Histology and Functions of Blood Vessels PDF

!&+#(:14383*&#-).#47)+4:3)1#3,# 3833.#6(11(81# !"#0329)392-#6!#%-%23#/%@@# ! Endocardium: ! Lines chambers of heart and covers heart valves ! Consists of: $ A layer of endothelial cells $ A sub-endothelial layer of c.t. (with veins, nerves and Purkinje fibres)...

!&+#(:14383*&#-).#47)+4:3)1#3,# 3833.#6(11(81# !"#0329)392-#6!#%-%23#/%@@# ! Endocardium: ! Lines chambers of heart and covers heart valves ! Consists of: $ A layer of endothelial cells $ A sub-endothelial layer of c.t. (with veins, nerves and Purkinje fibres) ! Myocardium: ! Consists of cardiac muscles ! Highly vascularized ! Epicardium: ! Also called visceral pericardium ! Made up of fibroelastic c.t. with blood vessels, lymphatics, nerves and adipose tissues ! Covered by a layer of mesothelium (simple squamous epithelium)  Striated involuntary muscle ! Centrally located nucleus ! Branched muscle fibres ! Intercalated disks connects adjacent cardiomyocytes ! Consists of 3 cell-cell junctions: gap junctions, adhering junctions and desmosomes allow rapid transmission of electrical impulses ! Form an syncytium for coordinated contraction 2. Purkinje Fibres ! Purkinje fibres: specialized cardiac muscle cells able to generate and conduct impulse (for synchronized contraction) ! Found in sub-endocardium ! Compared to cardiomyocytes: ! Larger ! Lots of glycogen and mitochondria ! Fewer myofibrils ! No T-tubules ! No intercalated discs (connected by desmosomes and gap junctions) ! Histology: paler myocyte cells (conferred by higher glycogen content) ! White pale region (glycogen) surrounded by pink cytoplasm (myofibrils) # D"#%/&36@6$/)%@#3%=-2&#6!#D@66E##-&&-@&# ! Three layers: ! Tunica intima ! Tunica media ! Tunica adventitia ! Wall is thicker in larger vessels than in small vessels ! Arterial wall is thicker than venous wall ! Composition of wall changes gradually along blood vessels ! Endothelium is continuous throughout all blood vessels 1. Tunica Intima ! Three layers (from innermost): ! Endothelium ! Subendothelial c.t. ! Internal elastic lamina ! Endothelium: simple squamous epithelium (on a basement membrane) ! Regulates transport of substances from lumen into vessel wall ! Controls local clotting through secretion of soluble factors (eg van Willebrand factor) ! Enables migration of WBC from blood (via P-selectin enables neutrophil migration into c.t.) ! Subendothelial layer of loose c.t. (with nerves and blood vessels) ! Internal elastic lamina: fenestrated membrane of elastic fibres separating intima from media ! Pathology: arteriosclerosis and atherosclerosis ! Arteriosclerosis: thickening and hardening of arterial walls ! Atherosclerosis: narrowing of arteries caused by buildup of fat and cholesterol in arterial walls $ Restriction of blood flow $ Formation of thrombus 2. Tunica Media ! Two layers: ! Smooth muscle layers ! External elastic lamina ! Smooth muscle layer: ! Circularly arranged smooth muscles ! Elastic fibres (elastin) ! Some collagen fibres ! External elastic lamina separating tunica media and tunica adventitia ! Pathology: ! Aneurysm: ‘ballooning’ out of arterial wall $ Smooth muscle weakness extensive dilation of arteries ! Marfan Syndrome (MFS): genetic c.t. disorder with mutated fibrillin 1 gene (essential for formation of elastic fibres) $ Separation between media and adventitia $ Media shows mucoid deposition and fragmentation of elastic lamellae 3. Tunica Adventitia ! Three layers: ! Loose c.t. ! Smooth muscles, collagen and elastic fibres (large vessels only) ! Vasa vasorum ! Connective tissue layer with many collagen and elastin fibres ! Continuous with surrounding c.t. ! Vasa vasorum: small nutrient arteries and veins providing blood supply to walls of larger blood vessels (! inadequate nutrient supply via diffusion) ! Importance: anchor blood vessel in c.t. # ;"#8/!!-2-+3#*=&-&#6!##-&&-@&# ! Total thickness of wall varies with pressure, distance from heart and type (artery vs vein) ! Thickness of each layer also varies: ! Tunica adventitia most distinguished layer in thin-walled vessels ! Tunica media most distinguished layer in thick-walled vessels ! 3-layered structure varies according to: $ Forces on vessel wall $ Size of vessel $ Function of vessels ! Types of blood vessels: ! Arteries: $ Elastic arteries $ Muscular arteries $ Arterioles ! Capillaries ! Veins: $ Venules $ Small veins $ Medium and large veins 1. Arteries %"#'@%&3/)#!23-2/-&# ! Also called conducting arteries ! Examples: aorta, pulmonary artery ! Function: transport blood away from heart ! Feature: ! Relatively thin wall: wall only amount to 10% of diameter ! Elasticity ! Importance of elasticity: ! Systole expansion ! Diastole elastic walls recoil to maintain blood pressure continue to drive blood through blood vessels ! Histological layers: ! Tunica intima: $ Thick $ Consists of endothelium with a relatively thick loose c.t. sublayer ! Tunica media: $ Consists of 50-75 concentrically arranged, fenestrated elastic laminae with smooth muscles and collagen fibre between them $ Internal and external lamina not readily recognizable ! Tunica adventitia: thin loose c.t. with vasa vasorum # :"#I9&)9@%2#!23-2=# ! Also called distributing arteries ! Examples: femoral artery, coronary arteries ! Function: distribute blood to various parts of body ! Features: ! Relatively thick wall (25% of diameter) ! Tunica media has a predominance of smooth muscles ( regulate blood flow) ! Histological layers: ! Tunica intima: $ Endothelium $ Flattened Subendothelial layer of collagen and elastic fibres $ Predominant internal elastic lamina  ! Tunica media: $ Thick smooth muscular layer with 10-40 circular layers of smooth muscle $ Much less elastin $ External elastic lamina present only in larger muscular arteries ! Tunica adventitia: $ Broad $ Mostly contains collagen, elastin and vasa vasorum *Comparison between elastic and muscular arteries # )"#!23-2/6@-&# ! Arterioles: small muscular arteries with diameter 0.04 – 0.4 mm ! Function: deliver blood and regulate blood flow to and in capillaries ! Retain general features of muscular arteries (but with much less layers of smooth muscles) ! Regulate blood flow through capillaries by metarterioles: ! Metarterioles: vessels connecting small arterioles to capillaries ! Metarterioles surrounded by precapillary sphincter smooth muscle at metarteriole-capillary junctions ! Contraction and relaxation of precapillary sphincter $ Control blood flow to capillary bed $ Alter BP by altering peripheral resistance to blood flow (peripheral resistance vessels) ! Histological layers: ! Tunica intima: very thin sub-endothelial layer of loose c.t. ! Tunica media: 1-3 layers of smooth muscle (easiest way to recognize an arteriole) ! Tunica adventitia: $ Fairly prominent $ Much thicker in venule than in arteriole $ Continuous with surrounding c.t. 2. Capillaries ! Usually 8-10 "m in diameter ! Connects arterioles to venules ! Wall consists of a single layer of endothelium surrounded by a basement membrane ! Scattered pericytes form discontinuous layer external to endothelium ! Found between basement membrane and endothelium ! Function: stem cells differentiating into smooth muscle cells or endothelial cells ! Function: allow exchange of nutrients and wastes between blood and tissues via passive diffusion and pinocytosis ! Diapedesis: WBC movement through intercellular junctions ! Function: repair, fight infection and cancer metastasis ! Three types of capillaries: ! Continuous capillaries ! Fenestratesd capillaries ! Discontinuous capillaries (sinusoids) # %"#;6+3/+969&#;%&/@@%2/-&# ! Most common type of capillaries ! Found in muscle, fat, c.t., lungs and brain ! Features: ! No pore or spaces between cells in walls ! Tight junctions between adjacent endothelial cells ! Continuous basement membrane ! Macromolecules pass through endothelial cells by pinocytic vesicles ! Cells move in and out by diapedesis ! Forms the blood-brain barrier in the brain (relatively impermeable) # :"#.-+-&32%3-E#;%&/@@%2/-&# ! Found in tissues where rapid exchange of substances occurs between tissues and blood ! Examples: kidney glomerulus, intestinal villi ! Have pores penetrating endothelial cells covered by a thin diaphragm (i.e. basement membrane, not found in kidney) ! Continuous basement membrane # )"#8/&)6+3/+969&#;%&/@@%2/-&#10/+9&6/E&2# ! Larger (30-40 "m) and more irregularly shaped ! Found in liver and haematopoietic organs (eg spleen and BM) ! Do not form a continuous lining between lumen and surrounding tissues (i.e. gaps found between adjacent endothelial cells without tight junctions) ! Basement membrane discontinuous or absent ! Endothelium poses no barrier to blood constituents ! Function: allow small and medium-sized proteins such as albumin to enter or leave blood ! Phagocytic cells (eg Kupffer cells in liver) are present 3. Veins ! Larger lumina (sing. lumen) and thinner walls than arteries ! Consists of 3 basic layers (tunica intima, tunica media, tunica adventitia) ! Less elastic and muscular components ! Collapsed wall (thinner and less elastic fibres) ! Medium and large veins with valves %"##-+9@-&# ! Formed from confluence of capillaries (0.2-1 mm diameter) ! Histological layers: inconspicuous ! Tunica intima: lacks subendothelial layer of c.t. ! Tunica media: 1 or 2 layers of muscle fibres ( with size) ! Tunica adventitia: fuses with surrounding tissues :"#0B%@@##-/+&# ! 1-9 mm in diameter ! Histological layers: ! Tunica intima: thin, very little supportive c.t. ! Tunica media: contains 2-4 layers of circular smooth muscles ! Tunica adventitia: $ Broadest, well-developed layer $ Rich in longitudinal collagen fibres # )"#I-E/9B#%+E#3%2$-##-/+&# ! All three tunics present ! Folds in endothelium forming valves ( prevents backflow of blood) ! Histological layers: ! Tunica intima: thin but well-developed ! Tunica media: $ Muscular $ A few circular layer of smooth muscles $ Thin but can still regulate diameter ( low BP in veins) ! Tunica adventitia: $ Thickest and best developed (predominant) layer $ Spirally arranged collagen and elastic fibres $ Longitudinal smooth muscles $ More vasa vasorum than arteries 4. Lymphatic Vessels ! Lymphatic vessels: thin, endothelial-lined tubes that drains interstitial fluid %"#3=B&*%3/)#;%&/@@%2/-&# ! Resemble blood capillaries but differ in the following aspects: ! Begin as blinding-ending vessels ! Incomplete or absence of basement membrane ! Absence of pericytes ! Larger, irregularly shaped lumina ! No tight junctions ! Anchoring filaments: contraction allows flow of tissue fluid into lymph vessels via opening between adjacent endothelial cells ! No RBCs ! Smaller vessels converge into larger vessels, returning tissue fluid back to veins ! Carries immune cells (eg lymphocytes) :"#I-E/9B#%+E#3%2$-#3=B&*%3/)##-&&-@&# ! Resemble venules and veins ! Consists of three basic layers ! With much less smooth muscles, elastic fibre and collagen fibres ! Have valves ! Difficult to distinguish from small or medium-sized veins except absence of RBC in lumen #