#### Transcript Neuron Structure and Function - University of British Columbia

```Blood Vessels and Flow Through the circulatory system
• Heart pumps blood by increasing local pressure
• As blood travels through circulatory system, it encounters
friction
Physics of Blood Flow
Physics of Blood Flow
What sets the resistance of a tube?
Think of drinking through a straw
• Is it easier to drink through a very long straw
or a shorter straw?
• What is the difference between drinking
through a skinny straw and a fat straw?
• What is the difference between drinking a
milkshake or water through a straw?
Poiseuille’s Equation
Q = P*r4 /8L
where:
Q = flow
L = length of the tube
 = viscosity of the fluid
r = radius of the tube
Physics of Blood Flow
Circulatory systems and electrical circuits
• Circulatory systems can be modeled as electrical circuits
• Heart = Power source
• Capillaries = Resistance
RT = R1 + R2
1/RT = 1/R1 + 1/R2
Blood flow distribution
Arteriolar & capillary branching reduces overall resistance
Physics of Blood Flow - velocity
• Flow – volume of fluid transferred per unit time
• Velocity – distance per unit time
• Blood velocity = Q/A
• A: cross-section area of the channels
• Velocity of flow is inversely related to total cross-sectional area
• Total cross-sectional area of capillaries is very large  velocity is
slow  allows time for diffusion
Physics of Blood Flow - Gravity
Hydrostatic pressure – pressure of a vertical column of fluid due to
gravity
P = pg  h
• p: density of the fluid
• g: acceleration due to gravity
•  h: height of the fluid column
Blood pressure varies in different parts of the body and when standing
or prone
Blood must move with and against the forces of gravity
Physics of Blood Flow - Gravity
• Measured blood
pressure when
standing includes a
hydrostatic (gravity)
component
Vertebrate blood vessels
3 layers
• Tunica intima
Endothelium
Basement membrane
• Tunica media
Smooth muscle
Elastin
• Tunica externa
Collagen
Vertebrate blood vessels
Capillaries
Lack tunica media and tunica externa
Walls vary among capillaries
• Continuous: cells are held
together by tight junctions;
found in skin and muscle
• Fenestrated: cells contain
pores; specialized for
exchange; found in the
kidneys, endocrine organs,
and the intestine
• Sinusoidal: cells are held
together by fewer tight
junctions; most porous for
exchange of large proteins;
found in liver and bone marrow
Capillary Beds
Regulation of blood flow
• Pressure, resistance, and flow vary throughout the
circulatory system and in response to changing
conditions
Capillary considerations
Role of capillaries
Facilitate exchanges between the blood & tissues
Capillary & rbc diameters are matched
Capillary blood velocity is the slowest
Capillary blood flow
Pressure pulsatility:
- Reduced by compliant arterial vessels
(aorta, bulbus, conus)
- Pulsatility is minimal in capillaries
Capillaries:
- Total vascular cross-sectional area is
vastly increased:
 reduced diffusion distance
Blood velocity:
- Proportional to total cross-sectional area
- Slowest in capillaries to aid exchange
Mean capillary blood pressure is appreciable
Blood Pressure
Relationship between velocity and cross-sectional area
Velocity = Q/A
Blood Pressure
• Pressure
fluctuations are
largest at the heart
• Degree of
fluctuation
decreases through
the circulation
Arteries- pressure reservoirs
Arteries dampen
the pressure
oscillations
caused by the
pumping action
of the heart
Mean Arterial Pressure (MAP)
MAP = 2/3 diastolic Pressure + 1/3 systolic Pressure
Distribution of blood to tissues
Vasoconstriction and Vasodilation
Distribution of blood to tissues
•
•
•
•
Tissue metabolic need sets blood distribution
Arterioles arranged in parallel
Blood can be redistributed as needed
Controlled by arterioles
Control of arteriolar diameter
•
•
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•
•
Myogenic autoregulation
Tissue metabolic activity
Paracrine signaling molecules
Endocrine system
Nervous system
Myogenic autoregulation
Stretch activated
Ca2+ channels
Effects of tissue metabolic activity
Metabolic Activity of Tissues
Paracrines (Nitric Oxide)
Nervous/Endocrine Regulation
Exchange at capillary governed by “Starling forces”
• Capillary hydrostatic pressure
• Interstitial hydrostatic pressure
• Capillary colloid osmotic pressure
• Interstitial colloid osmotic pressure
Capillary blood pressure & filtration
Capillary endothelial lining
• the barrier between blood & tissues
• varies according to tissue function
Circulatory Systems Zool 364
Net Filtration Pressure (NFP)
Blood pressure forces fluid out of capillaries
Starling principle: NFP = (Pcap – Pif) – (cap - if)
• Terms: hydrostatic pressure in the capillary (Pcap) and
interstitial fluid (Pif); osmotic pressure in the capillary
(cap) and interstitial fluid (if)
Figure 9.36
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