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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