Physio Ventilation (26)
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- getting air from the atmosphere INTO the alveoli
- aka transport of gas from the atmosphere to the alveolar surface
Blood Samples from Different Sites
- a arterial
- c capillary
- v venous
Variables Often Measured:
- P pressure
- V volume (gas)
- Q volume (blood)
- C content
- F fractional concentration
- S saturation
- eg. PaO2 = partial pressure of O2 in arterial blood
Sites Where Gases are Measured
- A alveolar
- I inspired
- D dead space
- E expired
- T tidal
- eg. VA = alveolar ventilation (volume of air/gas flow per unit time)
Dalton’s Law of Partial Pressures
- the total pressure (Ptotal) exerted by a mixture of gasses is the sum of the partial pressures (P1, P2, etc) exerted independently by each gas in
- the mixture
- Ptotal = P1 + P2 + P3 + ...
- the pressure of e/a gas contributes proportionally to its relative amount (if P1 = 25% of the total gas in a mixture, then P1 = Ptotal x 0.25)
- a mixture of gases: chiefly nitrogen, oxygen, water vapor, & carbon dioxide
- total pressure is the sum of e/a pressure contribution from the components of the mixture
- CO2 in atmosphere is actually low (doesn't take much of it to have adverse effects)
Using that chart, how would we get the partial pressure of each of these gasses?
multiply it's percentage (eg. 20.9 for O2) by the TOTAL pressure (760 mmHg)
DRY Atmospheric Air
- is ~79% nitrogen & 21% oxygen →
- PN2 = 597 mmHg & PO2 = 159 mmHg
What happens as we breath air IN?
- it passes over nasal turbinate → through the sinuses
- by the time it arrives in the trachea (then called tracheal or inspired air), the air has been HUMIDIFIED, PH2O = 47 mmHg (dilutes other gases)
- do this so air sent down to alveoli doesn't dry them out
- air is also warmed to body temperature at this point
Why do the partial pressures of the other components of air decrease when the partial pressure of water vapor (PH2O) increases at the trachea?
- b/c that increase in PH2O to 47 mmHg DILUTES the other components
*How to calculate Tracheal PO2:
- PO2 (tracheal) = PIO2 (inspired)
- = (Ptotal - PH2O) x 0.21
- atmospheric air - water component * .21 (b/c the partial pressure of O2 = 20.9)
- is FURTHER modified by the diffusion of O2 into the blood & CO2 from the blood into the alveoli
- O2 & CO2 numbers are CONSTANT
- PO2: 104 mmHg*
- PCO2: 40 mmHg*
- (eg. during exercise, you don't get TIRED from alveolar PO2 dropping)
- these values are only changes in disease
Partial PO2 & PCO2 Throughout Respiratory System
- blood comes into alveolar capillary from the R side of the heart DEoxygenated: has low O2 (40) & elevated CO2 (46)
- passing through the capillaries O2 is picked up in exchange for CO2 - diffuse down their CONCENTRATION gradients
- blood leaving capillary has high O2 (100) & lower CO2 (40)
Why is the PO2 of blood leaving alveolar capillaries slightly lower & not equal to the PO2 of air in the alveoli themselves?
- alveolar PO2: 105 mmHg
- blood exiting caps. PO2: 100 mmHg
- b/c the blood returning to the L atrium has some contribution from the venous blood of the BRONCHIOLE circulation - it has not been oxygenated in the lung
- the addition of bronchiole venous blood to this blood lowers the PO2
Partial PO2 & PCO2 Throughout Systemic Circulatory System
- systemic artery PO2: 100 mmHg
- systemic artery PCO2: 40 mmHg
- average cell PO2 < 40 (mt PO2 < 5!)
- average cell PCO2 > 46
- these are STEADY STATE VALUES
- as blood passes through capillaries surrounding normal cells, it emerges on the other side as venous blood (PO2 40, PCO2 46)
What type of blood is in pulmonary arteries & what type is in pulmonary veins?
- VENOUS blood flows through pulmonary arteries
- ARTERIAL blood flows through pulmonary veins
- it's the PO2/PCO2 composition that determines the TYPE of blood
- measured in (mL/min)
- is defined as:
- respiratory rate (breaths/min) * tidal volume (mL/breath)
- [conceptually analogous to C.O.; product of a rate * a volume]
amount of air you breath in & out in one breath
Minute Ventilation Typical Value
- respiratory rate * tidal volume
- respiratory rate at rest: ~12 breaths/min
- tidal volume: ~ 500 mL/breath
- = 12 bpm * 500 mLpb = 6000 mL/min (6 L/min)
- this is the typical amount of ventilation we supply our lungs w/
How much of the air we inhale gets to the alveoli?
- NOT ALL OF IT
- about 150 mL of inspired air DOESN'T reach the alveoli - it fills the conducting airways/bronchial tree
- it fills Anatomic Dead Space, aka where NO gas exchange occurs
- of the 500 mL tidal volume, only ~350 mL reaches the alveoli
Alveolar Ventilation (VA) Contributing to Gas Exchange
- 12 bpm * 350 mLpb = 4200 mL/min (4.2 L/min)
- (breaths per min) (mL per breath)
- as opposed to the 6000 mL/min calculated using the entire tidal volume
Total Alveolar Ventilation
- the tidal volume & alveolar ventilation are equal, but the air entering the alveoli contains the DEAD SPACE GAS from a previous breath + newly inspired air
- the air derived from the anatomic dead space does NOT contribute to gas exchange ("wasted")
Anatomic Dead Space
- @ end of expiration ~150 mL of used up alveolar gas sits in airways
- in next inspiration, 500 mL (tidal volume) of gas is inhaled
- sitting airways gas is returned to alveoli + ~350 mL of fresh air!
- the last 150 mL of inhaled gas sits in conducting airways & is exhaled in the NEXT breath w/o change
- 150 mL of inspired air filling conducting airways ~ filling anatomic dead space
Fowler N2 Washout Method
- an experiment that measures the anatomic dead space
- subject breathes pure 100% oxygen, then expires into a rapid nitrogen gas analyzer (analyzes N2 in expired air)
- 1st air blown out = pure O2 (N2 content will be 0)
- N2 concentration rises as the dead space gas is washed out by alveolar gas
- pure alveolar gas is indicated by a CONSTANT level of N2
- dead space volume is determined from a vertical line drawn such that areas A & B are equal
- dotted line ~1.5 L aka 150 mL (corresponds to anatomic dead space)
Respiratory (Alveolar) Dead Space
- results from alveoli that are ventilated but NOT perfused (gas entering such alveoli does NOT exchange w/ blood - same thing as in airways)
- healthy lungs have a very small amount of alveolar dead space
- it can be significant in pathologies such as Pulmonary Embolism
How would one potentially calculate the Alveolar (physiological) Dead Space (VD)?
- can do so if you know alveolar CO2 (PACO2) & expired CO2 (PECO2) & use this eqn
- VD/VE = (PACO2- PECO2) / PACO2
- VE = volume of expired air
- *won't ask about on test
- the proportion of dead space in each breath
- actual values:
- VD/VE = (40-28) / 40 = 0.3
- ~30% in this case
Alveolar Ventilation (VA)
- the flow of air into alveoli taking part in gas exchange
- of the 500 mL tidal volume, only ~350 mL of fresh air reaches the alveoli per resting breath, assuming NO alveolar dead space in a healthy individual
- therefore VA = resp rate * (tidal vol - anatomic dead space vol)
- VA = 12 bpm * 350 mLpb = 4200 mL/min
What must alveolar ventilation be adequate for?
- the removal of CO2 produced by cellular metabolism
- otherwise alveolar PCO2 & PACO2, will rise.
- therefore PACO2 at any point reflects the balance between CO2 production & removal by ventilation
PACO2 ~ VCO2 / VA
- means alveolar PACO2 is proportional to the rate at which you produce CO2 by metabolism divided by alveolar ventilation
- VCO2: the rate at of metabolic CO2 production
What will happen if alveolar ventilation fails (decreases)?
- alveolar CO2 (PACO2) will increase [based on the above relationship]
- this is what happens when certain disease processes negatively affect (suppress) alveolar ventilation
- high VCO2 (eg. in hyperthyroidism → elevated metabolic rate) will also result in ↑ PACO2
What value can be used to ESTIMATE alveolar ventilation (VA)?
- PaCO2: arterial CO2 (~ 40 mmHg)
- clinically, the partial pressures of CO2 in alveoli & arterial blood are assumed to be equal
- the partial pressures of CO2 in arterial blood is easier to measure therefore is used to estimate PACO2
What is another equation that can be used to calculate VA?
- VA = 0.863 * (VCO2 / PACO2)
- VA = 0.863 * (200 / 40) = 4.3 L/min (almost 4.2!)
- a change in PACO2 produces a compensatory change in VA
What is 0.863 in the equation VA = 0.863 * (VCO2 / PACO2)?
a proportionality constant that corrects for differences in the conditions for measuring VA, VCO2, & PACO2
top: decrease breathing rate increase breathing rate → ↑ alveolar CO2 (doubles! - morphine)
middle: normal PACO2 (40 mmHg)
bottom: increase breathing rate → ↓ alveolar CO2
What is the MOST important factor affecting air flow?
- an airway's RADIUS - a decrease of only 5% in bronchial radius reduces airflow by 20% (b/c there's an ↑ in Resistance)
- remember, Flow = ΔPπr4 / 8ηL
What is the total cross-sectional area of terminal versus initial airways?
- terminal airways have the LARGEST cross-sectional area
- this results in a DECREASE in airflow velocity
- the greatest resistance is actually in airways > 2 mm in diameter
Types of Air Flow
- laminar in straight sections of the bronchial tree but turbulent at branch points, where eddies occur
- laminar can also become turbulent at high velocities of airflow (rapid breathing)
- Transitional Flow: alternation between laminar and turbulent flows
What do Transitional & Turbulent flow require that Laminar DOESN'T for the same airflow?
a larger ΔP
What might happen to unsupported airways during expiration?
- they may collapse, particularly w/ increased intrapleural pressures during FORCED expiration
- small airways have no cartilage supporting their walls (only larger bronchi & bronchioles have cartilage supporting their branching)
- they're easily distended or compressed as the lungs inflate & deflate
- the distension during inspiration REDUCES resistance
What do parasympathetic cholinergic stimulation & sympathetic adrenergic stimulation cause?
- PSNS: bronchoconstriction
- SNS: bronchodilation
What agents cause bronchoconstriction?
1. chemical irritants (eg. ammonia, smoke, dust) - cause a reflex constriction involving neuropeptides (substance P)
2. immediate hypersensitivity response (histamine mediated)
3. inflammatory mediators: arachidonic acid metabolites (prostaglandins & leukotrienes)
What are the results of bronchoconstriction (& inflammation)?
- reduce radius
- increase resistance
- limit airflow
How can we measure the volumes of air we breath in & out?
- using open-circuit Spirometry
- basically have an inverted drum in a volume of water
- person holds a tube in their mouth that passes through fluid into the air in the drum
- inspiration: drum goes down
- expiration: drum goes up
- hooked up to pulley system that draws a picture of that breathing
- FRC: functional residual capacity
- RV: residual volume
- TLC: total lung capacity
- VC: vital capacity
- VT: tidal volume
Graph of Lung Volume v. Time
- 1st part: normal resting breathing; change in vol. = 500 mL
- 1st dip: occurs when person's instructed to blow out all the air they can (~1 L); called the Expiratory Reserve Volume
- 1st peak: difference between normal inspiration & maximum inspiration = Inspiratory Reserve Volume
What is it called when you move the maximum amount of air in & out voluntarily?
- Vital Capacity
- when you inhale maximally then exhale maximally
- in above graph, individual is moving ~5 L of air
Functional Residual Capacity (FRC)
- amount of air left in the lung after a normal breath (inhale → exhale)
- FRC = Expiratory Reserve Volume + Residual Volume
What is the one volume you can't measure directly using a spirometer?
- the Residual Volume (RV)
- even if you maximally expire, you can't expire the volume down to 0 [can't reduce lung volume to 0 - chest wall/thoracic cavity is too big]
So how can Residual Volume (RV) be acquired?
- using the Helium Dilution Method
- know volume of spirometer
- can add He to it - know the amount of He in spirometer drum is drum vol * He concentration
- open spicket - person breathes in air from tube
- this is started at the end of a normal exhalation - meaning air in lung corresponds to Functional Residual Capacity
- person breathes in & out air that contains He
- this dilutes He into total volume
- total volume is not original (V1) + lung volume (V2)
- …solve for V2
- I don't know
- C1 * V1 = C2 * V2 = C2 * (V1 + FRC)
What would you like to do?
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