Flow Poiseuille`s Law

10/8/12
Flow
Blood
Air
Other fluids
eg. lymph, exocrine secretions
Flow occurs from high to low pressure
Flow =
Pressure
Resistance
Flow is normally laminar; can be turbulent (BP
measurement; atherosclerosis)
BP measurement
Atherosclerosis
Plaque composed of
cholesterol, calcium etc.
Reduce blood flow
Can rupture and cause clots –
heart attacks/strokes
Treated by angioplasty or stent
implantation
Blood flow is complex: 2 phase system of liquid and
cells; blood vessels are not rigid
Can use physics to approximate physiological flow
Poiseuille’s Law
R=8Lη
πr4
Resistance = constant x length x viscosity
constant x radius of tube4
Role of Pressure in Pulmonary Ventilation
Air moves by bulk flow
Driven by P gradient (air moves from high to low P)
Flow = Patm – Palv /R
Atmospheric pressure constant (during breathing cycle)
Change in alveolar pressure creates/changes gradients
Length and viscosity normally constant; radius is
most important factor
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10/8/12
Forces for Air Flow
Boyle’s Law: pressure is inversely related
to volume (@ constant T)
Change alveolar pressure by changing volume
Resistance related to radius of airways and mucus
Trachea and bronchi: cartilage; little smooth muscle
Walls of bronchi and bronchioles: innervation by ANS
Muscarinic receptors: ACh: bronchoconstriction
Adrenergic receptors (β): bronchodilation
Histamine: bronchoconstriction (& mucus production)
LaPlace’s Law
In a sphere, P=2T
r
Inward Pressure = Constant x Tension
radius
The smaller the radius, the smaller the tension must be to
balance the pressure and avoid collapse
Alveolar surfactant: reduces surface tension (H bonds, lecture 2)
Concepts and problems assessment 17th Oct 2012
1) Describe the consequences of acute kidney failure on
physiological systems.
2) Describe the effects of Type II diabetes mellitus on physiological
systems. What are the recommended treatments? What is the
physiological rationale for these treatments?
3) A 20 year old, otherwise healthy, woman suffers an acute bout of
diarrhoea as a result of food poisoning. Describe the effects of this
bout of illness on physiological systems.
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