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The Cardiovascular System Overview The system consists of two separate loops The pulmonary loop is fed by the right heart and is the site of exchange of respiratory gases with the atmosphere. The systemic loop is fed by the left heart and serves blood to all the rest of the body, including the heart tissue itself. Systemic capillaries are the sites of exchange of respiratory gases, nutrients and wastes with the tissues. Obviously, the volume flow of blood through one loop must be equal to that of the other, as averaged over times longer than a few heartbeats. Systemic circulatory beds are arranged in parallel Arterial blood flows through only one set of capillaries before entering the venous side of the system - with a few exceptions: these exceptions involve what are called portal circulations. The two main ones that you will meet with in the postnatal circulation are the hepatic portal system that carries blood from the intestine to the liver, and the hypothalamic-hypophyseal portal system that carries blood from the hypothalamus of the brain to the anterior pituitary. These will be addressed in detail later. Some suggestions for how to think about this system • Think of the systemic arteries and systemic veins as two tanks. We will speak of them as ‘the venous side’ and ‘the arterial side’. The arterial side holds blood under high pressure – the venous side is a low-pressure system. • Arterioles are the sites of highest flow resistance in the system, so individually they determine the rate of blood flow to individual tissues and together they determine how rapidly blood ‘runs off’ from the arterial side to the venous side. • The arterial tank is small. Arteries are narrow and stiff. A single drop of blood spends only a few tens of seconds on the arterial side. At any instant, the volume of blood on the arterial side is determined by the interaction of the heart’s pumping action versus the ease of runoff. The mean arterial pressure is a direct function of the volume of blood that is on the arterial side. • The venous tank is large. Veins are large and compliant. At any instant, most of the blood volume is on the venous side of the circulation.. It may take an individual drop of blood quite a while (minutes) to pass through this part of the loop and return to the heart. The Heart Electrical Basis of Heart Activity Essential Features of Vertebrate Cardiac Muscle • • • • Striated Cells connected by gap junctions Dually innervated by ANS Spontaneously active – driven by specialized pacemaker cells. • In postnatal mammals: highly dependent on oxidative metabolism Differentiation of Functional Cell Types in the Heart • Nodal fibers- spontaneously active pacemakers that can initiate heartbeat • Conducting fibers - rapidly carry excitation from one part of the heart to another • Myocardial fibers - compose most of the mass of the heart and provide essentially all of the force. Nodal and conducting fibers in the heart Sequence of electromechanical events in a heartbeat • Spontaneous AP in SA nodal pacemakers • Excitation spreads throughout atria, followed by atrial contraction • Excitation reaches AV node, enters bundle of His, is conducted into both ventricles by branch bundles, and is rapidly spread throughout the ventricular myocardium by Purkinjie fibers - followed by ventricular contraction or systole. Agenda of topics to consider • • • • Nature of pacemaker action potentials Control of heart rate by the ANS Nature of myocardial action potentials Relationship between myocardial action potentials and the electrocardiogram • Myocardial excitation-contraction coupling • Control of myocardial contractile force by stretch and by autonomic inputs Action potentials recorded from the ventricular myocardium, SA node and atrial myocardium. The total time in each window is about 300 msec for the myocardial cells and about 150 msec for the SA nodal cell. Pacemaker potentials • Nodal cells (pacemakers) do not have stable resting potentials. • Instead, the cells undergo a spontaneous, slow depolarization (prepotential) until threshold is reached. • The upstroke of the AP is slow compared to nerve and skeletal muscle. • Each action potential leads to a temporary afterhyperpolarization that leads into the next prepotential. Prepotential Action potential The rate of pacemaker potentials is modulated by the autonomic NS • In the absence of any autonomic input, the natural rate of pacemaker potentials is about 100/min in human heart. • Cholinergic input slows the heart rate by slowing depolarization during the prepotential; adrenergic input increases the heart rate by increasing the rate of depolarization. This slide shows the effects of isoproterenol (a beta agonist; A), stimulation of the vagus nerve (B), and two concentrations of Ach. Before and after traces are overlain; c indicates control beats and * experimental beats. Key features of the myocardial action potential • Rapid upstroke • LONG plateau – Ca++ entry occurs during the plateau and triggers Ca++ induced Ca++ release • Potential relatively stable during the interbeat interval, except in disease. The electrocardiogram is an extracellular recording of the myocardial AP • Voltage is measured at several spots on the body surface - because body fluid is a conductor of electricity, these spots could be thought of as wires connected directly to the heart surface. • A voltage difference will exist only when some parts of the heart are depolarized while others are not. During the interbeat interval, OR when the ventricle is all depolarized, the EKG trace will return to baseline. If we measure the voltage difference between the right arm and left leg over a heart cycle, we will watch excitation start in the atria, spread through the ventricles, and end with ventricular repolarization Relationship between atrial and ventricular AP s and EKG waves Atrial Ventricular