Collin County Community College BIOL. 2402 Anatomy & Physiology WEEK 5 The Heart 1 The Heart Beat and the EKG 2 1
The Heart Beat and the EKG P-wave = Atrial depolarization QRS-wave = Ventricular depolarization T-wave = Ventricular repolarization PR-interval = time for signal to go from SA node to AV node 3 4 2
The Heart Beat and the EKG SA node sets the rhythm. if SA nodes becomes damaged, AV node will take over in setting the rhythm, but at a lower pace If AV node is damaged, Bundle of His sets the rhythm. However, that rhythm is too slow to be compatible with life Ectopic foci Abnormal conducting or contractile cells that generate AP s and override the impulse rhythm of SA or AV node Regular sinus rhythm : no rhythmic disturbances Sinus tachychardia : increased rhythm ( HR > 100/min) Sinus tachychardia : decreased rhythm ( HR < 50/min) Sinus arrhythmia : HR fluctuates in synchrony with respiration 5 The Heart Beat and the EKG Abnormal patterns large QRS wave may indicate an enlarged heart small T wave may indicate coronary ischemia or ion imbalances increased PR interval may indicate atrial scarring increased QT interval may indicate ventricular conduction problems First degree heart (AV) block increased PR interval Second degree heart (AV) block not every P wave is followed by a QRS wave Third degree heart (AV) block atria and ventricles beat independently Ventricular fibrillation No coherent contraction of the myocadrial muscle mass 6 3
Normal ECG: P waves (atrial depolarization) are followed faithfully by QRS (ventricular depolarization) and T waves (ventricular repolarization). 1st degree heart block ECG: PR segment is longer than normal 2nd degree heart block ECG: every other P wave fails to evoke QRST (partial atrioventricular block). 3rd degree heartblock ECG: P waves and QRST occur independently (full atrioventricular block). 7 The Heart Beat and the EKG The action potentials of the autorhythmic cells is passed on to the contractile cells where it starts the process of contraction. Since heart cells are connected via gap junctions, a wave of depolarization and contraction will be passed on from cell to cell. 8 4
The ECG and Heart Muscle Cells AP s The relationship between the electrocardiogram (ECG), recorded as the difference between currents at the left and right wrists, Note the much smaller scale for the ECG potential. and an action potential typical of ventricular myocardial cells. 9 Heart Muscle Cell AP and Contraction Heart muscle tension development starts right after the initiation of the myocardial action potential refractory Compared to skeletal muscles, cardiac muscles have a prolonged refractory period. ( a few msecs compared to 250 msec) 10 5
Heart Muscle Cell AP and Contraction The prolonged refractory period of cardiac muscle prevents tetanus contractions and is due to the opening of L- type Calcium channels. It allows the heart to relax and fill with blood again before the next contraction. Opening of the L-type Calcium channels provides the prolonged depolarization plateau, due to an influx of Calcium in to the myocardial cells. This influx of calcium is not only important for the prolonged depolarization of the myocardial cell but it is also important because it induces the release of Calcium from the SR. Without this influx, calcium can not be released from the SR and contraction is not possible. This event is referred to as Calcium - induced Calcium release 11 Heart Muscle Cell AP and Contraction Time when calcium induced calcium release starts Time when tension development (contraction) starts 12 6
13 Calcium induced contraction events in cardiac cells. Relaxation in heart muscle cells occurs when: Calcium is pumped back into SR by means of Ca-ATPase pumps Calcium is pumped out of the cell by means of Na-Ca antiports via a secondary active transport mechanism. 14 7
Heart Muscle Cell AP and Contraction 15 The Cardiac Cycle During a cardiac cycle, the heart goes through a sequence of phases The systolic phase The diastolic phase = contraction phase = relaxation phase A normal cycle starts when both atria and ventricles are in a relaxed state the two atria contract first while the ventricles relax the atria then relax while the ventricles contract 16 8
The Cardiac Cycle The cardiac cycle can be divided into 3 specific Periods The Ventricular Filling period Atrial Systole The Isovolumetric Contraction period The Ejection period The Isovolumetric Relaxation period Ventricular Systole Ventricular Diastole Since it is a cycle, we can look at this cycle from any point 17 The Cardiac Cycle 18 9
Atrial Systole : = ventricular filling pressure inside Ventricles drops and opens AV valve blood flows in ventricles and volume content of ventricle increases (pressure increases very slowly) ~ 80% of ventricular blood content is purely by pressure differential influx SA node fires and atria contract, forcing last 20% of blood into ventricles End of this period occurs when atria relax and electrical impulse reaches AV node AV valve SL valve 19 Ventricular Systole starts when AV node passes impulse on to bundle of His ventricles contract and pressure in ventricles increases dramatically immediately pushes the AV valves to close both AV and SL valves are closed now = iso volumetric contraction contraction 20 10
starts when pressure in this closed system becomes higher than aortic pressure pushes the SL valve open and blood is ejected into the aorta = ventricular ejection Ventricular Systole end point is reached when ventricles start to relax, ventricular pressure falls below aortic pressure and SL-valves shut close AV valve SL valve 21 Ventricular Diastole starts with the T-wave The Relaxation Period both atria and ventricles are relaxed blood pressure in aorta closes the SL-valve the AV-valve is still closed lack of contraction causes pressure to drop in ventricle and atria blood returns from vena cava into atria since both SL and AV valves are closed this period is referred to as Iso-Volumetric Relaxation period 22 11
The Cardiac Cycle Systole: ventricles contracting Diastole: ventricles relaxed 23 The Cardiac Cycle Pressure and volume changes in the left heart during a contraction cycle. 24 12
Pressure and volume changes in the left heart during a contraction cycle. 25 The Cardiac Cycle and Heart sounds 26 13
The Cardiac Cycle and Heart sounds Auscultation listening to heart sound via stethoscope Four heart sounds S 1 lubb caused by the closing of the AV valves S 2 dupp caused by the closing of the semilunar valves S 3 a faint sound associated with blood flowing into the ventricles S 4 another faint sound associated with atrial contraction 27 The Cardiac Cycle Heart beats on average 75 beats per minute That is 0.8 seconds per beat Time Atria Ventricles AV-valves SL-valves 0.4 sec DiaSt DiaSt Open Closed 0.1 sec Syst Diast Open Closed 0.3 sec DiaSt Syst Closed Open 28 14
The Cardiac Cycle Amount of blood in ventricles at end of Diastole = End-Diastolic Volume ( is maximum filling) Amount of blood left in ventricles at end of Systole = End- Systolic Volume ( what remains after ejection) 29 The Cardiac Cycle and Stroke Volume Stroke Volume = Volume ejected by a ventricle per beat = End Diastolic minus End Systolic Volume SV = EDV - ESV SV = 135 ml - 65 ml = 70 ml In a healthy heart, output of the right side must equal that of the left side! Left Ventricular SV = Right Ventricular SV 30 15
The Cardiac Cycle and Stroke Volume 31 The Cardiac Cycle and Cardiac Output Cardiac Output = Blood Volume ejected by a ventricle per minute = Blood Volume that goes into circulation per minute = Blood Volume ejected per beat times beats per minute Cardiac Output = SV x HR = 75 ml x 70 beats/min = 5,250 ml /min 32 16
Cardiac Output and Energy Demand The blood only holds a certain amount of oxygen During exercise, the body and tissues need more oxygen and nutrients to generate the required ATP The only way to increase the oxygen supply is to increase the blood supply = increase cardiac output Cardiac Output = SV x HR increase Stroke Volume increase Heart Rate 33 17