Cardiovascular

21Jan08

CARDIOVASCULAR SYSTEM II

  • heart à pump with 4 chambers.
  • Wall à 3 tunica : 1. endocardium
    • 2. myocardium
    • 3. epicardium
    • heart à surrounded by fibrous sac à pericardium
      • possesses the internal fibrous skeleton à
      • composed of dense CT.
  • Hyaline/ fibrocartilage/ cartilage plate à also located in various region of fibrous skeleton à play supporting role to myocardium n valves.
  • Fibrous skeleton:
  • consist of 4 fibrous rings surrounding the valves orifices, 2 fibrous trigones connecting the rings n the membranous part of the interventricular n interatrial septa.
  • fibrous rings à composed of dense irregular CT. they encircle the base of 2 arteries leaving the heart <aorta n pulmonary trunk> n the openings btw atria n the ventricles <right n left AV orifices>.
  • each annular arrangements of fibrous tissues serves for the attachment of the valves that allow blood to flow in one direction only thru the opening.
  • The fibrous skeleton also include an extension into the inter ventricular septum.
  • This small part of the i/ventricular septum is devoid of cardiac muscle.

ENDOCARDIUM

  • consist of an inner layer of endothelium.
  • 5 layers :
    • 1. innermost endothelium
    • 2. basal lamina
    • 3. subendothelial CT
    • 4. muscular elastic
    • 5. subendocardial CT
  • all those 5 layers are visible in thick wall only! Not in histo preparation.
  • the valves of the heart have a central sheet of fibrous tissue.
  • the surface of the valve that are exposed to blood are covered by endothelium.
  • fibrous thread-like cords called à chordae tendineae extends from the 3 edges of the AV valves.
  • 2 muscular projections from the wall of the ventricles à papillary muscles.
  • chordae tendineae are covered with endothelium.

MYOCARDIUM

  • thickest, basic tunica.
  • mainly consist of bundles of cardiac muscle fibers separated by loose CT layers with blood vessels n nerve elements.
  • most of cardiac muscle cells are striated n play the contractile function à typical cardiomyocytes.
  • the heart include impulse-generating /conducting system à composed of several specialized structures :
    • SA node
    • AV node
    • AV bundles <bundle of His,
    • immediately under endocardium>
  • the bundles are divided into smaller right n left branches, n then diffuse into Purkinje’s fibers.
  • aggregates of atypical fibers are located in the subendocardium layer of endocardium.
  • the diffused Purkinje fibers are sparsely dispersed in myocardium.
  • there are coordinated interactions with myocardium which regulate the heart beat.
  • the impulse-generating syst. Of the heart consist of modified/atypical cardiac muscle cells that are responsible for generating n conducting the electrical impulses.
  • structurally, they are characterized by poorly developed myofibers <no contraction coz contain few amounts of myofibers!!>
  • they make contact btw each other n with the typical cardiac muscle cells thru gap junctions, desmosomes n fascia adherents but no typical intercalated disc.
  • atypical cardiac muscle cells constitute typical cardiac muscle fibers.
  • the SA node, AV node, n AV bundle n their branches consist of modified cardiac muscle cells that are smaller than typical one n diffused Purkinje fibers à larger modified cells than typical cells n are characterized by highly developed glycogen + many mitochondria!
  • The SA node is the basic pace-maker of the heart in normal condition. So they are modified muscle cells named pace-maker cells.
  • the SA node generates electrical impulses n sends them to AV node.
  • normally the excitatory impulses are conducted to myocardium by the :
    • AV node
    • AV bundle + branches
    • diffused Purkinje fibers..
  • ..Successfully to initiate contraction of the heart.
  • after injury of SA node, the AV node become the 2ndary pace-maker!

ENDOCRINE FUNCTION OF THE HEART

  • its walls include atriocardiac muscle cells.
  • they are named “atrial” coz most of them are located in the wall of atria!
  • atrial cardiac muscle cells à contain atrial granules à source of 2 polypeptide hormones:
    • atrial natriuretic factor<ANF>
    • brain natriuretic factor <BNF>
  • both hormones are diuretics n inhibit renin secretions <kidney> n aldosterone <adrenal gland>.
  • both hormones also stimulate relaxation of smooth muscles which leads to dilatation of blood vessels n â blood pressure.

EPICARDIUM

  • It’s covered by layer of mesothelium cells.
  • under mesothelium, the EPI contains the CT with blood vessels n nerves.
  • normally EPI contains no. of adipose cells too!

INNERVETION OF CV SYSTEM

  • blood vessels àinnervated by sympathetic n sensory fibers.
  • sympathetic fibers à responsible for contraction of smooth muscle to reduce the luminal diameter.
  • generally vaso-constriction can be induced as nerve impulses n circulating hormones.
  • relaxation of smooth muscles á the luminal diameter. This phenomena is named vaso-dilation.
  • normally, vaso-dilation à passive n may occur in the absence of nerve impulses.
  • but artery of skeletal muscles also contain parasympathetic fibers Cholinoergic <Arethycholine> that inhibit nerve impulses.
  • coronary circulation possesses unique response to sympathetic influences as a rule it is á but reaction of different vessels is not uniform.
  • sympathetic nerve endings reduce the luminal diameter of large coronary blood vessels only n dilate the luminal diameters of arterioles.
  • the heart is innervated by sympathetic n paraS divisions of the autonomic nervous system n also sensory nerve fibers.
  • autonomic nerves of the heart modulate the rhythm of the heart beat but not initiate!!
  • sympathetic nerves accelerate the heart beat n parasympathetic slow the heart beat.
  • the sympathetic fibers supply SA n AV nodes, extend into myocardium n also pass into epicardium to reach coronary artery that supply the heart.
  • the parasympathetic fibers chiefly terminate above/about SA n AV nodes.
  • the PS fibers form synaptic contact with long axon neurons of intra-cardial ganglion in the SA n AV nodes.
  • those neurons send their axons within the SA n AV nodes n also into myocardium.
  • intra-cardiac ganglia include small intensively fluorescent cells <SIF cells>.
  • they are characterized by no. of large cytoplasmic vesicles < ≈ 200nm in d> à contain CATECHOLAMINES <collective name for epinephrine, nor epinephrine, n their precursors functioning as neurotransmitter.>
  • SIF cells receive both symp. n PS impulses n secrete catecholamines in extracellular environment.
  • they are believed to be interneuron-like cells.
  • the sensory nerve fibers form/are specialized receptors in the heart wall.
  • moreover, sensory nerve receptors for physiologic reflexes are located in the wall of large blood vessels near the heart.
  • F: baroreceptors n chemoreceptors.
    • BARORECEPTOR à sense general blood pressure. Located in the carotid sinus n aortic arch.
    • CHEMORECEPTOR à detect alterations in O2 n CO2 tension n in pH. These receptors are the CAROTID N AORTIC BODIES located at the bifurcation of the carotid arteries n in the aortic arch!
  • the CAROTID BODY consist of cords n irregular groups of epitheliod cells associated with these cells is the rich-supply of afferent n efferent nerve fibers.
  • the struct. Of AORTIC BODY ≈ similar to carotid body <in organization>.
  • both receptors function in neuro-reflexes that adjust cardiac output n respiratory rate.

INJURY N REPAIR

  • mature CM cells X divide n X be replaced by muscle cells.

an injury to CM tissue that results in the death of cells is repaired by the formation of fibrous CT with consequence lost of cardiac function.



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