13. CHARACTERISTIC PRESSURES IN THE BODY (PART 1)

Question 1

1. Pressures are a very common phenomenon in our lives. What are some examples of pressures?

Answer 1

• Atmospheric Pressure
• Tire Pressure
• Blood Pressure

Question 2

2. In which states is Pressure used for?

Answer 2

• gases
• liquid

Question 3

3. What is the quantity force per unit area for solids called?

Answer 3

• stress

Question 4

4. What is the definition for Pressure?

Answer 4

• the force per unit area
  (in a gas or a liquid)

Question 5

5. In which unit is pressure measured?

Answer 5

• it is measured in Newton’s per square metres
  (Pascals)

Question 6

6. What is the standard Atmospheric pressure at Sea Level?

Answer 6

• 101kPa
  (760 mm Hg)

Question 7

7. What is a common method of indicating pressure in medicine?

Answer 7

• by looking at the height of a column of Mercury
  (mm Hg)

Question 8

8. What is 1mmHg equal to in kPa?

Answer 8

• 0.133kPa

Question 9

9. What are the two broad categories that materials are classified as?

Answer 9

• solids
• fluids
  (gases and liquids)

Question 10

10. What are the distinguishing features between gases, solids and liquids?

Answer 10

• differences at their molecular levels
• differences at their macroscopic levels
• differences at their cosmic levels

Question 11

11. What is a fluid?

Answer 11

• a substance that is either in liquid or gas phase

Question 12

12. How do fluids react to deformation?

Answer 12

• a fluid deforms continuously under the influence of a sheer stress
• a fluid never stops deforming
• instead: it reaches a constant rate of strain

Question 13

13. What can be said about the Deformation occurring in this picture.

Answer 13

• 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

Question 14

14. How does a solid resist an applied shear stress?

Answer 14

• it deforms

Question 15

15. In solids, what is proportional to the strain?

Answer 15

• the stress

Question 16

16. In fluids, what is the stress proportional to?

Answer 16

• the strain rate

Question 17

17. How does a solid react to a constant shear force being applied to it?

Answer 17

• the solid will eventually stop deforming
• it will do this at a fixed strain angle

Question 18

18. What is the definition of stress?

Answer 18

• it is the force per unit area

Question 19

19. What is a Normal Stress?

Answer 19

• this is the normal component of a force acting on a surface per unit area
• it is associated with pressure

Question 20

20. What is Shear Stress?

Answer 20

• 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

Question 21

21. What is Pressure?

Answer 21

• the normal stress in a fluid at rest

Question 22

22. What is meant by Zero shear stress?

Answer 22

• a fluid at rest is at a state of zero shear stress

Question 23

23. What develops in a liquid when the liquid is tilted, or the container walls that the liquid was in are removed?

Answer 23

• a shear develops
• it does this as the liquid moves to re-establish a
  horizontal free surface

Question 24

24. What can be said about the shear stress and the pressure for fluids at rest?

Answer 24

• the shear stress is zero
• the pressure is the only normal stress

Question 25

25. How do we mathematically work out Normal Stress?

Answer 25

Question 26

26. How do we mathematically work out Shear Stress?

Answer 26

Question 27

27. How would we describe the molecular structure of a liquid?

Answer 27

- 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

Question 28

28. How would we describe the molecular structure of a gas?

Answer 28

- 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

Question 29

29. What can be said about the molecular arrangement of Solids?

Answer 29

- molecules are arranged in a pattern - this pattern is repeated throughout the solid

Question 30

30. What is the single most important difference between fluids and solids?

Answer 30

- a fluid must assume the shape of the container in which it is placed - a solid is able to sustain its own shape

Question 31

31. How does a body fluid respond when a shearing force is exerted on it?

Answer 31

- 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

Question 32

32. What causes the difference between fluids and solids?

Answer 32

- the intensity of the forces holding the molecules together - this forces form a coherent piece of material

Question 33

33. Can fluids be transformed into solids?

Answer 33

- yes - and vice versa - this is done by heating - or by changing the pressure of the ambient environment

Question 34

34. How is the pressure of a fluid column mathematically worked out?

Answer 34

- P= p.g.h - P= pressure in a liquid column - p = the fluid density - g = gravitational constant - h = height of the column

Question 35

35. 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

Answer 35

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

Question 36

36. What is Absolute Pressure (Pabs) ?

Answer 36

- it is the total force per unit area

Question 37

37. What is commonly cited when referring to pressures of the body?

Answer 37

- the Gauge Pressure (Pgauge) - the pressure relative to a standard - the standard in this case is atmospheric pressure

Question 38

38. How is Pgauge mathematically calculated?

Answer 38

- Pgauge = Pabs - 1 atm (atmospheric pressure)

Question 39

39. When we discuss blood pressure and the pressure of the lungs, what does the term P refer to?

Answer 39

- it refers to the gauge pressure relative to the local atmospheric pressure