Lecture 1 Blood & Lymphatic Blood Vessels PDF

Faculty of Medicine Academic Year: 2024-2025 Year: 1 Semester: 1 Module: BLOOD and body fluids (blf) 103  The circulatory system Lecture 1: BLOOD & LYMPHATIC VESSELS By: Doaa M...

Faculty of Medicine Academic Year: 2024-2025 Year: 1 Semester: 1 Module: BLOOD and body fluids (blf) 103  The circulatory system Lecture 1: BLOOD & LYMPHATIC VESSELS By: Doaa M Khaled Lecturer of histology , Faculty of Medicine , Helwan University Department: Histology & Cytology 12/29/2024 22  Objectives Outline general microscopic structure of blood vessels wall. Describe the microscopic structure of different types of arteries & veins. Mention special types of medium sized arteries (will be covered in practice). Compare between medium sized artery & vein. Classify types of arteriovenous connections. Describe histological features of different types of blood capillaries. Recognize the histological criteria of lymphatic vessels &capillaries. Compare between the histological structure of blood & lymphatic capillaries. 12/29/2024 BLF - 103 33  Introduction Circulatory system Blood vascular system Lymphatic vascular system (Cardiovascular system) Blood Lymphatic Lymphatic Heart vessels capillaries vessels 12/29/2024 BLF - 103 44 Types of blood vessels Arteries Veins Arteriovenous Venule Arteriole connections Small artery Small vein  General structure of blood vessels wall Except capillaries The walls of arteries and veins are composed of three layers called tunics Tunica intima: the innermost layer in contact with blood. Tunica media: the middle layer. Tunica adventitia: the outermost layer.  Tunica Intima The innermost layer of the vessel ❖ Consists of: ✓ Endothelium (simple squamous epithelium) resting on basal lamina & joined by tight occluding junction. ✓ Subendothelial CT: a thin layer of loose C.T ✓ Internal elastic lamina: - A fenestrated sheet of elastin separating tunica intima from media. - By H&E: appears as wavy pink line. - N.B. Fenestrations or holes allowing better diffusion of substances from blood deeper into the wall. Endothelial cells Internal elastic lamina: ▪ Well developed in muscular arteries and absent in small arterioles. ▪ Prevent occlusion of arteries. Endothelium Tunica Intima Internal elastic lamina Tunica Media ❖The middle layer of the vessel wall consists of: Concentric layers of 1-smooth muscle cells 2-Elastic fibers (Fenestrated elastic lamellae) Produced by vascular 3-Reticular fibers smooth muscle cells 4-Proteoglycans External elastic lamina: found in some arteries It is a layer of elastin that separates tunica media from adventitia.  Fenestrated elastic lamellae Smooth muscle cells Tunica media Tunica Adventitia (tunica externa) ❖ the outermost C.T layer ❖ Consists of longitudinally arranged collagen fibers (type I) & Fibroblasts Elastic fibers Vasa vasorum (vessels of the vessel) In adventitia of Nervi vasorum large vessels (vasomotor nerves)  Vasa vasorum (vessels of the vessel) - They are arterioles, capillaries & venules in tunica adventitia & outer part of tunica media of large vessels. - Provide these tunics with nutrients, oxygen & remove waste products. Vasa vasorum are more numerous in large veins than arteries (why??) Tunics of blood vessel wall Tunica intima Tunica media Tunica adventitia  Tissues found in vascular wall & their Functions Walls of arteries and veins contain: endothelial lining, connective tissue & smooth muscle Subendothelial C.T. Smooth muscle fibers Elastic fibers Collagen fibers T. media T. adventitia Permit vasoconstriction Expansion of the and vasodilation prevent over expansion vascular wall under regulating the blood of arterial wall beyond pressure. pressure. physiologic limits during systole of cardiac cycle. Simple squamous epithelium ??? Arteries  Large Elastic Artery (Aorta) diameter :>10 mm. Tunica intima: Well developed ▪ Endothelium with its basal lamina. ▪ Subendothelial C.T. ▪ Internal elastic lamina (present but not obvious) Tunica media: Thickest layer ▪ extremely elastic (have multiple sheets of fenestrated elastic lamellae) alternating with layers of smooth muscle cells. Tunica adventitia: Thinner than the media. ▪ Collagen & elastic fibers. ▪ Fibroblasts. ▪ Vasa vasorum & nervi vasorum.  Aorta Intima Fenestrated Elastic lamellae In adults, the aorta has Media 40-70 elastic lamellae The number & thickness of elastic lamellae are increased in individuals Adventitia with hypertension.  Aorta Nervi vasorum Vasa vasorum Adventitia  Medium-sized arteries (Muscular arteries, distributing arteries ) Muscular arteries have more smooth muscle cells & less elastin in tunica media. Outer diameter: 1- 10 mm. Example: (carotid & femoral arteries).  Muscular arteries Tunica intima: Thin Very thin subendothelial layer. Prominent internal elastic lamina. Tunica media: Endothelium ▪ More smooth muscle cells (concentrically arranged), up to 40 layers. ▪ Less elastic lamellae. ▪ An external elastic lamina (only in larger muscular arteries). Tunica adventitia: ▪ Is relatively thick, about the same thickness as the tunica media). Medium sized muscular artery  Arterioles Diameter:< 0.1 mm (10-100 µm) Tunica Intima: Endothelium, very thin subendothelial layer. NO internal elastic lamina. Tunica Media : thick ,1-3 layers of smooth muscle cells. Tunica adventitia: very thin.  Functions of arterioles The major determinants of systemic blood pressure: The lumen is kept narrow by a degree of tonus of smooth m cells in its wall, to maintain the blood pressure relatively high inside the arterial system peripheral resistance to blood flow. Increase arteriolar constriction Essential hypertension Control blood flow to capillaries Veins  ✓ Similar to structure of capillaries Postcapillary venules (endothelium + pericyte) but larger. ✓ No smooth muscle layer. ✓ Sites of leukocyte migration. ✓ Drainage of capillary bed Muscular venules Small veins 0.1-1 mm ❖All venules have Medium-sized 1-10 mm wide lumen & thin veins wall. Diameter: 10-100 µm Large veins >10 mm  Medium-sized veins No IEL/EEL Accompanied by corresponding muscular arteries. e.g., radial vein, tibial vein, popliteal vein. Intima: endothelium with its basal lamina & subendothelial C.T., with valves. Media: 3-5 layers of circular smooth muscle + reticular and few elastic fibers. Adventitia: thicker than media.  Venous Valves Valves: ✓Paired semilunar folds of the intima projecting into the lumen. ✓ In medium & large sized veins. ✓ More numerous in veins of lower limbs. ✓ Prevent retrograde movement (backflow) of blood & allow it to flow in one direction(toward the heart ).  Medium-sized artery and vein Medium sized artery Medium sized vein Wall thickness Thick Thin lumen Narrow wide Valves Absent Present T. intima Prominent IEL No IEL T. media Thick, made of MORE Thin, made of smooth muscles rather than smooth muscles, elastic fibers poor in elastic fibers T. adventitia Moderate in thickness Very thick (the (nearly the same thickness of thickest layer) media)  Large veins Endothelium Subendothelial CT EX. Superior vena cava, inferior vena cava Intima and portal vein. Media Smooth muscle fibers Intima: well-developed ,has valves (circular) Media: is relatively thin, with layers of circularly arranged smooth muscle cells (appose the action of gravity by its peristaltic pumping of blood up to Smooth muscle fibers the heart), some collagen & elastic fibers. (longitudinal) Adventitia Adventitia: - Thickest layer, It contains longitudinal Vasa vasorum bundles of smooth muscle fibers.(facilitate elongation & shortening of the vena cava with respiration).  Large vein (H & E) Endothelium Tunica media Tunica adventitia Longitudinal bundles of smooth muscle fibers  Remember Veins Wide lumen Thin wall No IEL No EEL Thick Valves in adventitia medium and large sized (Thickest layer) veins.  Arteriovenous connections Capillaries Arteriovenous anastomosis  Capillaries The smallest blood vessels that conduct blood from arterioles to postcapillary venules. Always function in networks called capillary beds. ✓ Tissues with high metabolic rates (kidney, liver & cardiac muscle) have rich capillary networks. But tissues with low metabolic rates (e.g. dense C.T) have fewer capillary networks.  Structure of blood capillaries: A single layer of endothelial cells that rest on a basal lamina & are surrounded by pericytes. Function: They allow metabolic exchange between blood & surrounding tissues. They adapted to their function due to: - Their thin walls & curved nucleus - Extensive surface area (network). - Slow pulsatile blood flow.  Types of capillaries According to the continuity of the endothelial cells & continuity of the basal laminae, capillaries are divided into 3 histologic types: 1.Continuous capillaries 2.Fenestrated capillaries 3.Sinusoidal capillaries  1-Continuous capillaries The most common type. Diameter: 4-10 μm Characterized by: - continuous endothelium - continuous basal lamina Surrounded by pericytes. BY EM: they show Numerous pinocytotic vesicles.& Tight junctions between endothelial cells. Site - Connective tissue. - Muscle tissue (cardiac, skeletal & smooth) - Blood-tissue barriers, such as blood-brain barrier (CNS).  2- Fenestrated capillaries Diameter: 4-10 μm Endothelial cells: are penetrated by fenestrations (holes). Basal lamina: is continuous.Pericytes: are rarely found. BY EM: Cells show Numerous pinocytotic vesicles& Tight junctions. Fenestrations by EM: - Are small circular openings within the endothelial cells. - Nearly 80 nm in diameter. - Some fenestrations are covered by diaphragms, others not covered by such diaphragms as those found in glomeruli of kidney (glomerular capillaries).. Diaphragm SEM of glomerular capillary showing fenestrations (arrows)  numerous pinocytotic Fenestrated capillary vesicles Endothelial cell cytoplasm Fenestrations covered by Fenestrations diaphragm (thin electron dense line) BL  Sites of Fenestrated Capillaries Because fenestrated capillaries are highly permeable, they are found in areas engaged in fluid transport and secretion of hormones, such as: ✓Kidneys: (blood filtrations) ✓Intestine: (absorption). ✓Endocrine glands: (transport of hormones).  Sinusoidal capillaries (Sinusoids) Diameter: 30 - 40 μm ( larger & irregular than other capillaries). Endothelial cells: ▪ Discontinuous (having large perforations without diaphragms). ▪ Separated by wide, irregular, intercellular gaps Basal lamina: discontinuous. Site: Liver, bone marrow & spleen. Sinusoids are specialized for ❑ Exchange of macromolecules (as plasma proteins). ❑ Easy movement of blood cells across the endothelium.  Sinusoidal capillaries Liver Spleen Bone marrow Carry plasma proteins to blood Carry stored blood cells to circulation Carry formed blood cells to circulation Adipocytes Sinusoids Haemopoietic cells Sinusoid  Macrophages are associated with sinusoidal capillaries (to engulf foreign bodies & defective or old erythrocytes) Kupffer cells in liver Macrophages in spleen Macrophages in bone marrow Reticular fibers Macrophages Macrophage Endothelial cells  Continuous capillaries Fenestrated capillaries Sinusoidal capillaries Diameter 4-10 μm 4-10 μm 30 - 40 μm Regularity of the Regular, narrow lumen Regular, narrow Irregular, wide wall Endothelium Continuous Fenestrations with diaphragm Large perforations without except in glomerular capillaries diaphragm Basal lamina Continuous Continuous Discontinuous Tight junction present present Wide intercellular gaps between endothelial cells Associated cells in pericyte pericyte macrophages the wall Sites C.T., Muscle, barriers Kidney, intestine, endocrine Liver, spleen, bone marrow glands  Arteriovenous (AV) anastomoses (AV shunts) ▪ Definition: direct routes between arterioles & venules bypassing capillaries. ▪ Structure: The shunt has three segments: ✓ Initial arterial segment (typical of an arteriole). ✓ Intermediate segment has: Thicker tunica media acting as a sphincter Thicker tunica adventitia acting as a capsule Richly innervated by autonomic fibers ✓ Terminal venous segment (typical of a postcapillary venule). ▪ Sites of AV shunts: Commonly found in skin of fingertips, nose, lips & erectile tissue of penis & clitoris. ▪ Mechanism of action: When the AV shunt is closed, due to contraction of smooth muscle in intermediate segment, blood flows Closed shunt normally through the capillary bed. When the shunt is open, due to relaxation of smooth muscle in intermediate segment, blood bypasses the capillary bed. AV shunts of skin serve in thermoregulation at the body surface open shunt  Lymphatic vascular system It is the system that drains excess interstitial fluid, the lymph, from tissue spaces & returns it to the blood. Each day, lymphatics return about 40% of total plasma proteins to veins. Conduct immune cells & lymph to lymph nodes. Lymphatics remove foreign substances from tissues. Transport lipid-containing particles It consists of lymphatic capillaries & lymphatic vessels.  Lymphatic capillaries ✓ Form networks close to blood capillaries. ✓ Begin as “blind-ended” tubes. ✓ Formed of single layer of endothelial cells with openings between them (NO tight junction)& NO pericytes. ✓ The basal lamina is discontinuous. ✓ Fine anchoring filaments extend from basal lamina to surrounding C.T , preventing collapse of lymphatic lumen. Lymphatic capillaries are more permeable than blood capillaries and collect excess protein-rich tissue fluid.  Intercellular gap Endothelium Lymphatic vessel lymphatic capillary  Lymphatic vessels Lymphatic capillaries converge into larger lymphatic vessels Structure of lymphatic vessels : Endothelial cells joined by tight junctions. Basal lamina is continuous. have valves like veins, but more numerous, to prevent backflow of lymph. Smooth muscle & connective tissue start to appear but without clear separation between tunics. Lymph nodes are distributed along the pathway of lymph vessels to filter the lymph. Lymphatic vessels converge as two large trunks, the thoracic duct & right lymphatic duct, which empty lymph back into the blood.  Interactive Question One of the following is FALSE about large elastic arteries? a) Have a thick wall & wide lumen. b) Have a thin tunica media. c) Rich in fenestrated elastic membranes. d) Contains vasa vasorum in the tunica adventitia. 12/29/2024 BLF- 103 52 52  Interactive Question 1. One of the following is a character of the sinusoidal capillary a) Has a pericytes along its wall. b) Has a blind ended lumen. c) Its endothelium has tight junction d) Has macrophages along its wall 2. The tunica media of the medium-sized artery is formed mainly of: a) Fenestrated elastic membranes b) Smooth muscle fibers c) Collagen fibers d) Elastic fibers 12/29/2024 BLF - 103 53 53  Interactive Question 3. The two types of fenestrated blood capillaries are a) ………………………………………………………………. Which are present in ………………………………………….. b) ……………………………………………………………….. Which are present in ………………………………………….. 12/29/2024 BLF - 103 54 54  Summary The walls of arteries & veins are composed of three layers called tunics Tunica intima, media & adventitia. Aorta (larg sized A), its tunica media is Thickest layer & extremely elastic. Muscular arteries have more smooth muscle cells & less elastin in the tunica media. Intima of Medium & large sized vein has valves , its media poor in elastic fibers. Adventitia of large veins is the Thickest layer &contains longitudinal bundles of smooth muscle fibers. According to the continuity of the endothelial cells & continuity of the basal laminae, capillaries are divided into 3 histologic types: Continuous capillaries, Fenestrated capillaries & Sinusoidal capillaries. Continuous capillaries has pericyte along their wall, but sinusoidal capillaries has macrophage. Av shunt, it is a direct routes between arterioles & venules bypassing capillaries & commonly found in skin of fingertips, nose, lips. Lymphatic capillaries begin with blinded end & has NO tight junction or pericyte. 12/29/2024 BLF - 103 55 55  References Junqueira's Basic Histology: Text & Atlas, 14 e > Chapter 11. The Circulatory System > 12/29/2024 BLF - 103 56 56