circulatory

• Cardiac Muscle Orientation


  • The tissues that comprise the myocardium, as well as the adjacent tissues of the endocardium and pericardium, are continuous, which means that the cardiac muscle is one single tissue that wraps around itself to form the heart
  • The myocardial tissue in a normal heart spirals up from the base to the apex, causing a series of clear intersections of cardiac muscle tissue
  • The double spiral information of the myocardial tissue allows a 60% increase in ejection fraction with a fiber shortening of 15%
• Orientation of myocardial fibers

  • Endocardium
  • Mid-wall
  • Epicardium
• Cardiac Muscle (longitudinal)


  • Consist of BRANCHED MUSCLE CELLS with one centrally placed nucleus
  • The nuclei are oval, rather pale and located centrally in the muscle cell which is 10-15 um wide
  • Cardiac muscle exhibits cross-striations
  • Cardiac muscle is for these reasons also called involuntary striated muscle
• Blood is carried in a closed system of vessels that begins and ends at the heart
• The three major types of vessels

  • Arteries
  • Capillaries
  • Veins
• Arteries


  Carry blood away from the heart
• Veins


  Carry blood toward the heart
• Capillaries


  Contact tissue cells and directly serve cellular needs
• Generalized structures of Blood Vessels

  • Arteries and veins are composed of three tunics: Tunica externa, Tunica media, Tunica interna
  • Capillaries are composed of endothelium with sparse basal lamina
  • Lumen - central blood containing space
• Tunica Intima


  • Innermost layer located next to the lumen
  • Contains endothelium with simple squamous lining and cells joined by zonula occludens junctions
  • Contains basal lamina of the endothelium
  • Contains subendothelial layer with few longitudinal layers of collagen and elastic fibers produced by smooth muscle cells in arteries and veins
• Tunica Media


  • Middle layer consisting of smooth muscle cells
  • Circumferentially or spirally oriented
  • Contains gap junctions between cells
  • Contains collagen fibers, elastic fibers and proteoglycans produced by and interspersed between smooth muscle cell layers
  • Contains Internal Elastic Lamina composed of elastin with holes that separates the intima from the media
• Tunica Adventitia


  • Outer Layer, consists mainly of fibroblast and collagen fibers, (partly type I)
  • Oriented longitudinally
  • Larger vessels contain unmyelinated sympathetic nerve fibers which releases norepinephrine
  • Generally, the thickest layer of venules and veins
  • Contains Vasa Vasorum (arterioles, capillaries, and venules in the adventitia, extending to the outer part of the media) that provide nourishment to larger vessels
• Major Properties of Endothelial Cells

  • Maintenance of selective permeability barrier
  • Maintenance of non-thrombogenic and thrombogenic barrier
  • Modulation of blood flow and vascular resistance
• Associated Functions of Endothelial Cells

  • Simple diffusion
  • Active transport
  • Pinocytosis
  • Receptor-mediated endocytosis
  • Secretion of anticoagulants, antithrombogenic agents and prothrombogenic agents
  • Secretion of vasoconstrictors and vasodilators
• Active Molecules Involved in Endothelial Functions

  • O2, CO2, Glucose, amino acids, electrolytes (Na, K, Ca2+), H2O, small molecules, soluble proteins, LDL, cholesterol, transferrin, growth factors, antibodies, MHC complexes
  • Thrombomodulin, Prostacyclin (PG/2), tissue plasminogen activator (TPA), antithrombin III, heparin, Tissue thromboplastin, von Willebrand factor, plasminogen-activator inhibitor
  • Endothelin 1, Angiotensin II, angiotensin-converting enzyme (ACE), Endothelial-derived growth factor (EDGF), Endothelial-derived relaxation factor (EDRF)/nitric oxide (NO), prostacyclin
• Other Endothelial Functions


  • Regulation of cell growth
  • Regulation of immune responses
  • Maintenance of extracellular matrix
  • Involvement in lipoprotein, cholesterol, metabolism
• Regulation of Cell Growth and Immune Responses by Endothelial Cells

  • Secretion of growth-stimulating and inhibiting factors
  • Regulation of leukocyte migration by expression of adhesion molecules
  • Regulation of immune functions
• Endothelial Involvement in Extracellular Matrix and Lipoprotein/Cholesterol Metabolism

  • Synthesis of basal lamina and glycocalyx
  • Production of free radicals, Oxidation of LDL
• Arteries


  • Originate with either the pulmonary trunk or aorta
  • Specializations of the walls of arteries relate mainly to the pressure pulses generated during contractions of the heart (systole) and the regulation of blood supply to the target tissues
  • The tunica media is the main site of histological specializations in the walls of the arteries
• Classification of Arteries


  • Conducting arteries (Elastic)
  • Distributing arteries (Muscular)
  • Arterioles
• Conducting (Elastic) Arteries


  • Examples: Pulmonary artery, Aorta, Brachiocephalic, Subclavian, Common Carotid, Common Iliac Arteries
  • Walls may be distinctly yellow in fresh state due to elastin abundance
  • Only part of the force of cardiac contraction is dissipated during diastole because of the distensible elastic walls
  • Continuous flow through the capillaries is ensured through wall recoil during diastole which serves as subsidiary pump maintaining flow
• Tunica Intima of Conducting Arteries

  • Thicker than in any other arteries
  • Endothelium involved in trans-endothelial transport
  • Subendothelial connective tissue allows the tunica intima to move independently from other layers as the elastic arteries distend
  • Distension of the walls is facilitated by concentric fenestrated lamellae of elastic fibers (elastin) in a thick tunica media
• Tunica Media of Conducting Arteries

  • 500um thick in large vessels
  • Has fenestrated laminae of elastin 5-15um apart
  • Thickness is affected by age and hypertension
  • Elastica interna is less prominent than it is in muscular arteries
  • The energy stored in the elastic fibers of the tunica media allows elastic arteries to function as a "pressure reservoir" which forwards blood during ventricular relaxation (diastole)
• Tunica intima


  • Thicker than in any other arteries
  • Endothelium involved in trans-endothelial transport of water, electrolytes and other macromolecules
  • Subendothelial connective tissue allows the tunica intima to move independently from other layers as the elastic arteries distend with the increase in systolic blood pressure
• Tunica media


  • 500um thick in large vessels
  • Has fenestrated laminae of elastin 5-15um apart
  • Thickness is affected by age and hypertension
  • Elastica interna is less prominent than it is in muscular arteries
  • The energy stored in the elastic fibers of the tunica media allows elastic arteries to function as a "pressure reservoir" which forwards blood during ventricular relaxation (diastole)
  • The space between elastic lamellae are occupied by long branching smooth muscle cells, fibroblast, and associated collagen fibers
• Tunica adventitia


  • Thin, consisting of fibroblast and collagen bundles of longitudinal orientation
  • Vasa vasorum - own vasculature for nourishment since thick walls of larger elastic arteries does not allow nourishment from vessel lumen
  • May penetrate some distance into the media
• Aorta


  • Endothelial lining: thin, corresponds to that of its vessels
  • Subendothelial layer of connective tissue: lower density of cells, fewer nuclei
  • Fibrous appearance of tissue, absence of a well-defined elastic layer
  • Elastic lamellae: visible in tunica media
  • Smooth muscle cells (SMC) are greater in number than collagen fibers found in between the layers of elastic fibers
  • Vasa vasorum: in the periphery, seen in the tunica adventitia and penetrating the outer part of the tunica media
• Veins


  • Walls are thinner, diameter is larger
  • Layering in the walls is not very distinct as compared to arteries
  • Tunica intima is very thin
  • Only the largest veins contain an appreciable amount of subendothelial connective tissue
  • Internal and external elastic laminae are absent or very thin
  • Tunica media appears thinner than tunica adventitia, the two layers tend to blend
  • Veins are thicker in the lower part of the body than the upper part and in tissues that provide structural support than unsupported veins
• Capillaries


  • Smallest diameter blood vessel
  • Wall may be formed by a single endothelial cell
  • Very large surface to volume ratio which favors movement of substances and exchange of gases and hormones
  • Humans contain approx. 50,000 miles of capillaries
  • Terminal branches of arterioles
  • Functions: selective permeability barrier, adsorption of nutrients, waste products and carbon dioxide, removal of waste products in the body, tremendous surface area in plexuses just below epidermis allows for cooling of blood by "radiator effect"
• Capillary wall


  • Only the tunica intima is present, consisting of single endothelium and basal lamina
  • Surrounded by pericytes - sparse network of retractile fibers, an incomplete layer of cells surrounding the capillary, have contractile properties and can regulate blood flow in capillaries, can also differentiate into endothelial and smooth muscle cells
  • Calibre average about 8 - 12um
• Fenestrated capillaries


  • The endothelial cell body forms small openings called fenestrations which allow components of the blood and interstitial fluid to bypass the endothelial cells
  • Fenestrations may be present or arise from pinocytic vesicles which open onto both the luminal and basal surfaces of the cell
  • Found in endocrine tissues, intestine, and renal glomeruli
  • Pore not closed by diaphragms, 3x thicker endothelially-derived basal lamina, allows the transfer of fluids
• Discontinuous/sinusoidal capillaries


  • Formed by fenestrated endothelial cells, which may not even form a complete layer of cells
  • Incomplete basal lamina
  • Form large irregularly-shaped vessels, sinusoids or sinusoidal capillaries
  • Found in liver, bone marrow, spleen, adrenal cortex
  • Much larger, has a diameter of up to 30um
  • Numerous, large fenestrae, no diaphragms
  • Cylindrical, conform in shape to the interstices among the epithelial sheets and cords of the organ they supply
  • Discontinuous basal lamina
  • Active in endocytosis, has a better lysosomal system
  • Classified as part of the reticuloendothelial system
• Types of capillaries


  • Continuous
  • Fenestrated
  • Discontinuous
• Continuous capillaries are found in muscles, connective tissue, lungs, exocrine glands, and nervous tissue
• Fenestrated capillaries are found in kidney, intestines, choroid plexus, and endocrine glands
• Discontinuous capillaries are found in lungs, adrenal glands, bone marrow, and spleen