Histology of the Vascular System.pdf

Histology of the Vascular System Dr. Mohammed Elsalanty Blood is the main medium of transporting molecules between organs Blood Vessels Form A Closed (Circulatory) System. (i.e. NOT like an irrigation system) Traffic Directions  Two main circulatory systems Cardioascular System:...

Histology of the Vascular System Dr. Mohammed Elsalanty Blood is the main medium of transporting molecules between organs Blood Vessels Form A Closed (Circulatory) System. (i.e. NOT like an irrigation system) Traffic Directions  Two main circulatory systems Cardioascular System: Transports oxygen, carbon dioxide, nutrients & metabolic breakdown products, cells of the immune & other defense systems, chemical messengers (hormones) & many other important substances (e.g. clotting factors) Lymphatic System: Drains extracellular fluid from the tissues back to the blood circulatory system after passage through lymph nodes, also involved with lipid absorption from the gut Types of Blood Vessels Arteries: Veins Elastic Capillaries Muscular Venules Arterioles Veins Capillaries Large veins  Arteries and Veins Wall composed of three layers: – Tunica intima (inner) – Tunica media (middle) – Tunica adventitia (outer) Tunica Intima (inner layer) Endothelium (simple squamous), in continuous contact with circulating blood cells Basal lamina and thin connective tissue: fibroelastic with myo-intimal cells (contractile CT cells that migrate towards the intima  phagocytic, contractile & synthesize CT) Separated from media by internal elastic membrane Tunica Media Fibroelastic connective tissue mixed with concentric layers of smooth muscle Contraction controls vessel diameter (and therefore its resistance) Often bounded by Internal & External Elastic Laminae Thicker in arteries Tunica Adventitia Fibroelastic connective tissue. Includes small blood vessels (vasa vasorum) & nerves (vasa nervosa) that supply the media Thicker in veins  Segments of the Vascular System Elastic Arteries (Aorta and PA) Muscular Arteries Arterioles Capillaries Venules Small and medium-sized veins Large veins (VC) file:///Users/elsalanty2018/Desktop/AandPch19LecturePearson.pdf  Elastic Arteries Aorta, PA, and their major branches The media (M) is very thick and contains considerable elastic fibers  the lumen expand with blood when the heart contracts  little rise in intravascular pressure during cardiac systole The intima (I) of the empty aorta is typically folded The adventitia (A) is thinner than the media. Figure 11-9 Copyright © McGraw-Hill Companies Muscular Arteries Include most of the named arteries of the body (e.g. ophthalmic artery) Highly contractile media, with layers of smooth muscles (SM) and distinct internal and external elastic laminae Figure 11-11 Copyright © McGraw-Hill Companies  Arterioles Diameter < 100 micron Adjustable lumen size regulates the systemic blood pressure as well as blood flow to a certain tissue (resistance vessels) Respond to several stimuli by dilation or constriction  Most capillary beds are supplied by arterioles and drain into venules In the skin, blood can bypass capillary beds altogether by passing through direct shunts to the venous system (e.g. in cold weather). In venous portal systems, one capillary bed drains into a vein that then branches again into another capillary bed. Figure 11-15 Copyright © McGraw-Hill Companies  Atherosclerosis Thickening of the arterial wall as a result of its invasion by WBCs  reducing elasticity and increasing pressure  Eventually affecting the blood flow Chronic inflammation underneath the intima, promoted by high LDL and low HDL  Capillaries Smallest vessels of the blood circulatory system (4-10 μm) & form a complex, interlinking network This is where exchange takes place between the blood stream and the tissues Wall composed of: – A single layer of endothelial cells (simple squamous) – A basal lamina (BL) – Occasional contractile cells called pericytes (P)  Types of Capillaries Continuous (somatic) Fenestrated (visceral) Sinusoidal Continuous (somatic) capillaries: The most prevalent type Tight and occluding junctions  minimal fluid leakage under normal conditions. Molecule exchanged must occur through the endothelial cells by diffusion or transcytosis. Figure 11-17 Copyright © McGraw-Hill Companies Fenestrated (visceral) capillaries Tight junctions between cells Perforations (fenestrations) through each cells, allowing greater exchange across the endothelium. The basement membrane is continuous as in continuous capillaries. Endocrine tissues, intestinal walls, glomeruli (kidney), and choroid plexus (brain, no tight junctions). Copyright © McGraw-Hill Companies Sinusoids (discontinuous) capillaries Wider lumen Gaps between the endothelial cells Large fenestrations through the cells Discontinuous basement membrane No diaphragm at the fenestra Allows exchange of macromolecules +/- cells Bone marrow, liver, lymph nodes, and spleen. Copyright © McGraw-Hill Companies  Venules A series of increasingly larger and more organized venules lie between capillaries and veins. Post-capillary venules are an important site where WBCs leave the circulation to become functional in the interstitial space of surrounding tissues when such tissues are inflamed or infected.  Venules Are unnamed Diameter of 0.2 - 1mm Intima show valves at sites of connection to other venules or larger veins (Direct flow towards deeper, larger veins) ~18% of blood volume is in the venules.  Veins Size