The Gas Laws and Ventilation

Question 1

what is the primary lung function?

Answer 1

Gas exchange:
Take in O2 for aerobic respiration (production of ATP from food stuff such (glucose, protein and fat) via oxidative phosphorylation).
CO2 is a bi-product which then needs to be exhaled.
(Primary lung function)

Question 2

what are the other functions of lungs?

Answer 2

Immunologic: alveolar macrophages secreting cytokines for protection
Defensive: particulate matters & microbial elements removed via mucociliary escalator
Endocrine & Metabolic: conversion & synthesis of surfactants, prostaglandins (powerful local vasodilators & inhibit aggregation of blood platelets) & angiotensin II
This is able to occur as lungs receive 100% of right-sided cardiac output
Haematologic: pulmonary capillaries filter small clots, fat & cancer cells
Phonation: production of speech from sound on expiration
Thermoregulatory & water-eliminative: The lungs have a smaller role in this domain than the skin and kidneys however the lungs become increasingly important in extreme conditions (i.e. extreme heat or cold)

Question 3

explain metabolic rate
what is it dependant on?

Answer 3

Metabolic rate is the rate of energy expenditure per unit time. It varies depending on needs.
The amount of O2 we consume and CO2 we produce varies depending on our metabolic rate
Metabolic rate can be measured as volume of O2 (VO2) consumed per minute (V̇O2)
- Average basal metabolic rate (BMR) = 250ml/min (of oxygen)
Basal means resting/ normal breathing
- Exercise: average = 3000ml/min
- Daily average V̇O2= 1000ml/min (1L/min)

Metabolic rate also be defined as volume of CO2 production per minute (V̇CO2) - linked to V̇O2 through Respiratory quotient.

Question 4

what is the equation for respiratory quotient?
explain what it means

Answer 4

Respiratory Quotient (RQ) = VCO2 produced/VO2 consumed
refers to a steady state & depends on the food stuff being metabolised (R used in non-steady state conditions)
If only carbohydrate: RQ = 1.0
1 molecule of glucose requires 6O2 which leads to the production of 6CO2 therefore 6/6 = 1
If only protein: RQ = 0.81
If only fat: RQ = 0.7
reason why its a smaller number = needs more O2 than carbohydrate
Mixed diet: RQ = 0.8

Lungs must accommodate up to 30-fold increase in gas transfer - any failure results in respiratory dysfunction (i.e. COPD) with associated increased morbidity & mortality

Question 5

what does movement of gases in the body occur through?

Answer 5

The movement of gasses between the environment and the alveoli occurs through a series of bifurcating airways.
You have the trachea at the start which links the lungs to the environment and the alveoli at the bottom where gas exchange occurs to the most extent.

The airways bifurcate:
- At generation 1 the trachea splits into the left and right primary bronchi. This splitting continues for a total of 23 generations.
- There are 23 generations of airways leading to the alveolar sac which then bud out to form alveoli.

Question 6

what is the conducting zone?

Answer 6

The first 16 generations.
This includes the trachea, bronchi, bronchioles and terminal bronchioles.
These are distinguished by not allowing gas exchange to occur across their surfaces as they are too thick for gas to move across them.

Question 7

what is the transitional and respiratory zone?

Answer 7

Transitional Zone (Respiratory bronchioles) & Respiratory Zone (alveolar ducts and alveolar sacs):
Generations 17-23.
Gas exchange occurs.
Gas exchange occurs increasingly further down.

Question 8

what are the characteristics of airway types?

Answer 8

300 million alveoli with a X Sect area of 50-100M2 (in a small volume of 6L)
The number of each air way type increases as you move down generations.
As airway types increase in number, they increase in exponentially in total cross-sectional area but they decrease in diameter
An initially increase in air way generation results in little change in air way surface area until we get to respiratory bronchioles (Gen 17) where we see significant increases in SA.
This relationship between airway generation and area has an impact on the speed at which air moves through the lungs.

Question 9

how is speed of air flow calculated?

Answer 9

Flow of gas (V̇) is measured in cm3/sec or ml/sec
Flow of gas (V̇) = u (speed, cm/sec) x A (area, cm2)
V̇ = u x A
The flow of gas remains constant.
Therefore, as total area (A) increases as you move down the airways the speed of air (u) in those airways must decrease.
I.e. the speed of air in the lower airways is lower than the speed in the upper airways.
Beyond generation 17 u has decreased to the speed of molecular diffusion and thus rate of gas exchange is ultimately determined by diffusion and not by convection
Look at graph for speed.

Question 10

how does gas movement occur in transitional and respiratory zone?

Answer 10

Gas movement occurs only by diffusion (due to differences is partial pressure i.e. concentration)
Therefore, CO2 and O2 only move in the direction determined by diffusion.
Therefore conc. of O2 & CO2 varies little with inspiration/expiration

Question 11

how does gas movement occur in conducting zone?

Answer 11

Gas moves via convection flow (due to difference in pressure)
Therefore concentration of O2 & CO2 varies a lot with inspiration/expiration.
E.g. when we breathe in the airways in the conducting zone are full of inspired air containing little CO2 but when we breathe out they become full of expired air with high CO2 conc.
But as gas exchange does not occur in conducting zone this has no impact on blood passing through those airways.

Question 12

what does diffusion depend on? what does this allow?

Answer 12

diffusion is the key principle that determines how gas flows between the environment and our bodies.
Diffusion depends upon partial pressure gradients – as gas will only diffuse down its chemical gradient.
The greater the gradient the greater the flow of gas.
Therefore to increase flow of oxygen across patient lungs we need to increase diffusion gradient and therefore increase partial pressure of oxygen in respiratory zone.

Question 13

define Barometric pressure (PB)

Answer 13

Barometric pressure (PB) = pressure exerted by weight of gas molecules in atmosphere above measurement point

Question 14

what would PB be at sea level and at half - altitude?

Answer 14

At sea level = 100kPa
Total pressure decreases exponentially with altitude due to increasingly less presence of gas molecules
Half altitude (half-life) = 5400m
PB = 100kPa at sea level
PB = 50kPa at 5450m
25kPa at 10900m

Question 15

what is partial pressure?
what law is it related to?

Answer 15

The pressure of any particular gas, whether alone or in a mixture of other gases.
It depends upon the number of molecules of that gas in the given volume and on the temperature.
This is related by Dalton’s Law of Partial Pressures: Total pressure = P1 + P2 + P3 + … + Pn (total pressure is sum of partial pressure of individual gases)
Partial pressure = fractional conc. of gas in a mixture x total pressure
i.e. PIO2 = FIO2 x PB

Question 16

what happens to the total pressure when you increase altitude?

Answer 16

As we increase altitude the total pressure decreases and therefore so does partial pressure of a gas.
Therefore, at higher altitudes, partial pressure of O2 reduced –> reduces pressure gradient –> diffusion in respiratory zone reduced
Given that movement of gases across lung depends on diffusion and diffusion depends on partial pressure it is therefore clear using the above equation we can alter partial pressure of oxygen by increase fractional concentration of gas or increasing total pressure.

Question 17

where would foreign objects more likely be lodged in the bronchus?

Answer 17

Right primary bronchus is in a far more direct line with the trachea than left primary bronchus so here inhaled foreign objects will get lodged rather than the left

Question 18

what is dead space?
difference between dead space and physiological dead space?

Answer 18

Dead space: Volume of gas within respiratory system not involved in gas exchange.
It consists of an anatomic dead space (which we all have) and a potential alveolar dead space (arises in some respiratory diseases - therefore reduces amount of gas volume patient has available for gas exchange)
Physiological dead space = anatomic dead space + alveolar dead space
Dead space can be measured using Fowler’s method (nitrogen washout)

Question 19

what is anatomic dead space?

Answer 19

Anatomic dead space: volume of the conducting zone
Anatomic dead space is the necessary consequence of internalising of lung.
When you take a single breath the last part of air we move in fills the conducting zone (this air is not exchanged). Inspired gas within it does not reach alveoli so unavailable for gas exchange.
Anatomic dead space = 150ml in adult male on average
Constant throughout life –> larger person = larger VD

Question 20

what is alveolar dead space?

Answer 20

Alveolar dead space: the volume of alveoli in the respiratory zone where gas exchange should but does not occur.
This is normally 0ml but can increase usually due to respiratory disease e.g. blockages to terminal bronchioles
Alveolar dead space is usually 0ml so physiological dead space is usually equal to anatomical dead space. If physiological dead space increases it is due to increase in alveolar dead space.

Question 21

what factors affect anatomic dead space (VD) (volume of dead space)?

Answer 21

Primarily the size of subject (larger the person, the larger the anatomical dead space)

Question 22

what factors affect alveolar dead space?

Answer 22

Too small to be measurable in normal as it is 0ml
Increased significantly in certain lung diseases - primarily due to under-perfusion (reduced blood flow) of affected alveoli eg - pulmonary hypotension due to haemorrhagic blood loss or pulmonary embolism
can increase usually due to respiratory disease e.g. blockages to terminal bronchioles
Increases in physiological dead space will decrease alveolar ventilation and therefore impact adversely upon blood oxygenation, blood C02 and blood pH.

Question 23

define minute ventilation
calculation?

Answer 23

Minute (or total) ventilation (V̇E) = volume of gas breathed out in 1 min.
It can be calculated as product of tidal volume and respiratory frequency (no. of breaths in a minute)
V̇E = VT x f (VT = tidal volume & f = respiratory frequency)
Average VT = 500ml & average f = 12 breaths/min
Minute ventilation = 500 x 12 = 6.0 L/min

Question 24

define alveolar ventilation
calculation?

Answer 24

Presence of anatomical dead space volume (VD) means that not all of V̇E reaches respiratory zone and therefore not all of it is useful for gas exchange so:
Alveolar ventilation = useful component of total ventilation (in terms of gas exchange.
V̇A = (VT – VD) x f

in image example 0.8L/min of total ventilation is wasted as anatomical dead space ventilation

Question 25

what factors affect alveolar ventilation?

Answer 25

Alveolar dead space VD will increase due to respiratory disease so the value of alveolar ventilation goes down
In addition patterns of breathing can alter V̇A
slow deep breaths provide a more efficient gas exchange than rapid, shallow breaths (rapid shallow breathing results restricted alveolar ventilation)
Understanding of pattern of breathing is how patients with COPD are trained to breathe. E.g.:
pursed lip breathing technique
increased alveolar ventilation
improves movement of air in & out of lungs
opens airways & removes trapped air
controls rate of breathing
reduces anxiety & promotes relaxation

Question 26

what are the 5 techniques to properly execute pursed lip breathing?

Answer 26

dont forget to do the questions at the end of hamzas notes!!
and read the key points re pk Lecture 1 on canvas