Physical Principals of Respiratory Care Flashcards
Gas Pressure
- All gases exert pressure
- Tension is often used to refer to pressure exerted by gases when dissolved in liquids
- The pressure or tension of a gas will depend upon kinetic activity
- Gases are attracted to the surface of the earth due to gravity
- Gravity increases the density of gases and increases the rate of molecular collisions and gas tension
- This explains why atmospheric pressure decreases with altitude
Atmospheric Pressure
Pressure exerted by gases in the atmosphere
The closer to the surface of the earth the higher the atmospheric pressure
Sea Level=760 mmHg
Calgary=670 mmHg
Mercury Barometer
The mercury barometer is the most common measure of atmospheric pressure
A column of mercury is open to a reservoir of mercury; the mercury in the column will try to return to the reservoir dur to the force of gravity. Atmospheric pressure will act upon the reservoir and oppose the force of gravity causing the mercury to move back up through the tube
The height of the mercury column represents the downward force of atmospheric pressure and is measures in inches or millimeters
Torrs
Torrs can also be used as a unit of measure
At sea level 1 torr equals 1 mmHg
A pressure reading of 772 mmHg is the same as 772 torr
Aneroid Barometer
The aneroid barometer measures the atmospheric pressure by equilibrating the atmospheric gas pressure with a mechanical force, or the expansion/compressive force of an evacuated metal container.
The aneroid barometer is a sealed, evacuated metal box with a spring and gear mechanism attached to an indicator dial. Atmospheric pressure pushes on the metal box, activating the spring mechanism moving the indicator dial to the corresponding pressure
Strain-Gauge Pressure Transducers
In this device a pressure change will expand and contract a flexible metal diaphragm which is connected to a electrical wire
The physical strain on the diaphragm changes the amount of electricity flowing through the wires
By measuring the change in electrical voltage we indirectly measuring pressure
Bourdon Gauges
Combined with a flow meter used on gas tanks
Measuring much psi in a tank and how much is coming out
When full every tank will reach 2200 psi regardless of the size of the tank
Accuracy is highest at high pressure
What types of pressure would we measure with a column of water (cmH2O)
1 atmosphere is ~1034 cmH20 high, that’s as tall as a two story building, not all that practical for high pressure readings
A water column (cmH2O) is used for pressures in the lungs (Peak and PEEP) – very small numbers 5-50cmH2O – much more manageable
What types of pressure would we measure with a column of mercury
Mercury is 13.6 times denser than water so using mercury to measure pressures that are larger is better because it is easier to read for the clinical measurements we need.
A mercury column (mmHg) is used for vascular pressures (BP 120/80) – using cmH2O would create even larger numbers, not feasible to work with clinically
Hydrostatic Pressure
Pressure in non moving liquids
The greater the height of a liquid, the greater the pressure
Pressure is greatest at the bottom
Pressure in Liquids is Porpotional to 3 Things
1) Height of the liquid (depth)
2) Density of the liquid (weight per volume)
Density of water= 1000 kg/ or 1000 g/L
3) Gravity
Pressure in Liquids Calculation
Pressure in Liquids=Height x Density x Gravity
It is important to note that this equation only works when fluids are not moving
Gauge Pressure
Will not take atmospheric pressure into account
Meaning we haven’t accounted for the column of air around the earth that is also putting pressure on that object
Absolute Pressure
Does takes atmospheric pressure into account
Work
The application of a force over a given distance
It is important to remember that the distance an object moves will only be considered work if it is in the same direction that the force is being applied
Work Calculation
Work (J)=Force (N) x Distance (m)
1 Joule (J)= 1 Newton acting on 1 kg to move it 1 meter
Pascal’s Principals
Any change in a pressure in anenclosedfluid is transmitted undiminished to all parts of that fluid and to the walls of the container
This is because the pressure of a liquid will equally act in all directions
Liquid pressure depends only on the height (h) and not on the shape of the vessel or the total volume of liquid.
Hooke’s Law
Hooke described the properties of elastic recoil
A material is said to be elastic if it will regain its length or shape after the removal of the force that caused the material to change length or shape
Springs are said to be compressed or extended
Ex. The human chest wall acts as a spring, acting to expand the thoracic cavity.The lung tissue itself is acting to reduce the volume of air in the lung. These two forces are opposing one another with one pulling in and one pulling out. At the end of exhalation, these two forces will be at equilibrium
After equilibrium is reached, the lungs continue to pull in, causing the chest wall “spring” to react and begin to pull the chest out resulting in air entering the lungs. As some point, the lungs begin to react and move to pull the chest wall in causing exhalation to begin.
Elastic Limit
Each material or object will have a limit that it can be changed above which it will not return to normal
Hooke’s Law Calculation
Mathematically, Hooke’s Law looks like: F = kx
F = deforming force
k = proportionality constant
x = amount of stretch
We can see that if the distorting force is doubled, then the amount of stretch is also doubled, to a certain limit.
Archimedes’ Principal
A body, immersed in fluid is buoyed up by a force that is equal to the weight of the fluid that has been displaced by that body. This means that in order for an object to float, the weight of the object is not the main concern it is the weight of the water that has been displaced by the object that will determine whether or not it will be able to float. In order for a boat to float it has to displace more water than the weight of the boat itself
Liquids exert a buoyant force due to the fact that the pressure below a submerged object will always exceed the pressure above it
This difference in pressure will create an upward supporting force
Gases can also can have a buoyant force that will keep solid particles suspended in gases
This suspension is known as aerosols
Urinalysis Hydrometer
A urinalysis hydrometer is an application of Archimedes’ Principal
How the urine analysis hydrometer works is that a glass tube of a known average density will be submerged in a liquid. The height to which it floats is calibrated as specific gravity
In some disease states there will be a change in urine composition which in turn means a change in specific gravity
Avogadro’s Hypothesis
Avogadro determines that an equal volume of gas at the same temperature and pressure (STP) will have an equal number of moles and therefore moleculesIn other word 1 GMW or mole of any gas at 22.4 L at a temperature of 0 degree Celsius and a pressure of 1 ATM
Ex. 1 mole of oxygen (GMW = 32g) will occupy 22.4 L and contains 6.02 x 10^23 molecules when measured at 0 degrees Celsius and 1 ATM
This is necessary when defining the densities of gases
1 mole of gas at this volume contains 6.023 x 1023 molecules (Avogadro’s Number)
It is important to note that one mole of any gas at STP will occupy 22.4 L
Different gases will have different weights depending on their makeup and the GMW of its constituents.
Density
Density is the measure of a substance’s mass per unit volume under specific conditions of pressure and temperature
Density (g/L)= Mass (GMW) / Volume (22.4 L/mole)
When look at the density of a gas that is combined with different types of gases we can use the following formula
[GMW (gas #1) * %] + [GMW (gas #2) * %]
22.4 L/mole
Specific Gravity
Specific gravity is the ratio of density of one fluid compared to the density of a reference substance
Specific Gravity is a relative term
When looking at a gas’s specific gravity it will be relative to air at 1.28 g/L
Specific gravity is a unit less number
The normal SG of urine is ~1.001 to 1.035
Gas Specific Gravity
Gas Specific Gravity=Density (g/L) / 1.28 g/L
Liquid Specific Gravity
When looking at a liquid’s specific gravity it will be relative to water at
Liquid Specific Gravity=Density (g/L) / 1000 g/L or 1000 kg/m3
Law of Mass Action
When two or more chemical substances react, one or more substances different from the original will form
A + B ⇔ C + D
As the arrow suggests the reaction is completely reversible
There is a definite relationship that exists between the speed and rate of the reaction and the quantity or concentration of the products
Other factors that will also play a role includes the chemical nature of the substance, the presence of a catalyst, and temperature
The rate of reaction is directly proportional to the concentration of the reactants.
Hydrolysis Equation
A practical example in the body of the Law of Mass Action is the Hydrolysis Equation
H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-
This equation will reach equilibrium at some point
Not all the content of any part will disassociate fully.
It is completely reversible. The equilibrium can be shifted in either direction with the addition of parts.
Adding CO2 to Hydrolysis Equation
Adding CO2 to the equation pushes it to the right. This will increase [H+] and therefore ↓ pH. This can occur from the following
Ex. Hypoventilation, COPD, renal failure, fever, etc.
Adding Bi-Carb to the Hydrolysis Equation
Adding bi-carb will push the equation to the left nullifying [H+] and increasing pH and eventually increasing CO2as well. This can occur from the following
Ex. Kidneys retaining bicarbonate due to chronic condition, certain acute conditions that result in the use of the bicarbonate drip (need to make sure there is adequate ventilation provided in this case
Gas
Weak attractive forces which results in a lack of restriction in their movement
There is a rapid random motion meaning their will be frequent collisions
Gas is a fluid meaning they have the ability to flow
Gases are very easy to compress and expand
Gases have the greatest amount of kinetic energy and the least amount of potential energy
Gases exist at temperatures above the substance’s critical temperature
Vapors
- A vapour is the gaseous form a substance that is below its boiling point and/or critical temperature
- Ex. Water vapor in the lungs, clouds
- Most gases that we use as RTs will be above their boiling points
- Above the boiling point, pressure will have to be added to return the gas to its liquid form until the critical temperature is reached
- Ex. CO2and N2O
A Vapor and Gas Pressure
A vapor will exert gas pressure just the same as a true gas
Vapor pressure will increase with temperature
A decreased temperature is known as a dew point
Head pressure is the pressure on the surface of a liquid
Vapor Pressure
The force the molecules exert as the hit the surface of a liquid and escape into the gaseous phase
When vapor pressure is equal to atmospheric pressure their will be rapid boiling
This is very powerful and can be explosive
Solids
Solids will maintain their shape due to their strong attractive forces this means that they are not easy to compress
These forces are known as Van der Waal forces
Solids have the greatest amount of potential energy and the least amount of kinetic energy
The molecules in solids have the shortest distance to travel unil they collide with one another
Liquids
Still have strong attractive forces that create a fixed volume but molecules are able to move more feely than solids which is why they take the shape of their containers
They have the ability to flow which I s why they are considered to be fluids
Like solids, they are still quite dense and not easy to be compressed
Phase Changes
The physical state of any substance is determined through its relation to kinetic energy and potential energy
Exothermic Reaction-Releases energy
Endothermic Reaction-Absorbs Energy
At the melting point of ice will begin to change into it’s liquid form
A full change into liquid for requires additional heat, this additional heat will change the state of water but not an immediate change in temperature
This extra heat that is needed is known as the latent heat of fusion
Boiling
Boiling: For each substance there is a temperature at which the substance will change from a liquid to a gas
The boiling point of a liquid is the temperature at which vapor pressure will equal/exceed the atmospheric pressure.
When atmospheric pressure is low liquid molecules will escape more easily and boiling will occur at a lower temperature . This means that the boiling point of a gas will decrease with an increased elevation
Condensation
Condensation is the reverse of boiling
Condensation is giving heat back into the surrondings
Vaporization
Vaporization: heat comes from the surroundings
Evaporization
Evaporization is a form of vaporization where heat is taken from the air surrounding the liquid and cooling the air
When water (or other substances) undergo a phase change from a liquid to a gas bellows its boiling point it is called evaporation. This will occur when we add enough kinetic energy.
The rate of evaporation will increase when there is a increase in temperature, surface area, or a decrease in pressure
Latent Heat of Vaporization
The energy that is required to vaporize a liquid is known as the latent heat of vaporization
This is the number of calories that are required to vaporize 1g of a liquid at its normal boiling point
Sublimation
When a solid changes directly to a gas it is called sublimation
Condensation and Evapoation
Both evaporation and condensation is very important in regards to breathing, as there is a delicate balance between evaporation and condensation that is needed so that the airway does not dry out and become irritated.
Mechanical ventilation will bypass the normal body processes that add heat and humidity into inspired air, which will compromise the balance between condensation and evaporation
Critical Temperature
The highest temperature at which a substance can exist as a liquid when under a specific pressure
Above this temperature the kinetic activity will be so great that the attractive forces of the substance are not enough to keep it a liquid
A true gas such as O2will have a critical temperature that is so low it can be store in room temperature without turning in to a liquid. In contrast a vapor is the gaseous state of a substance coexisting with its liquid or solid state at room temperature and pressure
This is why molecular water is referred to as water vapor
A gas can be liquefied by being cooled to below it’s boiling point or by being cooling to less than its critical temperature and being compressed
The more a gas is cooled below its critical temperature the less pressure that is needed to liquify it
However under no circumstances can pressure alone liquify a gas that exists above its critical temperature
Any gas with a critical temperature above ambient should be able to be liquefy simply by having pressure applied
Both CO2 and N2O have critical temperatures above normal room temperature
This means that both gases can be store as liquids when under pressure, usually strong metal containers
Critical Pressure
The pressure that is needed to maintain equilibrium between the liquid and gas phase of a substance
It is the force required to change a gas to a liquid when as critical temperature
critical point of a substance
Together the critical pressure and critical temperature represents the critical point of a substance
Triple Point
The set of circumstances needed to allow a substance to exist simultaneously as a solid, liquid, and gas
This happened with water at about 0.0006 atm and near 0 degrees Celsius
Bourdon Guage
Flowmeter is a pressure gauge calibrated in Lpm
UNCOMPENSATED FOR BACK PRESSURE-Meaning that when there there is back pressure the guage will read higher than what the patient is receiving
As pressure enters there will be a tube that straightens out indicating the pressure
Not position dependant
Obstruction to flow will result in back pressure
