histo lec

• The circulatory system pumps and directs blood cells and substances carried in blood to all tissues of the body. It includes both the blood and lymphatic vascular systems, and in an adult the total length of its vessels is estimated at between 100,000 and 150,000 kilometers.
• Components of the blood vascular system

  • Heart
  • Arteries
  • Capillaries
  • Veins

  
• Heart


  Propels blood through the system
• Arteries


  A series of vessels efferent from the heart that become smaller as they branch into the various organs, carry blood to the tissues
• Capillaries


  The smallest vessels, are the sites of O2, CO2, nutrient, and waste product exchange between blood and tissues. Together with the smallest arterial and venous branches carrying blood to and from them, capillaries in almost every organ form a complex network of thin, anastomosing tubules called the microvasculature or microvascular bed.

  
• Veins


  Result from the convergence of venules into a system of larger channels that continue enlarging as they approach the heart, toward which they carry the blood to be pumped again
• Endocardium


  • Consists of a very thin inner layer of endothelium and supporting connective tissue, a middle myoelastic layer of smooth muscle fibers and connective tissue

  
• Endothelium


  A simple squamous epithelium that line the internal surface of all components of the blood and lymphatic systems

  
• Myocardium


  • Consists mainly of cardiac muscle fibers arranged spirally around each heart chamber (Because strong force is required to pump blood through the systemic and pulmonary circulations)
  • Much thicker in the walls of the ventricles, particularly the left, than in the atrial walls

  
• Epicardium


  • A simple squamous mesothelium supported by a layer of loose connective tissue containing blood vessels and nerves
  • Corresponds to the visceral layer of the pericardium, the membrane surrounding the heart

  
• Layers of Pericardium


  • Visceral layer (Epicardium)
  • Parietal layer
• Where the large vessels enter and leave the heart, the epicardium is reflected back as the parietal layer lining the pericardium.

  
• During heart movements, underlying structures are cushioned by deposits of adipose tissue in the epicardium and friction within the pericardium is prevented by lubricant fluid (pericardial fluid) produced by both layers of serous mesothelial cells.

  
• Cardiac Skeleton


  • Dense irregular fibrous connective tissue
  • Forms part of the interventricular and interatrial septa
  • Surrounds all valves of the heart and extends into the valve cusps and the chordae tendineae to which they are attached

  
• Functions of the cardiac skeleton

  • Surrounding, anchoring, and supporting all heart valves
  • Providing firm points of insertion for cardiac muscle in the atria and ventricles
  • Helping coordinate the heartbeat by acting as electrical insulation between atria and ventricles
• Chordae tendineae


  A fibrous network that serves as an attachment for the cardiac skeleton and various valves

  
• Parts of the cardiac conducting system

  • Sinoatrial (SA) node (or pacemaker)
  • Atrioventricular (AV) node
  • AV bundle (bundle of His)
  • Subendocardial conducting network
• The SA node (pacemaker), in the right atrial wall, and the AV node, in the floor of the right atrium, consist of myocardial tissue that is difficult to distinguish histologically from surrounding cardiac muscle.

  
• The AV node is continuous with a specialized bundle of cardiac muscle fibers, the AV bundle (of His), which gives rise to right and left bundle branches that run along the interventricular septum to the apex of the heart.

  
• At the apex the bundle branches subdivide further as conducting (Purkinje) fibers which extend into myocardium of the ventricles.

  
• Heart Valves


  • Composed of connective tissue with overlying endocardium
  • Fibrosa: Core of the valve and contains fibrous extensions from the dense irregular connective tissue of the skeletal rings of the heart
  • Spongiosa: Loose connective tissue located on the atrial or blood vessel side of each valve, acts as a shock absorber to dampen vibration associated with the closing of the valve
  • Ventricularis: Adjacent to the ventricular or atrial surface of each valve and is covered with endothelium, continuous unto the chordae tendineae which are fibrous threadlike chords also covered w/endothelium
• Endothelium


  • Specialized epithelium that acts as a semipermeable barrier between two major internal compartments: the blood and the interstitial tissue fluid
  • Vascular endothelial cells are squamous, polygonal, and elongated with the long axis in the direction of blood flow
  • Presents a nonthrombogenic surface on which blood will not clot and actively secretes agents that control local clot formation (such as heparin, tissue plasminogen activator, and von Willebrand factor)
  • Regulates local vascular tone and blood flow by secreting various factors that stimulate smooth muscle contraction
  • Has several roles in inflammation and local immune responses, inducing specific white blood cells to stop and undergo transendothelial migration at sites of injury or infection
  • Secretes various growth factors, such as vascular endothelial growth factor (VEGF) to stimulate formation of the vascular system from embryonic mesenchyme, help maintain the vasculature in adults, and promote capillary sprouting and outgrowth from small existing vessels (angiogenesis) during normal growth, during tissue repair and regeneration

  
• Smooth Muscle


  • Occur in the walls of all vessels larger than capillaries and are arranged helically in layers
• Connective Tissue


  • Present in vascular walls in variable amounts and proportions based on local functional requirements

  
• Layers of the blood vessel wall

  • Tunica intima (endothelial layer)
  • Tunica media (smooth muscle)
  • Tunica adventitia (outside layer)

  
• Tunica intima


  Innermost layer of the vessel, consisting of endothelium, a basal lamina, and a subendothelial layer of loose connective tissue
• The subendothelial layer of the arteries and arterioles would usually contain a sheet like layer or a lamina of fenestrated elastic material called the internal elastic membrane, which allows expansion especially during high blood pressure.

  
• Tunica media


  • Consists primarily of circumferentially arranged layers of smooth muscle cells
  • In arteries, the media may have a thin external elastic lamina, separating it from the outermost tunic
  • Very thick for the arteries
• Tunica adventitia


  • Composed primarily of longitudinally arranged collagenous tissue (type 1) and a few elastic fibers
  • Contains the vasa vasorum ("vessels of the vessel") and nervi vascularis

  
• Vasa vasorum


  Blood supply of our blood vessel
• Nervi vascularis


  Nerve supply of the blood vessel
• Walls of both arteries and veins have three tunics called the intima, media, and the adventitia (or externa), which correspond roughly to the heart's endocardium, myocardium, and epicardium.

  
• An artery has a thicker media and relatively narrow lumen (more for distribution of blood). A vein has a larger lumen and its adventitia is the thickest layer (more for distribution of deoxygenated blood towards the tissue). The intima of veins is often folded to form valves.

  
• Capillaries have only an endothelium, with no subendothelial layer or other tunics.
• Elastic Arteries


  • The most prominent feature is the thick tunica media in which elastic lamellae alternate with layers of smooth muscle fibers
  • Tunica intima is well developed, with many smooth muscle cells in the subendothelial connective tissue, and often shows folds in cross section as a result of the loss of blood pressure and contraction of the vessel at death
  • The adventitia is much thinner than the media
• Function of elastic arteries


  The numerous elastic laminae contribute to making the blood flow more uniform

  
• Muscular Arteries


  • Also called distributing arteries
  • Distribute blood to the organs and help regulate blood pressure by contracting or relaxing the smooth muscle in the media
  • Intima has a thin subendothelial layer and a prominent internal elastic lamina
  • Media may contain up to 40 layers of large smooth muscle cells interspersed with a variable number of elastic lamellae (depending on the size of the vessel)
  • Adventitial connective tissue contains lymphatic capillaries, vasa vasorum, and nerves, all of which may penetrate to the outer part of the media
  • External elastic lamina is present only in the larger muscular arteries

  
• Arterioles


  • Smallest arteries branch
  • Only one or two smooth muscle layers, these indicate the beginning of an organ's microvasculature where exchanges between blood and tissue fluid occur
  • Subendothelial layer is very thin, elastic laminae are absent, and the media consists of the circularly arranged smooth muscle cells
  • In both small arteries and arterioles the adventitia is very thin and inconspicuous

  
• Precapillary sphincter


  Slight thickening of the smooth muscle at the origin of a capillary bed from an arteriole, where the smooth muscle fibers act as sphincters and produce periodic blood flow into capillaries
• Capillaries


  • Form blood vascular networks that allow fluids containing gases, metabolities, and waste products to move through their thin walls
  • Permit and regulate metabolic exchange between blood and surrounding tissues
  • Each consists of a single layer of endothelial cells and their basal lamina
  • Always function in groups called capillary beds