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The Circulatory System Chapter 49 Blood • Type of connective tissue composed of – Fluid matrix called plasma – Formed elements • Functions of circulating blood 1. Transportation 2. Regulation 3. Protection 2 Blood plasma • 92% water • Contains the following solutes – Nutrients, wastes, and hormones – Ions • Na+, Cl–, HCO3, and trace Ca2+, Mg2+, Cu2+, K+, Zn2+ – Proteins • Albumin, alpha (a) and beta (b) globulins • Fibrinogen – If removed, plasma is called serum 3 Formed elements • Red blood cells (erythrocytes) – About 5 million per microliter of blood – Hematocrit is the fraction of the total blood volume occupied by red blood cells – Mature mammalian erythrocytes lack nuclei – RBCs of vertebrates contain hemoglobin • Pigment that binds and transports oxygen 4 Formed elements • White blood cells (leukocytes) – Less than 1% of blood cells – Larger than erythrocytes and have nuclei – Can migrate out of capillaries into tissue fluid – Types • Granular leukocytes – Neutrophils, eosinophils, and basophils • Agranular leukocytes – Monocytes and lymphocytes 5 Formed elements • Platelets • Cell fragments that pinch off from larger cells in the bone marrow • Function in the formation of blood clots Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Prothrombin Thrombin Fibrinogen Thrombin Fibrin 1. Vessel is damaged, exposing surrounding tissue to blood. 2. Platelets adhere and become sticky, forming a plug. 3. Cascade of enzymatic reactions is triggered by platelets, plasma factors, and damaged tissue. 4. Threads of fibrin trap erythrocytes and form a clot. 5. Once tissue damage is healed, the clot is dissolved. 6 Formed elements • All develop from pluripotent stem cells • Hematopoiesis is blood cell production • Occurs in the bone marrow 7 Invertebrate Circulatory Systems • Sponges, cnidarians, and nematodes lack a separate circulatory system • Sponges circulate water using many incurrent pores and one excurrent pore • Hydra circulate water through a gastrovascular cavity (also for digestion) • Nematodes are thin enough that the digestive tract can also be used as a circulatory system 8 Invertebrate Circulatory Systems • Larger animals require a separate circulatory system for nutrient and waste transport • Open circulatory system – No distinction between circulating and extracellular fluid – Fluid called hemolymph • Closed circulatory system – Distinct circulatory fluid enclosed in blood vessels and transported away from and back to the heart 9 Vertebrate Circulatory Systems • Fishes – Evolved a true chamber-pump heart – Have a 2 chambered heart • Blood is pumped through the gills, and then to the rest of the body 10 Vertebrate Circulatory Systems • Amphibians – Advent of lungs required a second pumping circuit, or double circulation – Pulmonary circulation moves blood between the heart and lungs – Systemic circulation moves blood between the heart and the rest of the body 11 Vertebrate Circulatory Systems • Amphibian heart – 3-chambered heart • 2 atria and 1 ventricle – Incomplete seperation of two circuits – Amphibians obtain additional oxygen through their skin • Reptiles have a septum that partially subdivides the ventricle, further reducing mixing of blood 12 Vertebrate Circulatory Systems • Mammals, birds, and crocodilians – 4-chambered heart – 2 separate atria and 2 separate ventricles – Right atrium receives deoxygenated blood from the body and delivers it to the right ventricle, which pumps it to the lungs – Left atrium receives oxygenated blood from the lungs and delivers it to the left ventricle, which pumps it to rest of the body 13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lancelets Fish Amphibians Mammals Turtles Squamates Crocodilians Birds 4-chamber heart 4-chamber heart 3-chamber heart 2-chamber heart Evolution of the heart in vertebrates 14 The Cardiac Cycle • Heart has two pairs of valves – Atrioventricular (AV) valves • Maintain unidirectional blood flow between atria and ventricles • Tricuspid valve = On the right • Bicuspid, or mitral, valve = On the left – Semilunar valves • Ensure one-way flow out of the ventricles to the vessels • Pulmonary valve located at the exit of the right ventricle • Aortic valve located at the exit of the left ventricle 15 The Cardiac Cycle • Valves open and close as the heart goes through the cardiac cycle • Ventricles relaxed and filling (diastole) • Ventricles contracted and pumping (systole) • “Lub-dub” sounds heard with stethoscope – Lub – AV valves closing – Dub – closing of semilunar valves 16 The Cardiac Cycle • Heart contains “self-excitable” autorhythmic fibers • Most important is the sinoatrial (SA) node – Located in wall of right atrium – Acts as pacemaker – Autonomic nervous system can modulate rate 17 The Cardiac Cycle • Each SA depolarization transmitted – To left atrium – To right atrium and atrioventricular (AV) node • AV node is only pathway for conduction to ventricles – Spreads through atrioventricular bundle – Purkinje fibers – Directly stimulate the myocardial cells of both ventricles to contract 18 The Cardiac Cycle • Electrical activity can be recorded on an electrocardiogram (ECG or EKG) 19 Characteristics of Blood Vessels • Blood leaves heart through the arteries • Arterioles are the finest, microscopic branches of the arterial tree • Blood from arterioles enters capillaries • Blood is collected into venules, which lead to larger vessels, veins • Veins carry blood back to heart 20 Characteristics of Blood Vessels • Arteries and veins are composed of four tissue layers – Endothelium, elastic fibers, smooth muscle, and connective tissue – Walls too thick for exchange of materials across the wall • Capillaries are composed of only a single layer of endothelial cells – Allow rapid exchange of gases and metabolites between blood and body cells 21 Characteristics of Blood Vessels • Arteries and arterioles – Larger arteries contain more elastic fibers in their walls than other blood vessels • Recoil each time they receive blood from the heart – Contraction of the smooth muscle layer of the arterioles results in vasoconstriction • Greatly increases resistance and decreases flow • Chronic vasoconstriction can result in hypertension – Relaxation of the smooth muscle layer results in vasodilation • Decreasing resistance and increasing blood flow to an organ 22 Characteristics of Blood Vessels Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blood flows toward heart • Veins and venules – Thinner layer of smooth muscles than arteries – Venous pump helps return blood to heart • Skeletal muscle contractions and oneway venous valves Vein Open valve Contracting skeletal muscles Valve closed 23 The Lymphatic System • Significant amount of water and solutes in the blood plasma filter through the walls of the capillaries to form the interstitial (tissue) fluid • Most fluid leaves at the arteriole end of the capillary and returns at the venule end • Fluid that does not return to capillaries is returned to circulation by the lymphatic system 24 The Lymphatic System • Consists of lymphatic capillaries, lymphatic vessels, lymph nodes, and lymphatic organs (spleen and thymus) • Excess fluid in the tissues drains into blind-ended lymph capillaries • Lymph passes into progressively larger vessels with one-way valves • Eventually drains into subclavian veins 25 Cardiovascular Diseases • Leading cause of death in the United States • Atherosclerosis – Accumulation of fatty material within arteries – Impedes blood flow • Arteriosclerosis – Arterial hardening due to calcium deposition Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. 2000 mm b. 2500 mm a-b: © Ed Reschke; c: © Dr. Gladden Willis/Visuals Unlimited. c. 1000 mm 26 Blood Pressure • Measured as systole/diastole • Systole – ventricle contraction • Diastole – ventricle relaxation 27 Blood Flow and Blood Pressure • Blood pressure increases with blood volume • Blood volume is regulated by four hormones – Antidiuretic hormone (ADH) – Aldosterone – encourages kidney to excrete postassium and retain sodium – Atrial natriuretic hormone – increases sodium excretion and decreases blood pressure – Nitric oxide (NO) - vasodialator 28