week 5 Flashcards
what phases is pressure used for?
gases and liquids only
define pressure
the force per unit area in a gas or liquid
what is a fluid?
a substance in the liquid or gas phase
compare deformation in terms of solids and fluids
solids - resist an applied shear stress by deforming
fluids - deform continuously under the influence of a shear stress, no matter how small
compare the relationship of stress to strain in solids and fluids
solids - stress is proportional to strain
fluids - stress is proportional to strain rate
what is normal stress?
when a force acts perpendicular (or “normal”) to the surface of an object
what is shear stress?
when a force acts parallel to the surface of an object
what is a free surface
the surface of a fluid that is subject to zero paraller shear stress (the surface that is in contact with air)
what is gauge pressure (Pgauge)? why do we use it?
the amount by which the pressure measured in a fluid exceeds that of the atmosphere, it is used to measure just the pressure of our desired object (without the impact of the atmosphere)
what is a manometer used for?
it is commonly used to measure small and moderate pressure differences
what is a sphygmomanometer?
an instrument used to measure blood pressure, consisting of a cuff connected to a column of mercury next to a graduated cylinder than enables measuring systolic/diastolic pressure
how is pressure indicated by a mercury manometer?
the pressure is indicated by the height of a column of mercury inside a glass tube
how is pressure indicated by a aneroid manometer?
the pressure changes the shape of a sealed flexible container, which causes a needle to move on a dial
what are some examples of crude pressure indicators in the body?
- ex: a person in an airplane is often aware of the changes in atmospheric pressure on the ears
- ex: the size of the veins in the back of the hand
what is systolic pressure?
maximum blood pressure during the contraction of ventricles
what is diastolic pressure?
minimum pressure recorded just prior to the next contraction
what is transmural pressure?
the difference in pressure between two sides of a wall
the law of laplace relates
transmural pressure with wall stress
state the law of laplace
T = ∆ P x R
T - tension in the wall
P - pressure within the lumen of the vessel
R - the vessel radius
what is a limitation of the law of laplace?
it gives the average wall stress and thus it cannot give any information on the stress distribution across the wall
what is the main determinant of the wall stress?
the ratio r/h
how is hypertension caused?
- the cardiac muscles increase in thickness by building more contractile proteins in parallel, leading to concentric hypertrophy
- the thicker wall causes the systolic wall stress to return to presumably normal levels despite the higher pressure in systole
what is the equation of continuity?
a statement of the conservation of mass during flow (of blood)
what does the equation of continuity state?
when a fluid of a given density (ρ) moves with average speed (v), in a tube of cross-sectional area A, the product ρAv is constant
in steady state, _____ amount of mass flows in and leaves it, even with changes in cross-section
the same
since the flow through A1 (cross-section 1) must equal the flow through A2, one has
Q = A1v1 = A2v2
v1 and v2 are the average fluid speeds over A1 and A2 respectively
what does Q represent?
it represents volume per unit time
or (Av) in m^3/s
ρAv is
mass per unit time (kg/m^3 x m^3/s = g/s)
Bernoulli’s equation relates
blood pressure (P), and blood flow velocity (v)
what does Bernoulli’s equation express?
the conservation of energy in the flowing blood
if pressure losses due to friction or turbulence are neglected, Bernoulli’s equation states
the sum of
- fluid mechanical energy (pressure energy, P)
+
- fluid kinetic energy (1/2ρv^2)
+
- fluid potential energy (ρgh)
=
stays constant
what happens in a blood vessel in the supine human?
the term ρgh (fluid potential energy) can be neglected
what does bernoulli’s law tell us?
that when a fluid particle decelerates, pressure increases
example of a clinical application related to Bernoulli’s equation
estimating the severity of an aortic or mitral valve stenosis, arterial stenosis and aneurysms
what is an aneurysm?
a localized, balloon-like bulge in an artery
what does Poiseuille’s Law describe?
the relation between pressure drop (∆P) and fluid flow (Q), under steady flow conditions
