week 5

Question 1

what phases is pressure used for?

Answer 1

gases and liquids only

Question 2

define pressure

Answer 2

the force per unit area in a gas or liquid

Question 3

what is a fluid?

Answer 3

a substance in the liquid or gas phase

Question 4

compare deformation in terms of solids and fluids

Answer 4

solids - resist an applied shear stress by deforming
fluids - deform continuously under the influence of a shear stress, no matter how small

Question 5

compare the relationship of stress to strain in solids and fluids

Answer 5

solids - stress is proportional to strain
fluids - stress is proportional to strain rate

Question 6

what is normal stress?

Answer 6

when a force acts perpendicular (or “normal”) to the surface of an object

Question 7

what is shear stress?

Answer 7

when a force acts parallel to the surface of an object

Question 8

what is a free surface

Answer 8

the surface of a fluid that is subject to zero paraller shear stress (the surface that is in contact with air)

Question 9

what is gauge pressure (Pgauge)? why do we use it?

Answer 9

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)

Question 10

what is a manometer used for?

Answer 10

it is commonly used to measure small and moderate pressure differences

Question 11

what is a sphygmomanometer?

Answer 11

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

Question 12

how is pressure indicated by a mercury manometer?

Answer 12

the pressure is indicated by the height of a column of mercury inside a glass tube

Question 13

how is pressure indicated by a aneroid manometer?

Answer 13

the pressure changes the shape of a sealed flexible container, which causes a needle to move on a dial

Question 14

what are some examples of crude pressure indicators in the body?

Answer 14

• 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

Question 15

what is systolic pressure?

Answer 15

maximum blood pressure during the contraction of ventricles

Question 16

what is diastolic pressure?

Answer 16

minimum pressure recorded just prior to the next contraction

Question 17

what is transmural pressure?

Answer 17

the difference in pressure between two sides of a wall

Question 18

the law of laplace relates

Answer 18

transmural pressure with wall stress

Question 19

state the law of laplace

Answer 19

T = ∆ P x R

T - tension in the wall
P - pressure within the lumen of the vessel
R - the vessel radius

Question 20

what is a limitation of the law of laplace?

Answer 20

it gives the average wall stress and thus it cannot give any information on the stress distribution across the wall

Question 21

what is the main determinant of the wall stress?

Answer 21

the ratio r/h

Question 22

how is hypertension caused?

Answer 22

• 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

Question 23

what is the equation of continuity?

Answer 23

a statement of the conservation of mass during flow (of blood)

Question 24

what does the equation of continuity state?

Answer 24

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

Question 25

in steady state, _____ amount of mass flows in and leaves it, even with changes in cross-section

Answer 25

the same

Question 26

since the flow through A1 (cross-section 1) must equal the flow through A2, one has

Answer 26

Q = A1v1 = A2v2

v1 and v2 are the average fluid speeds over A1 and A2 respectively

Question 27

what does Q represent?

Answer 27

it represents volume per unit time
or (Av) in m^3/s

Question 28

ρAv is

Answer 28

mass per unit time (kg/m^3 x m^3/s = g/s)

Question 29

Bernoulli’s equation relates

Answer 29

blood pressure (P), and blood flow velocity (v)

Question 30

what does Bernoulli’s equation express?

Answer 30

the conservation of energy in the flowing blood

Question 31

if pressure losses due to friction or turbulence are neglected, Bernoulli’s equation states

Answer 31

the sum of
- fluid mechanical energy (pressure energy, P)
+
- fluid kinetic energy (1/2ρv^2)
+
- fluid potential energy (ρgh)
=
stays constant

Question 32

what happens in a blood vessel in the supine human?

Answer 32

the term ρgh (fluid potential energy) can be neglected

Question 33

what does bernoulli’s law tell us?

Answer 33

that when a fluid particle decelerates, pressure increases

Question 34

example of a clinical application related to Bernoulli’s equation

Answer 34

estimating the severity of an aortic or mitral valve stenosis, arterial stenosis and aneurysms

Question 36

what is an aneurysm?

Answer 36

a localized, balloon-like bulge in an artery

Question 37

what does Poiseuille’s Law describe?

Answer 37

the relation between pressure drop (∆P) and fluid flow (Q), under steady flow conditions