Physiology Year 2 - The Cardiovascular System PDF

Physiology , Year 2 1 THE CARDIOVASCULAR SYSTEM Department: Basic Sciences College of Dentistry...

Physiology , Year 2 1 THE CARDIOVASCULAR SYSTEM Department: Basic Sciences College of Dentistry University of Basrah By Assist. Prof. Dr.Areej H.S. Aldhaher Heart Anatomy DIVISIONS OF CIRCULATION Properties of Cardiac Muscle Action Potential of cardiac muscles Cardiac Cycle Heart Sound 2  The function of the cardiovascular system is to deliver oxygen and nutrients and to remove carbon dioxide and other waste products 3 Heart Anatomy v Approximately the size of your fist v Heart is a muscular organ that pumps blood throughout the circulatory system Superior surface of diaphragm Left of the midline Anterior to the vertebral column, posterior to the sternum 4 5 PERICARDIUM Pericardium is the outer covering of the heart. It is made up of two layers: Outer parietal pericardium which forms a strong protective sac around the heart Inner visceral pericardium or epicardium that covers myocardium. These two layers are separated by a space called pericardial cavity which contains a thin film of fluid. The pericardium: Protects and anchors the heart Prevents overfilling of the heart with blood Allows the heart to work in a relatively friction-free environment 6 MYOCARDIUM Myocardium is the middle layer of the wall of the heart and it is formed by cardiac muscle fibers. It forms the bulk of the heart and it is responsible for the pumping action of the heart. Myocardium is formed by three types of cardiac muscle fibers: v Muscle fibers which form the contractile unit of the heart v Muscle fibers which form pacemaker v Muscle fibers which form the conductive system.. 7  1. Muscle Fibers which Form the Contractile Unit of the Heart Cardiac muscle is striated, short, fat, branched, and interconnected The myofibrils are embedded in the sarcoplasm. The sarcomere of the cardiac muscle has muscle proteins namely, actin, myosin, troponin and tropomyosin. The important difference between skeletal muscle and cardiac muscle is that the cardiac muscle fiber is branched and the skeletal muscle is not branched Intercalated disk Intercalated disk is a tough double membranous structure situated at the junction between the branches of neighboring cardiac muscle fibers. The intercalated disks form adheres junctions which play an important role in contraction of the muscle as a Microscopic Anatomy of Heart Muscle single unit. The connective tissue endomysium ac tendon and insertion **Intercalated discs anchor cardiac cells together and Have striations (sarcomere organization allow free passage of ions 8 At each intercalated disc 1- gap junction The cell membranes fuse with one another in such a way that they form permeable “communicating” junctions (gap junctions).That allow almost totally free diffusion of ions. Therefore, from a functional point of view, ions move with ease , so that action Potentials travel easily from one cardiac muscle cell to the next. when one of these cells becomes excited, the action potential spreads from cell to cell Thus, cardiac muscle is said to act like syncytium and contract as a mass 2. desmosome help to hold cells together v Heart muscle behaves as a functional syncytium The syncytium in human heart has two portions, atrial syncytium and ventricular syncytium which are connected by atrioventricular ring. 9 Some of the muscle fibers of the heart are modified into a specialized structure known as pacemaker. The muscle fibers forming the pacemaker have less striation. Pacemaker is structure in the heart that generates the impulses for heart beat. It is formed by the pacemaker cells called P cells. Sinoatrial (SA) node forms the pacemaker in human heart a. S.A. node: node is composed of a group of specialized cardiac muscle cells (have almost no contractile muscle filaments Instead they are the cells that gained a property to generate spontaneous action potentials. Located at the junction of the superior vena cava with the right‫و‬atrium.,& attached to 3 bundles which connect S.A. node to A.V. node (internodal atrial fibers). A.V. node : Atrioventricular Located in the right posterior portion of the interatrial septum. 10 3-Muscle Fibers which Form the Conductive System The conductive system of the heart is formed by the modified cardiac muscle fibers. The impulses from SA node are transmitted to the atria directly. However, the impulses are transmitted to the ventricles, through various components of conducting system ex, atrioventricular bundle (bundle of His spread the action potential Bundle of His: Gives of a left bundle branch at the top of the interventricular septum & continue as the right bundle branch. d. Purkinje fibers : spread to all parts of the ventricle 11 12 RIGHT SIDE OF THE HEART Right side of the heart has two chambers, the upper right atrium and lower right ventricle. Right atrium is a thin walled and low pressure chamber. It has got the pacemaker known as sinoatrial node that produces cardiac impulses and atrioventricular node that conducts the impulses to the ventricles. It receives venous (deoxygenated) blood via two large veins: 1. Superior vena cava that returns the venous blood from the head, neck and upper limbs 2. Inferior vena cava that returns the venous blood from lower parts of the body (Fig. 1). Right atrium communicates with the right ventricle through the tricuspid valve. Venous blood from the right atrium enters the right ventricle through this valve. From the right ventricle, pulmonary artery arises. It carries the venous blood from right ventricle to the lungs. In the lungs, the deoxygenated blood is oxygenated. 13 LEFT SIDE OF THE HEART Left side of the heart has two chambers, the upper left atrium and lower left ventricle. Left atrium is a thin walled and low pressure chamber. It receives oxygenated blood from the lungs through pulmonary veins. This is the only exception in the body where an artery carries venous blood and vein carries the arterial blood. Blood from left atrium enters the left ventricle through the mitral valve (bicuspid valve). Wall of the left ventricle is very thick. Left ventricle pumps the arterial blood to different parts of the body through systemic aorta. SEPTA OF THE HEART Right and left atria of the heart are separated from one another by interatrial septum. The ventricles are separated from one another by interventricular septum. 14 15  Heart valves ensure unidirectional blood flow through the heart v Atrioventricular (AV) valves lie between the atria and the ventricles (2AV: Left AV and Right AV) Left atrioventricular valve is otherwise known as mitral valve or bicuspid valve. Right atrioventricular valve is known as tricuspid valve AV valves prevent backflow into the atria when ventricles contract v Aortic semilunar valve lies between the left ventricle and the aorta v Pulmonary semilunar valve lies between the right ventricle and pulmonary trunk Semilunar valves prevent backflow of blood into the ventricles 16 17 18  Major Vessels of the Heart v Vessels returning blood to the heart include: v Superior and inferior venae cavae v Right and left pulmonary veins v Vessels conveying blood away from the heart include: v Pulmonary trunk, which splits into right and left pulmonary arteries v Ascending aorta (three branches) – brachiocephalic, left common carotid, and subclavian arteries Pathway of Blood Through the Heart and Lungs Right atrium tricuspid valve right ventricle Right ventricle pulmonary semilunar valve pulmonary arteries lungs Lungs pulmonary veins left atrium Left atrium bicuspid valve left ventricle Left ventricle aortic semilunar valve aorta Aorta systemic circulation 19 Blood flows through two divisions of circulating system: 1. Systemic circulation 2. Pulmonary circulation. SYSTEMIC CIRCULATION It is otherwise known as greater circulation The blood pumped from left ventricle passes through a series of blood vessels of arterial system and reaches the tissues. Exchange of various substances between blood and the tissues takes place in the capillaries. After the exchange of substances in the capillaries, the blood enters the venous system and returns to right atrium and then the right ventricles. PULMONARY CIRCULATION It is otherwise called lesser circulation. Blood is pumped from right ventricle to lungs through pulmonary artery. The exchange of gases occurs between blood and alveoli of the lungs through pulmonary capillary membrane. The oxygenated blood returns to left atrium through the pulmonary veins. Thus, the left side of the heart contains oxygenated or arterial blood and the right side of the heart contains the venous blood 20 21 1- RHYTHMICITY Rhythmicity is the ability of a tissue to produce its own impulses regularly. It is more appropriately named as auto rhythmicity. It is also called self-excitation. heart has a specialized excitatory structure from which the discharge of impulses is rapid. This specialized structure is called pacemaker. From this, the impulses spread to other parts through the specialized conductive system. v Chambers can generate A.P. only when they are injured i.e normal cardiac muscle do not discharge. Automaticity & rhythmicity S.A node ,A.V node& other parts of the conductive system have capability of spontaneous genesis of A.P. & in a rhythmic manner 2- CONDUCTIVITY Human heart has a specialized conductive system through which the impulses from SA node are transmitted to all other parts of the heart The conductive system: a. S.A. node: b. A.V. node : c. Bundle of His: 22 d. d. Purkinje fibers : v The impulses from SA node are conducted throughout right and left atria. The impulses reach the AV node via some specialized fibers called internodal fibers. There are three types of internodal fibers: 1. Anterior inter nodal fibers of Bachman 2. Middle inter nodal fibers of Wenckebach 3. Posterior inter nodal fibers of Thorel v Impulse passes from atria to ventricles via the atrioventricular bundle (bundle of His AV bundle splits into two pathways in the interventricular septum (bundle branches) Bundle branches carry the impulse toward the apex of the heart Purkinje fibers carry the impulse to the heart apex and ventricular walls 23 3- EXCITABILITY Excitability is defined as the ability of a living tissue to give response to a stimulus. In all the tissues, the initial response to a stimulus is the electrical activity in the form of action potential. It is followed by mechanical activity in the form of contraction, secretion, etc. The action potential is transmitted from atria to ventricles through the fibers of specialized conductive system. Sequence of Excitation Sinoatrial (SA) node generates impulses about 75 times/minute Atrioventricular (AV) node delays the impulse approximately 0.1 second 24 ELECTRICAL CHANGES DURING MUSCULAR CONTRACTION When the muscle is in resting condition, the electrical potential is called resting membrane potential. When the muscle is stimulated, electrical changes occur which are collectively called action potential. RESTING MEMBRANE POTENTIAL Resting membrane potential is the electrical potential difference (voltage) across the cell membrane (between inside and outside of the cell) under resting condition. Resting muscle shows negativity inside and positivity outside. The condition of the muscle during resting membrane potential is called polarized state. ACTION POTENTIAL Action potential is defined as a series of electrical changes that occur when the muscle or nerve is stimulated. Action potential occurs in two phases: 1. Depolarization 2. Repolarization. Depolarization Depolarization is the initial phase of action potential in which the inside of the muscle becomes positive and outside becomes negative. That is, the polarized state (resting membrane potential) is abolished resulting in depolarization. Repolarization Repolarization is the phase of action potential when the potential inside the muscle reverses back to the resting membrane potential. That is, within a short time after depolarization the interior of muscle becomes negative and outside becomes positive. So, the polarized state of the muscle is re-established. 25 The resting membrane potential in: Single cardiac muscle fiber : – 85 to – 95 mV SA node: – 55 to – 60 mV Purkinje fibers : – 90 to –100 mV. Action potential in a single cardiac muscle fiber occurs in 4 phases: 1. Initial depolarization 2. Initial repolarization 3. A plateau – final depolarization 4. Final repolarization. Approximate duration of action potential in cardiac muscle is 250 to 350 msec. 26  Ventricular electrical activity: The R.M.P. =-90 mv. A. stage 0 (rapid depolarization) Initial Depolarization : Rapid & sharp ,because of opening of fast Na+ channels , which are voltage gated Na+ch. The M.P. changed to + 20 mv. it lasts for about 2 msec. B. Stage 1(Initial Repolarization Immediately after depolarization, there is an initial rapid repolarization for a short period of about 2 msec. Started from +20mv. It occurs due to opening of special K+ channels called transiently opened K+ →K+efflus. Simultaneously, the fast sodium channels close suddenly and slow sodium channels open resulting in slow influx of a low quantity of sodium ions C. Stage 2 Plateau phase– Final Depolarization Afterwards, the muscle fiber remains in the depolarized state for some time before further repolarization. It forms the plateau (stable period) in the action potential curve. 27 The plateau lasts for about 200 msec (0.2 sec) in atrial muscle fibers and for about 300 msec (0.3 sec) in ventricular muscle fibers. Due to the long plateau in action potential, the contraction time is longer in cardiac muscle by about 5 to 15 times than in skeletal muscle. very slowly due to opening of Ca++ channels (usually slowly & prolonged open channels)→Ca++ influx. Counterbalanced by delayed rectifier K+. channels →Efflux of K+. D. Stage 3 Final Repolarization (Late rapid repolarization): Final repolarization occurs after the plateau. Which takes the M.P. To -90 mv. ,due to Opening of multiple types of K+channels.(efflux of K).It is a slow process and it lasts for about 50 to 80 msec (0.05 to 0.08 sec) before the re- establishment of resting membrane potential. **stage 4: The R.M.P. 28 “f ” channels are more permeable to Na + than K + and there is high sodium ion concentration in the extracellular fluid outside the nodal fiber, positive sodium ions normally tend to leak to the inside between heartbeats through the “f ” channel , influx of positively charged sodium ions causes a slow rise in the resting membrane potential in the positive direction The prepotential is completed by the calcium current (I Ca ) due to the opening of T Ca channels. There are two types of Ca channels in the heart, the T (for transient) channels and the L (for long-lasting) channels When the potential reaches a threshold voltage of about −40 millivolts, the L-type calcium channels become “activated,” thus causing the action potential. So the action potentials in the SA and AV nodes are largely due to Ca 2+ , with no contribution by Na + influx. At the peak of each impulse, K channels opened and cause repolarization. 29 SPREAD OF ACTION POTENTIAL THROUGH CARDIAC MUSCLE The action potential spreads through the cardiac muscle very rapidly. It is because of the presence of gap junctions between the cardiac muscle fibers. The gap junctions are permeable junctions and allow free movement of ions. Due to this, the action potential spreads rapidly from one muscle fiber to another fiber. The action potential is transmitted from atria to ventricles through the fibers of specialized conductive system. 30 4- CONTRACTILITY Contractility is ability of the tissue to shorten in length (contraction) after receiving a stimulus. Various factors affect the contractile properties of the cardiac muscle. The contractile properties are: 1. ALL OR NONE LAW According to all or none law, when a stimulus is applied, whatever may be the strength, the whole cardiac muscle gives maximum response or it does not give any response at all. Below the threshold level, i.e. if the strength of stimulus is not adequate, the muscle does not give response. Cause for All or None Law All or none law is applicable to whole cardiac muscle. It is because of syncytial arrangement of cardiac muscle. In the case of skeletal muscle, all or none law is applicable only to a single muscle fiber. 2. SUMMATION OF SUBLIMINAL STIMULI When a stimulus with a subliminal strength is applied, the heart does not show any response. When few stimuli with same subliminal strength are applied in succession, the heart shows response by contraction. It is due to the summation of the stimuli 31  3. REFRACTORY PERIOD Refractory period is the period in which the muscle does not show any response to a stimulus. It is of two types Absolute Refractory Period It is the period during which the muscle does not show any response at all, whatever may be the strength of the stimulus. It is because, the depolarization occurs during this period. So a second depolarization is not possible. onset of phase 0 begin the absolute refractory period, and extends midway through phase 3 Relative Refractory Period It is the period during which the muscle shows response if the strength of stimulus is increased to maximum. It is the stage at which the muscle is in repolarizing state. occurs in the 2nd half of phase 3 Refractory Period in Cardiac Muscle Cardiac muscle has a long refractory period compared to that of skeletal muscle. The absolute refractory period extends throughout contraction period of cardiac muscle. It is for 0.27 sec and relative refractory period extends during first half of relaxation period which is about 0.26 sec. So, the total refractory period is 0.53 sec. Significance of Long Refractory Period in Cardiac Muscle Long refractory period in cardiac muscle has three advantages: 1. Summation of contractions does not occur 2. Fatigue does not occur 3. Tetanus does not occur. Most of the contraction is within the A.R. P.& this is very important to prevent tetanization of the cardiac muscle which can occur only if the stimulus is given during contraction. So it is impossible to produce tetanization of the heart ,& this is very safety factor to the heart 32 33 34 35  2- Ventricular Systole Ø A-Isovolumetric contraction ventricular contraction (Phase 2 on diagram) §Ventricles start to contract (systole) §Rising ventricular pressure > atrial pressure à AV valves close (1st heart sound – “lubb”) §Ventricular pressure not great enough to open semilunar valve( ventricular pressure < pulmonary + aortic arterial pressures) §So, no blood flowing=isovolumetric-same volume v When the Pr. In the left ventricle exceeds the Pr. Of the aorta (80 mmHg ) and the Pr. Of the right ventricle exceeds the Pr. of pulmonary artery (10 mmHg) Ejection phase start 36  B- Rapid ejection Ventricular Ejection (Phase 3 on diagram) Ventricles continue contracting Ventricular pressure > aortic pressure. The Pr. in the aorta =120 mmHg and Pr. in the pulmonary artery= 25 mmHg The blood will be ejected to the arteries. Ø C-Slow ejection 37  3- Ventricular Diastole A- Isovolumetric Relaxation (Phase 4 on diagram) Eventually ventricular pressure < aortic pressureà semilunar valves close—start of diastole All valves are closed so some blood is still in ventricles as they relax Ventricular pressure < atrial pressure so AV valves open and ventricle passively fill with blood until they contract again 38 diastasis: as blood flows from atria in smaller volume 39 40 Normal Volume of Blood in Ventricles Atrial systole pushes final 20-25 ml blood (20%) After atrial contraction, 110-120 ml in each ventricle (end- diastolic volume) Contraction ejects ~70 ml (stroke volume output) Thus, 40-50 ml remain in each ventricle (End- systolic volume) The fraction ejected is then ~60% (ejection fraction) 41 42