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AP Biology March 2008 Circulation Chapter 42 Circulation systems reflect phylogeny. 1) Some animals with simple body plans possess a gastrovascular cavity rather than a true circulatory system. i) Cnidarians ii) Hydras iii) Planarians iv) Other Flatworms A gastrovascular cavity serves both in digestion and distribution of substances throughout the body. 2) Both open and closed circulatory systems have: a) Blood b) Vessels c) Heart 3) In open circulatory systems, blood bathes the organs directly. a) The blood and lymph combined are called hemolymph. i) Hemolymph is a circulatory fluid, a mixture of blood and interstitial fluid. 4) In a closed circulatory system, the blood never leaves the heart or the vessels. 5) A human circulatory system is called the cardiovascular system. a) The heart has: i) The atrium: the chambers of the heart that receives blood. ii) The ventricles: pump blood into the arteries. 6) There are three kinds of blood vessels: arteries carry blood away from the heart, capillaries are where exchange with tissue fluid takes place, and veins return blood to the heart. a) Arteries i) Have thick walls and are resilient ii) Expand to accommodate the sudden increase in blood volume that results after heart contraction. iii) Divided into small arterioles b) Arterioles i) Constriction and dilation are regulated by the nervous system to regulate blood pressure. c) Capillaries i) Microscopic blood vessels with a wall formed of one layer of simple squamous cells. ii) Capillary beds are so prevalent that, in humans, all cells are within 60-80 µm of a capillary. iii) Only 5% are open at one time; if the animal eats, capillary beds of the digestive system open. iv) Capillaries are so narrow that red blood cells must pass through in a single file. v) Gas, nutrient, and waste exchange occur across thin walls. d) Venules i) Vessels that take blood from capillaries, and join to form a vein. e) Veins i) Transport blood toward the heart ii) Wall of a vein is much thinner than that of arteries; there is low blood pressure. iii) One way valves open in the direction of the heart; close to prevent backflow Transport in humans. The heart pumps blood. 1) Pumping of heart keeps blood moving in arteries. 2) Skeletal muscle contraction is responsible for blood movement in veins. 3) Heart is cone-shaped, muscular organ, ~~ the size of a fist. 4) It is located between the lungs directly behind the sternum and is tilted so that apex is directed to the left. 5) Myocardium is major portion of the heart consisting mostly of cardiac muscle; muscle fibers are branched and tightly joined together. 6) The heart lies within a pericardium sac that contains pericardial fluid, which provides cushioning. 7) Endocardium lines the inner surface of the heart; it consists of connective tissues and endothelial tissue. 8) The internal wall called the septum separates the heart into right and left halves. 9) The heart has two upper, thin-walled atria and two lower, thick-walled ventricles. a) The atria receive blood from the venous portion of the cardiovascular system. b) Atria are so much smaller and weaker than the muscular ventricles, but they hold the same volume of blood c) Ventricles pump blood into the arterial portion of the cardiovascular system. 10)Heart valves direct flow of blood and prevent backward movement. a) Valves are supported by strong fibrous tendons (chordae tendineae) attached to muscular projections of ventricular walls; they prevent valves from inverting. b) Atrioventricular valves located between atria and ventricles prevent back flow from ventricle to atrium. c) Right atroventricular (tricuspid) valve on the right side of the heart consists of three cusps or flaps. d) Left atrioventricular (bicuspid or mitral) valve on the left side consists of two cusps or flaps. e) Semilunar valves resembling half-moons are located between a ventricle and an artery that prevents back flow from artery to ventricle. i) The pulmonary semilunar valve lies between the right ventricle and the pulmonary trunk. ii) The aortic semilunar valve lies between the left ventricle and the aorta. Path of Blood Through the Heart 1) Deoxygenated blood enters the right atrium from both the superior vena cava and the interior vena cava. 2) Right atrium sends blood through the right atrioventricular valve to the right ventricle. 3) Right ventricle sends blood through the pulmonary semilunar valve into the pulmonary trunk and arteries to lungs. 4) Oxygenated blood returns from the lungs through the pulmonary veins and is delivered to the left atrium. 5) The left atrium sends blood through the left atrioventricular (bicuspid or mitral) valve to the left ventricle. 6) Left ventricle send blood through the aortic semilunar valve into the aorta and to the body. The heart is therefore a double pump serving the lungs and body circulations simultaneously. The Heartbeat 1) The heart contracts (beats) about 70 times/min 2) Heartbeat (cardial cycle) consists of phases: a) Systole – contraction of heart chambers b) Diastole – relaxation of heart chambers 3) The atria contract first when ventricles relax (0.15 sec), then ventricles contract while atria relax (0.3 sec), then all chambers rest (0.40 sec). 4) The heart is in diastole about 50% of the time. 5) The short systole of the atria is needed only to send blood into ventricles. 6) The term “systole” refers to the left ventricle systole. 7) When the heart beats, the familiar “lub-dub” sound is heart as the valves of the heart close. a) Lub – vibrations of the heart when the atrioventricular valves close. b) Dub – vibrations due to the closing of the semilunar valves. 8) Pulse is a wave effect that passes down walls of arterial blood vessels when the aorta expands and then almost immediately recoils following ventrical systole. 9) Since there is one arterial pulse per ventricular systole, arterial pulse rate can be used to determine heart rate. 10)Heart contraction is intrinsic; heart will beat without any stimulation from nervous system because it contains nodal tissue with both muscular and nerve characteristics. a) Sinoatrial (SA) node i) “Pacemaker” found in upper dorsal wall of the right atrium. ii) Initiates heartbeat by sending out an excitatory impulse every 0.85 sec to cause atria to contract b) Atrioventricular (AV) node i) Found in base of right atrium very near septum ii) When stimulated by impulses from SA node, it sends out impulses through septum to cause ventricles to contract. Physical principles govern blood circulation. 1) Blood pressure refers to the hydrostatic pressure that blood exerts against the wall of a vessel and that propels the blood. a) It is measured and recorded as two numbers separated by a dash. i) The first number is the systolic pressure, when the heart contracts. ii) The second number is the diastolic pressure, when the heart is relaxed. 2) The lymphatic system is responsible for returning lost fluid and proteins to the blood in the form of lymph. a) Along a lymph vessel are lymph nodes that filter lymph and attack viruses and bacteria, playing an important role in immunity. Blood is a connective tissue with cells suspended in plasma. 1) Plasma is mostly water, but it also contains ions, electrolytes, and plasma proteins. a) It transports: i) Nutrients ii) Metabolic wastes iii) Gases iv) Hormones b) It carries: i) Red blood cells (erythrocytes) (1) Transports oxygen via hemoglobin (an ironcontaining protein) ii) White blood cells (leukocytes) (1) Part of the immune system iii) Platelets (1) Fragments of cells responsible for blood clotting. 2) Blood contains a soluble plasma protein called fibrinogen. a) This protein forms clots when it is converted to its active form, fibrin.