardiac Contractions Obtained by the mitochondrial breakdown of fatty acids and glucose when there is oxygen

readily available Cardiac muscle cells maintain their own reserves of oxygen, molecules bound to the heme units

of myoglobin molecules.

o Angina pectoralis- chest pain caused by partial obstruction of coronary blood flow which leads to ischemia or lack of oxygen in heart tissue. Angina is usually activity related and will subside when activity is stopped.

o Myocardial infarction- complete obstruction of coronary arteries leads to complete lack of oxygen in heart tissues which results in death of cardiac cells in the affected area. Usually presents as pain or pressure in the chest that radiates down the left arm.

o Nerve supply to the heart: sympathetic nerves from: upper thoracic spinal cord directly to ventricular myocardium. Can raise heart rate to 230 bpm, and parasympathetic nerves: right vagal nerve to SA node, left vagal nerve to AV node, vagal tone- normally slows HR to 70-80 bpm

o Properties of the cardiac conduction system: myogenic- heartbeat originates within heart, autorythmic- regular, spontaneous depolarization

o Components of the cardiac conduction system: SA node- pacemaker, initiates heartbeat, sets heart rate, AV node- electrical gateway to ventricles, AV bundle- pathway for signals from AV node (right and left bundle branches- divisions of AV bundle that enter interventricular septum), and purkinje fibers- upward from apex spread throughout ventricular myocardium to carry signal

o Cardiac rhythm: systole-ventricular contraction, diastole- ventricular relaxation, sinus rhtym- set by SA node at 60-100 bpm, adult at rest is 70 to 80 bpm (vagal inhibition)

o Premature ventricular contraction- caused by hypoxia, electrolyte imbalance, stimulants, or stress

o Ectopic foci- region of spontaneous firing (not SA node)o Arrhythmia- abnormal cardiac rhythm, failure of conduction system (heart block)o Depolarization of SA node: SA node has no stable resting membrane potential, gradual

depolarization from -60mV, slow influx of Na+. Action potential occurs at threshold of -40mV, depolarizing phase to 0mV, Ca2+ channels open, influx of Ca2+, repolarizing K+ channels open, K+ our, at -60 mV K+ channels close and potential starts over, each depolarization is one heartbeat, resting SA node fires every .8 sec (75 bpm)

o