13. CHARACTERISTIC PRESSURES IN THE BODY (PART 1) Flashcards
1
Q
- Pressures are a very common phenomenon in our lives. What are some examples of pressures?
A
- Atmospheric Pressure
- Tire Pressure
- Blood Pressure
2
Q
- In which states is Pressure used for?
A
- gases
- liquid
3
Q
- What is the quantity force per unit area for solids called?
A
- stress
4
Q
- What is the definition for Pressure?
A
- the force per unit area
(in a gas or a liquid)
5
Q
- In which unit is pressure measured?
A
- it is measured in Newton’s per square metres
(Pascals)
6
Q
- What is the standard Atmospheric pressure at Sea Level?
A
- 101kPa
(760 mm Hg)
7
Q
- What is a common method of indicating pressure in medicine?
A
- by looking at the height of a column of Mercury
(mm Hg)
8
Q
- What is 1mmHg equal to in kPa?
A
- 0.133kPa
9
Q
- What are the two broad categories that materials are classified as?
A
- solids
- fluids
(gases and liquids)
10
Q
- What are the distinguishing features between gases, solids and liquids?
A
- differences at their molecular levels
- differences at their macroscopic levels
- differences at their cosmic levels
11
Q
- What is a fluid?
A
- a substance that is either in liquid or gas phase
12
Q
- How do fluids react to deformation?
A
- a fluid deforms continuously under the influence of a sheer stress
- a fluid never stops deforming
- instead: it reaches a constant rate of strain
13
Q
- What can be said about the Deformation occurring in this picture.
A
- this picture shows the deformation of a rubber block placed between two parallel plates
- it is experiencing the influence of shear force
- the shear force shown is acting in the rubber
- there is also an equal shear stress/force of opposite direction acting on the upper plate
- the stretch is acting in one direction
14
Q
- How does a solid resist an applied shear stress?
A
- it deforms
15
Q
- In solids, what is proportional to the strain?
A
- the stress
16
Q
- In fluids, what is the stress proportional to?
A
- the strain rate
17
Q
- How does a solid react to a constant shear force being applied to it?
A
- the solid will eventually stop deforming
- it will do this at a fixed strain angle
18
Q
- What is the definition of stress?
A
- it is the force per unit area
19
Q
- What is a Normal Stress?
A
- this is the normal component of a force acting on a surface per unit area
- it is associated with pressure
20
Q
- What is Shear Stress?
A
- it is the tangential component of a force acting on a surface per unit area
- it is always written as Force per unit
TANGENTIAL= along a tangent
TANGENT= a straight line that touches a curve or a
curved surface at a point
21
Q
- What is Pressure?
A
- the normal stress in a fluid at rest
22
Q
- What is meant by Zero shear stress?
A
- a fluid at rest is at a state of zero shear stress
23
Q
- What develops in a liquid when the liquid is tilted, or the container walls that the liquid was in are removed?
A
- a shear develops
- it does this as the liquid moves to re-establish a
horizontal free surface
24
Q
- What can be said about the shear stress and the pressure for fluids at rest?
A
- the shear stress is zero
- the pressure is the only normal stress
25
Q
- How do we mathematically work out Normal Stress?
A
26
Q
- How do we mathematically work out Shear Stress?
A
27
Q
- How would we describe the molecular structure of a liquid?
A
- the molecules can move relative to each other
- the volume remains relatively constant
- this is because of the strong cohesive forces between the molecules
- the liquid takes the shape of the container it is in
- it forms a free surface in a larger container that is in a gravitational field
- molecules in a liquid can rotate
- they can translate freely
- they have weaker intermolecular bonds than solids
- they have stronger intermolecular bonds than liquids
- they have flexibility between the bonds of the
molecules - there is enough intermolecular force to still keep the
molecules together
28
Q
- How would we describe the molecular structure of a gas?
A
- a gas expands until it encounters the walls of the container
- it fills up the entire available space
- this is because the gas molecules are widely spaced
- the cohesive forces between the molecules are very small
- a gas in an open container CANNOT form a free surface
- molecules are far apart from each other
- there is no existent molecular ordering
- the molecules have Brownian motion
- they have random, unorganised motion
- the intermolecular bonds between gas molecules are the weakest
29
Q
- What can be said about the molecular arrangement of Solids?
A
- molecules are arranged in a pattern
- this pattern is repeated throughout the solid
30
Q
- What is the single most important difference between fluids and solids?
A
- a fluid must assume the shape of the container in which it is placed
- a solid is able to sustain its own shape
31
Q
- How does a body fluid respond when a shearing force is exerted on it?
A
- it is not able to resist the shearing force
- it must keep deforming when it is subjected to this force
EG: the motion in the water in a lake is caused by an
overpassing wind
: it cannot resist the wind
: it can only keep deforming
32
Q
- What causes the difference between fluids and solids?
A
- the intensity of the forces holding the molecules together
- this forces form a coherent piece of material
33
Q
- Can fluids be transformed into solids?
A
- yes
- and vice versa
- this is done by heating
- or by changing the pressure of the ambient environment
34
Q
- How is the pressure of a fluid column mathematically worked out?
A
- P= p.g.h
- P= pressure in a liquid column
- p = the fluid density
- g = gravitational constant
- h = height of the column
35
Q
- What are the densities of the following fluids:
35.1: Mercury
35.2: Water
35.3: Blood Plasma
35.4: Circulating blood cells
35.5: Whole blood
A
35.1: 13 560 kg/m³
35.2: 1000 kg/m³
35.3: 1025 kg/m³
35.4: 1125 kg/m³
35.5: 1060 kg/m³
36
Q
- What is Absolute Pressure (Pabs) ?
A
- it is the total force per unit area
37
Q
- What is commonly cited when referring to pressures of the body?
A
- the Gauge Pressure
(Pgauge) - the pressure relative to a standard
- the standard in this case is atmospheric pressure
38
Q
- How is Pgauge mathematically calculated?
A
- Pgauge = Pabs - 1 atm
(atmospheric pressure)
39
Q
- When we discuss blood pressure and the pressure of the lungs, what does the term P refer to?
A
- it refers to the gauge pressure relative to the local atmospheric pressure
