Pressure and Compliance

Question 1

what volumes are capacities made up of ?
(there are 6)

Answer 1

Total lung capacity (TLC) (6L)
Functional Residual capacity (FRC)
Tidal volume (TV)
Inspiratory reserve volume (IRV)
Inspiratory capacity (IC)
Expiratory reserve volume (ERV)

Question 2

what is total lung capacity (TLC)?
what is it made up of?

Answer 2

is the maximum capacity of our lungs.
TLC made up of:
Vital Capacity (VC) (4.8L): The total amount of air exhaled after maximal inhalation. VC = TV+IRV+ERV.
Residual volume (RV) (1.2L): the volume of air that is left in your lungs after maximal exhalation. (forced expiration)

Question 3

what is Functional Residual capacity (FRC)?

Answer 3

(2.7L)
It is the amount of air remaining in the lungs at the end of a normal exhalation. FRC = RV+ERV.

Question 4

what is Tidal volume (TV)?

Answer 4

(0.5L)
It is the amount of air that can be inhaled or exhaled during one breath.

Question 5

what is Inspiratory reserve volume (IRV)?

Answer 5

(2.8L)
It is the amount of air that can be forcibly inhaled after a normal tidal volume. IRV is usually kept in reserve, but is used during deep breathing.

Question 6

what is Inspiratory capacity (IC)?

Answer 6

(3.3L)
It is the maximum volume of air that can be inhaled following a resting state. IC = IRV+TV

Question 7

what is Expiratory reserve volume (ERV)?

Answer 7

(1.5L)
It is the volume of air that can be exhaled forcibly after exhalation of normal tidal volume.

Question 8

what volume do we breathe normally and when demands increase?

Answer 8

So typically, we breathe at our tidal volume
But as demands increase (during exercise) we increase our tidal volume by eating into IRC and ERV up to a maximum of our vital capacity.
This involves using accessory muscles of respiration.

Question 9

what factors have a correlation between itself and increased lung volume?

Answer 9

Increased height (taler you are the large your lungs)
Male: have large lungs than women of same height (as they have larger thoracic cage)
Lung volumes initially increase with age but then decrease in older age
Race – European/North A. > Asian
Respiratory Disease (Lung volume & flow rate reduced in respiratory disease)
These factors are taken into account when determining if someone’s lung volumes are normal

Question 10

what needs to happen in order for air to move in and out of lungs?

Answer 10

air moves from high to low pressure
Air flows down pressure gradients - difference needed between PA (alveolar pressure) & PB (barometric pressure) in order for air to move into & out of the lungs

Question 11

what is Boyle’s Law?

Answer 11

PB can’t be altered (constant at a set altitude) therefore air flow in/out of lungs results due to changes in PA.
This can be explained due to Boyle’s law:
Boyle’s Law – inverse relationship between pressure & volumes of gasses
P1 x V1 = P2 x V2 - if volume doubled, pressure halved (inverse relationship)
So, if we alter the thoracic volume the PA must also change and air will flow (PB = constant)
- If we increase thoracic volume the alveolar volume increase –> PA decreases (below PB) - therefore air moves into lungs (inspiration) down a pressure gradient until PA=PB.
- If we decrease thoracic volume the alveolar volume decreases –> PA increases (above PB) - therefore air moves out of the lungs (expiration) down a pressure gradient until PA=PB.

Question 12

what happens during quite Inspiration?

Answer 12

Diaphragm contracts (flattens down) –> increases vertical dimension of thoracic cavity (leading to inc in thoracic vol) –> decrease PA

Question 13

what happens during quite expiration?

Answer 13

Diaphragm relaxes –> decrease in vertical dimension of thoracic cavity (leading to dec in thoracic vol) –> Increase PA

Question 14

what happens during forced inspiration?

Answer 14

Requires the contraction of the accessory muscles of inspiration (in addition to diaphragm):
External intercostals
Scalene
Sternocleidomastoids
Contraction of these muscles results in an increase in the lateral and anterior-posterior dimension of the thoracic cage –> greater decrease in PA

Question 15

what happens during forced expiration?

Answer 15

Occurs following forced inspiration
Requires contraction of accessory muscles for expiration
abdominal muscles
internal intercostal muscles.

Question 16

Respiratory muscles do work to…

Answer 16

stretch the elastic components of the respiratory system
to overcome the resistant to flow

Question 17

what are lungs surrounded by?

Answer 17

Lungs are surrounded by visceral pleura & the inner surface of the thorax is lined by parietal pleura
Between these two layers of pleura lies the pleural space containing small amount of cohesive, lubricating & non-expanding pleural fluid (usually no air in pleural space)
Each lung has its own pleural coverings so they each act separately from each other

Question 18

how does the work of respiratory muscles acting on the thoracic volume get transmitted into a change in lung volume?

Answer 18

Negative pressure (lower than atmospheric pressure) is created in the pleural space:
At every lung volume healthy lung tends to recoil inwards (collapse) and at most lung volumes the thoracic cage tends to recoil outwards and pull away from the lungs.
[Explanation: As a result, they are trying to increase the volume of the pleural cavity which in turn results in a decrease intrapleural pressure => negative]

This creates negative intrapleural pressure (which is exerted on the pleural fluid) which allows for mechanical coupling of the lungs and thorax (the visceral & parietal pleura push against one another)
Therefore, if the chest wall pulls outwards the lung will come with it
Pleural layers don’t actually touch –> repulsive forces between several layers of phospholipids on mesothelial surfaces carrying same charge

Question 19

what happens when respiratory muscles are relaxed?

Answer 19

At the Functional residual capacity (when respiratory muscles are relaxed) the outward and inward recoils are equal and opposite - there is a negative intrapleural pressure (sub - atmospheric pressure) but the respiratory system is at equilibrium.
The act of inspiration involves the decrease in alveolar pressure relative to barometric pressure. The change in PA is brought about by the activity of inspiratory muscles that contract to increase the thoracic cage volume and as the lungs are ‘mechanically’ coupled to the thoracic cage, via the intrapleural pressure, lung volume is increased causing the fall in PA and hence airflow into the lungs until PA=PB.
Expiration is the reverse of this process – but may not require expiratory muscle contraction.

Question 20

explain the pressure and volume changes during a respiratory cycle

Answer 20

Air flow: Movement of air in and out of the lungs
When Air flow = 0 –> Alveolar pressure = Barometric Pressure (=0)
When we breathe in, air flow becomes more negative => signifies inspiration
When we breath out, air flow becomes more positive => signifies expiration
Volume: Volume of air in the lungs
Recoil Pressure (Pel): Lung Recoil pressure –> Tendency of lung to try and collapse
Alveolar Pressure: Pressure within the alveoli (Relative to barometric pressure)
PA is negative in inspiration and positive in expiration
Intrapleural Pressure: pleura pressure
Intrapleural pressure is always negative during quite breathing
Don’t usually see positive intrapleural pressure

Question 21

explain each stage of the respiratory cycle

Answer 21

1. Time = 0 (1st Line)
   Lung volume = FRC
   Air flow = 0 as alveolar pressure is 0 relative to barometric pressure
   Negative intrapleural pressure allows for coupling of lungs and thorax (-0.5kPa)
2. Time = Half way through inspiration (2nd Line)
   We are now at peak inspiratory air flow - results in an increase in lung volume
   Air flow is at its peak here as we have the greatest difference between alveolar pressure and barometric pressure
   Intrapleural pressure becomes more negative during inspiration:
   The chest wall moves outwards and the lungs recoil pressure becomes greater - as a consequence, this makes intrapleural pressure more negative (coupling lung more strongly to chest wall)
3. Time = end of inspiration (3rd line)
   Defined when Air flow = 0
   Occurs as alveolar pressure is now equal to barometric pressure
   Lung volume is at its maximum
   Intrapleural pressure is now at its most negative:
   As Lung is at its greatest recoil –> greatest volume and greatest tendency to collapse.
4. Time = half way through expiration (4th line)
   We are now at peak expiratory air flow - results in an decreasing in lung volume
   Air flow is at its peak here as we have the greatest difference between alveolar pressure and barometric pressure
   Intrapleural pressure also rises back to resting FRC value
5. End of expiration (5th Line)
   Lung volume = FRC
   Air flow = 0 as alveolar pressure is 0 relative to barometric pressure

Question 22

Quick Notes
1. Airflow profile follows PA changes
2. During expiration, both Ppl and PA rise
3. PA is always > Ppl
4. In quiet breathing, Ppl is always negative whilst PA is negative in inspiration and positive in expiration
5. Airflow ceases when PA is zero
6. At high ventilation Ppl and PA changes are increased.
a. The curves are the same shape but peak values are greater
7. Ppl can be +ve in forced expiration (eg +8kPa in coughing/sneezing)
a. So if you can’t produce +ve Ppl you won’t be able to cough and sneeze so can’t clear mucus from air ways

Question 23

define compliance

Answer 23

Compliance (C) = measure of distensibility of an elastic structure

Question 24

what is the equation for compliance?
why is it a static measurement?

Answer 24

Compliance (L/kPa) = Change in volume/change in distending pressure.
In a more compliance structure, there will be a greater change in volume for a fixed change in P.
Inspiratory muscles work in part to stretch the elastic component of the respiratory system.
Compliance is a static measurement:
In order to measure the compliance of lung or Thoracic cage we need to remove air way and air flow resistance.
Therefore, compliance is measured during static manoeuvres – when air is not moving.

Question 25

what are the lungs comparable to and why?

Answer 25

The lungs are comparable to a balloon. The lung will inflate/distend as long as Pin (pressure inside) is greater than Pout (pressure outside) i.e. positive distending pressure The lung will deflate to volume very close to 0 when distending pressure = 0 Therefore, in order to measure compliance of the lung we need to measure lung volume against a distending pressure Lung distending pressure: Pressure across the lung (Pressure inside lung minus pressure outside the lung) Pin - Pout

Question 26

what is the thoracic cage comparable to and why?

Answer 26

The Thoracic Cage is comparable to a tennis ball with a hole in it. When it is not being compressed or distended (i.e. when distending pressure is 0) its volume will not equal 0 (like a balloon). This is because of the integrity of its structure i.e. the muscles and ribs hold thoracic cage at a certain volume when no pressure is present. Thoracic cavity Distending pressure (pressure across thoracic cage): Pressure inside thoracic cage minus pressure outside the thoracic cage Pin - Pout

Question 27

what is distending pressure?

Answer 27

Pin - Pout is the difference between the airway pressure and the pleural pressure

Question 28

what is the distending pressure for lungs and chest wall?

Answer 28

For the lungs: - Pressure in = PA - Pressure out = Ppl Therefore, distending pressure is PA- Ppl For the chest wall: - Pressure in = Ppl - Pressure out = PB Therefore, distending pressure is Ppl-PB

Question 29

what would positive and negative distending pressure mean?

Answer 29

Positive Distending pressure: Pin greater than Pout so this the structure is being distended Negative distending pressure: Pin smaller than Pout so the structure is being compressed.

Question 30

what does volume on the y-axis show?

Answer 30

Volume = 0 --> complete compression Volume = 6L --> TLC NB: It is not possible for lung volume to reach 0 due to the presence of the residual volume.

Question 31

what does the lung compliance curve show?

Answer 31

This curve shows the relationship between pressure across the lung (distending pressure) and the volume of the lung The curve lies to the right of the 0 distending pressure mark, therefore, for all lungs volumes, the lungs are always being distended As distending pressure increases (Pressure in becomes greater than pressure outside) the volume of the lungs also increases This continues to TLC – where we reach elastic limit of the lungs is reached. NB: A lung volume of 0L occurs when there is no distending pressure. This is the equilibrium volume of the lung (=0L) Equilibrium volume of structure is found at zero distending pressure – this is where elastic structure would like to be as there isn't distending pressure on it. Where elastic structure is attempting to recoil towards.

Question 32

what does the thoracic cage compliance curve show?

Answer 32

Most of it lies to the left of the 0 distending pressure point, therefore the chest wall for most of its time is being compressed. Increasing distending pressure towards more positive values results in an increase is volume of thoracic cage. At high volumes the thorax is being distended. If we take a deep breath in to TLC at this point we stretch thoracic cage beyond its equilibrium volume so it will now have the tendency to collapse. The chest walls equilibrium volume is around 5L. When we are breathing in the chest wall is moving towards where it wants to go (being helped) but after large inspiration chest wall wants to collapse.

Question 33

what part of the curve would refer to the compliance? explain what it means at different stages

Answer 33

Compliance is simply the slope of the line. Compliance (slope) of both structures is greatest around FRC: This is convenient as this is where the region where we breathe So this is the region that is easiest to breathe – requires the least amount of energy. Lung compliance is lowest towards TLC – if you take a deep breath in and try to breathe again it is very difficult to breathe. Normal compliance = 1.5L.kPa-1 Lung and thoracic cage compliance similar around FRC - slop of line is identical --> so chest wall and lung are similar in there stretchiness. The chest wall compliance curve crosses zero distending pressure where as lung does not. When we are breathing in the chest wall is moving towards where it wants to go (being helped) but after large inspiration chest wall wants to collapse.

Question 34

what is specific compliance?

Answer 34

Specific compliance = compliance/FRC used and useful when we compare object of different sizes e.g lung of baby vs lung of adult will have similar compliances but different specific compliances.

Question 35

what is pulmonary fibrosis?

Answer 35

Causes scaring of lung as the lung responds abnormally during healing process This scar tissue results in low lung compliance (stiffer lung) --> harder to breathe. Curve on the graph for some one suffering from pulmonary fibrosis will be much flatter Because it is harder to breathe you get less oxygen in blood stream. First sign of pulmonary fibrosis is shortness of breath.

Question 36

what is total system compliance?

Answer 36

The lungs lie within the thoracic cage and both need to be stretched together during inspiration so we need to consider the total system compliance. We get a decrease in compliance when we put the two structures together. 1/total system compliance (Ctot) = 1/lung compliance + 1/thoracic wall compliance This equation shows total system is twice as stiff (half as compliant) as its individual components

Question 37

explain how the total system compliance curve would look

Answer 37

Minimum volume: Residual volume Maximum volume: TLC The slope of total system compliance curve at FRC is less than thoracic cavity and Lung compliance curve (about half) The Zero distending pressure equilibrium position of total system compliance = FRC = 2.7L This therefore must mean that the distending pressure of lung at FRC is equal and opposite to distending pressure of thoracic cage at FRC. So FRC is determined by both elasticity (compliance) of both lung and thorax. So any change in lung or thoracic cage compliance will change FRC.

Question 38

what is pneumothorax?

Answer 38

a collapsed lung a pneumothorax happens when air leaks into the pleural cavity

Question 39

how does pneumothorax occur?

Answer 39

Occurs if lung perforates or if there is a penetrating wound of chest wall --> air enters and fills pleural space from lung or from atmosphere --> causes loss of -ve intrapleural pressure - Distending pressures lost --> PA = PPl = PB = 0kPa (relative). Therefore, lung can’t be held open and thoracic cage can’t be compressed so the lung collapses to its equilibrium volume (0L) and the chest wall expands to its equilibrium volume (5L) As a result, breathing becomes uncoupled (lungs no longer move as chest wall moves) thoracic cage can still move if innervation intact but breathing is painful, difficult or impossible. Other lung continues breathing normally due to separation of two pleural membranes.

Question 40

what are the types of Pneumothorax?

Answer 40

Primary spontaneous pneumothorax: most common occur spontaneously occur often in tall thin younger individuals. Small blebs on lung --> ruptures --> internal lung perforations outpatient treatment Secondary pneumothorax: Associated with respiratory disease (COPD, asthma) Requires intercostal tube drainage Traumatic pneumothorax: Following blunt or penetrating chest trauma Or through mechanical ventilation (too high for patient) Requires Intercostal tube drainage Tension pneumothorax: Occurs due to mechanical ventilation or trauma Results in mediastinal shift (movement of heart & trachea due to air rushing in – great veins are blocked off) Medical emergency, requiring drainage with a 14-G needle to allow air to escape from pleural space. so much air flowing into pleural space; wound acts as a valve preventing outflow of air look at hamzahs notes for questions