Circulatory System 1 PDF

General Structure of Blood Vessels Generally the wall of any blood vessel is formed of three layers: 1. Tunica Intima, which lies next to the lumen, is the innermost layer. 2. Tunica Media is the middle layer. 3. Tunica Adventitia (the outermost layer) is usually continuous with the surrounding conn...

General Structure of Blood Vessels Generally the wall of any blood vessel is formed of three layers: 1. Tunica Intima, which lies next to the lumen, is the innermost layer. 2. Tunica Media is the middle layer. 3. Tunica Adventitia (the outermost layer) is usually continuous with the surrounding connective tissue (C.T.). 1. Tunica Intima It is composed of: 1- simple squamous epithelium (endothelium) , 2-subendothelial C.T.& 3- Internal elastic lamina. Endothelium Endothelial cells are flattened cells resting on a basal lamina. Its nucleus causes the cell to bulge into the vessel lumen. The cytoplasm contains few organelles (a small Golgi, mitochondria, free ribosomes &few RER). Endothelial cells are connected together by tight junctions. Functions of endothelial cells: 1-They provide smooth surface 1 2-They secrete collagen, lamin, 2 endothlin and nitric oxide. 3-They have membrane bound enzymes for bradykinin, serotonin, prostaglandins, thrombin, norepinephrin and lipoprotein lipase. 2. Tunica Media It is mainly composed of helically arranged smooth muscle cells with some elastic &collagen fibers interspersed between them. An external elastic lamina between tunica media &tunica adventitia may be present. Capillaries &postcapillary venules do not have a tunica media, in these small vessels pericytes replace the tunica media. 3. Tunica Adventitia It is composed of C.T. layer. The tunica media &tunica adventitia of larger vessels are supplied by small arteries called vasa vasorum. The thickness of these vessels prevents their constituent cells from being nourished by diffusion from the lumen of the vessel. Vasa vasorum are more found in the walls of veins than arteries. Arteries -Arteries carry out blood from the heart to the capillary network in tissues &organs. -Arteries are classified into: 1. Large, elastic or conducting arteries. 2. Medium, muscular or distributing arteries. 3. Small arteries or arterioles. Veins Veins are vessels that return blood to the heart. Veins are classified into: 1. Large veins. 2. Medium veins. 3. Small veins. 1- Large Elastic Arteries (Aorta) The aorta Histological Features: a. Tunica intima: It is composed of an endothelium supported by a narrow sub endothelial layer of C.T. The internal elastic lamina is present but indistinct. b. Tunica media: (the thickest layer): It consists of many fenestrated lamellae of elastin alternating with circularly oriented layers of smooth muscle cells. Smooth muscle cells are less abundant in elastic arteries. c. Tunica Adventitia:It is thin layer of loose fibroelastic C.T. containing vasa vasorum. 3- Large Veins Inferior venae cavae, Tunica intima: Has endothelium and thick subendothelial C.T. containing collagen fibers & few elastic fibers. A delicate internal elastic lamina may be present. Tunica media: It is very thin and contains few layers of smooth muscle cells & abundant C.T. No external elastic Lamina. Tunica adventitia: It is the thickest layer. Contains many collagen fibers and vasa vasorum & few elastic fibers. In addition to has longitudinally arranged bundles of smooth muscle fibers. Contraction of these muscle fibers helps in propelling blood towards the heart against gravity. Differences Between Medium Sized Arteries &Veins Medium Sized Artery Medium Sized Vein Thicker wall Thinner wall Narrow regular lumen Wide collapsed lumen No valves are present Valves may be present Well developed internal &external No internal &no external elastic elastic laminae laminae Thick tunica media, containing elastic fibers Thin tunica media, with no elastic fibers Tunica adventitia is equal in thickness to tunica media Tunica adventitia is much thicker than tunica media Med sized art Med sized v Differences Between Medium Sized Arteries &Veins Vein valves: Bag-like protrusion of tunica intima, which prevents the blood flow from running to opposite direction. Blood Capillaries Capillaries form anastmosing network between arterioles & venules. Their diameter is (8-10 um). slightly larger than the diameter of RBCs. General structure: -Formed of single L. of endothelial cells. Pericytes present outside capillaries and share the basal lamina of the endothelial cells. -The large number of pinocytotic vesicles on the plasma membranes & in the cytoplasm is an identifying characteristic of capillaries. These vesicles are responsible for transport of macromolecules in both directions across the endothelial cytoplasm They regulates blood flow through the capillaries as they contain tropomyosin which is related to the contractile process. After injury, pericytes can differentiate to form new blood vessels & C.T. cells, thus participating in the repair process. Types of blood capillaries 1- Continuous 2- Fenstrated 3- Sinasoidal 1- Continuous capillaries: Structure: They are characterized by Continous endothelium & continuous basal lamina. Occluding junction between endothelial cells represent the main structural component of the barrier. Site: Muscles, C.T. &nervous tissue. 2- Fenestrated capillaries: Structure: They are characterized by interrupted endothelium. The endothelial cells have pores (60-80 nm), usually covered by pore diaphragm. The basal lamina is continuous. Site: Intestine, endocrine glands &glomerular capillaries of the kidney. They are leakier than continuous capillaries and allow regulated exchange through the pores. 3- Sinusoidal capillaries: Structure: They have an enlarged diameter of 3040um & The endothelial wall and the basal lamina are discontinuous. The fenestrae in the endothelial cells are without diaphragm. Site: Present in bone marrow, liver& spleen. Function: Allow more facilitated exchange between blood and surrounding tissue than other types of capillaries. Structure of the Heart Endocardium – Endothelial layer – Subendothelial layer - veins, nerves and Purkinje cells Myocardium – Cardiac muscle cells Epicardium – Visceral layer of the pericardium mesothelium – Subepicardial layer of loose connective tissues - veins, nerves Cardiac skeleton The central supporting structure of the heart is the "cardiac skeleton," composed of dense white fibrous (collagenous) connective tissue into which the cardiac muscle fibers of the atria and ventricles insert and to which the heart valves are attached. This skeleton provides structure and support for the heart, as well as isolating the atria from the ventricles. Cardiac valves Are endocardial folds supported by internal plates of dense collagenous and elastic connective tissue continuous with the fibrous ring of cardiac skeleton. The right atrioventricular valve has three cusps; hence, it is called the tricuspid valve. The left atrioventricular valve has two cusps and is called the bicuspid, or mitral valve. Semilunar valves located at the ventricular entrance to the aorta and pulmonary arteries. The valves are arranged to prevent retrograde or reverse blood flow. Cardiac Valves Purkinje fibers Are modified cardiac muscle cells, electrically excitable cells, also called conducting myofibers. They are distal branches of atrioventricular node bundles to both ventricles. Histologically:The fibers are larger than cardiac muscle fibers, have one or two central nuclei and lightly stained glycogen filling most of the cytoplasm. The Purkinje fibers conduct the electrical impulses for heart's contraction.