C7

• Circulatory system


  Transportation system that serves to pump, transport and distribute required elements to the cells, tissues and different parts of the body and permits them to exchange their particular products with the cells in other parts
• Two subsystems of the circulatory system

  • Cardiovascular system
  • Lymphatic vascular system
• Cardiovascular system


  • Comparable to a closed system of plumbing, through which the blood circulates with the aid of an in-line pump
  • Has four components: heart, arteries, veins, capillaries
• Lymphatic vascular system


  • Comprises an additional set of vessels, in which lymph moves in one direction only
  • Lacks separate pump
  • Starts as blind-ending tubules or lymphatic capillaries
  • Main function is to collect the excess interstitial fluid, filter it through various lymph nodes and return extracellular fluid as lymph to the blood vascular system
• Three types of lymph vessels

  • Lymphatic capillaries
  • Lymphatic vessels
  • Lymphatic ducts
• Lymphatic capillaries


  • Blind-ending endothelial tubes that collect lymph (excess tissue fluid, cellular debris and lymphocytes) from the intercellular spaces
• Lymphatic vessels


  • Collect lymph from lymphatic capillaries
• Lymphatic ducts


  • Collect lymph from smaller lymphatic vessels and empty it into the large jugular and sub-clavian veins
• Cardiovascular elements


  • Composed of three layers or tunics: tunica intima, tunica media, tunica adventitia
• Tunica intima


  Innermost layer that borders the lumen, composed of endothelium, subendothelial coat, and internal elastic membrane
• Endothelium


  • Squamous cells that line the lumen of the organ, a common and consistent feature of all blood vessels and the heart
• Subendothelial coat


  • Consists of loose connective tissue, fibroblasts and some smooth muscle fibers
• Internal elastic membrane


  • A condensation of elastic fibers that separates the tunica intima and tunica media
• Tunica media


  • Middle layer, consists of a mixture of smooth muscle cells, collagen fiber, elastic fibers and fibroblasts
  • Arteries generally have a thicker media containing more muscle and elastic fibers, than that of veins and lymphatic vessels
  • Large arteries often exhibit an external elastic lamina between the tunica media and adventitia
  • The media of the heart (myocardium) is several times thicker than that of the large artery (aorta) and is composed of cardiac muscle
• Tunica adventitia


  • Outermost layer, consists chiefly of Type I collagen and elastic fibers that anchors the vessel in the surrounding tissue
  • In veins, the adventitia is the thickest layer and may contain longitudinal bundles of smooth muscle
  • In large vessels, the adventitia contains small blood vessels (vasa vasorum) that supply oxygen and nutrients to the cells in the vessel wall too far from the lumen to be nourished by diffusion
• Capillaries


  • The smallest vascular channels in the body, with an average diameter of about 8 um
  • Their walls consist of a single layer of simple squamous epithelium (endothelium) rolled into a tube covered on the outer surface by a thin basal lamina
• Capillary beds


  • A profusion of anastomosing (interconnecting) capillary channels
• Cells of capillaries


  • Endothelial cells
  • Pericytes or adventitial cells
• Endothelial cells


  • The chief structural component of capillaries, simple squamous epithelial cells of mesenchymal origin joined by intercellular junctions to form an epithelial tube
• Pericytes or adventitial cells

  • Small mesenchymal cells scattered along capillaries, surrounded by their own basal lamina and cling by long cytoplasmic processes to the outside of capillaries, may or may not be contractile, can differentiate into a variety of cell types
• Three types of capillaries

  • Continuous capillaries
  • Fenestrated capillaries
  • Sinusoidal capillaries
• Continuous capillaries


  • Characterized by the absence of interruptions in the continuity of the endothelial cells, have smooth, nonporous endothelial lining and the cells attached tightly to each other by junctional complex, most common type found in connective tissues, all muscle tissues, the central nervous system, and other organs
• Fenestrated capillaries


  • Characterized by the presence of pores (fenestrae) in the endothelial lining, allow a more rapid exchange of molecular substances between blood and tissues, found primarily in the endocrine organs, the small intestine, and the glomeruli of the kidneys
• Sinusoidal capillaries


  • Have unusually wide lumens (30-40 m), follow a tortuous path, have gaps between their endothelial cells often large enough to allow cells to pass, have abundant fenestrations, have phagocytic cells interspersed among their endothelial cells, surrounded by a discontinuous basal lamina, found in the liver, spleen, and bone marrow
• Arteries


  • Have thicker tunica media than veins, distinguished by refractile eosinophilic internal and external elastic laminae, usually accompanied by veins, the major function is the distribution of the blood to the capillary beds of the body
• Three major categories of vessels in the arterial system

  • Arterioles
  • Small arteries
  • Large arteries
• Arterioles


  • The smallest branch of the arterial vessels (0.5 mm or less in diameter) having relatively narrow lumens and relatively thick walls, the terminal arterioles give rise to capillaries, their walls consist of one to five layers of smooth muscle, regulate the flow of the blood into the capillary bed by autonomic constriction or dilation of their lumen
• If the tonus of the smooth muscle cells in arterioles increased above the normal range, hypertension (high blood pressure) results
• Four sizes of arteriole

  • Large arteriole
  • Small arteriole
  • Pre-capillary or terminal arteriole
• Small arteriole


  • Continues as the pre-capillary or terminal arteriole, has a thinner wall and smaller lumen than large arteriole, internal elastic membrane is very thin, has only 2 to 3 layers of smooth muscle cells, tunica adventitia consists mainly of collagenous fibers
• Arterioles


  Regulate the flow of blood into the capillary bed
• If the tonus of the smooth muscle cells in arterioles increases above normal

  Hypertension (high blood pressure) results
• Arterioles


  • Act as sphincters and control the amount of blood flowing through the central or thoroughfare channel which courses through the capillary bed and joins the venous side of the circulation
• Sizes of arterioles


  • Large arteriole
  • Small arteriole
  • Precapillary or terminal arteriole
  • Metarteriole
• Large arteriole


  • Direct continuation of small artery, branches and becomes a small arteriole
• Small arteriole


  • Continues as the pre-capillary or terminal arteriole, has a thinner wall and smaller lumen, very thin internal elastic membrane, 2-3 layers of small smooth muscle cells, tunica adventitia mainly of collagenous fibers
• Precapillary or terminal arteriole

  • Lumen about the size of an RBC, no internal elastic membrane, tunica media only one layer of smooth muscle cells surrounded by connective tissue
• Metarteriole


  • Small branch of terminal arteriole, constriction can regulate blood flow in capillaries, smooth muscle cells replaced by perivascular cells/pericytes (Rouget cells)
• Precapillary sphincters


  • Rings of smooth muscle cells around the metarterioles at capillary origins, can halt or control blood flow through the arterial capillaries and into the capillary bed proper
• Muscular arteries (distributing arteries or small and medium arteries)

  • Branches of elastic arteries, majority of arteries in the body, control blood flow and blood pressure through vasoconstriction or vasodilation, innervated by sympathetic nervous system