Chapter 32 Circulation BSC 1005 Tidwell All circulatory systems have 3 parts o A pump the heart that keeps the blood circulating o A liquid blood that serves as a medium of transport o A system of tubes blood vessels that conduct the blood throughout the body Open circulatory systems are found in many invertebrates including arthropods and mollusks o These animals have one or more simple hearts a network of vessels and series of interconnected spaces within the body called a hemocoel o Tissues and organs in hemocoel are bathed by a fluid called hemolymph which acts as both blood and extracellular fluid that bathes all cells Closed circulatory systems are present in some invertebrates including the earthworm and very active mollusks such as squid and octopuses and in all vertebrates including humans o In these systems blood is confined to the heart and blood vessels which branch through the organs and tissues of the body o Closed systems allow more rapid blood flow more efficient transport of dissolved substances and higher blood pressure than is possible in open systems o It has many functions Transport of O2 from the lungs or gills to the tissues and transport of CO2 from the tissues to the lungs or gills Distribution of nutrients from the digestive system to all body cells Transport of waste products and toxic substances to the liver where many of them are detoxified and to the kidneys for excretion Distribution of hormones from the glands and organs that produce them to the tissues on which they act Regulation of body temperature by adjustments in blood flow Wound healing and blood clotting to prevent blood loss Protection against disease by circulating white blood cells and antibodies o Chambers called atria collect blood o Atrial contractions send blood into ventricles chambers whose contractions circulate blood through the lungs and to the rest of the body o Blood usually first enters through the atria o Oxygen depleted blood enters the right atrium through two large veins vessels that carry blood toward the heart called the superior and inferior vena cava o After filling with blood the right atrium contracts forcing blood into the right ventricle o Contraction of the right ventricle sends the oxygen depleted blood to the lungs through the pulmonary arteries vessels that carry blood away from the heart Page 1 of 6 Chapter 32 BSC 1005 Tidwell o The other pump consisting of the left atrium and left ventricle deals with oxygenated blood Oxygen rich blood enters the left atrium through the pulmonary veins and is then squeezed into the left ventricle Contraction of the left ventricle sends the oxygenated blood through the aorta to the rest of the body o Valves maintain the direction of blood flow Pressure in one direction opens them easily but reverse pressure forces them closed o Semilunar valves allow blood to flow from the atria into the ventricles o Atrioventricular valves but not the reverse when the ventricles contract but prevent blood from returning as the ventricles relax allow blood to enter the pulmonary artery and the aorta Right side deals with deoxygenated blood o Systemic circuit o Pulmonary circuit Left side deals with oxygenated blood Cardiac muscle cells are small branched and striated o They are linked to one another via intercalated discs which appear as bands between the cells o Adjacent cell membranes are attached to one another by junctions called desmosomes which prevent the strong heart contractions from pulling the muscle cells apart o Intercalated discs also contain gap junctions that allow the electrical signals that trigger contractions to spread from one muscle cell to another producing synchronous heart muscle contractions The heart beats in a coordinated fashion o Both atria contract and pump blood into the ventricles o Both ventricles contract and pump blood into the arteries that exit the heart o All chambers relax briefly before the cycle repeats o The cardiac cycle lasts for less than 1 second o Systolic pressure Blood Pressure measured during ventricular contraction between contractions o Diastolic pressure the higher of the two readings when taking blood pressure is the minimum pressure in the arteries as the heart rests o A blood pressure reading of less than 120 80 is considered healthy a pressure of 140 90 or higher is defined as high blood pressure o High blood pressure or hypertension is caused by the constriction of small arteries which causes resistance to blood flow and strain on the heart Page 2 of 6 Chapter 32 BSC 1005 Tidwell 2 Chambers Fish 1 Atrium 1 Ventricle 3 Chambers Amphibians Snakes Lizards Turtles 2 Atria 1 Ventricle 4 Chambers Crocodiles and their relatives Birds Mammals 2 Atria 2 Ventricles The contraction of the heart is initiated and coordinated by a pacemaker a cluster of specialized heart muscle cells that produce spontaneous electrical signals at a regular rate o The heart s pacemaker is the sinoatrial SA node located in the upper wall of the right atrium o Electrical signals from the SA node pass freely into the connecting cardiac muscle cells and then throughout the atria o The electrical signal then passes from the right atrium to a specialized group of muscle cells between the right atrium and right ventricle called the atrioventricular AV node o From the AV node the signal to contract spreads along specialized tracts of rapid conducting muscle fibers called the atrioventricular bundle AV bundle which sends branches to the lower portion of both ventricles o Here the bundles branch further forming Purkinje fibers that transmit the electrical signal throughout the ventricle When the pacemaker fails rapid uncoordinated weak contractions called fibrillation occurs o Fibrillation may be treated with a defibrillating machine which applies a jolt of electricity to the heart synchronizing the contractions of the ventricular muscle cells and the pacemaker resumes its normal coordinating function The nervous system and hormones influence heart rate o On its own the SA node pacemaker would maintain a heart rate of about 100 beats per minute however nerve impulses and hormones significantly alter the heart rate At rest the parasympathetic nervous system slows the heart rate to about 70 beats per minute During exercise and stress the sympathetic nervous system increases the heart rate to meet the demand for greater blood flow to the muscles Blood has 2 major components o A liquid called plasma which comprises about 55 of