42 Internal Transport

Lecture Outline

I. Types of circulatory systems A. Some invertebrates have no circulatory system 1. Small invertebrates have a gastrovascular cavity that functions in circulation as well as digestion 2. Diffusion through the body of flatworms allows for gas exchange 3. Fluids in a pseudocoelom act in circulation B. Many invertebrates have an open circulatory system 1. Blood and interstitial fluid intermingle and are called hemolymph a) Hemolymph spills into sinuses that comprise the hemocoel 2. In the molluskian open circulatory system, the heart has two atria and one ventricle a) Hemolymph is pumped from the gills to the atria, then to the ventricle b) Hemolymph in the ventricle is pumped to sinuses collectively called the hemocoel, then back to the gills c) Some mollusks have hemocyanin, a copper-containing pigment that aids in transportation of oxygen 3. In arthropods, a tubular heart pumps hemolymph into vessels that connect to the sinuses of the hemocoel a) In insects, the circulatory system has a minimal respiratory function b) The tracheal system pipes oxygen to the cells of insects C. Some invertebrates have a closed circulatory system 1. Annelids, cephalopods, and echinoderms have a closed circulatory system a) Blood flows through a continuous circuit of blood vessels b) Thin-walled capillaries permit the exchange of materials 2. The proboscis worms have a primitive closed circulatory system, but lack a heart a) Pumping is sluggish and is based on movement of the worm and the contraction of blood vessels 3. Earthworms and other annelids have a complex, closed system a) The dorsal blood vessel carries blood anteriorly; the ventral vessel carries blood posteriorly b) Both vessels are connected by lateral vessels in each segment c) Five contractile vessels ("hearts") pump blood d) Earthworms have hemoglobin dissolved in the plasma 4. Cephalopods have a closed circulatory system, including accessory "hearts" at the base of the gills D. All vertebrates have a closed circulatory system 1. All vertebrates have a ventral muscular heart pumping blood in a system of closed vessels 2. Functions of the vertebrate circulatory system include: a) Transportation of nutrients b) Transportation of respiratory gases c) Transportation of wastes d) Transportation of hormones e) Maintenance of fluid balance f) Internal defense g) Thermoregulation h) Maintenance of appropriate pH

II. Vertebrate blood consists of plasma, blood cells, and platelets A. The human circulating blood volume is about 5.6 L 1. Approximately 55% of blood volume is plasma 2. Approximately 45% of blood volume is cells and platelets B. Plasma is the fluid component of blood 1. Plasma is mostly water, with dissolved proteins, salts, gases, nutrients, wastes, and hormones 2. Composition of the plasma varies with location of the body, as material fluxes change 3. Plasma proteins have a variety of functions a) Fibrinogen is involved in clotting (1) When clotting proteins have been removed from plasma, it is called serum b) There are three types of globulins: alpha, beta, and gamma c) Alpha globulins include some hormones and proteins that transport hormones d) Beta globulins include some lipoproteins, as well as proteins that transport some vitamins and minerals (1) High-density lipoproteins transport triglycerides and cholesterol e) The gamma globulins include many antibodies f) Albumins are important in maintaining the appropriate osmotic balance of the blood C. Red blood cells transport oxygen 1. Erythrocytes are the most abundant of the cellular elements 2. Erythrocytes are flexible, biconcave discs that are packed with hemoglobin 3. As they lack most cellular organelles, their lifespan is only about 120 days a) Erythrocytes are produced in the red bone marrow in vertebrae, the sternum, ribs, and some skull and long bones b) Old erythrocytes are removed by the spleen and liver, and various components are recycled 4. Anemia is a deficiency of hemoglobin, or the number of red blood cells, or both a) Anemia may be due to bleeding, decreased production of hemoglobin or RBCs due to lack of iron, or destruction of RBCs (hemolytic anemias such as sickle cell anemia) D. White blood cells defend the body against disease organisms 1. Leukocytes (WBCs) function in immunity, and may be classified by the presence or absence of conspicuous cytoplasmic granules 2. Granular leukocytes are manufactured in bone marrow, and have lobed nuclei and granules visible under the light microscope a) Neutrophils are the most abundant WBCs, and are primarily phagocytic (1) The cytoplasmic granules contain digestive enzymes b) Eosinophils stain red in an eosin dye and function during allergic reactions and parasitic worm infections c) Basophils stain in basic dyes, and are also involved in allergic reactions and produce histamines and heparin 3. The agranular leukocytes do not have conspicuous granules, and their nuclei are round or kidney-shaped a) Lymphocytes produce antibodies b) Monocytes are the largest WBCs, but typically stay within the circulatory system for a very short time (1) Monocytes leave the circulatory system and complete development within connective tissue, turning into a macrophage, a type of phagocyte 4. A differential count analyzes the different proportions of WBCs, which is used as a diagnostic aid 5. Leukemia is a cancer of the white blood cells; an overabundance of these immature cells leads to weakness and impaired clotting E. Platelets function in blood clotting 1. Thrombocytes in most vertebrates have nuclei; in mammals, the thrombocytes are small cell fragments lacking nuclei 2. Platelets form a temporary clot, or platelet plug, at the site of a cut 3. Next, a complex process involving a series of reactions forms a more permanent clot of thrombin and fibrin fibers a) Hemophilia is a genetic mutation in which the person lacks one of the clotting factors, which may lead to uncontrolled bleeding

III. Vertebrates have three main types of blood vessels F. Arteries carry blood away from the heart, and typically carry oxygenated blood 1. Arteries are characterized by branching, forming smaller arterioles 2. Arterioles ultimately form microscopic capillaries G. Capillaries merge to form venules, and then veins H. Blood vessels have three layers 1. The innermost layer consists mainly of endothelium 2. The middle layer is composed of connective tissue and smooth muscle 3. The outer layer is composed of elastic and collagen fibers I. The thick walls of arteries and veins prevent passage of materials; the walls of capillaries are so thin that materials pass relatively easily J. Smooth muscle in the walls of arteries results in vasoconstriction or vasodilation, necessary to maintain appropriate blood pressure and blood supplies 1. Precapillary sphincter muscles are located between metarterioles and true capillaries and allow a fine control of blood flow

IV. Each vertebrate class exhibits different adaptations of the heart and circulation K. The vertebrate heart has one or two atria, and a single or divided ventricle L. In fish, blood flows in a single circuit from heart to gills to capillaries, which supply the tissues of the body 1. This circuit is a low-pressure system and permits a slow metabolic rate in fishes 2. The heart of a fish has a sinus arteriosus leading to a single atrium followed by one ventricle, and blood ultimately flows out a non-muscular conus arteriosus M. In amphibians, the pulmonary circuit flows to the lungs and skin, and the systemic circulation flows to the remainder of the body 1. The atria are separate, and the ventricle is undivided, but oxygenated and unoxygenated blood are prevented from mixing by the fold in the conus arteriosus, which keeps the blood apart N. In most reptiles, a partially divided ventricle further prevents mixing 1. In crocodiles, the heart is truly four chambered O. In birds and mammals, the heart is four chambered 1. The conus arteriosus has split and forms the bases of the aorta and the pulmonary artery 2. The sinus venosus is no longer a separate chamber, but is present as a vestige in the wall of the right atrium (the sinoatrial node) 3. The systemic circuit is a higher-pressure circuit, delivering blood to the body more efficiently, allowing endothermy and high metabolic rates

V. The human heart is well adapted for pumping blood P. The heart, beating over 2.5 billion times in an average lifetime, can vary its output from 5 to more than 20 L of blood 1. The heart is surrounded by the pericardium, composed of dense connective tissue a) The inner and outer pericardium form a small space between them, the pericardial cavity, filled with pericardial fluid 2. The wall of the heart is composed of cardiac muscle fibers and collagen fibers 3. The interatrial septum divides the atria; the interventricular septum divides the ventricles a) The fossa ovalis is located in the interatrial septum, and is the remnant of the fetal foramen ovale b) The externally visible flap in the wall of the atrium is the auricle 4. Valves prevent backflow of blood a) Atrioventricular valves are located between the atria and the ventricles (1) The right AV valve is called the tricuspid valve as it has three cusps or flaps (2) The left AV valve is called the bicuspid or mitral valve (3) AV valves are closed during ventricular contraction (4) AV valves are prevented from everting by the chordae tendinae b) Semilunar valves are found between the ventricles and the great vessels of the heart (1) The pulmonary semilunar valve is located between the pulmonary artery and the right ventricle (2) The aortic valve is located between the aorta and the left ventricle Q. Each heartbeat is initiated by a pacemaker 1. The SA (sinoatrial) node is the pacemaker, located in the wall of the right atrium a) When the SA node fibers fire, an action potential is spread across both atria simultaneously, via the gap junctions in the intercalated discs between cells b) The action potentials last longer in cardiac muscle than in skeletal muscle cells 2. The AV (atrioventricular) node, located in the lower right atrium, acts as a delay and relay node 3. After a slight delay, transmission continues into the Purkinje fibers, which make up the atrioventricular (AV) bundle 4. The AV bundle splits, sending branches upward over both ventricles, resulting in ventricular contraction 5. The cardiac cycle lasts about 0.8 sec, consisting of atrial systole and ventricular diastole—then atrial diastole and ventricular systole R. Two main heart sounds can be distinguished 1. The "lub-dup" heart sounds mark the beginning of ventricular systole, then ventricular diastole 2. A heart murmur is caused by valves that do not close properly and results in hissing heart sounds S. Heart rate is regulated by the nervous system 1. Cardiac centers in the medulla communicate with the heart via autonomic fibers 2. Hormones from the adrenal glands speed the heart rate 3. Norepinephrine and adrenaline act on the ion channels a) Beta blockers are used clinically to treat hypertension, as they target the ion channels T. Stroke volume depends on venous return 1. Starling’s law states that the greater the venous return, the more forceful the contraction of the heart (and the greater the cardiac output) 2. Sympathetic stimulation increases the force of contraction U. Cardiac output varies with the body's need 1. The amount of blood pumped by one ventricle in one beat is the stroke volume 2. The stroke volume x the number of ventricular contractions per minute = the cardiac output (about 5 L per minute) 3. Changes in cardiac output are based on changes in either stroke volume or heart rate (or both)

VI. Blood pressure depends on blood flow and resistance to blood flow V. When cardiac output increases, blood pressure increases 1. Peripheral resistance is the resistance to blood flow by the viscosity of the blood, itself, and the friction between the blood and the walls of the vessels 2. Blood pressure varies during ventricular systole and diastole, varying from 120 to 80 mm Hg 3. Blood pressure is expressed as a fraction, with systolic as the numerator and diastolic as the denominator 4. When diastolic pressure is typically 80 to 89 mm Hg and systolic pressure is 120 to 139 mm Hg, hypertension may be problematic W. Blood pressure is highest in arteries 1. Arteries have higher pressure due to ventricular systole and their smaller diameters as compared with veins 2. Because pressure is lower in veins, most large veins have valves to prevent backflow X. Blood pressure is carefully regulated 1. Complex mechanisms regulate blood pressure as we move to different positions 2. Baroreceptors in several large arteries and the heart wall sense changes in blood pressure 3. When blood pressure rises, medullary centers send parasympathetic impulses to the heart to lower blood pressure by slowing the heart 4. Angiotensins are hormones that are vasoconstrictors a) The hormone renin causes production of angiotensins

VII. Blood is pumped through pulmonary and systemic circuits Y. The pulmonary circulation oxygenates the blood 1. Blood is pumped to the lungs by the right side of the heart 2. The pulmonary trunk carries blood from the right ventricle and then splits into two arteries that lead to each lung 3. Respiratory gases diffuse into capillaries in the alveoli of the lungs 4. The pulmonary veins carry blood back to the left atria, carrying oxygenated blood Z. The systemic circulation delivers blood to the tissues 1. Blood leaves the left ventricle and flows into the aorta 2. Most of the blood supplies the tissues of the body a) The carotid arteries supply the brain b) The subclavian arteries supply the upper appendages c) The mesenteric arteries supply the intestines d) The renal arteries supply the kidneys e) The iliac arteries supply the lower appendages 3. Veins return blood to the right side of the heart a) Jugular veins return blood from the brain b) The subclavian veins return blood from the upper appendages c) Renal veins return blood from the kidneys d) Iliac veins return blood from the lower appendages e) Hepatic veins return blood from the liver f) Renal, iliac, and hepatic veins empty into the inferior vena cava g) Jugular and subclavian veins empty into the superior vena cava 4. The coronary circulation delivers blood to the heart a) A branch of the aorta supplies blood to the wall of the heart b) Coronary arteries branch into coronary capillaries, where nutrient exchange takes place c) Coronary veins form the coronary sinus, which empties into the right atrium 5. Four arteries deliver blood to the brain a) Two internal carotid arteries and two vertebral arteries supply blood to the brain b) These arteries form a circuit called the Circle of Willis c) Blood from the brain returns to the superior vena cava via the internal jugular veins 6. The hepatic portal system delivers nutrients to the liver a) A portal system is a capillary system with veins on both ends b) Blood flows from the intestinal veins to the hepatic portal vein, and this breaks up into a second capillary system within the liver for processing of the blood

VIII. The lymphatic system is an accessory circulatory system AA. The functions of the lymphatic system include return of interstitial fluid to the circulatory system, immunity, and absorption of lipids from the gastrointestinal tract BB. The lymphatic system consists of lymphatic vessels and lymph tissue 1. Lymph vessels conduct lymph, derived from interstitial fluid 2. Lymph tissue is an organ composed of connective tissue with many lymphocytes a) Lymph nodes and nodules are small masses of lymph tissue (1) Lymph nodes function to filter the lymph b) The spleen, tonsils, and thymus are larger organs (1) Tonsils are masses of lymph tissue in the pharyngeal region that filter out pathogens 3. Lymph capillaries are one-way vessels, which join and merge to form larger lymphatics (= lymph veins) a) Lymph vessels ultimately empty into the subclavian veins via the larger thoracic duct and the right lymphatic duct b) Lymph is moved in mammals by differences in pressure, pulsation of vessel walls, and contraction of skeletal muscles CC. The lymphatic system plays an important role in fluid homeostasis 1. Fluid is forced out of capillaries and must ultimately return to the circulatory system 2. The ends of the lymph capillaries have tiny valves that allow one-way flow of interstitial fluid into the lymph capillary 3. Obstruction of lymph vessels leads to edema 4. Some parasitic nematodes cause extreme edema, followed by development of connective tissue, which leads to grotesque swelling of affected parts

Research and Discussion Topics

 Describe the evolution of the heart from ancestral fish, to amphibians, to reptiles, to the four-chambered heart seen in birds and mammals.  Discuss the three different types of arteriosclerosis, including atherosclerosis, which is the most common form. What are the various proposed causes of arteriosclerosis?  How might a differential count give a diagnosis for a patient? For example, if you had a patient who had an elevated eosinophil count, what might you suggest as a diagnosis? Specifically, what questions might you ask the patient?  Think about the various stresses on the four valves of the heart. Which is under the most stress, and is therefore the most involved in problems? (Hint: What is MVP? Answer: mitral valve prolapse.)