Histology of the Cardiovascular System PDF

Histology of the Cardiovascular System  HISTOLOGY OF THE CIRCULATORY SYSTEM The cardiovascular system is subdivided into two functional parts 1. Blood vascular system a. The cardiovascular system distributes nutrients, gases, hormones to all parts of the body; collects wastes produced dur...

Histology of the Cardiovascular System  HISTOLOGY OF THE CIRCULATORY SYSTEM The cardiovascular system is subdivided into two functional parts 1. Blood vascular system a. The cardiovascular system distributes nutrients, gases, hormones to all parts of the body; collects wastes produced during cellular metabolism. b. The blood vascular system consists of blood vessels (arteries, arterioles, capillaries, venules, veins) and a muscular pump (heart). c. Blood is the fluid found within the blood vascular system. 2. Lymph vascular system a. The lymph vascular system collects tissue fluid from tissues and returns it to the blood vascular system. b. The lymph vascular system consists of blind-ended capillaries (lymphatic capillaries) connected to venous vessels (lymphatic vessels) and various lymphoid organs (e.g., lymph nodes). c. The fluid found within the lymph vascular system is lymph.  1. The heart wall can be viewed as a three-layered structure. a. Inner layer = endocardium b. Middle Layer = myocardium c. Outer layer = epicardium (also called the pericardium) 2. Except for the smallest vessels, blood and lymphatic vessel walls can also be viewed as three-layered structures. a. Inner layer = tunica intima b. Middle layer = tunica media c. Outer layer = tunica adventita  Structure of the heart wall 1. The endocardium is the inner layer of the heart wall and consists of the endothelial lining and the underlying connective tissue layers. a. The lumen of the heart is lined by an endothelium consisting of a typical simple squamous epithelium. b. A connective tissue region consisting of three layers separates the endothelium from the myocardium - in humans consist of:- (1) A thin layer of loose fibroelastic connective tissue FECT (containing mainly fine collagen fibers) referred to as subendothelial layer which is next to the endothelium. (2) A thicker layer of moderately dense FECT (with many elastic fibers) and some smooth muscle forms the center of the connective tissue region. (3) A thin layer of loose FECT (often referred to as the subendocardial layer) containing many blood vessels joins the endocardium to the myocardium Purkinje fibers run in this layer in the interventricular septum.  2. The myocardium is the middle layer of the heart wall and contains the cardiac muscle throughout most of the heart. a. Cardiac muscle cells in the myocardium are arranged in strands whose ends attach to the dense connective tissue which surrounds the valves. b. Loose FECT holds bundles of cardiac muscle cells/fibers together and contains numerous blood vessels. c. Dense FECT (heavily collagenous) replaces the cardiac muscle in region around each of the major heart valves This connective tissue frame around each valve is called the cardiac skeleton  The epicardium is the outer layer of the heart and consists of a connective tissue region covered by a mesothelium on its outer surface. a. The connective tissue region consists : (1) The inner two regions are referred to collectively as the subepicardial layer and contain large blood vessels (coronary vessels), nerves, and varying amounts of adipose tissue. (2) A thin layer of loose FECT with many elastic fibers connects the connective tissue layers of the epicardium to the mesothelial covering. Epicardium Adipose Tissue Coronary vessels and cardiac nerves Mesothelium A mesothelium (simple squamous epithelium) covers the outer surface of the heart (except where the arteries leave and the great veins enter the heart). This covering epithelium closely resembles the mesothelial covering of the other thoracic and abdominal organs. The thickness of the heart wall and the thickness of the layers within the heart wall varies with location. 1. The myocardium is thickest in the ventricular region, especially the left ventricle, and contains more cardiac muscle in the ventricles than in the atrium. The myocardium around the valves contains only dense collagenous CT which forms the cardiac skeleton. 2. The endocardium and epicardium are thinner in the ventricles than in the atria  Special features of the heart A heart valve normally allows blood to flow in only one direction through the heart. The four valves commonly represented in a mammalian heart determine the pathway of blood flow through the heart. A heart valve opens or closes incumbent on differential blood pressure on each side The four main valves in the heart are: The two atrioventricular (AV) valves, the mitral valve (bicuspid valve), and the tricuspid valve, which are between the upper chambers (atria) and the lower chambers (ventricles). The two semilunar (SL) valves, the aortic valve and the pulmonary valve, which are in the arteries leaving the heart. The mitral valve and the aortic valve are in the left heart; the tricuspid valve and the pulmonary valve are in the right heart.  "Pacemakers" in the heart are modified cardiac muscle cells. a. Cardiac muscle cells in the myocardium of the sinoatrial (SA) node are modified to serve as the pacemaker region. The plasma membrane of the cells has a high leakage rate, giving them the fastest intrinsic contraction rate among the populations b. Cardiac muscle cells in the atrioventricular (AV) node have a similar histological appearance, but have a lower intrinsic rate of contraction, so these cells do not normally act as a pacemaker region. These cells receive the wave of excitation from the cardiac muscle of the atria and pass the excitation on to the bundle of His.  The impulse-conducting system which connects the atria with the ventricles serves several functions. a. The impulse conducting system is made up of a series of Purkinje fibers which are specialized cardiac muscle cells. (1) Purkinje fibers are organized into a branched bundle (Bundle of His) which extends from the atrio-ventricular (AV) node, through the interventricular septum down to the apex of the ventricles. (2) Purkinje fibers are attached (by intercalated disks) to cardiac muscle cells in the myocardium at the apex of the ventricles and along outer walls of both ventricles b. The impulse conducting system improves heart function in two ways  Purkinje Fibers Muscle Conduction System = AV Bundle of His + Purkinje Fibers  Microanatomy of Blood Vessels Most larger blood vessel walls contain three major layers with sublayering. 1. The tunica intima is the luminal layer. a. The lumen is lined by an endothelium of simple squamous epithelium. b. A subendothelial layer of loose FECT is present in most medium to large vessels and may contain scattered smooth muscle in larger vessels. 2. An internal elastic lamina (elastica interna) marks the boundary between the tunica intima and the tunica media. 3. The tunica media contains layers of either elastic laminae/lamellae (fenestrated sheets) or FECT alternating with layers of smooth muscle. 4. If present, the external elastic lamina (elastica externa) marks the boundary between the tunica media and the tunica adventita. 5. The tunica adventita contains loose to moderately dense FECT, +/- scattered smooth muscle cells. Small and medium arteries and veins are present in the tunica adventitia of large arteries and veins  Large arteries (also called elastic arteries or conducting arteries) include the aorta and its largest main branches. (a. Tunica intima - thin (relative to other layers in this type of vessel) (1) Endothelium (2) Subendothelial layer contains some smooth muscle, elastic fibers, collagen fibers b. Internal elastic lamina - not as distinct as in other arteries c. Tunica media - thick (1) 40 - 60 distinct, concentrically arranged elastic laminae (2) Between elastic laminae - fibroblasts, elastic fibers, collagen fibers, spiral (to circular) smooth muscle d. Tunica adventita - thin; consists mainly of collagen fibers, blood vessels, nerves; some elastic fibers, fibroblasts, macrophages may also be present 2. Function = to conduct blood from the heart to smaller arteries and to even out blood pressure and flow. The presence of elastic laminae gives these vessels elastic properties. They expand as the heart contracts (to modulate blood pressure and store energy) and recoil during ventricular relaxation (to maintain more even pressure in large arteries).  Medium to small arteries (also called muscular arteries) a-Tunica intima - thin (1) Endothelium (2) Thin subendothelial layer consisting of scattered fine collagen and elastic fibers and a few fibroblasts b. Internal elastic lamina - very distinct, usually folded c. Tunica media - thick (1) Circular smooth muscle, 5 - 40 layers (2) Small amount of CT with collagen fibers and elastic fibers (longitudinal orientation) between muscle (3) Thickness decreases as diameter of vessel decreases d. External elastic lamina (May be indistinct in smaller muscular arteries) e. Tunica adventita - thick; loose FECT Function - to distribute blood to smaller arterial vessels. The muscular wall resists damage due to relatively high blood pressure in these vessels  Arterioles 1. Structure a. Tunica intima - very thin consisting only of endothelium b. Internal elastic lamina - usually present except in smaller arterioles c. Tunica media - 1 to 5 layers of smooth muscle, some elastic fibers d. Tunica adventita - thin, consisting of longitudinally arranged collagen and elastic fibers Function - to redistribute blood flow to capillaries and to alter blood pressure by altering peripheral resistance to blood flow. Arterioles can change diameter very drastically therefore affecting blood pressure and flow patterns. Arterioles are referred to as peripheral resistance vessels.  Capillaries 1. Structure - consist only of endothelium. Three types of capillaries may be distinguished a. Continuous (type I) capillaries have relatively thick cytoplasm and the capillary wall is continuous b. Fenestrated (type II) capillaries have extremely thin cytoplasm c. Sinusoidal capillaries are larger in diameter than the other. 2. Functions a. Capillaries are the site of normal exchange of materials between blood and tissue fluid. b. Capillaries may be a site of exit of WBCs from blood into tissue under some conditions, although this is probably more frequent in venules.  Venules Size varies from 10 microns (post-capillary venules) to 1 mm (muscular venules) 2. Post-capillary venules a. Structure - larger diameter than capillaries; consist of endothelium surrounded by pericytes b. Functions (1) Collect blood from capillaries (2) Respond to vasoactive agents (e.g., histamine, serotonin) by altering permeability (3) Also a site of exchange of materials between tissue fluid and blood (4) Site of exit of WBCs from blood into tissue  Larger muscular venules a. Structure (1) Tunica intima - thin; endothelium surrounded by outer sheath of collagen fibers (2) Tunica media - thin; 1 - 3 layers of smooth muscle (circular) with collagen and elastic fibers between muscles (3) Tunica adventita - thick; loose FECT containing longitudinal collagen fibers and scattered elastic fibers and fibroblasts b. Function - to collect blood from post-capillary venules  Small to medium veins 1. Structure a. Tunica intima - thin (1) Endothelium (2) Thin subendothelial layer (3) May be folded to form valves b. Tunica media - thin; circular smooth muscle, collagen fibers, some elastic fibers c. Tunica adventita - well developed; loose FECT with longitudinally arranged collagen and elastic fibers, bundles of longitudinal smooth muscle 2. Function - to collect blood from smaller venous vessels  Large veins - vena cavae and larger branches 1. Structure a. Tunica intima - thicker (1) Endothelium (2) Thin subendothelial layer b. Internal elastic lamina - usually distinguishable c. Tunica media - thin, poorly developed; mostly FECT; little smooth muscle d. Tunica adventita - very thick; moderately dense FECT with spirally arranged collagen fibers, elastic laminae, longitudinal smooth muscle 2. Function - to collect blood from medium sized veins and return it to heart  Microanatomy of Lymphatic Vessels A. Lymph capillaries 1. Structure - blind-ended tubules; consist only of endothelium (which lacks cell junctions); similar to post capillary venules of blood vascular system 2. Function - to collect excess tissue fluid Actual slides: 1 Artery 2 Vein 3 Nerve fascicles 4 Skeletal Muscle Tissue 5 Lumen of the vein (with RBC’s) 6 Vein Tunica Media- smooth muscle layer 7 Vein Tunica Externa- connective tissue layer 8 Lumen of the Artery (with RBC’s) 9Artery Tunica Externa- connective tissue layer 10 Artery Tunica Media- smooth muscle layer 11 Artery Tunica Interna- endothelial cells (simple squamous) 12 Artery Tunica Interna- Internal elastic lamina 13 Capillary Tunica Interna- endothelial cells 14 Vein Tunica Interna- endothelial cells (simple squamous)  3 4 2 3 1 16 12 10 8 15 2 9