Chapter IV Circulation
The circulation system consists of the heart and the vessels.
Functions of the Heart
The heart consists of right and left atria and right and left ventricles. The atrioventricular valves prevent backflow of blood from ventricles to atria; the pulmonary and aortic valves prevent backflow of arterial blood into the ventricles.
Heartbeat coordination The sinoatrial (SA) node generates the current that leads to depolarization of all other cardiac muscle cells. The SA node manifests a pacemaker potential, which brings its membrane potential to threshold and initiates an action potential. The impulse spreads from the SA node throughout both atria and to the atrioventricular (AV) node, where a small delay occurs. The impulse then passes, in turn, into the bundle of His, right and left bundle branches, Purkinje fibers, and contractile ventricular fibers.
Cardiac muscle cells must undergo action potentials for contraction to occur. The rapid depolarization of the action potential in contractile atrial and ventricular cells is due mainly to a positive-feedback increase in sodium permeability. Following the initial depolarization, the membrane remains depolarized (the plateau phase) almost the entire duration of the contraction because of prolonged entry of calcium into the cell through slow plasma membrane channels. Calcium also released from the sarcoplasmic reticulum, functions as the excitation-contraction coupler in cardiac muscle. The amount of calcium released does not usually saturate all troponin-binding sites, and so the number of active cross bridges can be increased if cytosolic calcium is increased still further. Cardiac muscle cannot undergo summation of contractions because it has a very long effective refractory period.
Mechanical events of the cardiac cycle The cardiac cycle is divided into systole (ventricular contraction) and diastole (ventricular relaxation). At the onset of systole, ventricular pressure rapidly exceeds atrial pressure, and the AV valves close. The aortic and pulmonary valves are not yet open, and so no ejection occurs during this isovolumetric ventricular contraction. When ventricular pressures exceed aortic and pulmonary trunk pressures, the aortic and pulmonary valves open, and ventricular ejection of blood occurs. When the ventricles relax at the beginning of diastole, the ventricular pressures fall significantly below those in the aorta and pulmonary trunk, and the aortic and pulmonary valves close. Because the AV valves are also still closed, no change in ventricular volume occurs during this isovolumetric ventricular relaxation. When the ventricular pressures fall below the pressures in the right atrium and the left atrium, the AV valves open, and the ventricular filling phase of diastole begins. Filling occurs very rapidly at first so that atrial contraction, which occurs at the very end of diastole, usually adds only a small amount of additional blood to the ventricles. The amount of blood in the ventricles just prior to systole is the end-diastolic volume. The volume remaining after ejection is the end-systolic volume. Pressure changes in the systemic and pulmonary circulation have similar patterns, but the pulmonary values are much lower. The first heart sound is due to the closing of the AV valves, the second to the closing of the aortic and pulmonary valves.
Cardiac output The cardiac output is the volume of blood pumped by each ventricle and equals the product of stroke volume and heart rate. It is about 5.0L/min in a resting man. The average rate of the heartbeat in normal adult is 75 beats/min, it varies from 50 to 100 beats/min in different conditions. The amount of blood pumped out of each ventricle per beat is called stroke volume, which is about 70 ml in a resting man. Heart rate is increased by the sympathetic nerves to the heart and by epinephrine. It is decreased by the parasympathetic nerves to the heart. Stroke volume is increased by an increase in venous return (preload), by a decrease in arterial blood pressure (afterload) and by an increase in contractility due to sympathetic nerve stimulation or to epinephrine.
Functions of Vascular System
Arterial blood pressure Blood pressure (BP) means the force exerted by the blood against unit area of the vessel wall. BP in the arteries is called the arterial pressure. The value of BP changes continuously throughout each cardiac cycle. The pressure rises during cardiac systole and falls during diastole. The peak pressure value reached during systole is termed as the systolic pressure, while the minimum pressure value reached during diastole is termed as the diastolic pressure. The difference between the systolic and diastolic pressure is called the pulse pressure. The mean of the pressure value during the entire cardiac cycle is called the mean arterial pressure. Mean arterial pressure equals the product of cardiac output and total peripheral resistance. Mean arterial pressure can be estimated as diastolic pressure plus 1/3 pulse pressure.
BP can be affected by the stroke volume of the left ventricle, heart rate ,peripheral resistance, elasticity of the large vessel walls and the quantity of blood in the circulation.
Function of capillaries The capillaries are the sites of exchange of nutrients and waste products between blood and tissues. Capillary blood flow is determined by the resistance of the arterioles supplying the capillaries and by the number of open precapillary sphincters. Blood flows through the capillaries more slowly than in any other part of the vascular system because of the huge cross-sectional area of the capillaries. Diffusion is the mechanism by which nutrients and waste products exchange between capillary plasma and interstitial fluid. Lipid-soluble substances move across the entire endothelial wall, whereas ions and polar molecules move through water--filled channels that exist between the cells or as fused-vesicle channels. Plasma proteins move across most capillaries only very slightly, either by diffusion or vesicle transport.
Formation of interstitial fluid The formation of interstitial fluid is determined by the effective filtration pressure (EFP). EFP consists of the capillary pressure, the interstitial fluid pressure, the plasma colloid osmotic pressure and the interstitial fluid colloid osmotic pressure. Filtration from plasma to interstitial fluid is favored by the hydrostatic pressure difference between the capillary and the interstitial fluid. Absorption from interstitial fluid to plasma is favored by the plasma protein concentration difference between the plasma and the interstitial fluid. Filtration and absorption do not change the concentrations of crystalloid in the plasma and interstitial fluid because these substances move together with water. There is normally a small excess of filtration over absorption.
The lymphatic system provides a one-way route for movement of interstitial fluid to the cardiovascular system. It returns the excess fluid filtered from the capillaries, as well as the protein that leaks out of the capillaries. Lymph flow is driven by the skeletal muscle pump, the respiratory pump, and contraction of smooth muscle in the larger lymphatic vessels.
Regulation of Cardiovascular Functions
The cardiovascular basis center is in the medullary. The neurons in this center receive input from the various baroreceptors. This input determines the outflow from the center along axons that terminate upon the cell bodies and dendrites of the vague to the heart and the sympathetic neurons to the heart, arterioles, and veins (to see baroreceptor reflexes) .
The sympathetic branch of the autonomic nervous system, which mainly causes vasoconstriction, generally mediates extrinsic control of vascular smooth muscle. Most blood vessels are innervated only by sympathetic adrenergic fibers. Vasoconstriction, the predominant adrenergic effect, is mediated by a1-receptors. In some tissues such as skeletal muscle, vasodilation is mediated by B1 adrenergic receptors and by cholinergic sympathetic fibers. Epinephrine causes vasoconstriction or vasodilation, depending on the organ or tissue. Angiotensin II and vasopressin cause vasoconstriction. Some vasodilator inputs act by releasing endothelium-derived relaxing factor from endothelial cells.
Baroreceptor reflexes The primary baroreceptors are the arterial baroreceptors-- the two carotid sinuses and the aortic arch. Nonarterial baroreceptors are located in the systemic veins, pulmonary vessels, and walls of the heart. The firing rates of the arterial baroreceptors are proportional to mean arterial pressure and to pulse pressure. The medullary cardiovascular center integrates inputs from baroreceptors in the walls of the carotid sinus and aortic arch. Decreased mean arterial pressure, indicated by decreased baroreceptors firing, results in decreased vagal tone to the heart, increased sympathetic input to heart and vessels, and increased release of epinephrine from the adrenal medulla. The increased heart effectiveness and vasoconstriction return arterial blood pressure towards the setpoint value. An increase in firing due to an increase in pressure causes, by way of the medullary cardiovascular center, an increase in parasympathetic outflow to the heart and a decrease in sympathetic outflow to the heart, arterioles, and veins. The result is a decrease in cardiac output and total peripheral resistance and, hence, a decrease in mean arterial pressure. The baroreceptor reflexes are short-term regulators of arterial pressure but adept to a maintained change in pressure. The most important long-term regulator of arterial pressure is the blood volume.
