Cardiovascular Physiology Part Three

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Cardiovascular PhysiologyPart Three

• Capillaries Microcirculation
• Immune System
• Regulation

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Capillaries and Microcirculation

• most tissues have extensive capillary bed no
  cell is gt 3-4 cells away from a capillary very
  NB as transfer of gases, nutrients wastes by
  diffusion is exceedingly slow process
• just large enough for RBCs (erythrocytes) to
  squeeze through (sometimes WBCs leukocytes get
  stuck)

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Microcirculatory beds

• fig 12-35 p. 507 - microcirculatory bed
  arterioles are surrounded by smooth muscle which
  becomes discontinuous in metarteries ends in a
  smooth muscle ring called precapillary sphincter
• capillaries single layer of endothelial cells
  surrounded by a basement membrane of collagen
  mucopolysaccharides
• a few elongated cells with the ability to
  contract wrap around caps pericyte cells

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Microcirculatory bedscont

• innervated smooth muscle of arterioles (i.e. the
  smooth muscle sphincter at junction of arteries
  arterioles) control blood distribution to each
  cap bed most arterioles innervated by
  sympathetic /NS a few in lungs innervated by
  para NS
• precapillary sphincter appears to not be
  innervated but under local control its opening
  closing alters blood flow through cap. bed in
  certain specific places cap. either open most
  of time (e.g. brain) or closed (e.g. skin) for
  considerable times
• all cap beds combined 14 of BV but with the
  opening/closing only 30-50 of all caps are
  open 5-7 total BV

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Transfer of substances fig 12.36

• endothelium much more permeable than epithelial
  cell layers allows substances to move with
  relative ease in out of caps. - caps in
  different tissues differ in permeability ( this
  difference in permeability is related to
  differences in structure!!) 1.Continuous least
  perm muscle, neuronal tissue, lungs, CT
  exocrine glands
• 2. Fenestrated intermediate perm glomerulus,
  intestines endocrine glands
• 3. Sinusoidal most permeable liver, bone
  marrow, spleen, lymph nodes adrenal cortex

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Transfer of substances cont

• 1. continuous capillaries least permeable in
  muscle, neuronal tissue, lungs, CT and exocrine
  glands substances move across either through or
  between endothelial cells (i.e. lipid soluble
  substances diffuse through cell membrane H2O
  ions diffuse through water-filled clefts between
  cells in brain, there are carrier mechanisms
  for transport of glucose some aas research
  suggests, substances are packaged in vesicles for
  transport) lower perm of brain caps is
  considered to result from tight junctions between
  endothelial cells

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Transfer of substances cont

• 2. fenestrated capillaries intermediate perm
  glomerulus, intestines endocrine glands perm
  to nearly everything except large proteins RBCs
  ultrafiltrate is formed across such a
  endothelial barrier very few vesicles
• 3. sinusoidal capillaries most permeable
  liver, BM, spleen, lymph nodes adrenal cortex
  these cap beds have large paracellular gaps that
  extend through basement membrane no vesicles
  most transfer occurs between cells fluid
  surrounding caps in liver has much same comp as
  plasma
• inflammation or certain substances e.g. histamine
  increase size of openings at venous end of cap
  network making it very perm

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The Lymphatic System

• Lymph transparent, slightly yellow or milky
  fluid is collected from interstitial fluid in all
  parts of body returned to blood via lymphatic
  system (contains no RBCs only WBCs)
• L system begins with blind-ending lymphatic caps
  that drain interstitial spaces join to form a
  treelike structure with branches converging from
  all tissues
• Lymphatic trunks (larger lymphatic vessels)
  resemble veins empty via a duct into blood
  circulation at point of low pressure

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The Lymphatic System cont

• in mammals and other vertebrates, lymph vessels
  drain via a thoracic duct into very low pressure
  region of venous system usually close to heart
• Returns excess fluid proteins that filter
  across cap walls into interstitial spaces back to
  blood large molecules (e.g. fats absorbed from
  gut some hormones) reach blood via lymphatic
  system
• Walls of lymph vessels single layer of
  endothelial cells basement membrane absent or
  discontinuous large paracellular gaps between
  adjoining cells

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The Lymphatic System cont

• Pressure lowers than surrounding tissue
  interstitial fluid passes easily into lymph
  vessels vessels are valved one way flow
  some large lymph vessels do have smooth muscle
  contract rhythmically driving fluid away from
  tissues squeezing by contractions in gut
  skeletal muscles ( general body movement)
• Fats taken up from gut by lymph system rather
  than directly into blood (folds in gut wall
  villi each contain a lymphatic vessel central
  lacteal into which fats

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The Lymphatic System cont

• fat-soluble nutrients (e.g. Vit A, D, E K)
  pass from lumen of gut
• Lymph flow NB in draining tissues of excess
  interstitial fluid (if lymph production exceeds
  lymph flow, severe edema can result)
• Interesting variations in other species
  reptiles many amphibians have lymph hearts
  which aid in movement of fluid within lymphatic
  system fishes appear to either lack lymphatic
  system or only very rudimentary one

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Circulation and the Immune Response

• circulatory lymphatic system contribute to
  bodys immune response
• lymphocytes (type of leukocyte aka WBC unique
  ability to recognize foreign substances
  (antigens) including those on surface of invading
  pathogens, virus-infected cells tumor cells
• 2 main types of lymphocytes B lymphocytes
  (B-cells) T lymphocytes (T-cells subdivided
  helper T TH) cytotoxic T (TC) (see
  Pathophysiology Assignment posted notes for more
  details)

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Circulation and the Immune Response cont

• other leukocytes neutrophils macrophages both
  of which can engulf microorganisms foreign
  particulate matter by phagocytosis these cells
  are mobile
• Immune Response recognizing invader, marking
  destroying it recognition carried out
  exclusively by lymphocytes either lymphocytes
  or phagocytes can destroy must distinguish self
  from non-self failure to do so autoimmune
  diseases

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The Immune Response cont

• Lymphocytes respond in 3 ways fig 12-40 p. 511
• 1. B cells develop into plasma cells secrete
  antibodies that bind to pathogens marking them
  for degradation by phagocytes
• 2. TC cells recognize tumor cells, including
  those infected by pathogens the recognition
  stimulates TC to mature into active cytotoxic T
  lymphocytes (CTLs) which destroy altered
  self-cells

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The Immune Response cont

• recognition of antigen by TH cells stimulates
  them to secrete cytokines, which in turn promote
  growth responsiveness of B cells, TC cells
  macrophages increasing strength of immune
  response to a pathogen
• lymphocytes in lymph blood large s in lymph
  nodes located along lymphatic vessels lymph
  nodes filter lymph help bring antigens into
  contact with lymphocytes

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The Immune Response cont

• to get to areas where there is an infection,
  lymphocytes must be able to leave lymphatic
  circulatory systems process called
  extravasation vessels become inflamed at sites
  of infection producing signals inducing synthesis
  activation of adhesive proteins on blood side
  of endothelium as pas inflamed endothelium
  molecule called P-selection on blood-facing
  surface binds to slows a

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The Immune Response cont

• passing leukocyte interaction stimulates
  leukocytes to produce integrin receptors (e.g.
  LFA-1) which then bind with intracellular
  adhesion molecules (ICAMs) on surface of
  endothelium once adhered, leukocyte can move
  between endothelial cells migrate into infected
  tissues
• Fig 12-41 p. 512

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Regulation of Circulation

• 3 central priorities of CVS
• 1. adequate blood supply to brain heart
• 2. to other
  organs after brain heart supply assured
• 3. control capillary pressure to maintain tissue
  volume composition of interstitial fluid within
  reasonable ranges

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Control of Central CVS

• Baroreceptors monitor BP info from
  baroreceptors info from chemoreceptors
  (monitoring CO2 O2 concentrations pH of
  blood) is transmitted to brain other sensory
  receptors are involved in reflex effects on the
  CVS including mechanoreceptors (respond to
  mechanical distortion pressure)
  thermoreceptors (responsive to temperature
  changes) all this info is integrated in a
  collection of brain neurons called medullary CV
  center (at the level of medulla/pons)

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Control of Central CVS cont

• Medullary CV center receives info also from
  medullary respiratory center, hypothalamus,
  amygdala nucleus cortex output from medullary
  CV center feeds into sym para autonomic motor
  neurons that innervated heart smooth muscle of
  arterioles veins
• Stim. of sympathetic nerves increases rate
  force of heart contraction causes
  vasoconstriction marked increase in arterial BP
  CO in general, the reverse happens when stim
  para nerves ending in reduction of BP CO

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 Medullary CV Center

• 2 functional regions with opposing effects on BP
• 1. stimulation of pressor center results in
  sympathetic activation rise in BP
• stimulation of depressor center in parasym.
  activity drop in BP fig 12-42 p. 513
• Role played by baroceptors which are widely
  distributed in arterial system show increased
  rates of firing with increase in BP

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Arterial Baroreceptors

• unmyelinated barorecptors (mammals, amphibians
  reptiles) respond only to pressures above normal
  initiating reflexes that reduce arterial BP
• myelinated baroreceptors (only mammals) respond
  only to pressures below normal initiating
  reflexes that raise BP - many baroreceptors are
  located in carotid sinus in mammals, carotid
  sinus is a dilation of internal carotid artery at
  its origin buried in the thin walls are finely
  branched nerve endings function as baroreceptors
  ( inc. in BP stretches wall of carotid sinus
  causing an increase in discharge frequency)

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Arterial Chemoreceptors

• arterial chemoreceptors located in carotid
  aortic bodies NB in ventilation (later) but also
  have some effect on CVS when blood perfusing
  carotid aortic bodies has high levels of CO2 or
  low O2 pH, arterial chemoreceptors respond with
  increase in discharge frequency which results in
  peripheral vasoconstriction slowing of HR if
  animal is not breathing (e.g. submersion)
• CO is reduced while birds mammals are diving

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Arterial Chemoreceptors cont

• Under these conditions, peripheral
  vasoconstriction ensures maintenance of arterial
  BP thus blood flow to brain (diving response)
• Many interactions between control systems
  associated with respiratory CVS

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Cardiac Sensory Receptors

• atrial receptors (esp. mechanoreceptors in atrial
  walls)
• ventricular receptors (nerve endings of both
  myelinated mechanoreceptive chemoreceptive
  unmyelinated sensory afferent fibers imbedded in
  ventricles) together monitor venous pressure
  HR to ensure activity of heart is correlated with
  blood inflow from venous system blood outflow
  into arterial system

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Additional Points

• Neuronal control of microcirculation operates
  under a priority system if arterial pressure
  falls, blood flow to gut, liver muscles is
  reduced to maintain flow to brain heart (most
  arterioles are innervated by sympathetic nerves,
  whose terminals release the catecholamine
  norpinepherine)
• vasopressin (ADH) rennin-angiotensin-aldosterone
  system operate in conjunction with neuronal
  reflexes to maintain BV (review this material)
• many special circumstances to accommodate e.g.
  inflammation, strenuous exercise, diving
  hemorrhage