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The Circulatory System
- The circulatory system is the body’s transport system. It delivers oxygen, nutrients, metabolites, hormones, neurochemicals, proteins, and blood cells through the body and carries metabolic wastes to the kidneys and lungs for excretion.
- The circulatory system consists of the heart and blood vessels and is made up of two separate, serially connected systems: the pulmonary circulation and the systemic circulation.
- The pulmonary circulation is driven by the right side of the heart; its function is to deliver blood to the lungs for oxygenation.
- The systemic circulation is driven by the left side of the heart, and its function is to move oxygenated blood throughout the body.
- The lymphatic vessels collect vessels collect fluids from the interstitium and return the fluids to the circulatory system
- The heart consists of four chambers (two atria and two ventricles), four valves (two atrioventricular valves and two semilunar valves), a muscular wall, a fibrous skeleton, a conduction system, nerve fibers, systemic vessels (the coronary circulation), and openings where the great vessels enter the atria and ventricles.
- The heart wall, which encloses the heart and divides it into chambers, is made up of three layers: the pericardium (outer layer), the myocardium (muscular layer), and endocardium (inner lining).
- The myocardial layer of the two atria, which receive blood entering the heart, is thinner than the myocardial layer of the ventricles, which have to be stronger to squeeze blood out of the heart.
- The right and left sides of the heart are separated by portions of the heart wall called the interatrial septum and the interventricular septum.
- Deoxygenated (venous) blood from the systemic circulation enters the right atrium through the superior and inferior vena cavae. From the atrium, the blood passes through the right atrioventricular (tricuspid) valve into the right ventricle. In the ventricle, the blood flows from the inflow tract to the outflow tract and then through the pulmonary semilunar valve (pulmonary valve) into the pulmonary artery, which delivers it to the lungs for oxygenation.
- Oxygenated blood from the lungs enters the left atrium through the four pulmonary veins (two from the left lung and two from the right lung). From the left atrium, the blood passes through the left atrioventricular valve (mitral valve) into the left ventricle. In the ventricle, the blood flows from the inflow tract to the outflow tract and then through the aortic semilunar valve (aortic valve) into the aorta, which delivers it to systemic arteries of the entire body.
- The heart valves ensure the one-way flow of blood from atrium to ventricle and from ventricle to artery.
- Oxygenated blood enters the coronary arteries through an opening in the aorta, and unoxygenated blood from the coronary veins enters the right atrium through the coronary sinus.
- The pumping action of the heart consists of two phases: diastole, during which the myocardium relaxes and the ventricles fill with blood, and systole, during with the myocardium contracts, forcing blood out of the ventricles. A cardiac cycle consists of one systolic contraction and the diastolic relaxation that follows it. Each cardiac cycle constitutes one “heartbeat”.
- The conduction system of the heart generates and transmits electrical impulses (cardiac action potentials) that stimulate systolic contractions. The autonomic nerves (sympathetic and parasympathetic fibers) can adjust heart rate and systolic force, but they do not stimulate the heart to beat.
- The normal electrocardiogram is the sum of all action potentials. The P wave represents atrial depolarization; the QRS complex is the sum of all ventricular cell depolarizations. The ST interval occurs when the entire ventricular myocardium is depolarized.
- Cardiac action potentials are generated by the sinoatrial node at the rate of about 75 impulses per minute. The impulses can travel through the conduction system of the heart, stimulating myocardial contraction as they go.
- Cells of the cardiac conduction system possess the properties of automaticity and rhythmicity. Automatic cells return to threshold and depolarize rhythmically without outside stimulus. The cells of the sinoatrial node depolarize faster than other automatic cells, making it the natural pacemaker of the heart. If the sinoatrial node is disabled, the next fastest pacemaker, the atrioventricular node, takes over.
- Each cardiac action potential travels from the sinoatrial node to the atrioventricular node to the bundle of Hiss (atrioventricular bundle), through the bundle branches, and finally to the Purkinje fibers. There the impulse is stopped. It is prevented from reversing its path by the refractory period of cells that have just been polarized. The refractory period ensures that diastole (relaxation) will occur, thereby completing the cardiac cycle.
- Adrenergic receptor number, type, and function govern autonomic (sympathetic) regulation of heart rate, contractile force, and the dilation or constriction of coronary arteries. The presence of specific receptors (α1, α2; β1, β2) on myocardium and coronary vessels determines the effects of the neurotransmitters norepinephrine and epinephrine.
- Unique features that distinguish myocardial cells from skeletal cells enable myocardial cells to transmit action potentials faster (through intercalated disks), synthesize more ATP (because of a large number of mitochondria), and have readier access to ions in the interstitium (because of an abundance of transverse tubules). These combined differences enable the myocardium to work constantly, which skeletal muscle is not required to do.
- Cross-bridges between actin and myosin enable contraction. Calcium and its interaction with the troponin complex facilitate the contraction process. With troponin release of calcium, myocardial relaxation begins.
- Cardiac performance is affected by preload, afterload, myocardial contractility, and heart rate.
- Preload, or pressure generated in the ventricles at the end of diastole, depends on the amount of blood in the ventricle. Afterload is the resistance to ejection of the blood from the ventricle. Afterload depends on pressure in the aorta.
- The Frank-Starling law of the heart states that the myocardial stretch determines the force of myocardial contraction (the greater the stretch, the stronger the contraction).
- Contractility is the potential for myocardial fiber shortening during systole. It is determined by the amount of stretch during diastole (i.e., preload) and by sympathetic stimulation of the ventricles.
- Heart rate is determined by the sinoatrial node and by components of the autonomic nervous system, including cardiovascular control centers in the brain, neuroreceptors in the atria and aorta, hormones, and catecholamines (epinephrine, norepinephrine).
The Systemic Circulation
- Blood flows from the left ventricle into the aorta and from the aorta into arteries that eventually branch into arterioles and capillaries, the smallest of the arterial vessels. Oxygen, nutrients, and other substances needed for cellular metabolism pass from the capillaries into the interstitium, where they are available for uptake by the cells. Capillaries also absorb products of cellular metabolism from the interstitium.
- Venules, the smallest veins, receive capillary blood. From the venules, the venous blood flows into larger and larger veins until it reaches the venae cavae, through which it enters the right atrium.
- Vessel walls consist of three layers: the tunica intima (inner layer), the tunica media (middle layer), and the tunica externa (the outer layer).
- Layers of the vessel wall differ in thickness and composition from vessel to vessel, depending on the vessel’s size and location within the circulatory system. In general, the tunica media of arteries close to the heart contains a greater proportion of elastic fibers because these arteries must be able to distend during systole and recoil during diastole. Distributing arteries farther from the heart contain a greater proportion of smooth muscle fibers because these arteries must be able to constrict and dilate to control blood pressure and volume within specific capillary beds.
- Blood flow into the capillary beds is controlled by the contraction and relaxation of smooth muscle bands (precapillary sphincters) at junctions between metarterioles and capillaries.
- Endothelial cells from the lining or endothelium of blood vessels. The endothelium is a life-support tissue and functions as a filter, altering permeability, changes in vasomotion (constriction and dilation), and is involved in clotting and inflammation.
- Blood flow through the veins is assisted by the contraction of skeletal muscles (the muscle pump), and one-way valves prevent backflow in the lower body, particularly in the deep veins of the legs.
- Blood flow is affected by blood pressure, resistance to flow within the vessels, blood consistency (which affects velocity), anatomic features that may cause turbulent or laminar flow, and compliance (distensibility) of the vessels.
- Poiseuille law describes the relationship of blood flow, pressure, and resistance as the difference between pressure at the inflow end of the vessel and pressure at the outflow end divided by resistance within the vessel.
- According to Poiseuille formula, resistance depends on the vessel’s length and radius and on the viscosity of the blood. The greater the vessel’s length and the blood’s viscosity and the narrower the radius of the vessel’s lumen, the greater the resistance within the vessel.
- Total peripheral resistance, or the resistance to flow within the entire systemic circulatory system, depends on the combined lengths and radii of all the vessels within the system and on whether the vessels are arranged in series (greater resistance) or in parallel (lesser resistance).
- Poiseuille law and Poiseuille formula are based on physical laws governing the behavior of fluids in a straight tube. In the body, blood flow is also influenced by neural stimulation (vasoconstriction or vasodilation) and by autonomic features that cause turbulence within the vascular lumen (e.g., protrusions from the vessel wall, twists and turns, bifurcations).
- Arterial blood pressure is influenced and regulated by factors that affect cardiac output (heart rate, stroke volume), total resistance within the system, and blood volume.
- Antidiuretic hormone, renin-angiotensin system, natriuretic peptides, Adrenomedullin, and insulin can all alter blood volume and thus blood pressure.
- The tissue renin-angiotensin system is activated in response to tissue injury. This system is gaining importance in the maladaptive alterations, such as ventricular and vascular remodeling, alterations in renal function, and atherosclerosis.
- Particularly significant is an increased recognition of the role of angiotensin II for causing the systemic effects of vasoconstriction, hypertension, activation of the sympathetic nervous system, and retention of sodium and fluids.
- Venous blood pressure is influenced by blood volume within the venous system and compliance the venous walls.
- Blood flow through the coronary circulation is governed not only by the same principles as flow through other vascular beds but also by adaptations dictated by cardiac dynamics. First, blood flows into the coronary arteries during diastole rather than systole, because during systole, the cusps of the aortic semilunar valve block the openings of the coronary arteries. Second, systolic contraction inhibits coronary artery flow by compressing the coronary arteries.
- Autoregulation enables the coronary vessels to maintain optimal perfusion pressure despite systolic effects, and myoglobin in heart muscle stores oxygen for use during the systolic phase of the cardiac cycle.
The Lymphatic System
- The vessels of the lymphatic system run in the same sheaths with the arteries and veins.
- Lymph (interstitial fluid) is absorbed by lymphatic venules in the capillary beds and travels through ever larger lymphatic veins until it is emptied through the right or left thoracic duct into the right or left subclavian vein.
- As lymph travels toward the thoracic ducts, it is filtered by thousands of lymph nodes clustered around the lymphatic veins. The lymph nodes are sites of immune function.
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