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The Circulatory System

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The Circulatory System Powered By Docstoc
					The Cardiovascular system
Functions of the circulatory system
• Transport
   – Erythrocytes (red blood cells, RBC’s) carry oxygen from lungs, remove
     CO2 from tissues
   – Nutrients, hormones etc. all carried by the fluid portion of blood (NOT
     RBC’s)
   – Metabolic wastes from body tissues delivered to renals
• Protection
   – “White” cells (immune cells)
   – Antibodies, inflammatory mediators (cytokines), blood clotting factors
• Regulation
   – Constant flow helps to stabilize fluid and fluid ingredient distribution
     (mixes everything equally)
   – Buffers pH changes in tissue
   – Buffers temperature changes
The Heart
                                  The heart
•   Two pump: Right (pulmonary system)
    and left pump (organ system)
•   Pumps  5 L/min (2.5+ million liters
    year, roughly 60-70 bpm,
           • An RBC takes  1 minute to
             travel from the heart to your
             finger or toe and back to the
             heart!
•   Roughly size of your fist
                      Heart - pericardium
•   Contained within pericardium
     – Parietal pericardium:
         • Outer dense fibrous connective
            tissue
         • Inner serous parietal
            pericardium
     – Visceral pericardium  serous layer
       = epicardium
     – Produces serous fluid that surrounds
       heart proper
                                Heart – the wall
• 3 layers form the heart “proper” (organ within
   the parietal pericardium)
    – Epicardium (visceral pericardium)
         • Where cardiac vasculature is located
         • Where you look for the root cause of
           damage during myocardial infarction
    – Myocardium (muscle layer)
         • Cardiac muscle cells
              – Arranged so that during contraction,
                chambers squeeze in a particular manner
         • Thickness reflects amount of force required
           to pump
              – Thickest in region of the Left Ventricle
              – Thinnest in the atrial walls
    – Endocardium (endothelium within the
      heart)
         • Inner lining of the heart chambers
         • Not very porous (acts as a “bag” to
           retain blood and prevent leakage
           between muscle layers)
          Cardiac muscle
• Most of your heart is made of cardiac
  muscle
  – Striated (like skeletal muscle in contractile
    protein arrangement)
     • Each cell is much shorter, and usually more thick as
       well
     • Each cell joined by an intercalated disc
            » An area of cell-cell adhesion, as well as a
              gap junction complex to permit 1 cell to
              stimulate the next
 Intercalated disc
with a gap junction
                        Cardiac muscle
• Cardiac muscle cells have less
  developed sarcoplasmic
  reticulum
   – Less ability to store calcium
       • Have larger transverse tubules
         (the tunnels of the plasma
         membrane/sarcolemma) to
         permit extracellular calcium to
         enter more easily
   – Damage is repaired by fibrosis
     only
       • Reason why heart attack
         patients have such a hard time
         recovering…they’ve lost heart
         contractility and will never
         regain/repair it
                            Cardiac muscle
• NO neural stimulus for contraction
    – Has “pacemaker cells” that set off rhythmic
      depolarizations (electrical pulses) to trigger your
      heartbeat
        • The autonomic nervous system feeds into these
          pacemaker cells to increase or decrease your heart
          rate, but these cells operate without neural control
          to make sure you heart ALWAYS beats
    – Known as “autorhythmic” because your heart does
      not need your brain to tell it to beat



• These pace-maker cells are distinct from the cardiac
  muscle cells. They do not contract
               Cardiac muscle
• External heart structure:
   – Coronary sulcus: divides atria from ventricles
      • Think: “circumferential” or coronal plane sulcus
   – Interventricular sulci: divides left & right
     ventricles
   – Look for “adipose lines”
               Cardiac muscle
• External heart structure:
   – Various sulci serve as “routes” for cardiac blood
     vessels
      • Cardiac muscle reliant on cardiac blood vessels for
        blood supply (endocardium does not allow for much
        fluid or gas exchange within the heart)
                                      Heart
• Within the heart proper, 4
  chambers & valves
   – Atria contract simultaneously,
     emptying into each respective
     ventricle
       • Atrial walls “reinforced” by
         pestinate muscles (gives the look
         of a wicker basket)
       • Interatrial septum = thin,
         muscular membrane separating
         left & right atria
       • Atrioventricular valves separate
         atria from ventricles
   – Ventricles = much more muscular
     (have to pump blood further)
   – Valves maintain one-way flow of
     blood through heart
                     Heart wall
• Within ventricles endocardium = specialized
  formation
  – Trabeculae carnae: ridges & “mesh-like structure”
    within the ventricle to prevent “suction”
     • If the inner wall of the ventricle were flat, as the
       ventricle contracted, it would have difficulty relaxing as
       the two flat surfaces would adhere together
                        The heart valves
• Atrio-ventricular valves (A/V
  valves): separate atria from
  ventricles:
   – Right side: Tricuspid valve
   – Left side: Bicuspid or Mitral valve
• Semi-lunar valves:
   – Pulmonary semi-lunar valve within the
     pulmonary trunk
   – Aortic semi-lunar valve, at the
     beginning of the aorta
• The long cords, the chordae
  tendinae, hold the valves in place
  during contraction and prevent
  reflux
     Coronary vessels




Angiogram
                  Coronary arteries
• Blood is brought to the
  cardiac muscle by the
  coronary arteries which
  originate at the base of
  the aorta:
   – Left coronary artery split
     into the circumflex artery
     and the left anterior
     descending (LAD) artery
   – The right coronary artery
     split into the marginal
     artery and the right
     posterior descending
     artery
                    Coronary veins
• The blood from the cardiac
  muscle drains into the venous
  system.
• It is collected by the Great
  cardiac vein, the small and
  middle cardiac veins.
• The blood empties in the right
  atrium at the coronary sinus
  (not to be confuse with the S/A
  node or the pace-maker)
• “The “widow maker”
                               Heart Conduction
•   Conduction system create and spread a
    wave of depolarization.
•   This wave is then followed by cardiac
    muscle contraction
•   SA node = “pacemaker” due to cyclic
    depolarization of specialized neurons
     – Located in right atrium, near insertion of
       SVC
     – Depolarization spreads across BOTH atria
          • Recall both atria contract simultaneously
•   Impulse then passes down to
    atrioventricular node (AV node)
     – Inferior side of the interatrial septum 
       atrioventricular bundle at most superior
       end of the interventricular septum  L/R
       ventricles
     – In the ventricles, conduction fibers
       (Purkinje fibers) carry impulse throughout
       both ventricles
                Heart conduction
• This electrical activity
  sustained by the
  conduction system can be
  followed by the EKG
  (ECG)

• The corresponding heart
  muscle contraction results
  in rhythmic contraction
  and movement of blood
  throughout the heart
• See Human Physiology
             “lub”
             “systole”




             “dub”
“diastole”
Blood vessels
                              Blood vessels
• Tubular network for blood flow
   – Blood flow is a closed system
     (components of blood do not
     readily leave the blood vessels)
• 3 layers to every blood vessel
   – Tunica externa (adventitia)
       • Most superficial layer of loose
         connective tissue
   – Tunica media
       • Smooth muscle layer
           – In arteries, tunica media layer
             contains large density of elastic
             fibers
   – Tunica interna (endothelium)
       • Simple squamous epithelial tissue
         with elastic fibers
                                    Arteries
Elastic fibers within the tunica
   media
   – Allows artery to expand when
     heart expels blood
   – Elastic fibers permit recoil to
     original shape following
     expulsion of blood from heart
   – Expansion & recoil (elasticity)
     acts to smooth out the blood flow
     (less pulsing)
   – As arteries reduce in size, they
     become less elastic
       • Small arteries, arterioles are less
         elastic than arteries
       • Capillary = 7-10 m diameter
                                 Capillaries
• Nutrient and gas exchanges are
  only possible across capillaries
       • Endothelium in arteries is quite
         “tight”
   – Over 40 billion capillaries in your
     body (1800+ square kilometers of
     coverage)
       • No cell is more than a few m
         from a capillary
       • Despite large surface area and
         extensive network, only 250 ml
         blood is within the entire capillary
         network at any one time!
   – Walls are unique
       • Simple squamous endothelium
                                    Capillaries
• 3 subtypes of capillaries:
   – Continuous capillary: tight pores
      between squamous cells
        • Muscle, lungs, adipose & CNS
        • Remember that in CNS, this is the
          basis for the blood-brain
          barrier…incredibly tight capillary
          network
    – Fenestrated capillary: fenestrations
      = windows/pores
        • Renals, endocrine organs & GI tract
        • Wide pores permits fast transfer of gas
          and nutrients/waste
        • Covered by a mucoprotein diffusion
          barrier
    – Discontinuous capillary: widest
      pore size
        • Bone marrow, liver & spleen
        • Pores so wide = sinusoids (sinus-like
          pores)
In discontinuous capillaries,
the endothelial cells do not
physically connect to one
another. This permits pores
so wide they’re called
sinusoids.
Discontinuous capillaries are
restricted to organs that
process LARGE volumes of
blood
                                       Veins
• Carry blood from capillaries BACK
  to heart
   – From capillary – venule – vein

• Very LOW pressure (0.02 psi)
   – Arteries can hold up to 5 psi (in
     some areas, even more)
   – At this low pressure, blood
     cannot return to the heart
        • Relies on 1-way valves (venous
          valves) and skeletal muscle
          contractions to propel blood
          back to heart
        • Only find valves in veins…never in
          arteries
•   Varicose veins = veins stretched from
    standing (stretched veins = pulled valves that
    don’t work correctly)
                  Circulation
• Coronary circulation

• Cardiac blood flow

• Pulmonary circulation

• Systemic circulation

• Fetal circulation
Coronary circulation: blood circulation in myocardium
• Ascending aorta  aortic/semilunar valve
  L/R-coronary arteries
   – Left coronary artery  anterior
     atrioventricular artery  anterior region
     of both ventricles  circumflex artery
       • Circumflex artery  L-atrium & L-
         ventricle
   – Right coronary artery  posterior
     interventricular sulcus  Posterior region
     of both ventricles
       • From capillaries in myocardium 
         cardiac veins
           – Anterior interventricular vein
              (drains from anterior region of
              heart)
           – Posterior interventricular vein      Do not confuse coronary sinus
              (drains from posterior heart)       and sinus atrial node or S/A node
           – Merge into coronary sinus  R-
              atrium
                         Cardiac Blood Flow
•   L-atrium receives oxygenated blood from
    lungs via 2X right/left pulmonary veins
                 – ONLY veins that carry
                    oxygenated blood
•   The right atrium receives blood from the vena
    cavaes.

•   R and L-atrium contract and pump blood into
    the ventricles via the atrio-ventricular valves
    (A/V valves), the “bicuspid” or “mitral” for
    the left side and the tricuspid valve for the
    right side
      – Valve opening dependent upon ventricle
          RELAXATION
      – Ventricle contract & pumps blood into
          the systemic circulation:
            Through the aortic semilunar valve into       3
               ascending aorta on the left side and
               the pulmonary semi-lunar valve         4
               into the pulmonary trunk on the
               right side.
                “lub”  Cardiac contraction is
                       systole. Cardiac
                       relaxation is diastole.
                       The atria relax  blood
                       flows min, passively.
                       Both atria contract at
             “systole”
                       the same time to force
                       blood into the
                       ventricles. Both
                 “dub”
                       ventricles then contract
                       at the same time to
                       propel blood towards
                       the pulmonary artery or
                       the aorta.
“diastole”
              Pulmonary circulation
• Blood vessels that
  transfer blood between
  heart & lungs
      • Blood “vessel-way”
          – R-ventricle  pulmonary
            valve  pulmonary trunk
             L/R-pulmonary arteries
             pulmonary capillaries
            (in lungs)  pulmonary
            veins  L-atrium
      • Note how the coronary
        circulation begins at the
        right ventricle and ends at
        the left atrium
                 Systemic circulation
• Everything OUTSIDE the
  pulmonary circulation
     • Includes the coronary circuit
       as well
  – From:
     • Left ventricle  aortic valve
        ascending aorta 
       systemic vasculature 
       capillaries (not within the
       lungs)  venous apparatus 
       right atrium
                     Portal circulation
• Portal circulation: vein-
  capillary-vein
   – Recall portal circulation in
     the adenohypophysis
     (anterior pituitary)
      • Carries venous blood from
        hypothalamus into the
        capillary bed of the
        adenohypophysis
   – Hepatic portal blood circuit
      • Drains blood from the
        gastrointestinal viscera via
        hepatic portal vein, into the
        liver (hepatic) system before
        emptying into the IVC via the
        hepatic vein
         Principle Arteries
• Aorta = major systemic artery
  – Aortic arch
     • Directly from left ventricle = ascending aorta
        – Right & left coronary arteries are the ONLY branches at this
          point
     • Aortic ARCH
        – Brachiocepalic trunk
            » Further branches into right subclavian artery & right
              common carotid artery
        – Next branch = Left common carotid artery
        – Third branch = Left subclavian artery
               Principle Arteries
• Towards upper appendage via subclavian artery
  – Arterial blood from subclavian artery has a number
    of “choices”
     • Vertebral artery = towards cranium via transverse
       foramen of the cervical vertebrae & enters cranium via
       foramen magnum
     • Thyrocervical trunk = destined for thyroid
     • Internal thoracic artery = destined for thymus &
       pericardium
     • Costovertebral trunk = destined for intercostal muscles &
       spinal meninges
                Principle Arteries
• Towards upper appendage via subclavian artery
  – Arterial blood from subclavian artery has a number
    of “choices”
  – If destined for the appendage:
     • Subclavian artery = axillary artery between 1st rib &
       median edge of the humerus
        – Past medial side of humerus = Brachial artery
        – Around humerus = anterior & posterior humeral circumflex
          arteries
            » Ring of arteries around humeral muscles
        – Bifurcates into radial & ulnar arteries proximal to cubital fossa
            » Radial = pulse at the wrist
                  Principle Veins
• From upper appendage
  – In order to return arterial blood that has passed out
    of the capillary beds throughout the upper
    appendage:
     • Combination of superficial & deep veins
        – Superficial veins often quite variable in location
        – Deep veins usually follow arteries
     • Radial & ulnar veins draw blood from palmar region
        – Both anastomize into brachial vein
     • Superficial basilic vein draws blood ulnar & medial veins
        – Eventually anastomize with brachial vein  axillary vein
     • Superficial cephalic vein draws blood from superficial
       radial region of arm
                  Principle Veins
• From upper appendage
  – Once all upper appendicular veins have anastomized
    into axillary vein:
  – Axillary vein  subclavian vein
     • Receives venous drainage from cranium as well
        – External jugular vein (“external” skull)
        – Internal jugular vein (brain, meninges etc.)
            » Physically larger than external jugular
            » Adjacent to common carotid artery
     • Where internal jugular vein merges/anastomizes with
       subclavian vein = brachiocephalic vein
                  Principle Arteries
• Abdominal:
  – 4 branches from the dorsal/descending/abdominal aorta
     • Celiac trunk
         – Splenic artery
         – L-gastric artery
         – Common hepatic artery
             » Further bifurcates into gastroduodenal artery & “proper” hepatic
               artery
     • Superior mesenteric artery
         – Branches throughout mesentery (small intestine, upper 2/3 large
           intestine, pancreas)
     • Left & right renal arteries
     • Inferior mesenteric artery
         – Branches throughout distal/terminal mesentery (termnal colon, rectum)
                 Principle Arteries
• Abdominal:
  – Mesentery = “mes” “enteric”
     •   “reflection/fold” of the “peritoneal cavity”
     •   “enteric” usually infers gastrointestinal
     •   “middle of the gastro or intestinal tract”
     •   Mesenteric artery = artery that branches within the
         mesentery
                  Principle Veins
• Abdominal veins:
  – Absorptive viscera do not directly drain into the
    inferior vena cava
     • Lower extremities, renals & reproductive organs are the
       only organs that directly drain into the IVC
     • Absorptive viscera drain into hepatic portal vein
        – All venous blood from GI tract drains into liver via hepatic portal
          vein
        – Liver processes venous blood, then delivers back to inferior vena
          cava (cranial to diaphragm) via hepatic vein (NOTE: not the
          hepatic PORTAL vein)
Important note: in following
the major arteries, do not
make the mistake that
capillary beds are only at
the “ends” of these arteries.
Along literally the entire
length of many arteries are
branches that provide
arterioles / capillary beds
for practically every tissue
“along the way”.
Remember that practically
every cell in your body is
mere microns (m) from a
capillary bed.
         Circulatory “collaterals”
• Throughout your circulatory system, there are
  “collaterals”
  – Pools or “supplies” of blood that can be “mobilized”
    when called for
     • GI tract retains 50-70% blood volume during rest
        – During high activity/trauma, GI tract innervated by sympathetic
          nervous system is triggered to vasoconstrict (provide more blood
          for vitals and skeletal muscle)
     • Within brain: “circle of Willis” provides a similar
       function for the brain
        – Paired carotid arteries, paired vertebral arteries provides at 4
          different pathways for arterial blood to enter the brain
  – Pairs of vessels span many joints
     • Allows flexion of the joint while maintaining blood flow
Within the mesenteric vasculature,
all of the capillaries are “gated” by
precapillary sphincters. When
called upon by the sympathetic
nervous system, these sphincters
will constrict the amount of blood
entering the capillary bed,
restricting the bloodflow and
permitting more arterial blood to
be shunted to the vitals.
 Follow the Vertebral arteries and the Internal carotid
arteries (not labeled). Note the “circle of Willis”. Any
 of the 4 arteries can feed into the circle of Willis and
       keep the brain supplied with arterial blood.
              Circulatory pathophysiology
•   Atherosclerosis: scar tissue of the arteries
     – Recall elasticity of arteries (tunic media)
           • Despite elasticity, or in cases of
             exaggerated stretching, the endothelial
             layer tends to suffer damage
           • Circulating immune cells then sense this
             damage and act to form a “scar”
                   –   Actually start to attack endothelium &
                       place fatty deposits under the scar
                   –   Once scar & fatty deposits begin to
                       calcify = atherosclerotic plaque
                           » Plaque then inhibits/prevents
                               “stretch” response
                           » Inability for artery to respond to
                               stretch = inability to control blood
                               pressure (blood pressure usually
                               rises…velocity rises significantly)
     – Should really be thought of as a chronic
        inflammation of the arterial system
• Boundary layer/unstirred layer effect: as the velocity of
  the fluid within a tube increases, there is a decrease in
  velocity at the very edge of that tube
   – Boundary layer does not move (actually an “unstirred layer of
     fluid)…makes it very easy for these plaques and particles to
     collect
      • When these particles collect, the plaque formation can increase faster
          – A vicious cycle: formation of initial restriction = increased velocity
            through that region = greater boundary layer = greater ability for plaque
            to “take hold”
          – Plaque formation can eventually starve flow
Angioplasty: insertion of a balloon into the region of restriction in order to
restore flow. Newer techniques couple a balloon with a stent (wire structure to
hold artery open) that is usually “coated’ with anticoagulants/anti-plaque
chemicals
              Fetal Circulation

• Fetus receives
  maternal blood &
  nutrients
  – Transition occurs at
    placenta
     • Umbilical cord =
       between placenta &
       fetus
         – Umbilical vein + 2
           umbilical arteries
     • Umbilical vein carries
       oxygen-rich blood
       towards liver
                    Fetal Circulation
• Foramen ovale is
  therefore the first “shunt”
  to close following
  parturition

• Ductus arteriosus closure
  = more gradual
   – Usually remains partially
     open for  6 weeks
      • Increasing levels of vascular
        oxygen stimulates arterial
        smooth muscle contraction
   – Ductus remnant atrophies
     & becomes non-functional

				
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