Chapter 19 The Cardiovascular System: Blood Vessels G.R. Pitts, J.R. Schiller, and J. F. Thompson

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Chapter 19 The Cardiovascular System Blood
VesselsG.R. Pitts, J.R. Schiller, and J. F.
Thompson
Use the video clip CH 19 - Anatomy of the
Blood Vessels for a review of vessel structure
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Vessel Structure

• Structure/function relationships change as one
  moves through the cardiovascular tree
• Tunic thickness and composition of the three
  layers are variable

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Capillary Beds

• Flow regulated by smooth muscle valves

• Metarterioles
• from arterioles to venules through capillary bed
• allows flow through capillary bed w/out flow
  through caps

• True capillaries
• pre-capillary sphincter
• ring of smooth muscle
• open/close to control flow
• regulated by chemicals
• intermittent vasomotion, open for flow 5-10 times
  each minute

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Capillaries

• Allow exchange of nutrients and wastes between
  the blood and the tissue cells
• Capillary structure simple squamous epithelium
• basal lamina - connective tissue
• endothelial cells
• Details of structure determine specific functions
• 3 types continuous, fenestrated, sinusoidal

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Vascular Anastomoses

• Arterial Anastomoses
• - provide collateral supply to some organs and
  tissues, e.g., skeletal muscles
• Arteriovenous Anastomoses
• - thoroughfare channels
• Venous Anastomoses
• - most common, e.g., deep and superficial veins
  in limbs and head

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Vessel Structure - Histology
Vein Artery
Vein Artery
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Varicose Veins
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Vessel Structure/Function

• At rest
• 60 of blood volume is located in veins and
  venules
• venous system serves as reservoirs for blood
• particularly veins of the abdominal organs and
  the skin
• ANS regulates volume distribution
• vasoconstriction
• vasodilation
• diverts blood to areas with increased metabolic
  needs

Spleen 1L
Compare to Cardiac Output figures
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Blood Distribution at Rest
? 0.75 L/min
Rest
CO 5 L/min
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Blood Distribution -- Exercise
Using cardiac reserve
CO 25 L/min
Heavy Exercise
? 20 L/min
? 0.75 L/min
Rest
CO 5 L/min
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Physiology of Circulation

• Flow ?P/R
• or CO MAP/R
• MAP mean arterial pressure
• higher pressure to lower pressure with decreasing
  resistance (R)
• Blood pressure
• pressure of the blood on the vessel wall
• measure the pressure of a volume in a space
• systole/diastole - 120/80 (mm Hg)
• BP falls progressively from the aorta to
  essentially 0.0 mm Hg at the right atrium (RA)

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Physiology of Circulation

• Resistance - opposes blood flow because of the
  friction produced by the vessel walls
• Factors that affect resistance (R)
• (1) resistance (R) is proportional to viscosity
  ?V ? ?R
• thickness of the blood
• e.g., dehydration, elevated plasma proteins,
  polycythemia (?RBCs), leukemias (?WBCs)
• (2) resistance (R) is proportional to vessel
  length
• obesity increases the route lengths within
  connective tissue
• (3) resistance (R) is inversely proport. to
  vessel width
• decrease the radius by 1/2 and R increases by 16X
• most important in vessels that can change their
  size actively
• changes in diameter affect flow
• vessel wall drag blood cells dragging against
  the wall
• laminar flow layers of flow

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Physiology of Circulation

• Systemic Vascular Resistance (SVR) Total
  Peripheral Resistance (TPR)
• all vascular resistance is offered by the
  systemic vessels
• which vessels change size?
• resistance is highest in arterioles
• largest pressure drop is in the arterioles
• Relationship of the radius to resistance in the
  arterioles is due to smooth muscle
  contraction/relaxation

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Systemic Blood Pressure

• Arterial Blood Pressure
• Pulsatile in arteries due to the pumping of the
  heart
• Systolic/diastolic values
• Pulse pressure systolic (minus) diastolic
• Q- What does the Windkessel effect have on pulse
  pressure?
• Q- What is the effect of hardening of the
  arteries on pulse pressure?

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Systemic Blood Pressure

• Capillary Blood Pressure
• relatively low blood pressure
• low pressure is good design for capillaries
  because
• capillaries are fragile - high pressure would
  tears them
• capillaries are very permeable - high pressure
  forces a lot of fluid out

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Systemic Blood Pressure

• Venous return
• the volume of blood flowing back to heart from
  systemic veins
• depends on pressure difference from beginning of
  venules (16 mmHg) to heart (0 mmHg)
• any change in right atrial (RA) pressure changes
  venous return

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Venous Return/Valves

• Assistance for venous return
• skeletal muscles act as pumps
• contracting muscles squeeze veins
• force blood back to the heart
• valves prevent back flow
• respiratory pump
• inhaling causes a lowered pressure in the
  thoracic cavity
• primarily to pull air into the lungs
• helps to draw blood into thorax via pulmonary
  circulation

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Velocity of Blood Flow

• Velocity of blood flow - inversely proportional
  to the total cross sectional area (CSA) of
  vessels
• Aorta
• total CSA 3-5 cm2
• velocity 40 cm/sec
• Capillaries
• total CSA 4500-6000 cm2
• velocity 0.1 cm/sec
• Vena Cava
• total CSA 14 cm2
• velocity 5-20 cm/sec

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Capillary Function

• Capillary Function
• site of exchange between blood and tissues
• delivery of nutrients and removal of wastes
• slow flow allows time for molecules to diffuse
• Mechanisms of nutrient exchange
• diffusion - O2, CO2, glucose, AA's, hormones,
  electrolytes -- diffuse down gradients
• lipid soluble molecules can pass through cell
  membrane easily
• water soluble molecules generally require
  transport mechanisms to enter/exit cells

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Capillary Function

• Fluid movement
• Fluid diffuses out and is reabsorbed across the
  capillary walls
• Starlings law of the capillaries

• Forces driving the movement of fluid
• Hydrostatic pressure capillary (HPc)
• Hydrostatic pressure interstitial fluid (HPif)
• Osmotic pressure capillary (OPc)
• Osmotic pressure interstitial fluid (OPif)
• Net filtration pressure (NFP) is the net effect
  of all four forces at any point along the
  capillary

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Net Filtration Pressure (NFP)

• NFP (HPC - HPIF) - (OPC - OPIF)
•        Pushing forces - Pulling forces

• On average, 85 of fluid entering the tissues on
  the arteriole side is reabsorbed on venous end

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Maintaining Blood Pressure Short Term
Mechanisms - CNS

• Neural Control - Cardiac Centers in medulla
• Vasomotor center
• medullary area dedicated to control of blood
  vessels
• sends sympathetic output to blood vessels
• Vasoconstricts or vasodilates as needed
• Vasomotor tone - normal amount of
  vasoconstriction or vasodilation
• ANS can vary the vasomotor tone which varies the
  delivery of blood to particular regional
  capillary beds
• receives sensory input from different sources
• baroreceptors (blood pressure)
• chemoreceptors (O2, CO2, H, HCO3-)

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Maintaining Blood Pressure Short Term
Mechanisms - CNS

• Baroreceptor initiated reflex
• located at carotid sinuses and aortic arch
• monitors blood pressure
• regulates the activity of the sympathetic nervous
  system (vascular tone)

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Maintaining Blood Pressure Short Term
Mechanisms - CNS

• Chemoreceptor initiated reflexes
• Carotid bodies, aortic bodies
• Monitor changes in indicator chemicals (O2, CO2,
  H, HCO3-)
• ? CO2, ? H, ? O2 (stresses) result in ?
  sympathetic activity and ? BP

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Maintaining Blood Pressure Short Term
Mechanisms - CNS

• Influence of higher brain centers (areas above
  medulla) - cortex and hypothalamus
• not involved in minute-to-minute regulation
• influence vasomotor center depending on
  conditions
• temperature changes
• stressful emotional situations

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Maintaining Blood Pressure Short Term
Mechanisms - Chemicals

• Renin - Angiotensin - Aldosterone
• Renin/ACE
• enzymes from kidney/lung
• catalyze formation of Angiotensin I/II
• Angiotensin II
• vasoconstrictor
• stimulates ADH, thirst
• stimulates aldosterone release for Na H2O
  reabsorption
• why/how would these things affect blood pressure?

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Maintaining Blood Pressure Short Term
Mechanisms - Chemicals

1. diverts blood from the skin and abdominal organs
   to the skeletal muscles
2. increases heart rate, stroke volume and,
   therefore, cardiac output blood pressure

• Adrenal medulla releases epinephrine and
  norepinephrine in coordination with activity from
  the Sympathetic Division of the ANS

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Maintaining Blood Pressure Short Term
Mechanisms - Chemicals

• Antidiuretic Hormone (ADH) or Vasopressin
• osmoreceptors in hypothalamus trigger release
  from the neurohypophysis
• ADH targets kidneys to retain water (ADH action
  is inhibited by alcohol)
• ADH also stimulates vasoconstriction at high
  levels
• why/how would this affect blood volume and
  pressure?

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Maintaining Blood Pressure Short Term
Mechanisms - Chemicals

• Atrial Natriuretic Peptide (ANP)
• released from atrial cells in response to ?blood
  vol ? BP
• stimulates vasodilation, ?Na and water loss,
  antagonizes Aldosterone, inhibits thirst
• why/how would this affect blood volume and
  pressure?

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Maintaining Blood Pressure Long Term Regulation

• Renal mechanism
• control blood volume
• nervous control - ANS
• hormones
• regulation in the short term by adjusting blood
  pressure and adjusting blood flow to different
  capillary beds
• regulation in the long term by adjusting blood
  volume
• target the kidneys
• ? BP, ? urine flow to ? BP
• ? BP, ? urine flow to ? BP

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Control of Blood Flow

• Autoregulation (local control) - local automatic
  adjustment of blood flow to match specific local
  tissue metabolic needs
• Physical changes
• Warming - ? vasodilation
• Cooling - ? vasoconstriction
• Chemical changes in local tissues generate
  metabolic byproducts
• vasodilators or vasoconstrictors
• Myogenic control
• smooth muscle controls resistance
• ? stretch ? contraction ? stretch ? contraction

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Blood Flow in Special Areas

• Skeletal Muscle
• fine tuned control with wide variation in rate of
  flow
• brain directs the sympathetic division for NE
  release in response to the degree of muscular
  activity
• a receptors - vasoconstriction
• ß receptors - vasodilation
• metabolic regulation in tissue
• low O2 causes vasodilation, increasing flow
• high O2 cause vasoconstriction, decreasing flow
• Brain
• minimal variation in rate of flow
• minimal nutrient storage, so adequate flow must
  be maintained!
• local metabolic changes adjust local
  autoregulation

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Blood Flow in Special Areas

• Skin
• adjusting rate of flow aids in temperature
  regulation
• controls skins capacity as a blood reservoir
• sympathetic and local metabolic regulation
• Lungs
• low pressure (25/10 mm Hg), low resistance system
• flow regulated by O2 availability in the lungs
• high O2 ? vasodilation to increase flow
  opposite of muscle
• low O2 ? vasoconstriction to decrease flow
  opposite of muscle
• Heart
• variable flow depending on metabolic/pumping
  activity
• sympathetic and local metabolic regulation

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Regulation of Blood Pressure
CO MABP/R
MABP CO x R
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The CirculationLearn specific vessels and routes
in lab
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The CirculationLearn specific vessels and routes
in lab
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Hepatic Portal System
a portal system transfers venous blood from one
capillary bed to another capillary bed before the
blood is returned to the heart

• HPS collects venous blood from five abdominal
  organs and routes the blood to the liver for
  specific processing of transported molecules
• - stomach toxins (ethanol)
• - small intestine nutrients, toxins
• - large intestine nutrients, toxins
• - pancreas insulin, glucagon
• - spleen RBC breakdown products

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Fetal Circulation

• Umbilical veins bring oxygen and nutrients from
  the placenta to the liver and then to the heart
  of the fetus

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Fetal Circulation

• ductus venosus bypasses liver
• 3 Right ? Left shunts
• because oxygenated blood is derived from the
  placenta
• ductus arterious ? ligametum arteriosum
• foramen ovale ? fossa ovalis
• interventricular shunt ? no remnant

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Circulatory Shocksudden dramatic loss in blood
pressureor sudden decrease in circulatory flow

• Hypovolemic Shock
• Acute hemorrhage (or other sudden fluid loss as
  from vomiting or diarrhea)
• Vascular Shock
• Loss of vasomotor tone as from anaphylaxis,
  neural malfunction, or poisons (septicemia)
• Cardiogenic Shock
• Loss of cardiac output due to heart failure

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End Chapter 19