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Pitt NUR 0013 - Chapter 18 The Heart

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NUR 0013 1st Edition Lecture 9 Outline of Current Lecture 1. Physiology and Cardiac Cyclea. Anatomy of heart cellsi. Cells are striated and uninuclear- (1 nucleus) They are short, and branched.b. Action Potential Generation2. Electrical eventsa. Intrinsic Conducting Systemb. Extrinsic Conducting Systemc. Electrocardiography (the EKG or ECG)3. Mechanical Events: The Cardiac Cycle4. Heart Sounds5. Cardiac Output Current LectureChapter 18 the heart I. Physiology and Cardiac CycleA. Properties of Cardiac Muscle Cells1. Anatomy Cells are striated and uninuclear- (1 nucleus)  They are short, and branched. Connected to each other at the intercalated discs, which are special anchors that hold the cells together and allow them to communicate via gap junctions. The entire myocardium behaves like a single cell or unit and is called a functional Action Potential Generation Rapid depolarization – phase where opening of fast sodium channels occurs. Partial repolarization (rapid) - phase when slow calcium channels open and prevent rapid repolarization. Repolarization - calcium channels shut and membrane is again most responsive to potassium.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Speed of generation Skeletal muscles  2 millisecs for AP generation Cardiac muscles - 300-500 millisecs for AP generation.Refractory period has both absolute and relative periods.- refractory period is prolonged so that fatigue does not set in, when cardiac muscle beats.II. Electrical events1. Intrinsic Conducting System Composed of special self-depolarizing autorhythmic cells. These cells have unstable resting membrane potentials and will depolarize spontaneously. The pacemaker cells are those found in the right atrium as part of the SA node. SA node- Initiates depolization.A small mass of specialized cardiac muscle fibers located in the posterior wall of the right atrium of the heart that acts as a pacemaker of the cardiac conduction system by generating at regular intervals the electric impulses of the heartbeat. AV node- delays the impulsbrifly before it moves on to the ventricleis an area of specialized tissue between the atria and the ventricles of the heart, specifically in the poster inferior region of the interatrial septum near the opening of the coronary sinus, which conducts the normal electrical impulse from the atria to the ventricles. AV bundle (bundle of HIS)- a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the AV node (located between the atria and the ventricles) to the point of the apex of the fascicular branches via the bundle branches. bundle branches- is a defect in the heart's electrical conduction system. During a right bundle branch block, the right ventricle is not directly activated by impulses travelling through the right bundle branch. Purkinje fibers- are a unique cardiac end-organ. Further histologic examination reveals that these fibers are split in atria and ventricles walls. The electrical origin of atrial Purkinje fibers arrives from the sinoatrial node.2. Extrinsic innervations Sympathetic innervation: sympathetic nerves run to the heart muscle and to the SA and AV nodes to accelerate the heart. Parasympathetic innervation: Travels to the heart mainly via the vagus nerve and impacts the SA and AV nodes. Hormones – Epi, NE3. Electrocardiography (the EKG or ECG) Only measures electrical events, NOT MECHANICAL. P wave –depolarization of atria (leads to atrial contraction) QRS complex – depolarization of ventricles T wave – repolarization of ventricles PR interval– contraction of the atria QT interval- ventricle wave length  PQ segment-  ST segment- ventricle contraction C. Mechanical Events: The Cardiac Cycle Systole – ventricles are contracting. when heart is pumping  Diastole –resting pressure. Ventricles are filling.  Entire cycle generally lasts about .8 secondsPhases1. Ventricular filling to Atrial Systole2. Ventricular Systole2a isovolumetric contraction-to an event occurring in early systole, during which the ventricles contract with no corresponding volume change. This short-lasting event takes place when both the AV valve and SL valve are closed.2b. Ventricular ejection-fraction (LVEF) is the measurement of how much blood is being pumped out of the left ventricle of the heart (the main pumping chamber)with each contraction.3. Ventricular Diastole-is the period during which the ventricles are filling and relaxing, while atrial diastole is the period during which the atria are relaxing. The term diastole originates from the Greek word διαστολη, meaning dilation.D. Heart Sounds Lub – closure of the AV valves Dup – closure of the semilunar valves.III. Cardiac OutputA. Cardiac Output The amount of blood pumped out by each ventricle in 1 minute. CO = SV*HR SV = stroke volume: the volume of blood ejected each time the heart contracts. HR = heart rate: the resting heart rate.B. Obtaining SV and preload SV = End Diastolic Volume – End Systolic Volume SV = 120ml-50ml = 70ml/beat. Preload – is the degree of stretch in cardiac muscle fibers before they contract. Increasing the stretch in the ventricles by increasing venous return will increase SV. D. Other effectors of Cardiac OutputANS regulation Parasympathetic innervation will decrease the heart rate and lower CO. Sympathetic innervation will increase the heart rate and raise


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