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Lecture 17: Cardiovascular System Electrical Activity and EKG The Cardiac Action Potential Has a Prolonged Refractory Period Heart action potential has a prolonged spike (depolarized) Membrane is refractory for a long time This prevents summation and gives the heart time to fill All Parts of the Heart Beat Spontaneously Heart muscle does not require stimulation by a nerve Nerves usually inhibit the heart beat; cutting the nerves -> heart speeds up Beat originates as a depolarization in the heart muscle cell itself (self stimulation) All parts of the heart can beat spontaneously Advantage: if one part of the heart is damaged another part can still produce a beat Risk: beats originating outside of the pacemaker can produce life-threatening arrhythmias Normally the Heart Beat Originates in the SA Node The heart beat originates from the part of the heart with the fastest beat Normally this is the SA (sinoatrial) node of the right atrium The SA node is called the pacemaker Ectopic beats are those originating outside of the normal pacemaker The Atria Are Electrically Insulated From the Ventricles The upper part of the heart (the 2 atria) is insulated from the lower part Electrical excitation can pass from the atria to the ventricles only at the AV node The Heart Has Special Electrical Conducting Tissue Electrical excitation is passed through special conducting tissue from the AV node to the ventricles Route: bundle of His -> bundle branches -> Purkinje fibers 1 Excitation is Delayed in the AV Node The excitation starts in the SA node and spreads across the atria When the excitation reaches the AV node there is a delay of about 0.1 seconds before it passes into the bundle of His The delay allows the atria to contract before the ventricles are stimulated This results in better filling of the ventricles From the AV node impulses enter the Bundle of His and then travel along the right and left bundle branches in the septum between the right and left ventricles In the ventricles the impulse spreads through the Purkinje fibers Gap Junctions in the Intercalated Discs Spread Electrical Excitation Between Heart Cells Gap junctions are low resistance connections between 2 cells When the impulse reaches cardiac muscle cells it is rapidly passed from one muscle cell to the next because of gap junctions in the intercalated discs Excitation of the Heart Can be Followed From the Body Surface With the EKG EKGs are not measured across the membranes of cardiac cells EKGs are measured by electrodes attached to the skin of the body surface These electrodes record the average activity of millions of heart cells EKG voltages are very small because the electrodes are far from the heart and most of the electrical activity cancels out The most common set of connections is with electrodes connected to both arms and the left leg (Einthoven's triangle). This produces the 3 limb leads (I, II, III). Alternate connections give 3 more limb leads (AVL, AVR, AVF)- the 6 limb leads give electrical views of the heart in the frontal plane 2 6 chest leads are also used (V1, V2, V3, V4, V5, V6)- these give views of the heart in a transverse plane A Typical EKG Record Contains P, QRS and T Waves The P wave is caused by depolarization (excitation) of the atria The QRS is produced by depolarization (excitation) of the ventricles The T wave represents repolarization (recovery) of the ventricles The small atrial recovery wave cannot be seen- it is swamped out by the large QRS wave Different electrodes give different views of these waves- the wave below is typical for lead II (lead II has the same direction as the axis of the normal heart) The EKG Gives Information on Heart Rate, Rhythm, Orientation and Pathology 3 If 2 QRS waves are close together the heart is beating at a fast rate; if they are far apart the heart has a slow rate If the heart is functioning properly each P wave is followed by a QRS wave If electrical conduction between the atria and ventricles is partially or completely blocked there will be a disturbance of the heart rhythm- the atria and ventricles may beat independently of each other The orientation of the heart can be determined from the sizes of the waves coming from different electrodes Damage to the heart caused by poor coronary circulation can cause the waves to widen (slower conduction). There are also other changes in wave shape, such as depression of the ST segment The systemic circulation provides the functional blood supply to all body tissue. It carries oxygen and nutrients to the cells and picks up carbon dioxide and waste products. Systemic circulation carries oxygenated blood from the left ventricle, through the arteries, to the capillaries in the tissues of the body. From the tissue capillaries, the deoxygenated blood returns through a system of veins to the right atrium of the heart. The coronary arteries are the only vessels that branch from the ascending aorta. The brachiocephalic, left common carotid, and left subclavian arteries branch from the aortic arch. Blood supply for the brain is provided by the internal carotid and vertebral arteries. The subclavian arteries provide the blood supply for the upper extremity. The celiac, superior mesenteric, suprarenal, renal, gonadal, and inferior mesenteric arteries branch from the abdominal aorta to supply the abdominal viscera. Lumbar arteries provide blood for the muscles and spinal cord. Branches of the external iliac artery provide the blood supply for the lower extremity. The internal iliac artery supplies the pelvic viscera. Major Systemic Arteries 4 All systemic arteries are branches, either directly or indirectly, from the aorta. The aorta ascends from the left ventricle, curves posteriorly and to the left, then descends through the thorax and abdomen. This geography divides the aorta into three portions: ascending aorta, arotic arch, and descending aorta. The descending aorta is further subdivided into the thoracic arota and abdominal aorta. Major Systemic Veins After blood delivers oxygen to the tissues and picks up carbon dioxide, it returns to the heart through a system of veins. The capillaries, where the gaseous exchange occurs, merge into venules and these converge to form larger and larger veins until the blood reaches either the superior vena cava or inferior vena cava, which drain into the right atrium. Fetal Circulation Most circulatory pathways in a fetus are like those in the adult but there are some notable differences because the lungs, the gastrointestinal tract, and the kidneys are not functioning before birth. The fetus obtains its oxygen and nutrients from the mother and also depends on maternal circulation to carry away the carbon dioxide and waste products. The umbilical cord contains two umbilical arteries to carry fetal blood to the placenta and one umbilical vein to carry oxygen-and-nutrient-rich blood from the placenta to the fetus. The ductus venosus allows blood to bypass the immature liver in fetal circulation. The foramen ovale and ductus arteriosus are modifications that permit blood to bypass the lungs in fetal circulation. 5