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Transcript
Cardiovascular Physiology Dr. Faten zakareia Associate. Prof. Physiology KSU The cardiovascular system consists of 1-The heart which acts as a pressure pump.The heart is a muscular organ enclosed in a fibrous sac, the pericardium, and located in the chest The wall of the heart is composed of cardiac muscle cells “myocardium” The heart is divided to 2 halves each has one atrium and one ventricle 2- a group of blood vessels (arteries, arterioles, capillaries, venules, and veins) all contain blood which is ever circulating throughout life. Valves of the heart : There are 4 sets of valves 1- Tricusped valve (between right atrium & right ventricle). 2- Mitral valve (between left atrium & left ventricle). Tricuspid and mitral valves are called Atrioventricular valves ( A-V valves). *They prevent backflow of blood from ventricle to atria during ventricular systole. They close when ventricular pressure > atrial pressure and produce 1st heart sound. *They open when the atrial pressure > ventricular pressure. 3- Pulmonary valve (between right ventricle & pulmonary artery). 4- Aortic valve (between left ventricle & Aorta). Pulmonary and aortic valves are called semilunar valves. They prevent backflow of blood from aortic & pulmonary vessels into ventricles during diastole. They close when pressure inside Aortic and pulmonary vessels is > ventricular pressure and produce the 2nd heart sound. They open when ventricular pressure > Aortic & pulmonary pressure Cardiac properties The properties of cardiac muscle are: *Excitability *Rhythmicity *Conductivity *Contractility Excitability It is the ability of cardiac muscle to respond to a threshold stimulus by developing an action potential followed by contraction(or the amount of inward current required to bring a myocardium to the threshold potential) It has 2 phases absolute refractory period ( during the depolarization- systole) relative refractory period ( repolarization- Refractory Periods: - Absolute refractory period (APR): For most of the duration of AP, the myocardium refractory to fire another action potential -It includes: upstroke, plateau and a portion of repolarization (to -50mv) Cardiac Action Potential The Spreading of Action Potential SA node is the pacemaker of the heart where the AP is initiated Atrial internodal tracts and atria helps distributing of AP to atria AV node has the slowest conduction velocity Bundle of His and purkinje system distribute the action potential to the ventricles very fast to perform an efficient contraction Rhythmicity It is the ability of cardiac muscle to contract regularly independent of nerve supply ( myogenic). It is inherent spontaneous rhythmcity. 1-Sino-atrial node ( S-A node): It posses the greatest rhythmicity & initiates the beat of heart. It is the main pace maker of the heart. It lies in posterior wall of right atrium near opening of superior vena cava (S.V.C). it develops action potential without stimulation. Origin of Cardiac Action Potential & its Spreading 2-Atrial internodal tracts help distributing of AP to atria. 3-AV node has the slowest conduction velocity where delay of the impulse occur (why?). 4-Bundle of His and purkinje system distribute the action potential to the ventricles very fast to perform an efficient contraction Regulation of Pacemaker Activity ) regulation of heart rate) The SA node displays intrinsic automaticity (spontaneous pacemaker activity) at a rate of 100-110 action potentials ("beats") per minute. This intrinsic rhythm is primarily influenced by autonomic nerves, A- vagal influences being dominant over sympathetic influences at rest. This "vagal tone" reduces the resting heart rate down to 60-80 beats/min. The rate of SA nodal firing can be altered by changes in autonomic nerve activity (sympathetic and vagal). circulating hormones (thyroxine, adrenaline) serum ion concentrations cellular hypoxia drugs Rhythmicity in different cardiac fibers: S-A node is the normal pace maker of heart i.e. it initiates the excitation wave that drives whole heart and makes the pace (speed) of heart. . S-A node generates the highest frequency of impulse and discharge spontaneously at a rate of 75 impulse/min(due to vagal tone) If the S-A node is damaged, A-V node becomes the new pace maker and heart follow it but at a slower rate ( 40 impulse/min).( A-V nodal rhythm). Cont.. If S-A node & A-V node are damaged, or if their conduction is blocked, A-V bundle & purkinje fibers become the pacemaker with a rhythm of 25 impulse /mn ( idioventricular rhythm). Any pace maker other than the S-A node is called an ectopic focus. Rhythmicity is high in S-N node> A-V node> purkinje fibers & ventricles. Conductivity It is the ability of the cardiac muscle fiber to conduct cardiac impulses that are initiated in the S-A node & transmitted in a specialized conducting system formed from specialized muscle fibers The conducting system of the heart is composed of S-A node, internodal pathway, A-V node, bundles of Hiss& purkinje fibers. I. S-N node It is located at the posterior wall of right atrium near opening of the SVC. It consists of modified cardiac muscle fibers. It is normal pacemaker. II. The internodal pathway. -Action potential travels from S-A node through atrial muscle to A-V node. -Action potential can travel through small bundles of atrial muscle fibers - internodal pathway- (anterior, middle, posterior) at higher velocity. III. A-V node & bundle of Hiss -A-V node is situated in right atrium at posterior part of interatrial septum. -Receives impulses from S-A node & transmits it to ventricles through A-V bundle. -At the A-v node, impulse is delayed briefly to give the atria time to finish contraction before ventricular contraction starts. IV- Right & left bundle branches. They pass along sides of interventricular septum to reach apex, then reflect to ventricular wall. V.Purkinje fibers The fibers penetrate about 1/3 of muscle wall & terminate on muscle fibers of the ventricle.& When impulses reaches end of purkinje fibers they are transmitted through ventricular muscle fibers. This allows both ventricles to contract at the same time for their effective pumping (synchronous contraction). Contractility It is the ability to convert chemical energy into mechanical form of energy. Contraction is called systole and relaxation is called diastole. Factors affecting myocardial contractility Are called to have inotropic effect. That increase contractility +ve inotropic factors That decrease myocardial contractility –ve inotropic factors. These as follow -Starling Law of the heart. -Cardiac innervations ( sympathetic , parasympathetic. Cont.. Factors affecting contractility Hormonal and chemical factors ( adren. ,NE, , alkali , digitalis, ca++ increase. While Ach, acids, ether, bacterial toxins, chloroform decrease. Physical factors ( warming increase, cooling decrease). Oxygen supply ( hypoxia decrease.).sympathetic(decrease) ¶symparhetic(increase) Cardiac Cycle Is the mechanical and electrical events occurring from the beginning of one heart beat to the next one Consist of a period of - Relaxation “diastole” - Contraction “systole” Normal Cardiac cycle time: 60 / heart rate = 60/75 = 0.8 sec The cardiac cycle Each cycle is initiated by depolarization of S-A node, followed by contraction of atria. Then atria starts to relax, the signal is transmitted to ventricles through A-V node & hiss bundle to cause ventricular contraction. Cardiac cycle time *This is time required for one complete cardiac cycle. When heart rate is 75 beats/min, the time will be 0.8 sec. *The time is inversely proportional with the heart rate. - Cardiac cycle starts by systole of both atria (0.1 sec), followed by systole of both ventricles (0.3 sec), then diastole of whole heart (0.4 sec). N.B Atrial and ventricular systole do not occur at same time, but their relaxation occurs at the same time during diastole of the whole heart which lasts foe 0.4 Sec. Events occurring during the cardiac cycle are 1-Mid to late diastole: -Here the heart is in complete relaxation. Blood is accumulating in the atria. -Pressure in the atria becomes higher than pressure in the ventricles. - Blood flow passively from atria to ventricles through A-V valves which are open. -Semilunar valves are closed. -The atria contract and force the blood remaining in their chambers into the ventricles. 2- Ventricular systole -The ventricles contract and the pressure within them increase rapidly, closing the A-V valves. -When intraventricular pressure becomes higher than aortic and pulmonary pressure, the semilunar valves are forced to open. And blood rushes through them out of ventricles. - During ventricular systole, the atria are relaxed and their chambers are again filled with blood. 3- Early Diastole. -At the end of systole, the ventricles relax the intraventricular pressure drop. - The semilunar valves close to prevent backflow of blood, and for a moment the ventricles are completely closed chambers (A-V and semilunar valves are closed). -When the interventricular pressure drops below atrial pressure (which has been increasing as blood is filling their chambers), the A-V valves are forced open and the ventricles again begin refill rapidly with completing the cycle. Heart sounds Using a stethoscope, 2 heart sounds can be heard during each cardiac cycle. First heart sound: Longer and louder than the second heart sound, like the word ( LUP). And is caused by closure of the A-V valves at the beginning of ventricular systole. Second heart sound: Short and sharp like the word (DUP) and is caused by closure of semilunar valve at the beginning of ventricular diastole. The sequence is like that Lup-dup, pause, lup –dup, pause and so on. Cardiac output Stroke volume:- It is the volume of blood pumped by each ventricle per bear( each cardiac cycle). It= 70 ml. Cardiac output (CO): It is the volume of blood pumped by each ventricle per minute It = HR x Stroke volume = 75 beats/min x 70 ml ≈5L/min. This volume will vary with the size of individual. *Women have smaller CO than men. *Children have smaller CO than adults. Since the normal adult blood volume is 5L, the entire blood supply passing through the body each minute is 5L. Factors affecting cardiac output There are 2 major factors which determine CO. These are The stroke volume and The heart rate Regulation of stroke volume According to starling's law, the more the cardiac muscle fibers are stretched the stronger will be the contraction. Provided that the fibers are not excessively stretched. *The important factor stretching the cardiac muscle is the venous return, the amount of blood entering the heart are distending the ventricles. C.O.P *When increased quantities of blood flow into right atrium as during exercise, this stretches the walls of heart chambers. As a result, cardiac muscle contracts with increased force and extra blood flowing into heart is automatically pumped into aorta and flows again through systemic circulation. *On the other hand, low venous return such as result from severs blood loss or extremely rapid heart rate, decreases stroke volume causing the heart to contract less forcefully. Regulation of heart rate Physical factors Age, sex, body temperature and exercise influence heart rate Resting HR is faster in fetus and then gradually decreases throughout life. HR is faster in females (72-80 beats/min) than in males (64-72 beats/min). Heat increases HR as occur in high fever. Cold has the opposite effect. Exercise acts through sympathetic nervous system to increase HR. Autonomic nervous system *Sympathetic nerves stimulate SA node and cardiac muscle itself and increase HR as occur during stress. * Parasympathetic nerves (vagus nerve) slow HR. 3-Hormones and drugs Adrenaline and thyroxin increase HR Increased calcium level in blood causes prolonged contraction that the heart stops entirely. Reduced calcium level in blood depresses the heart. Excess or reduced sodium and potassium modify heart activity. Arterial blood pressure (ABP) Blood pressure is the pressure the blood exerts against the inner walls of the blood vessels, and is the force that keeps blood circulating continuously even between heart beats. Normal values of blood pressure: In normal adults at rest, blood pressure rises and falls during each beat. Systolic BP is the pressure in the arteries at the peak of ventricular contraction. It varies between 110-140 mmHg. The diastolic BP is the pressure in the arteries when ventricles relax. It varies between 75-80 mmHg. Blood pressure 120/80 refers to systolic pressure of 120 mmHg and diastolic pressure of 80 mmHg. It can be measured by auscultatory method using the stethoscope. Variations in blood pressure Blood pressure varies with age, weight, race, mood, physical activity, posture. The venous return depends on:1-blood pressure gradient The blood flows from the ventricle into large, thick-walled elastic arteries expand as blood is pushed into them, then blood flows into smaller arteries, arterioles, capillaries, venules, veins and finally back to the large vena cava entering the right heart. The pressure is highest in large arteries and continuously drop throughout the pathway, reaching zero or –ve pressure at venae cava High pressure in the arteries force the blood to continually move into where pressure is low (blood flows along a pressure gradient from high to low pressure). Continual blood flow depends on the stretching of a large arteries and their ability to recoil and keep the pressure on the blood as it flow in circulation. 2-The presence of valves in the large veins 3- The skeletal muscle pump activity that squeeze the blood vessels between muscle fibers. 4- Pressure changes in the thorax 5- effect of gravity. Arterial blood pressure= cardiac output x peripheral resistance. So, ABP is directly related to cardiac output and peripheral resistance. Peripheral resistance It is the amount of friction encountered by the blood as it flows through blood vessels especially the arterioles. It depends on:Diameter of arterioles, their narrowing by sympathetic stimulation and atheroscelerosis increases PR. Blood volume. Increase blood volume will increase PR. Blood viscosity. Increase blood viscosity also raises PR. Factors affecting blood pressure Neural factors (autonomic nervous system) Parasympathetic system has little or no effect on blood pressure. Sympathetic system is important and its major action is to cause vasoconstriction 9 narrowing of blood vessels) which increases blood pressure. Examples: During standing up suddenly after lying down, the effect of gravity cause blood to pool in the vessels of legs and blood pressure drops. -This activates the baroreceptors in large arteries of the neck and chest. -They send off signals that result in reflex vasoconstriction which increases blood pressure to normal this is called (Baroreceptor reflex). B- When blood volume suddenly decreases as in hemorrhage, blood pressure drops and the heart begins to beat rapidly. But because the venous return reduced by blood loss, the heart beats weakly. In such cases, sympathetic nerves cause vasoconstriction to increase blood pressure so that venous return increase. c- During vigorous exercise, there is generalized vasoconstriction except blood vessels of skeletal muscles, heart and brain. 2- Renal factors The kidneys play a major role in regulating ABP by changing blood volume. *As blood pressure or volume increases above normal, the kidney allows more water to leave the body in urine. This decrease blood volume which in turn decreases blood pressure to normal. When blood pressure falls, the kidneys retain body water, increasing blood volume and blood pressure rises. In addition when ABP falls the kidney release renin enzyme that helps the formation of angiotensin II, a potent vasoconstrictor. Angiotensin also stimulates the release of aldosterone hormone that enhances sodium ion reabsorption by the kidney into blood. As sodium moves into blood, water follows. Thus both volume and pressure rises. Temperature Cold has a vasoconstricting effect. Heat has a vasodilator effect. Chemicals Adrenaline increases heart rate and blood pressure. Nicotine increases blood pressure by causing vasoconstriction. Alcohol and histamine cause vasodilatation and decreases blood pressure. 4- Diet A diet low is salts, saturated fats and cholesterol help to prevent hypertension. **A brief elevation in blood pressure is a normal response to fever, physical exertion and emotions upset such as fear and anger. Persistent hypertension, or high blood pressure, is pathological, and defined as a condition of sustained elevated arterial blood pressure of 140/90 or higher. Electrocardiogram ECG (EKG) Fig 14 20 The electrocardiogram (ECG) It is the record of action potential generated by myocardial fibers during cardiac cycle. It can be recorded by placing electrodes on body surface on opposite sides of the heart. The principle: -Body fluids are good conductors, so that when cardiac impulses pass through the heart, electrical currents spread into tissues surrounding the heart & to surface of the body. -If electrodes are placed on body surface on opposite sides of heart we can record the potential differences, the record is ECG. Normal ECG It shows at least five different waves, 3 above the isoelectric line (positive waves) & 2 are below it (negative). These waves are called P, Q, R, S, and T. Analysis of normal ECG The P-Wave -It represents atrial depolarization. It starts 0.02 Sec before atrial contraction. The QRS complex It represents the process of ventricular depolarization. It includes 3 waves Q-wave: It starts 0.02 sec before mechanical response of ventricles (systole). It is caused by depolarization of ventricular septum. R- wave: It is the most constant wave having the tallest amplitude. It represents depolarization of main ventricular muscles. S- wave: It represents depolarization of base of ventricles. T- wave: It represents process of ventricular repolarization. N.B: The manifestations of atrial repolarization are not normally seen because they are obscured by the more powerful QRS complex.