Blood Flow

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Lecture 18

• Blood Flow
• Control of BP

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Factors Affecting Blood Flow

• Flow rate
• F a DP/R
• Resistance to flow
• R 8Ln/(pr4)
• Velocity of flow
• v F/cross sectional area
• Terms
• F flow rate volume of blood passing point in
  system per unit time (L/min)
• DP pressure gradient
• R resistance to flow vasoconstriction/dilation
  
• n fluid viscosity
• r vessel radius
• v velocity of flow

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Outline

• Blood Flow
• Blood Vessels
• Blood Pressure
• Factors Affecting BP
• Capillary Exchange
• Lymphatics
• Regulation of BP

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Outline

• Blood Flow
• Blood Vessels
• Blood Pressure
• Factors Affecting BP
• Capillary Exchange
• Lymphatics
• Regulation of BP

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Blood Flow

• Blood Vessels
• Circulatory system
• Structure of blood vessels
• Made of layers of smooth muscle connective
  tissue
• Allows vasoconstriction and vasodilation
• Inner lining endothelium, permeability barrier

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Types of Blood Vessels

• Arteries
• Thick smooth muscle layers
• Large amounts of elastic and fibrous tissue
• Allows them to distend as they fill with blood
• As they become smaller? less elastic, more
  muscular
• Capillaries
• Smallest vessels
• 1 layer of endothelial cells on a basement
  membrane
• Usually have leaky junctions between cells
• Veins
• More numerous than arteries
• Large diameter
• Thinner walls than arteries with less elastic
  tissue

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Blood Pressure

• Pressure created by ventricles
• Driving force for blood flow
• As blood leaves ventricles
• Aorta and arteries expand to accommodate
• After SL valve closes, elastic recoil in arteries
  propels blood forward
• Flow in arterial side is pulsatile
• Flow in veins is continuous
• Blood pressure values
• Highest in arteries
• Falls continuously as blood flows
• Decrease in pressure occurs due to resistance of
  vessels to flow

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Blood Pressure

• Pulse pressure
• Highest pressure occurs in aorta
• Systole 120 mm Hg
• Diastole 80 mm Hg
• Difference is pulse pressure PPDP - SP
• Mean arterial pressure
• MAP (2DPSP)/3 DP (2/3)PP
• Diastole lasts twice as long as systole
• Mean pressure and pulse pressure fall throughout
  circulatory system
• Veins
• Have one way valves to prevent backflow
• Venous return is aided by muscle and respiratory
  pumps

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Factors Affecting Blood Pressure

• Mean arterial pressure
• Main determinant of flow
• MAP a CO x Rarterioles
• Arterioles are the main site of variable
  resistance
• More than 60 of total circulatory resistance
• Main determinant of hypertension
• Changes in blood volume
• Increase BV ? Increase MAP

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Capillary Exchange

• Capillary Structure
• Once blood reaches capillaries, plasma cells
  exchange material
• Most cells in body are within 100 microns of
  capillary
• Two types of capillaries
• Continuous capillary
• Endothelial cells are closely joined
• Some small molecules pass thru cell junctions
• Bigger molecules (eg proteins) are transported in
  vesicles by transcytosis
• Fenestrated capillary
• Have large pores that allow high volume of fluid
  to pass rapidly
• Found primarily in intestines and kidney

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Capillary Exchange

• Flow in capillaries
• Blood velocity is lowest in capillaries
• Flow is proportional to cross sectional area

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Capillary Exchange

• Diffusion
• Source of most exchange between plasma
  interstitial fluid
• Blood cells and most proteins are too big to get
  through
• Bulk flow
• Mass movement of water and solutes as a result of
  hydrostatic or colloid osmotic pressure
• Out of capillary filtration
• Into capillary absorption
• Most capillaries show transition
• net filtration to net absorption
• as you move from arterial to venous side
• Regulated by two forces
• Hydrostatic pressure lateral pressure component
  that pushes fluid through capillary pores
• Colloid osmotic pressure Determined by solute
  concentration differences between two
  compartments (PROTEINS)

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Capillary Exchange

• Net pressure
• NP Hydrostatic press. grad. Colloid osmotic
  press. grad.
• NP (Pcap PIF) - (Pcap-PIF)
• Pcap decreases along length of caps
• PIF very low
• Pcap 25 mm Hg
• PIF 0
• Arterial side
• NP 32 25 7 mm Hg ? Filtration
• Venous side
• NP 15 25 -10 mm Hg ? Absorption
• Filtration gt Absorption
• Bulk flow 3 L/day
• How do we get this fluid back?

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Lymphatics

• Three main functions
• Return fluid and proteins filtered out of
  capillaries
• Pick up fat absorbed by small intestine
• Serve as filter to capture and destroy foreign
  pathogens
• Designed for one way movement of IF from tissues
  to circulation
• Structure
• Closed end lymph capillaries lie close to all
  blood capillaries
• Single layer of flattened endothelial cells
• Walls anchored to surrounding connective tissue
  by fibers that hold them open
• Large gaps between cells allow fluid, protein,
  bacteria, to be swept in by bulk flow

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Lymphatics

• Lymphatic system
• Lymph caps join to form larger collecting vessels
• Does not have a single pump like circulatory
  system
• Depends on SMC contraction in larger lymph
  vessels
• Skeletal muscle pump

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Edema

• Edema
• Accumulation of fluid in interstitial space
• Two causes
• Inadequate drainage of lymph
• Capillary filtrationgtgtcapillary absorption
• Increase in capillary hydrostatic pressure
• e.g. heart failure ? increased venous pressure
• Decrease in plasma protein concentration
• e.g. liver failure, malnutrition
• Increase in interstitial protein concentration
• e.g. inflammation ? histamine makes capillary
  walls leaky, allows proteins to escape

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Outline

• Blood Flow
• Blood Vessels
• Blood Pressure
• Factors Affecting BP
• Capillary Exchange
• Lymphatics
• Regulation of BP

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Regulation of Blood Pressure

• Overview
• CNS coordinates reflex control of BP
• Occurs in medulla oblongata
• Little is known about medullary CV control center
• Primary goal is to maintain adequate blood flow
  to brain and heart

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Regulation of Blood Pressure

• Baroreceptor reflex
• Baroreceptors
• Stretch sensitive receptors
• Located in wall of carotid artery, aorta
• Monitor pressure of blood to brain and body
• Fire APs continuously at normal BP
• Reflex
• When BP increases? Firing rate increases
• When BP decreases? Firing rate decreases
• APs travel to brain via sensory neurons
• CNS determines appropriate response
• change CO or peripheral resistance

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Outline

• Blood Flow
• Blood Vessels
• Blood Pressure
• Factors Affecting BP
• Capillary Exchange
• Lymphatics
• Regulation of BP

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Poem of the Day
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Due Dates

• Tuesday, November 9th
• Homework 9
• Project Memo (Hard copy)
• Tuesday, November 16th
• Homework 10 (LAST ONE!)
• First draft of website due (e-mail url)
• Tuesday, November 23rd
• Exam Three
• Wednesday, November 24th
• Project Due (e-mail url)