LECTURE 10 (Hydrostatics) Flashcards
What are the properties of Fluid?
- Cannot maintain equilibrium condition under various influences
- Deforms/begins movement due to influence
- Fluid compression is very low
What is Pressure?
The force per unit area applied in a direction perpendicular to the surface of an object
What is Liquid pressure?
The force per unit area of hard substance from liquid layer or liquid itself
P = F/S
P - Fluid pressure
F - Pressure force
S - Surface area
What is the equation for Pressure force?
F = PS
F - Pressure force
P - Fluid pressure
S - Surface area
What is pressure measured in?
Pascal (Pa)
Explanation: Pressure equals 1 Pascal, if per 1 m2 area surface is influenced by 1 Newton force
What does Pascal’s law state?
Pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid
What is Fluid hydrostatic pressure?
The pressure of non-moving (static) fluid determined by exertion of gravity forces on fluid (fluid mass)
Hydrostatics is based on two principles:
- Pascal’s law
- The pressure ratio remains the same in all points of horizontal plane
What is the difference between water horizontally and vertically?
Horizontally = water pressure is the same (P1 = P2 = P3)
Vertically = water pressure increased with depth (pgh = density x gravity x height)
What is the U-pipe principle?
Pressure increase in one bend of U-pipe causes pressure increase in another bend with the same value
What does Atmospheric pressure equal?
The weight of mercury
What is the standard atmosphere?
A unit of pressure defined as 101325 Pa or 760 mmHg
What is a Manometer?
- Comprised of a bulb containing a gas and a U-shaped tube partially filled with mercury
- Measure static pressure
- If both ends of U-pipe fluid level equal -> object’s pressure is equal to atmospheric pressure
- If open bend of manometer fluid level raises -> object’s pressure is higher than atmospheric pressure
What is the difference between fluid levels in manometer bends determined by?
Δp value
[Δp = h]
Explanation: Δp value shows how object pressure differs from atmospheric pressure
What is Decompression sickness?
Also called “the bends”, Caisson sickness or “diver’s disease” is a life threatening condition caused by a buildup of nitrogen bubbles in the bloodstream and body tissues
CAUSE:
As a diver descends, pressure increases -> Nitrogen is absorbed into the body tissues -> Diver must ascend SLOWLY to allow nitrogen to escape out of the body -> If diver ascends too fast, nitrogen becomes bubbles in the tissue + bloodstream
SYMPTOMS:
- Unusual fatigue
- Skin itch
- Pain in joints and/or muscles of arms, legs or torso
- Dizziness, vertigo, ringing in ears
- Numbness, tingling and paralysis
- Shortness of breath
TREATMENT:
Recompression in a HIGH-PRESSURE HYPERBARIC CHAMBER in which the patient is put back under pressure and receives 100% oxygen -> reverses pressure changes that allows gas bubbles to form + drives back into liquid form that can be gradually cleared
Pressure in a mixture of gases follows which principles?
- Each gas exerts the same pressure that it would if it alone occupied the container
- Pressure each gas exerts is called its “Partial pressure” and the total pressure is “the sum of the partial pressure of the constituent gases”
- Ratio of the partial pressure of any component to the total pressure equals the ratio of its partial volume to the total volume
- Gasses dissolved in water also exert pressure (since move randomly + have kinetic energy)
What is the Ideal gas equation?
PV = nRT
P - pressure
V - volume
n - number of moles
R - universal gas constant
T - absolute temperature
What is Dalton’s law of partial pressures?
The total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases
What does a pressure of 50mmHg in a blood vessel mean?
It means that force of blood is sufficient to push a column of mercury up level 50mm
What are the difference reference points for measuring blood pressure?
- Atmospheric pressure is used as zero reference point (90mmHg means 90mmHg above atmospheric pressure)
- Position of heart (patient positioning must be at cardiac level)
What is Blood pressure?
The pressure exerted by the blood at right angles to the walls of blood vessels (Pi) minus the environmental or ambient pressure (Pa)
BP = Pi - Pa
Which equation allows us to convert BP in mmHg into pascals?
P= ρHggh/1000 (Pa)
ρHg - specific density of Hg
g - gravitational acceleration
h - value of BP in mmHg
What are the factors that influence blood pressure?
- Cardiac constriction power (increase cardiac constriction power -> increase BP)
- Blood volume (increase cardiac output -> increase BP)
- Resistance (increase resistance -> increase BP)
- Blood viscosity (increase viscosity -> increase resistance -> increase BP)
What factors increase resistance in blood vessels?
- Vessel size (larger vessel -> decrease resistance)
- Vessel wall smoothness (fatty plaques form -> resistance increased -> flow becomes turbulent)
- Vasoactive compounds (alters vessel diameter)
What are the properties of BP?
- Varies during the day (Circadian rhythm)
- Changed in response to stress, nutrition factors, drug influence and during various diseases
Describe the pathways of blood pressure in the vessel network
Blood pressure is high when leaving via aorta (around 100mmHg) -> As heart performs pulse work, arterial pressure is changed between systolic (120mmHg) and diastolic (80mmHg) pressures -> Progressively decreases and at penetrating into right ventricle is zero
What is Stroke volume?
Volume of blood in mm ejected from each ventricle due to the contraction of the heart muscle which compresses these ventricles
SV = EDV - ESV
EDV - End diastolic volume
ESV - End systolic volume
measured in ml/m2
What is cardiac output?
Amount of blood the heart pumps from each ventricle per minute
CO = HR X SV (heart rate x stroke volume)
What are the factors affecting heart rate?
- Autonomic innervation
- Hormones
- Fitness levels
- Age
What are the factors affecting stroke volume?
- Heart size
- Fitness levels
- Gender
- Contractility
- Duration of contraction
- Preload (EDV)
- Afterload (resistance)
What is Venous pressure?
Blood pressure in a vein or in the arteries of the heart. It’s less than arterial BP, with 5mmHg in the right atrium and 8 mmHg in the left atrium
What is Central venous pressure?
The pressure of blood in the thoracic vena cava near the right atrium to provide information about the body volume status and right ventricular function
Normal CVP = 0-8 mmHg
Less than 0 = Hypovolemia
More than 0 = Hypervolemia
What are the properties of Central venous pressure?
- Reflects the amount of blood returning to the heart and the ability of the heart to pump the blood into the arterial system
- Good approximation of right atrial pressure -> major determinant of right ventricular end diastolic volume
What does gravity in the upright human body cause?
A vertical gradient of venous pressures
How does lying supine increase pulse pressure?
Increased central venous pressure -> increased end-diastolic volume -> increased stroke volume -> increased pulse pressure -> blood evenly distributed in veins
What is Mean arterial pressure?
The average arterial pressure during a single cardiac cycle
MAP = (stroke volume x systemic resistance) + CVP
MAP = Diastolic + 1/3 pulse pressure
What is Pulse pressure?
Systolic minus diastolic blood pressure
Pulse pressure = systolic - diastolic
Low values: error of measurement or if really low -> congestive heart failure and or shock
High values: stiffness of major arteries, aortic regurgitation, hyperthyroidism -> can accelerate the normal ageing of body organs
What is Orthostatic hypotension?
Gravity reduces the rate of blood return from the body veins below the heart back to the heart, reducing stroke volume and cardiac output
[a few seconds are needed for recovery otherwise pt will suffer reduced blood flow to brain resulting in dizziness + potential blackout]
