Chapter 18 / The Cardiovascular System: Blood Vessels and Circulation I. INTRODUCTION A. One main focus of this chapter considers hemodynarnics, the means by which blood flow is altered and distributed and by which blood pressure is regulated. B. The histology of blood vessels and anatomy of the primary routes of arterial and venous systems is surveyed. II. ANATOMY OF BLOOD VESSELS A. Arteries 1. Arteries carry blood away from the heart to the tissues. a. The wall of an artery consists of a tunica interna, tunica media (which maintains elasticity and contractility), and tunica extema. b. Large arteries are referred to as elastic (conducting) arteries, and medium sized arteries are called muscular (distributing) arteries. 2. Many arteries anastomose--the distal ends of two or more vessels unite. a. Anastomoses between arteries provide alternate routes for blood to reach a tissue or organ. b. Thus, if a vessel is blocked by disease, injury, or surgery, circulation to a part of the body is not necessarily stopped. 1) An alternate blood route from an anastomosis is called collateral circulation. 2) Arteries that do not anastomose are called end arteries. Occlusion of an end artery interrupts the blood supply to a whole segment of an organ, producing necrosis (death) of that segment. B. Arterioles 1. Arterioles are very small, almost microscopic, arteries that deliver blood to capillaries. 2. Through vasoconstriction (decrease in the size of the lumen of a blood vessel) and vasodilation (increase in the size of the lumen of a blood vessel), arterioles assume a key role in regulating blood flow from arteries into capillaries and in altering arterial blood pressure. C. Capillaries 1. Capillaries are microscopic blood vessels through which materials are exchanged between blood and tissue cells. a. They usually connect arterioles and venules. b. Capillary walls are composed of only a single layer of cells (endothelium) and a basement membrane. 2. Capillaries branch to form an extensive capillary network throughout the tissue. This network increases the surface area, allowing a rapid exchange of large quantities of materials. a. The flow of blood through capillaries is regulated by vessels with smooth muscle in their walls. b. Rings of smooth muscle fibers (cells) called precapillary sphincters regulate blood flow through true capillaries. c. Some capillaries are continuous, whereas others are fenestrated . d. Microscopic blood vessels in organs such as the liver, spleen, and bone marrow are called sinusoids ; they are wider than capillaries, more tortuous, and specialized for the functions of the specific organs. D. Venules 1. Venules are small vessels that are formed from the union of several capillaries; venules merge to form veins. 2. They drain blood from capillaries into veins. E. Veins 1. Veins consist of the same three tunics as arteries but have a thinner tunica interna and media and a thicker tunica externa; they have less elastic tissue and smooth muscle and are therefore thinner-walled than arteries . a. They contain valves to prevent backflow of blood. b. Weak valves can lead to varicose veins. 2. Vascular (venous) sinuses are veins with very thin walls with no smooth muscle to alter their diameters. Examples are the brain's superior sagittal sinus and the coronary sinus of the heart. F. Blood Distribution 1. The volume of blood in various parts of the cardiovascular system varies considerably. 2. At rest, the largest portion of the blood is in systemic veins and venules, collectively called blood reservoirs. a. They store blood and, through venous vasoconstriction, can move blood to other parts of the body if the need arises. b. In cases of hemorrhage, when blood pressure and volume decrease, vasoconstriction of veins in venous reservoirs helps to compensate for the blood loss. 3. The principal reservoirs are the veins of the abdominal organs (liver and spleen) and skin. III. CAPILLARY EXCHANGE A. Substances enter and leave capillaries by diffusion, vesicular transport (endocylosis and exocytosis), and bulk flow (filtration and absorption). B. The movement of water and dissolved substances (except proteins) through capillaries is dependent upon hydrostatic and osmotic pressures. 1. The near equilibrium at the arterial and venous ends of a capillary by which fluids exit and enter is called Starling's law of the capillaries 2. Occasionally, the balance of filtration and reabsorption between interstitial fluid and plasma is disrupted, allowing an abnormal increase in interstitial fluid called edema. Edema may be caused by several factors: a. Increased blood hydrostatic pressure in capillaries due to an increase in venous pressure. b. Decreased concentration of plasma proteins that lower blood colloid osmotic pressure. c. Increased permeability of capillaries, allowing greater amounts of plasma proteins to leave the blood and enter tissue fluid. d. Increased extracellular fluid volume as a result of fluid retention. e. Blockage of lymphatic vessels postoperatively or due to filarial worm infection. IV. HEMODYNAMICS: PHYSIOLOGY OF CIRCULATION A. Velocity of Blood Flow 1. The volume that flows through any tissue in a given period of time is blood flow. 2. The velocity of blood flow is inversely related to the cross-sectional area of blood vessels; blood flows most slowly where cross-sectional area is greatest. 3. Blood flow decreases from the aorta to arteries to capillaries and increases as it returns to the heart. B. Volume of Blood Flow 1. Blood flow is determined by blood pressure and resistance. 2. Blood flows from regions of higher to lower pressure; the higher the resistance the lower the blood flow. C. Blood pressure (BP) is the pressure exerted on the walls of a blood vessel; in clinical use, BP refers to pressure in arteries. 1. Factors that affect blood pressure include cardiac output, blood volume, viscosity, resistance, and elasticity of arteries. 2. As blood leaves the aorta and flows through systemic circulation, its pressure progressively falls to 0 mm Hg (Mercury) by the time it reaches the right atrium. D. Resistance refers to the opposition to blood flow as a result of friction between blood and the walls of the blood vessels. 1. Resistance depends on blood viscosity, blood vessel length, and blood vessel radius. 2. Systemic vascular resistance (SVR) (also known as total peripheral resistance) refers to all of the vascular resistances offered by systemic blood vessels; most resistance is in arterioles, capillaries, and venules due to their small diameters. E. Venous Return 1. A number of factors aid venous return, the volume of blood flowing back to the heart from the systemic veins, by increasing the magnitude of the pressure gradient between the venules and the right atrium. 2. Blood return to the heart is maintained by several factors, including skeletal muscular contractions, valves in veins (especially in the extremities), and pressure changes associated with breathing. V. CONTROL OF BLOOD PRESSURE AND BLOOD FLOW A. Cardiovascular Center 1. The cardiovascular (CV) center is a group of neurons in the medulla that regulates heart rate, contractility, and blood vessel diameter. 2. The CV center receives input from higher brain regions and sensory receptors (baroreceptors and chemoreceptors). 3. Output from the CV center flows along sympathetic and parasympathetic fibers. a. Sympathetic impulses along cardioaccelerator nerves increase heart rate and contractility. b. Parasympathetic impulses along vagus (X) nerves decrease heart rate. c. The sympathetic division also continually sends impulses to smooth muscle in blood vessel walls via vasomotor nerves. The result is a moderate state of tonic contraction or vasoconstriction, called vasomotor tone. B. Neural Regulation of Blood Pressure 1. Baroreceptors (pressoreceptors) are important pressure-sensitive sensory neurons that monitor stretching of the walls of blood vessels and the atria. a. The cardiac sinus reflex is concerned with maintaining normal blood pressure in the brain and is initiated by baroreceptors in the wall of the carotid sinus. b. The aortic reflex is concerned with general systemic blood pressure and is initiated by baroreceptors in the wall of the arch of the aorta or attached to the arch. c. If blood pressure falls, the baroreceptor reflexes accelerate heart rate, increase force of contraction, and promote vasoconstriction. d. The right heart (atrial) reflex responds to increases in venous blood pressure and is initiated by baroreceptors in the right atrium and venae cavae. 2. Receptors sensitive to chemicals are called chemoreceptors. a. They monitor blood levels of oxygen, carbon dioxide, and hydrogen ion concentration. C. Hormonal Regulation of Blood Pressure 1. Several hormones affect blood pressure and flow by acting on the heart, altering blood vessel diameter, or adjusting the total blood volume. a. Among the hormones that help regulate blood pressure are epinephrine, norepinephrine (NE), antidiuretic hormone (ADH), angiotensin, atrial natriuretic Deptide (ANP), histamine, and kinins. D. Autoregulation of Blood Pressure 1. Autoregulation refers to local, automatic adjustments of blood flow in a given region to match the particular needs of the tissue. 2. In most body tissues, oxygen is the principal, though not direct, stimulus for autoregulation. 3. Researchers have identified two general types of stimuli that cause autoregulatory changes in blood flow– physical and chemical. VI. SHOCK AND HOMEOSTASIS A. Shock is an inadequate cardiac output that results in failure of the cardiovascular system to deliver adequate amounts of oxygen and nutrients to meet the metabolic needs of body cells. As a result, cellular membranes dysfunction, cellular metabolism is abnormal, and cellular death may eventually occur without proper treatment. B. Signs and symptoms of shock include clammy, cool, pale skin; tachycardia; weak, rapid pulse; sweating; hypotension (systemic pressure < 90 mm Hg); altered mental status; decreased urinary output; thirst; and acidosis. C. Stages of shock are all characterized by inadequate perfusion of tissues. 1. The stages of shock are described in the textbook as they apply to hypovolemic shock, since it has been studied so extensively. Hypovolemic shock refers to decreased blood volume resulting from loss of blood or plasma due to acute hemorrhage or excessive fluid loss (as in excess vomiting, diarrhea, sweating, dehydration, urine production, and burns). 2. The development of shock occurs in three principal stages, which merge with one another. a. Stage I is compensated (nonprogressive) shock, in which negative feedback systems restore homeostasis. 1) If the initiating cause does not get any worse, a full recovery follows. 2) Compensatory adjustments include activation of the sympathetic division of the ANS, the renin-angiotensin pathway, release of antidiuretic hormone (ADH), and release of vasodilator factors in response to hypoxia. b. Stage II is decompensated (progressive) shock, in which positive feedback cycles intensify the shock and immediate medical intervention is required. 1) It occurs when there has been a reduction in blood volume of 15-25%. 2) Among the positive feedback cycles that contribute to decreased cardiac output and blood pressure are depression of cardiac activity, depression of vasoconstriction, increased permeability of capillaries, intravascular clotting, cellular destruction, and acidosis. c. Stage III is irreversible shock, in which there is rapid deterioration of the cardiovascular system than cannot be helped by compensatory mechanisms or medical intervention. VII. CHECKING CIRCULATION A. Pulse 1. Pulse is the alternate expansion and elastic recoil of an artery wall with each heartbeat. It may be felt in any artery that lies near the surface or over a hard tissue, and is strongest in the arteries closest to the heart; the radial artery is most commonly used, to feel the pulse. 2. A normal resting pulse (heart) rate is between 70 and 80 beats per minute. a. Tachycardia means a rapid resting heart or pulse rate (> 100 beats/min). b. Bradycardia indicates a slow resting heart or pulse rate (< 60 beats/min). B. Measurement of Blood Pressure (BP) 1. Blood pressure is the pressure exerted by blood on the wall of an artery when the left ventricle undergoes systole and then diastole. It is measured by the use of a sphygmomanometer, usually in one of the brachial arteries. a. Systolic blood pressure (SBP) is the force of blood recorded during ventricular contraction. b. Diastolic blood pressure (DBP) is the force of blood recorded during ventricular relaxation. c. The various sounds that are heard while taking blood pressure are called Korotkoff sounds. d. The normal blood pressure of a young adult male is 120/80 mm Hg (8- 10 mm Hg less in a young adult female). The range of average values varies with many factors. 2. Pulse pressure (PP) is the difference between systolic and diastolic pressure. It normally is about 40 mm Hg and provides information about the condition of the arteries. VIII. CIRCULATORY ROUTES A. Introduction 1. The blood vessels are organized into parallel routes that deliver blood throughout the body. 2. The largest circulatory route is the systemic circulation. 3. Two of the several subdivisions of the systemic circulation are coronary (cardiac) circulation, which supplies the myocardiurn of the heart; the cerebral circulation, which supplies the brain; and the hepatic portal circulation, which extends from the gastrointestinal tract to the liver. 4. Other routes include pulmonary circulation and fetal circulation. B. Systemic Circulation 1. The systemic circulation takes oxygenated blood from the left ventricle through the aorta to all parts of the body, including some lung tissue (but does not supply the air sacs of the lungs) and returns the deoxygenated blood to the right atrium. a. The aorta is divided into the ascending aorta, arch of the aorta, and the descending aorta. b. Each section gives off arteries that branch to supply the whole body. 2. Blood returns to the heart through the systemic veins. All the veins of the systemic circulation flow into the superior or inferior venae cavae or the coronary sinus, which in turn empty into the right atrium. C. Hepatic Portal Circulation 1. The hepatic portal circulation collects blood from the veins of the pancreas, spleen, stomach, intestines, and gallbladder and directs it into the hepatic portal vein of the liver before it returns to the heart. 2. A portal system carries blood between two capillary networks, in this case from capillaries of the gastrointestinal tract to sinusoids of the liver. 3. This circulation enables nutrient utilization and blood detoxification by the liver. D. Pulmonary Circulation 1. The pulmonary circulation takes deoxygenated blood from the right ventricle to the air sacs of the lungs and returns oxygenated blood from the lungs to the left atrium. 2. It allows blood to be oxygenated for systemic circulation. E. Fetal Circulation 1. The fetal circulation involves the exchange of materials between fetus and mother. 2. The fetus derives its oxygen and nutrients and eliminates its carbon dioxide and wastes through the maternal blood supply by means of a structure called the placenta. 3. At birth, when pulmonary (lung), digestive, and liver functions are established, the special structures of fetal circulation are no longer needed. IX. AGING AND THE CARDIOVASCULAR SYSTEM A. General changes associated with aging and the cardiovascular system include loss of compliance (extensibility) of the aorta, reduction in cardiac muscle fiber (cell) size, progressive loss of cardiac muscular strength, reduced cardiac output, a decline in maximum heart rate, and increased systolic blood pressure. B. The incidence of coronary artery disease (CAD), congestive heart failure (CHF), and atherosclerosis increases with age. X. DEVELOPMENTAL ANATOMY OF BLOOD VESSELS AND BLOOD A. Blood vessels develop from isolated masses of mesenchyme in the mesoderm called blood islands. 1. Spaces soon appear in the islands and become the lumens of the blood vessels. 2. Some of the mesenchymal cells immediately around the spaces give rise to the endothelial lining of the blood vessels. B. Blood plasma and blood cells are produced by the endothelial cells of blood vessels, a function later assumed by the liver, spleen, bone marrow, and lymph nodes. XI. DISORDERS: HOMEOSTATIC IMBALANCES A. Hypertension, or high blood pressure, is the most common disease affecting the heart and blood vessels. It is classified as primary (essential) or secondary. 1. Primary (essential) hypertension (approximately 90-95% of all hypertension cases) is a persistently elevated blood pressure that cannot be attributed to any particular organic cause. 2. Only secondary hypertension has causes that are identifiable. Two of these are kidney disease and adrenal hypersecretion. 3. A number of treatment strategies are effective in lowering blood pressure. B. An aneurysm is a thin, weakened section of the wall of an artery or a vein that bulges outward, forming a balloonlike sac of the blood vessel. Left untreated, it may burst, causing massive hemorrhage with shock, severe pain, cardiovascular accident (CVA, or stroke), or death. Some may be surgically repaired. C. Coronary artery disease (CAD) is a condition in which the heart muscle receives inadequate oxygen due to blockage of its blood flow. It may be caused by atherosclerosis, coronary artery spasm, and other factors. D. Deep-venous thrombosis (DVT) is a blood clot (thrombus) in a deep vein, especially in the lower extremities. It has two serious complications: pulmonary embolism and postphlebitic syndrome.