MED-202 Histology I Cardiovascular System Fall 2024 PDF

Cardiovascular System MED-202 Histology I Fall 2024 Annita Achilleos, PhD  Reading Material Histology: A Text and Atlas (Pawlina): Chapter 13 Wheater’s Functional Histology: Chapter 8 Lecture Material  Learning Objectives Describe t...

Cardiovascular System MED-202 Histology I Fall 2024 Annita Achilleos, PhD  Reading Material Histology: A Text and Atlas (Pawlina): Chapter 13 Wheater’s Functional Histology: Chapter 8 Lecture Material  Learning Objectives Describe the histology and function of the different layers of the heart. Outline the different microscopic features of arteries and veins. Identify the differences between different types of arteries (from elastic artery to arteriole) and veins (from large vein to venule).  Overview of the Cardiovascular System Lungs – Cardiovascular system gas exchanged - carries blood and lymph to and from the tissues of the body - it includes: 1. Heart Aorta and 2. Blood vessels branches 3. Lymphatic vessels Two circuits distribute blood in the body: – Pulmonary circulation: conveys blood from the heart to the lungs and O2 rich, CO2 poor blood back O2 poor, CO2 rich blood – Systemic circulation: conveys blood from the heart to the other tissues in the Body tissue – body and back gas exchange Overview of the Cardiovascular System Assuming 75 beats/minute, a human heart would contract: ~108,000 times in one day >39 million times in one year nearly 3 billion times during a 75-year lifespan Each of the major pumping chambers of the heart ejects approximately 70 mL blood per contraction in a resting adult. This would be equal to 5.25 liters of fluid per minute 14,000 liters per day Over one year, that would equal 10,000,000 liters or 2.6 million gallons of blood sent through roughly 60,000 miles of vessels. The Heart The Heart Fibrous skeleton 1. Fibrous skeleton - anchors the heart valves and gives attachment to the myocardium above and below 2. Heart wall - responsible for contraction and protection Conduction system 3. Conduction system - composed of modified cardiac muscle cells (Purkinje fibers) initiation and propagation of rhythmic contractions Coronary vasculature 4. Coronary vasculature - 2 coronary arteries - 2 cardiac veins  Fibrous skeleton The fibrous skeleton of the heart (cardiac skeleton): - dense irregular connective tissue - anchors the heart valves and provides support preventing them from stretching - gives attachment to the myocardium of the ventricles and atria - it consists of: - 4 fibrous rings - 2 fibrous triangular structures connecting the rings - the membranous part of the interventricular septum Left fibrous trigone Right fibrous trigone Membranous part of the interventricular septum  Heart Valves The heart consists of four chambers - two atria - two ventricles Pulmonary Aortic - the blood passes through a valve before valve valve leaving each chamber of the heart - the valves prevent the backward flow of blood 1. Tricuspid valve: between the right atrium and the right ventricle Mitral Tricuspid valve valve 2. Pulmonary valve: between the right ventricle and the pulmonary artery 3. Mitral valve: between the left atrium and the left ventricle Each valve has three flaps, except 4. Aortic valve: between the left ventricle the mitral valve, which has two flaps. and the aorta  Heart Wall - 3 Layers The heart wall: is composed of: - connective tissue - cardiac muscle - endothelium Pericardium Epicardium is divided into three layers: 1. Epicardium − the outer protective layer of the heart 2. Myocardium: Myocardium − muscular middle layer wall of the heart Endocardium 3. Endocardium: − the inner layer of the heart Pericardium: − a fibrous sac that encloses the heart and the great vessels  Histology of the Heart - Pericardium Pericardium - a double-walled sac containing the heart and the roots of the great vessels - encloses the pericardial cavity which contains pericardial fluid - provides the lubrication for the heart - has two layers a. a serous layer – inner layer b. a fibrous layer – outermost layer Pericardial cavity Fibrous pericardium Myocardium Parietal layer of Endocardium serous pericardium Visceral layer of serous pericardium (epicardium)  Histology of the Heart - Epicardium Epicardium - consists of: a. single layer of mesothelial cells simple squamous epithelial cells (of mesodermal origin) b. underlying loose and dense connective and adipose tissue - contains the blood vessels and nerves that supply the heart Pericardial cavity Fibrous pericardium Myocardium Parietal layer of Endocardium serous pericardium Epicardium (visceral layer of serous pericardium)  Histology of the Heart - Myocardium Myocardium - the middle layer of the heart wall - is composed of cardiac muscle fibers - enable heart contractions - is the thickest layer of the heart wall (thickness varies in different parts of the heart) - the myocardium of the left ventricle is the thickest Pericardium Endocardium Myocardium Epicardium Histology of the Heart - Myocardium  Histology of the Heart - Endocardium Endocardium - the thin inner layer of the heart wall - covers the heart valves - composed of - is continuous with the endothelium of the large blood vessels - endothelium - location of the conducting system - connective tissue - smooth muscle cells Pericardium Endocardium Myocardium Epicardium  Conducting system of the heart Purkinje fibres - found in the sub-endocardium - specialized conducting fibers composed of electrically excitable cells - their primary role is in rapid conduction of the electrical impulse larger than cardiac muscle cells have fewer myofibrils increased glycogen and mitochondria - connected together by desmosomes and gap junctions, but not by intercalated discs  Conducting system of the heart Purkinje fibres - facilitate fast propagation of signal partially due to the different connexins in the gap junctions in these cells Cx40 - a connexin protein responsible for high conductance - is at least three fold greater in Purkinje fibers than in myocardial cells  Coronary vasculature of the heart Left coronary artery Right coronary artery Circumflex artery Left anterior ascending artery Cardiac veins  Coronary vasculature of the heart Coronary arteries are about 4 mm wide and gradually taper as they descend on the heart Cardiac veins may measure up to 6 mm  Vascular System Blood travels from the heart: arteries arterioles capillaries - oxygen and nutrients pass from the blood into tissues - carbon dioxide and waste products pass from tissues into blood Then blood passes: capillaries venules veins to return to the heart Arteries and arterioles: - have relatively thick muscular walls blood pressure is high they must adjust their diameter to maintain blood pressure and to control blood flow Veins and venules: - have thinner muscular walls the pressure is much lower veins may dilate to accommodate increased blood volume  General Features of Arteries and Veins Arteries: Veins: - Large (Elastic) arteries - Large Vein - Medium (Muscular) arteries - Medium Vein - Arteriole - Venule  Layers of the vascular wall Tunica intima - endothelium simple squamous epithelial cells Tunica Tunica media - basal lamina externa (adventitia) Tunica intima - sub-endothelial layer loose connective tissue Tunica media Smooth muscle Endothelium - smooth muscle cells - external elastic membrane External elastic membrane Internal elastic membrane Tunica adventitia (externa) - connective tissue - collagen - elastic fibers (few)  Vascular Endothelium - Tunica intima Functions of the endothelium artery 1. Barrier function 2. Blood clotting prevention 3. Inflammation 4. Angiogenesis 5. Vasoconstriction and vasodilation tunica endothelium intima connective tissue internal elastic membrane tunica involuntary media muscle fiber elastic fiber tunica adventitia external elastic membrane connective tissue  Tunica Media artery Tunica media is made up of: a. smooth muscle cells vasodilate and vasoconstrict b. elastic tissue tunica intima endothelium flexibility connective tissue internal elastic membrane tunica involuntary media muscle fiber elastic fiber tunica adventitia external elastic membrane connective tissue  Tunica Adventitia (externa) Tunica adventitia artery - the outermost layer - a tough layer consisting mainly of collagen fibers (support) tunica intima endothelium - large arteries have a much connective tissue thicker tunica adventitia and internal elastic tunica media than medium-sized membrane arteries tunica involuntary media muscle fiber elastic fiber tunica adventitia external elastic membrane connective tissue  Arteries Three main types of vessel in the arterial system: 1. Elastic arteries - the major distribution vessels - include: - the aorta - the innominate (brachiocephalic trunk) - common carotid - subclavian arteries - most of the large pulmonary arterial vessels 2. Muscular arteries - medium-sized arteries - draw blood from an elastic artery and branch into small arteries and arterioles - contains a lot of smooth muscles (contraction and expansion) - are the main distributing branches of the arterial tree - include: - radial artery - femoral artery - coronary artery - cerebral artery 3. Arterioles - the terminal branches of the arterial tree - supply the capillary beds Arteries  Common Disorders of the arteries - Formation of thrombus Atherosclerosis - elastic (large) and muscular arteries are involved - a common disease in which lipid material infiltrates the tunica intima and accumulates macrophages induces proliferation of fibroblasts and myointimal cells producing a plaque thickening of the intima reduction of the lumen the blood flow is limited if ruptured, the lumen narrows further aggregation of platelets and fibrin can form a thrombus further increase of the plaque size and further reduction of the vessel lumen  Common Disorders of the arteries - Formation of aneurysm Aneurysm - an abnormal swelling or bulge on the wall of a blood vessel - can be a consequence of severe atheroma in elastic arteries the muscle cells of the tunica media are replaced by non-contractile and non-elastic collagen weakening of the arterial wall leading to bulging and possibly rupture  Large Veins Tunica Media - relatively thin Tunica Adventitia - thickest layer of the vessel wall - longitudinal bundles of smooth muscle cells Endothelium Tunica externa Tunica media Tunica interna  Medium Veins Tunica Media - relatively thin Tunica Adventitia - thickest layer of the vessel wall - collagen and elastic fiber - very few (if any) smooth muscle cells  Venules Venule - a very small blood vessel in the microcirculation - allows blood to return from the capillary beds to drain into the veins - range from 7μm to 1mm in diameter - veins contain approximately 70% of total blood volume, 25% of which is contained in the venules - many venules unite to form a vein  Arteries vs Veins Small arteries - have 3-25 muscle layers in the tunica media Small veins - found adjacent to small arteries - have thinner walls - a larger lumen - more irregular in shape A. small artery B. small vein  Arteries vs Veins Aorta Large Vein Arteries vs Veins  Arteries vs Veins - Arteries carry blood away from the heart - Veins return blood to the heart - Veins are: - generally larger in diameter - carry more blood volume - have thinner walls in proportion to their lumen - Arteries are: - smaller in diameter - have thicker walls in proportion to their lumen - carry blood under higher pressure than veins - Arteries and veins often travel in pairs using the same connective tissue pathways Capillaries  Continuous capillaries Continuous capillaries - most common - have a continuous endothelial lining the least permeable - have tight junctions with intercellular clefts (spaces) for passage of small molecules blood–brain barrier  Continuous capillaries Tight junction Tight junction TEM – cross-section of a continuous capillary in smooth muscle  Fenestrated capillaries Fenestrated capillaries - the endothelium has pores of 60–80 nm in diameter (fenestrae) - spanned by a diaphragm of radially oriented fibrils that allows small molecules and limited amounts of protein to diffuse - found in areas that require a lot of exchange between blood and tissues kidney  Fenestrated capillaries TEM – cross-section of a monocyte in a fenestrated capillary SEM – fenestrated capillary in the glomerulus (kidney)  Sinusoidal (Discontinuous) capillaries Discontinuous capillaries or Sinusoidal capillaries - a special type of open-pore capillary - wider openings in the endothelium, 30–40 μm diameters - no diaphragm surrounding the pores - discontinuous basal lamina - red and white blood cells (7.5 μm – 25 μm diameter) and some proteins can go through mainly found in the liver, bone marrow, spleen, and brain circumventricular organs  Sinusoidal capillaries TEM – hepatic sinusoid  Lymphatic System Lymph - a clear-to-white fluid - made of: 1. plasma 2. white blood cells - It carries the waste products and destroyed bacteria back into the bloodstream Lymphatic System  Lymphatic Vessels Lymphatic vessels - convey fluid from the tissues to the bloodstream - unidirectional Lymphatic capillaries - more permeable than blood capillaries - tubes of endothelium with no continuous basal lamina - converge into larger lymphatic vessels Lymphatic Vessels  Summary 1. Basic architecture of the human heart 1. Valves 2. Endocardium 3. Myocardium 4. Pericardium 2. Clinical correlation of diseases affecting the heart/vessels. 3. Vascular system 1. Organisation of blood vessels 2. Comparison between the wall structure of Veins and Arteries 3. Key difference between arteries/veins and lymphatic vessels 4. Different types capillaries - Continuous, Fenestrated, Discontinuous 4. Lymphatic vessel and key difference with arterioles/venules Thank you!