Biology 2672a: Comparative Animal Physiology

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Biology 2672a Comparative Animal Physiology

• Circulation II
• Regulation of Circulation

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What happens in the systemic circuit?
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The Systemic circuit

• Blood is delivering oxygen and nutrients and
  picking up waste products (and delivering them to
  the kidneys)
• Organs muscles
• Linear flow rate is reduced
• Cross-sectional area increases

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Major Arteries

• Muscular, elastic thick walls (smooth muscle and
  elastin)
• Pressure of 10-20 kPa
• Elastic
• Dampen pressure differences
• Store some elastic energy
• e.g. Aorta, carotid artery, femoral artery
• 16/10 kPa

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Terminal arteries

• 12/8 kPa

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Arteriole 8/3.5 kPa
Capillaries c. 2.5-3 kPa
Venules c. 1.3-2 kPa
Right ventricle then increases pressure to c. 1.9
kPa for passage through lungs
Fig. 24.11
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• Precapillary sphincter
• Can close off flow to capillary bed

• Arteriolar-venular anasomosis
• Allows blood to bypass the capillary bed

Fig. 23.11
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Veins

• Low pressure
• Have a system of one-way valves
• Much thinner-walled than arteries

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Pressure drop across vascular system
Fig. 24.12b
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What about birds?

• Broadly similar
• Heart pumps more and faster to meet greater
  oxygen demands
• Higher pressures

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What about birds?

• Jugular Anastamosis

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Gravity also affects pressure
Acceleration due to gravity
?P ?g?h
Height difference across the system
Fluid density (mercurygtseawatergtwatergtoil)
See Fig 24.7
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The problem of being a giraffe

• The brain of a standing giraffe is 2m above its
  heart
• To maintain a pressure of c. 13 kPa in brain
  arteries, needs an aortic pressure of c. 29 kPa
  (!)

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• Tight skin on legs
• Muscular arteries
• High interstitial fluid pressure, efficient
  return of venous blood

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Giraffes have a drinking problem
?P ?g?h

• Very high pressure blood into brain
• Blood can pool in brain

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Solving the giraffe drinking problem

• Vasodilation in lower body reduces blood pressure
• Elastic arteries near brain absorb some increased
  pressure
• One-way valves in jugular vein prevent backflow
  of blood into head

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Brain
Need blood flow to be maintained
Heart
Kidneys Require a regulated blood pressure to
function
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Animals with a closed circulatory system are able
to regulate

• Where blood goes
• How much of it goes there
• Need to respond to central requirements
• e.g. fight-or-flight
• Also need to respond to local conditions
• O2 demand, localised damage

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How to regulate blood flow?
Change tube diameter
Change Energy input
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Energy input Cardiac Output
Can be modulated by both endocrine and nervous
systems
stroke volume
Cardiac Output
heart rate
Modulated by nervous activity (via
norepinephrine) and circulating epinephrine
(adrenaline)
Equation 24.1
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Myogenic autoregulation (stretch response)
Increased blood flow
Increased pressure on arteriole wall
Smooth muscle stretched
Decreased blood flow
Smooth muscle contracts
Increased resistance
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Neural control of vasoconstriction

• Sympathetic nervous system can be activated to
  induce vasoconstriction
• Thermoregulation
• Fight or flight
• Mediated by Norepinephrine released by
  sympathetic neurons

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Neural Regulation of vasomotor tone

• sympathetic nerves
• noradrenaline smooth muscle
• ????? receptors
• ? constriction
• ? dilation
• Relative receptor population density

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Smooth Muscle Cell
Viagra inhibits cGMP breakdown
NO Produced
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Endocrine control of vasomotor tone

• Epinephrine (Adrenaline) from Adrenal medulla
  induces vasodilation
• Fight or flight
• Vasopressin (ADH) Angiotensin II
  vasoconstriction
• Activate Adrenergic receptors on smooth muscle
• Not necessarily mediated by nerves

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Paracrine control of vasomotor tone

• NO
• Produced by endothelial cells to maintain
  vasomotor tone in response to hormonal cues
• Same mechanism as for parasympathetic activation
• Nitroglycerine
• Also responses to local factors indicating
  hypoxia and damage

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Local regulation

• Hormonal
• Adrenaline (Epinephrine)
• ????? receptors
• Local Control
• low O2, pH, ATP
• high CO2, K
• dilates vessels locally
• override neural hormonal control

Fig. 23.11
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Reading for Tuesday

• Intro to Gas Exchange breathing in water
• pp 533-560