Cardiovascular System: The Heart PDF

Cardiovascular System The Heart Histology of Cardiac Muscle  Chambers of the Heart  Right Atrium  Receives blood from 3 sources (superior vena cava, inferior vena cava and coronary sinus)  Tricuspid valve: blood flows through into right ventr...

Cardiovascular System The Heart Histology of Cardiac Muscle  Chambers of the Heart  Right Atrium  Receives blood from 3 sources (superior vena cava, inferior vena cava and coronary sinus)  Tricuspid valve: blood flows through into right ventricle  Right Ventricle  Pulmonary semilunar valve: blood flows into pulmonary trunk  Left Atrium  Receives blood from lungs - 4 pulmonary veins (2 right + 2 left)  Bicuspid valve (Mitral): blood passes through into left ventricle  Left Ventricle  Aortic semilunar valve: blood passes through into the ascending aorta  just above valve are the openings to the coronary arteries  Blood Circulation  Pulmonary circulation  right side of heart pumps deoxygenated blood to lungs  right ventricle pumps blood to pulmonary trunk branches into pulmonary arteries, which carry blood to lungs for exchange of gases  oxygenated blood returns to heart in pulmonary veins  Systemic circulation  left side of heart pumps blood through body  left ventricle pumps oxygenated blood into aorta which branches into many arteries into arterioles into capillaries for exchange of gases and nutrients to the tissues  deoxygenated blood begins its return in venules that merge into veins and return to right atrium  Heart cells  Cardiac muscle cells are autorhythmic cells (10%) and contractile cells (90%).  Because autorhythmic cells are self- excitable. They repeatedly generate spontaneous action potentials (pacemaker potential) that then trigger heart contractions. Conduction System of Heart  SA node fires spontaneously 90-100 times per minute  Signals from the autonomic nervous system and hormones, such as epinephrine, do modify the heartbeat (in terms of rate and strength of contraction), but they do not establish the fundamental rhythm.  100 msec delay at AV node due to smaller diameter fibers- allows atria to fully contract filling ventricles before ventricles contract Why are fibers of the conducting system autorhythmic? Action Potential in Cardiac Muscle  Depolarization, plateau, repolarization Action Potential in Cardiac Muscle  Physiology of Contraction  Refractory period  the time interval when a second contraction cannot be triggered  It is longer than the contraction itself, so heart can fill. Therefore tetanus cannot occur. Electrocardiogram (ECG or EKG)  Impulse conduction through the heart generates electrical currents that can be detected at the surface of the body.  A recording of the electrical changes that accompany each cardiac cycle (heartbeat) is called an electrocardiogram.  The ECG helps to determine if the conduction pathway is abnormal, if the heart is enlarged, and if certain regions are damaged.  Electrocardiogram (ECG or EKG)  P wave  Atrial depolarization: spread of impulse from SA node over atria  QRS complex  Ventricular depolarization: Spread of impulse through ventricles  T wave  Ventricular repolarization  The Cardiac Cycle  one complete sequence of contraction and relaxation of the heart , one cycle requires 0.8 sec.  systole (contraction) and diastole (relaxation) of both atria, plus the systole and diastole of both ventricles  End diastolic volume (EDV)  volume in ventricle at end of diastole, about 130ml  End systolic volume (ESV)  volume in ventricle at end of systole, about 60ml  Stroke volume (SV)  the volume ejected per beat from each ventricle, about 70ml  SV = EDV - ESV Phases of Cardiac Cycle  Ventricular Pressures  Blood pressure in aorta is 120mm Hg  Blood pressure in pulmonary trunk is 30mm Hg  Differences in ventricle wall thickness allows heart to push the same amount of blood with more force from the left ventricle  Stroke volume: the volume of blood ejected from each ventricle with each contraction (70ml)  Cardiac Output  Body cells need specific amounts of blood each minute to maintain health and life.  Cardiac output (CO) is the volume of blood ejected from the left ventricle (or the right ventricle) into the aorta (or pulmonary trunk) each minute.  Cardiac output equals the stroke volume multiplied by the heart rate …………………. (CO = SV X HR).  at 70ml stroke volume & 75 beat/min = 5.25 liters/min  Influences on Stroke Volume  Preload (affect of stretching)  According to the Frank-Starling law of the heart, a greater preload on cardiac muscle fibers just before they contract increases their force of contraction during systole.  Preload is proportional to EDV (more blood more force of contraction).  EDV is determined by length of ventricular diastole and venous return.  Contractility  Myocardial contractility, the strength of contraction at any given preload, is affected by positive and negative inotropic agents.  Positive inotropic agents increase contractility  Negative inotropic agents decrease contractility.  autonomic nerves, hormones, Ca+2 or K+ levels  Afterload  The pressure that must be overcome before a semilunar valves can open is the afterload  high blood pressure creates high afterload Starling's Law (length - tension relationship): With increased stretch force of contraction increases. (Increased EDV leads to increased SV) EF=SV/EDV. An increase in EF reflect an increase in contractility.  Afterload  In congestive heart failure, blood begins to remain in the ventricles increasing the preload and ultimately causing an overstretching of the heart and less forceful contraction  Left ventricular failure results in pulmonary edema  Right ventricular failure results in peripheral edema.  Influences on heart rate  Cardiac output depends on heart rate & stroke volume.  Changing heart rate is the body’s principal mechanism of short-term control over cardiac output and BP.  Several factors contribute to regulation of heart rate.  Nervous control from the cardiovascular center in the medulla oblongata  Hormones  epinephrine, norepinephrine. Stroke Volume and Heart Rate