Cardiovascular System PDF

Cardiovascular System Kristine Krafts, M.D. Cardiovascular System Lecture Objectives • Describe the histologic structure of the 3 layers or tunics making up the wall of blood vessels and the heart. • Discuss and compare the variation in structure and function of capillaries, arterioles, muscular a...

Cardiovascular System Kristine Krafts, M.D. Cardiovascular System Lecture Objectives • Describe the histologic structure of the 3 layers or tunics making up the wall of blood vessels and the heart. • Discuss and compare the variation in structure and function of capillaries, arterioles, muscular arteries, elastic arteries, veins, and venules. • Discuss the structure and function of the lymphatic vascular system. Cardiovascular System Lecture Outline • Introduction • Blood vessels • Basic structure • Types of blood vessels • Heart • Gross structure • Three layers of the heart • Embryological development of the heart Cardiovascular System Lecture Outline • Introduction Function of the Cardiovascular System The main job of the cardiovascular system is to transport (and exchange) nutrients, oxygen, body fluids, waste material, heat, and blood cells around the body. Components of the Cardiovascular System • The cardiovascular system is basically a closed system of plumbing: blood circulates through pipes (blood vessels) and is propelled by an inline pump (the heart). • Blood flows through the system from: heart → elastic artery → muscular artery → arteriole → capillaries → venules → vein → heart Vessels of the Cardiovascular System Cardiovascular System Lecture Outline • Introduction • Blood vessels • Basic structure Blood Vessels: Three-Layered Structures Vein Artery Blood Vessels: Three-Layered Structures Tunica Intima • The innermost layer of the vessel wall, next to the lumen. • Composed of endothelium and subendothelium. • Surrounded by basal lamina and loose connective tissue. • Arteries have an internal elastic lamina that separates the tunica intima from the tunica media. The internal elastic lamina is made of elastin and has holes to allow diffusion. Tunica Media • The middle layer of the vessel wall. • Consists of concentrically arranged layer(s) of smooth muscle. • Arteries have a much thicker tunica media than veins due to more smooth muscle. • Large arteries typically have an external elastic lamina separating tunica media and tunica adventitia. Tunica Externa (Adventitia) • The outermost layer of the vessel wall. • Composed of collagen and elastic fibers. Blends into the connective tissue surrounding the blood vessel. • The tunica externa (adventitia) is the thickest layer in veins. • Large veins may have smooth muscle bundles running longitudinally in this layer. Layers are different in different types of vessels! Elastic artery Muscular artery Arteriole Layers are different in different types of vessels! Large vein Medium-sized vein Venule Vasa Vasorum • Large arteries and veins have small blood vessels called vasa vasorum in the adventitia and outer media. • Vasa vasorum nourish the adventitia and outer tunica media. • Veins have more vasa vasorum than arteries because veins have a lower content of oxygen and nutrients in blood in the lumen. Tunica intima Tunica media Tunica externa Vasa vasorum Vasa vasorum in adventitia of muscular artery Cardiovascular System Lecture Outline • Introduction • Blood vessels • Basic structure • Types of blood vessels Capillaries • Smallest diameter of all blood vessels (7-9 μm). • Allow exchange of metabolites and waste between blood and surrounding tissue. • Composed of a single layer of endothelial cells rolled up into a tube. • Endothelial cells rest on a basal lamina. • Endothelial cells are connected by junctions of the zonula occludens type. Other Cool Things Capillaries Do • Convert angiotensin I to angiotensin II • Inactivate things like prostaglandins and norepinephrine • Produce factors that constrict and relax vessels Pericytes • Small cells adjacent to capillaries and venules. • During injury and repair, pericytes can transform into other cells, forming new blood vessels and connective tissue cells. Three Types of Capillaries Differences are in structure of endothelium and basal lamina. • Continuous capillaries • Fenestrated capillaries • Sinusoidal capillaries or sinusoids Continuous (Tight) Capillaries • Most common type of capillary. • Found in muscle, connective tissue, glands and nerve tissue. • No fenestrae (openings) are present, resulting in less exchange of substances across capillary wall. • Some continuous capillaries have numerous pinocytotic vesicles in endothelium, which transport substances across endothelial cells. Continuous Capillary Fenestrated Capillaries • Present in organs needing rapid exchange of substances between tissues and blood (e.g. kidney, intestine, and endocrine organs). • Fenestrations are perforations or openings typically covered by thin diaphragms. • Continuous basal lamina is present. • Substances easily cross capillary wall. Fenestrated Capillary Continuous basal lamina Fenestration covered by diaphragm Endothelial cell nucleus Fenestrated capillary and diaphragms in endothelial cell Sinusoidal Capillaries (Sinusoids) • Very large diameter and twisting course (so blood moves slowly). • Located in liver, bone marrow, spleen and some endocrine organs. • Very leaky! Substances move freely across capillary wall and into tissues because: • There are spaces between endothelial cells • The endothelial cells have large fenestrae without diaphragms • The basal lamina is discontinuous Sinusoid Macrophages Sinusoidal capillaries in spleen Capillary Bed • Capillaries branch extensively, forming a microvasculature network (capillary bed) with a huge surface area. • Capillary beds are great places to exchange water, ions, and macromolecules between blood and tissue because: • Blood flows slowly • Surface area is huge Exchange of substances between blood and tissue occurs in capillary beds. So…what determines whether stuff moves across capillary walls? • Size and charge of molecules • Structure of capillary: • Types of intercellular junctions • Presence of spaces between epithelial cells • Presence of fenestrations • Presence of diaphragms • Presence and number of pinocytotic vesicles Arteries and Arterioles vs. Veins and Venules • Walls of arteries/arterioles thicker than walls of veins/venules of the same diameter • How to tell: examine wall-to-lumen ratio • Arteries/arterioles: larger wall-to-lumen ratio (because of all that smooth muscle in the wall) • Veins/venules: smaller wall-to-lumen ratio Arteries and Arterioles vs. Veins and Venules Arterioles • Small lumen • Tunica intima thin, with typical endothelial cells, often without internal elastic lamina • Tunica media has 1-3 layers of smooth muscle and no external elastic lamina • Tunica externa is thin Venules • Very thin walls • Smaller venules are similar to capillaries in structure and function • All 3 tunics are thin, but externa is relatively the thickest • Tunica media contains a few smooth muscle cells and/or pericytes (not concentric rings of smooth muscle) Arterioles and venules Arterioles and capillaries Arterioles and venules. Note complete rings of smooth muscle in arterioles. Arteries • Arteries conduct blood to different regions and organs of the body. • Muscular arteries are medium to large size arteries. • Elastic arteries are the largest arteries. Large elastic arteries close to heart minimize changes in blood pressure. Muscular (Distributing) Arteries • Make up most of the named arteries of the body. • Prominent internal elastic lamina. • Thick tunica media (up to 40 layers of smooth muscle!). • The larger the artery, the more elastin is present with the smooth muscle. Large muscular artery Tunica intima Tunica media Tunica externa Vasa vasorum Vasa vasorum in adventitia of muscular artery Small muscular artery Elastic Arteries • Largest-diameter arteries in the body • Include the aorta and its largest branches • Stretch during systole and contract during diastole (good thing, otherwise blood pressure would drop to zero during diastole) Elastic Arteries • Tunica intima thicker than that of muscular arteries due to thicker subendothelial layer • Tunica media has numerous elastic laminae which increase with age. In between these are smooth muscle fibers, reticular fibers, and ground substance. • Tunica externa relatively thin, with loose connective tissue, nerves, and vasa vasorum. • Internal and external elastic lamina present but not usually visible. Tunica intima Tunica media with elastic fibers and smooth muscle Tunica externa with vasa vasorum Elastic artery Veins • Veins return blood to the heart • Considered capacitance vessels because at any one time they hold more than 70% of the total blood volume. Small and Medium-Sized Veins • Have thin intima and media, but a relatively thick adventitia. • The media contains a few small bundles of smooth muscle. • The externa consists of collagen fibers without smooth muscle. Valves in Veins • Valves consist of folds of tunica intima that project into the vein lumen. • Most numerous in the limbs. • Ensure that blood flows toward the heart and does not pool in lower extremities. • Blood is propelled toward the heart by contractions of the heart and skeletal muscles surrounding veins. Small vein with valves Small vein and small muscular artery Medium muscular artery and medium vein Large Veins • Tunica media is relatively thin and contains several layers of smooth muscle and abundant connective tissue. • Tunica adventitia relatively thick and contains abundant collagen and some bundles of smooth muscle that run longitudinally. • The purpose of the adventitial smooth muscle is to strengthen the vessel wall to prevent excessive distension. The smooth muscle also has a peristalsis-like action to squeeze the blood up to the heart. Large vein Lymphatic Capillaries • Lymphatic system collects interstitial fluid (lymph) that leaks from blood vessels into connective tissue, and return it to blood • Flow is unidirectional • Lymphatic capillaries consist of one layer of endothelial cells and a discontinous basal lamina • Lymphatic capillaries merge, forming lymphatic vessels (with very thin walls) Lymphatic capillary Lymphatic vessel with valve Lymphatic vessel and blood venule Cardiovascular System Lecture Outline • Introduction • Blood vessels • Basic structure • Types of blood vessels • Heart • Gross structure Cardiovascular System Lecture Outline • Introduction • Blood vessels • Basic structure • Types of blood vessels • Heart • Gross structure • Three layers of the heart The heart is composed of three layers and surrounded by a pericardial cavity Endocardium • Inner layer of heart (analogous to intima of blood vessels). • Consists of endothelium and basal lamina resting on a thin layer of connective tissue. • Subendocardium is a layer of loose connective tissue between endocardium and myocardium. It contains nerves, blood vessels, and branches of the conducting system of the heart. • Valves are considered part of the endocardium. Endocardium right ventricle: thin layer of endothelial cells Subendocardium and endocardium, left atrium (endocardium thicker on this side because of high pressure) Tricuspid valve Right atrium Right ventricle Epicardium Coronary artery Endothelium Dense connective tissue Endothelium Tricuspid valve, high power Myocardium • Muscle fibers are arranged in a spiral pattern around the chambers of the heart. When the fibers contract, the heart twists and wrings out blood from the chambers. • Cardiac muscle fibers are inserted into a dense fibrous connective tissue skeleton of the heart. • Impulse-generating and conducting cells are also present in the myocardium. Myocardium Epicardium • Outer layer of the heart. • Consists of a surface of mesothelium (simple squamous epithelium) supported by a thin layer of connective tissue. • Epicardium contains loose connective tissue, large amounts of adipose tissue, nerves, and blood vessels, including the coronary vessels. Myocardium Epicardium: outer mesothelial cells; underlying dense connective tissue Conducting System • Consists of modified cardiac muscle cells specialized for initiation and conduction of electrochemical impulses. • Cells are distributed in a pattern that coordinates contraction of myocardium. • Purkinje fibers are enlarged cardiac muscle fibers specialized for conducting impulses. Purkinje fibers: larger and lighter-staining (have more glycogen, less myofibrils than regular myocytes) Cardiovascular System Lecture Outline • Introduction • Blood vessels • Basic structure • Types of blood vessels • Heart • Gross structure • Three layers of the heart • Embryological development of the heart Endocardial Heart Tubes • Cardiovascular system starts developing in week 3. • Mesoderm gives rise to two endocardial heart tubes. • Tubes fuse and connect to form the primitive heart tube. Neural groove Amniotic cavity Left dorsal aorta Left dorsal aorta Differentiating myocytes Day 20 Endocardial tubes Neural groove Amniotic cavity Foregut Myocardium Day 21 Left dorsal aorta Myocardium Cardiac jelly Endocardial tubes Two endocardial heart tubes Tubes fuse, forming primitive heart tube Four Chambers of the Early Heart The primitive heart tube folds upon itself, forming dilations and constrictions, producing four chambers: 1. Sinus venosus 2. Primitive atrium 3. Primitive left ventricle 4. Primitive right ventricle (bulbus cordis) Heart tube lengthens and folds Neural fold Neural fold 1st aortic arches Foregut Future left ventricle Bulbus cordis ventricle Outflow tract Future right ventricle (bulbus cordis) Primitive atrium Pericardial cavity Sinus venosus Yolk sac Day 24 Day 25 Heart tube lengthens and folds Neural groove Outflow tract Future left ventricle Atrioventricular sulcus Future right ventricle Future atrium Sinus horns Sinus venosus Bulboventricular sinus Day 25 Heart tube lengthens and folds Day 29 Day 26 Continued folding of heart tube First Heartbeat • The first heartbeat occurs at day 21 or 22. • Originates in the muscle, forming peristaltic waves in the sinus venosus. • By day 28, blood flows in coordinated fashion: • Flows into sinus venosus, then primitive ventricle • Ventricle contracts, pushing blood into bulbus cordis, truncus arteriosus (future aortic sac), and then to rest of embryo Division into Four-Chambered Heart • Between weeks 4 and 7, the true chambers form. • First, endocardial cushions form in walls of atrioventricular canal. • At week 5, they approach, fuse, and form right and left canals. Endocardial cushion formation Formation of Atria • Atria are divided first by the septum primum, then by the septum secundum. • Septum secundum division is incomplete and leaves a hole called the foramen ovale. • A valve (derived from degeneration of the septum primum tissue) covers the foramen ovale. • This is an excellent design because it lets blood flow from right to left ventricle, bypassing lungs. At birth, the foramen usually closes permanently. Septum primum and secundum formation Septum primum and secundum formation Formation of Ventricles • At the end of week 7, the interventricular septum closes, dividing the primitive ventricle into two ventricles (left and right). • The right ventricle communicates with the pulmonary trunk and the left ventricle with the aorta. Initial septation of the ventricles Fetal vs. Postnatal Circulation • Fetal circulation is designed to carry oxygenated blood from the placenta to fetal circulation, bypassing the lungs. • At birth, the infant’s lungs expand and oxygenate the blood. Circulation of blood through the placenta ceases. • After birth, the foramen ovale and umbilical vessels are no longer needed and they close. Cardiovascular System Lecture Outline • Introduction • Blood vessels • Basic structure • Types of blood vessels • Heart • Gross structure • Three layers of the heart • Embryological development of the heart