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Kristina C. Erasmo, M.D. Function • Transport – oxygen, carbon dioxide, nutrients, hormones, waste products • Immune defense CIRCULATORY SYSTEM • Cardiovascular system – Heart – 2 systems of blood vessels: • Systemic circulation • Pulmonary circulation • Lymph vascular system Heart • Muscular pump that propels blood to the arteries of both systemic and pulmonary circulation • Systemic circulation – brings blood from the heart to the rest of the body then back to the heart • Pulmonary circulation – brings blood from the heart to the lungs and then back to the heart Heart • Located in the central mediastinum of the thoracic cavity • Four chambers: – Left ventricle – systemic circulation – Right ventricle – pulmonary – Left atrium – Right atrium Pericardium • Connective tissue sac covering the heart – Outer sac (fibrous pericardium) – Inner sac (serous pericardium) • Parietal pericardium • Visceral pericardium • Pericardial cavity – with fluid Pericardium • Fibrous and parietal pericardia – dense CT (collagen and elastic fibers, fibroblasts, other CT elements) • Inner aspect of parietal pericardium – simple squamous epithelium (mesothelium) • Visceral pericardium a.k.a. epicardium Histologic Layers of Heart Wall • Epicardium • Myocardium • Endocardium Epicardium • a.k.a. visceral pericardium • Outermost layer of heart wall • Lining epithelium of external surface: mesothelium Myocardium • • • • Thickest (thickness varies for different parts) Thickest: left ventricle Thinnest: atria Mainly cardiac muscle fibers Purkinje Fibers • Modified cardiac muscle cells • Initiate and conduct the electrical impulse that controls the contraction of the heart • Form the impulse-conducting system of the heart • Larger, contain more glycogen than ordinary cardiac muscle cells Myocardium • Atrial natriuretic peptide (ANP) – polypeptide hormone secreted by cardiac muscle cells in atria and interventricular septum – Causes natriuresis – excretion of sodium by the kidneys Endocardium • Thinnest • Lining epithelium: endothelium • Continuous with innermost layer of the great blood vessels • Lines all internal surfaces of heart Endocardium • Subendothelial layer – loose CT • External to subendothelial layer – dense CT, thickest portion of endocardium Blood Vascular System • Refers to the system of blood vessels within which blood circulates • 3 types of blood vessels: – Arteries – Veins – Capillaries Blood Vascular System • Arteries – carry blood from the heart to the capillaries • Veins – carry blood from the capillaries to the heart Endothelium • Lining of the luminal surface of all blood vessels • Functions: – Lining blood vessels to facilitate blood flow – Secretes components of underlying CT Endothelium • Functions: – Secrete prostacyclin – inhibits platelet aggregation, promotes vasodilation – Nitric oxide and endothelin I – vasodilator – Von Willebrand factor – blood coagulation Vasa Vasorum • Network of small blood vessels that supply large blood vessels • Veins: present in the tunica adventitia and media • Arteries: confined to adventitia Capillaries • Smallest blood vessels (5-10 um) • Connect arterioles and venules • Function: Allow exchange of gases and materials between blood and surrounding tissue Capillaries • Wall consists of single layer of endothelial cells and its associated basal lamina • Surrounding connective tissue elements Pericytes • Slender, elongated, perivascular cells that usually accompany capillaries • Look like fibroblasts • Not part of the endothelium • Contain tropomyosin • Function: contractile cells that influence the luminal size of capillaries Types of Capillaries • Continuous (Type I) • Fenestrated (Type II) • Sinusoids Continuous Capillary • Found in muscles, lungs, CNS, skin • The endothelial cell and the basal lamina do not form openings (which would allow substances to pass the capillary wall without passing through both the endothelial cell and the basal lamina) Fenestrated Capillary • Mucous membranes of GIT, renal glomerulus, pancreas, some endocrine glands • Endothelial cell body forms small openings (fenestrations) which allow components of the blood and interstitial fluid to bypass the endothelial cells Sinusoids • a.k.a. discontinuous capillaries or sinusoidal capillaries • Formed by fenestrated endothelial cells, (which may not even form a complete layer of cells) • Basal lamina is also incomplete • Found in liver, spleen, red bone marrow (free exchange of substances or even cells between bloodstream and organ is advantageous) Histologic Layers of Arteries and Veins • Tunica adventitia • Tunica media • Tunica intima Tunica Adventitia • Outermost coat of arteries and veins • Chiefly made up of connective tissue with cells and fibers arranged longitudinally Tunica Media • Middle coat of arteries and veins • Concentrically arranged smooth muscle fibers • Interspersed between the muscle cells are connective tissue elements (collagen and elastic fibers) Tunica Media • In large arteries: – External elastic lamina/membrane – outer layer of elastin which demarcates the tunica media from the tunica adventitia Tunica Intima • Innermost coat of arteries and veins • Consists of a layer of endothelium that rest on a basal lamina • Subendothelial layer of loose CT • In arteries: – Internal elastic lamina/membrane – demarcates tunica intima from tunica media Arteries • Classification: – Small (arteriole) – Medium (muscular or distributing) – Large (elastic or conducting) • Decrease in size but increase in number as they go farther from the heart Arterioles • Smallest arteries (40 – 400 um) • Tunica intima: only endothelium, no IEL or subendothelial CT • Tunica media: no EEL • Tunica adventitia: loose CT Arterioles • Relatively thick walls and narrow lumens – offer considerable resistance to blood flow (thus able to deliver blood to the capillary beds under greatly reduced pressure) Medium Arteries • Muscular arteries – well-developed tunica media • Distributing arteries – distribute blood to the different parts of the body • Biggest: brachial and femoral arteries Medium Arteries • Tunica intima: 3 layers – Endothelium, subendothelium, and IEL • Tunica media: Up to 40 layers of circularly arranged smooth muscle fibers, EEL present • Tunica adventitia: loose CT, vasa vasorum Large Arteries • Elastic arteries – high elastic tissue content • Conducting arteries – major conduits in the transport of blood away from the heart • E.g. aorta • Recoil is responsible for the flow of blood through the blood vessels during diastole Large Arteries • Tunica intima: – Polygonal endothelial cells, – Subendothelial layer – loose CT – Indistinct IEL Large Arteries • Tunica media: – 40-60 layers of elastic laminae arranged concentrically – Indistinct EEL • Tunica adventitia: – Thin, merges with surrounding CT – Contains vasa vasorum Sensory Organs Associated with Arteries • Carotid bodies and aortic bodies – Sensitive to changes in oxygen and CO2 tension of blood • Carotid sinus – Sensitive to changes in arterial blood pressure Veins • Classification: – Small (venules) – Medium – Large Veins • Carries blood from the capillaries to the heart • More numerous than arteries • Bigger calibers, more irregular lumens, thinner and less elastic walls • Increase in caliber and walls become thicker as they travel toward the heart Small Veins (Venules) • The 3 histologic layers of the vessel wall can only be appreciated in venules with a luminal diameter of at least 50 um • Like capillaries, they are sites for exchanage of gases and substances Medium Veins • Tunica intima: – Polygonal endothelial cells – Subendothelial layer • Tunica media: small bundles of circularly arranged smooth muscle cells • Tunica adventitia: bulk of the wall Large Veins • E.g. inferior and superior vena cava, portal vein • Tunica intima: thicker • Tunica media: poorly developed, few smooth muscle fibers Large Veins • Tunica adventitia: very thick, consists of 3 poorly-defined zones – Internal zone – dense CT – Middle zone – longitudinally arranged muscle fibers – Outer zone – collagenous and elastic fibers Arteries vs. Veins Arteries Veins Lumen Patent, RBC not seen Lumen may be collapsed, RBC usually seen Endothelial lining Well-defined Not so well-defined Internal elastic lamina (IEL) Distinct Poorly defined Tunica media Forms 2/3 of thickness of wall 1/3 External elastic lamina (EEL) Distinct Poorly defined Tunica adventitia Forms 1/3 of thickness 2/3