Pulmonary: Physiology Review Flashcards
Definition of Respiration
Partial Pressure Abbreviations and Definitions
Other Respiratory Definitions
What are the funcitons of the upper respiratory tracts?
Air conditioning: humidification and warming (mouth, nose, nasal cavity, pharynx)
Filtration: nose, nasal cavity, pharynx
Olfaction
Phonation
What are the functional characteristics of the lower respiratory system?
Cilia to move pathogens out
Mucus to entrap pathogens
Surfactant to maintain patency, opsonize pathogens, modify T-cells
Smooth muscle to regulate bronchomotor tone
Cartilage to keep trachea and bronchi open and to protect
NOTE: see attachment to see WHERE each begins and ends.
What is the driving force for gas exchange in the lungs?
The difference in gas partial pressures
How do we determine the partial pressure of a gas in the lungs?
Dalton’s Law of Partial Pressures
In dry gas (inspired air), the partial pressure of x is the barometric pressure times the fraction of the gas: Px = Pb * F
In humidified gas (bronchial air), the partial pressure of x is the difference between the barometric and water vapor pressures times the fraction of the gas:
Px=(PB - PH2)) * F
What is the fraction of Oxygen in the air?
What is the barometric pressure?
The fraction of oxygen in the air 21% anywhere on earth.
The barometric pressure changes according to altitude. It s 760 mm Hg at sea level
What is the point of measuring and calculating partial pressures?
When a patient is experiencing dyspnea,
knowing the partial pressures helps figure out where the problem is.
What are the two ways air flows in the respiratory system and where do they happen?
Convection (bulk flow) in the airways
(May be turbulent or laminar/smooth)
Diffusion in the alveoli
Since there is not mucus or cilia in the alveoli,
how are they protected from pathogens?
Macrophages phagocytize pathogens
Surfactant opsonizes them and modulates T-cells.
What seven barriers must gases move through for gas exchange?
Alveolar fluid layer, epithelium, basement membrane
Interstitial space
Capillary basement membrane and endothelium
What affects the concentration of gas in the body fluids?
The concentration of gas in the body fluids
is proportional to its partial pressure Px and its solubility
Henry’s Law:
Cx =Px * solubility
Compare the solubility of oxygen versus carbon dioxide.
The solubility coefficient for carbon dioxide (.57) is WAY higher
than for other gases including oxygen (.024)
(CO2 likes fluids!)
Note: At equilibrium, the partial pressure of the gas in liquid phase
equals the partial pressure in the gas phase.
What determines how easily a gas diffuses across cell membranes?
Rate of transfer is
proportional to the tissue area and the difference in partial pressure of the gas
and is inversely proportional to the tissue thickness.
Frick’s Law of Diffusion:
Flux = Area/thickness * pressure difference * diffusion coefficient
V = [A/Δx] · D · (P1 – P2)
What are the limiting factors for gas movement in the body?
Diffusion through tissue water:
Thickness (inversely proportional)
Surfact area (proportional)
Note that both of these can be affected by disease states.
What is the diffusion coefficient? (D)
The relative diffusion coefficient is based on oxygen (D=1).
Carbon Dioxide (D=20.3) likes to move through fluid
How does surface area change as air moves down the airways?
While the surface area of individual airways
decreases as you go deeper into the lungs,
the total surface area increases dramatically.
What is the total surface area available for gas exchange?
75.4 m2
However, this is the absolute maximum.
At rest we do not use this much, but during exercise we can recruit unopened alveoli.
In certain diseases, the alveoli are damaged or destroyed which limits the ability to recruit more when needed.
What is the V/Q ratio?
The V/Q ratio is the ration of ventilation to perfussion
At equilibrium, gas exhange is perfusion limited and requires increased blood flow to increase gas exhange (give the patient some fluids or blood)
When equilibrium is not reached, gas exchange is diffusion-limited and requirees increased pressure gradient to increase gas exchange (give the patient some oxygen)
What is the diffusing lung capacity?
How is it used?
Diffusing Lung Capacity DL combines the factors that affect diffusion: the diffusion coefficient (D), the surface area (A) and the membrane thickness (Δx).
DL = D · A/Δx
So Fick’s Law becomes:
V (flux) = DL · (P1 – P2)
This accounts for the time it takes for gas (oxygen)
to combine with blood proteins (Hb)
How do we measure lung diffusion capacity and what affects it?
We use carbon monoxide to measure it: DLCO
It is limited in patients with thickened respiratory membranes like the edema in heart failure and the firbrosis in sarcoidosis.
It is limited in patients with decreased surface area for gas change like emphysema
How is oxygen transported in the blood?
- 5% dissolved in plasma
- 5% bound to Hb (improving carrying capacity by 70 fold)
Oxygen content in the blood = dissolved oxygen + oxygen bound to Hb
Oxygen bound to Hb = binding capacity * %Hb saturation
What factors decrease Hb saturation?
(What causes the Hb to unload Oxygen?)
Increased temperature
Increased PCO2
Increased 2,3 DPG
Increased acidity (decreased pH)
How is Carbon Dioxide transported in the blood?
7% dissolved in plasma
23% bound to Hb in the RBCs
70% in bicarb buffer system in RBCs
What muscles assist with ventilation during rest?
During exerise or disease?
Inspiration: Diaphram and external intercostals at rest
Accessory muscles added during exercise/disease.
Expiration: passive at rest (recoil)
Abdominal and internal intercostals used during exercise or disease.
What is the net driving pressure for air movement during ventilation?
Driving pressure for ventilation:
The force of muscle contraction
Lung compliance (ΔP/ΔV)
Airway resistance
What is the general gas law and how does it apply to ventilation?
PV=nRT
At constant temperature, P*V is constant so:
P1V1 = P2V2
Eg: increased volume in the lungs during inspiration
leads to decreased pressure in the lungs, pulling air inside.
Which volumes and capacities cannot be measured by spirometry?
You cannot measure Residual Volume (RV)
Therefore you also cannot measure Functional Residual Capacity (FRC)
or Total Lung Capacity (TLC) because these are calculated using RV.
Know and understand the attached graph!!!
What happens to the pressure in the lungs when the volune in the lungs increases?
It decreases. See attached.
What is physiologic dead space?
Physiological dead space is the volume of lungs that
does not participate in gas exchange.
Physiologic dead space = anatomic dead space (150 mL) + functional dead space
Functional dead space increases due to disease,
thereby increasing physiological dead space.
What is the difference between pulmonary ventilation (breathing)
and alveolar ventilation?
Minute ventilation (amount of air expired in a minute) = Tidal Volume * Respiratory Rate
(VE) = VT (mL) x Resp Rate (breaths/min)
Alveolar ventilation is the amount of air entering/leaving the alveoli in a minute, accounting for the physiologic dead space
(VA) = (VT –VD) x RR
What is happening to the volume of air when the lungs are at rest?
The lungs are at rest at the Functional Residual Capacity,
when the elastic recoils of the lung and chest wall are equal but opposite.
See attached.
What are compliance and elastance and how are they related?
Compliance is lung stretchability: change in volume over change in pressure
Compliance = Δ volume /Δ pressure
Elastance is tendencey to recoil: change in pressure over change in volume
Elastance = Δ pressure/ Δ volume
Compliance is inversely proportional to elastance.
What affects compliance and elastance?
Collagen and elastin fibers
Surface tension of small airways and alveoli
Elastic recoil is the pressure difference required
to inflate the lung to a specific volume
What role does Surfactant play in pulmonary compliance?
Surfactant reduces surface tension in the alveoli, thereby increasing compliance.
Without surfactant, the attractive forces of fluid would collapse the alveoli.
Since each alveoli has the same amount of surfactant,
smaller alveoli have a higher concentration,
thereby balancing the volume difference
so that there is no pressure gradient
(gases do not move from one alveoli to another).
How does airway resistance affect the flow of air?
Flow (Q) = pressure gradient (ΔP)/resistance (R)
The pressure gradient can be the difference between inside and outside pressure in ventilation or between alveoli and blood in gas exchange.
What is the key factor that affects airway resistance?
According to Poiseuille’s Law: Resistance (R) = 8ηl / πr4
Most of these variables are constant in the body, so they key factor is the
radius of the airway
Resistance is inversely proportional to r4
so a tiny change in radius will have a huge impact on resistance.
Eg: if you cut the radius in half, you increase the resistance 16-fold!
How does lung volume affect resistance?
Increased volume decreases resistance
(Inversely proportional)
How does the automomic system regulate airway resistance?
Sympathetic: bronchodilation decreases resistance
(via circulating catecholamines on bronchial smooth muscle)
Parasympathetic: bronchoconstriction increases resistance
(via Ach from vagus nerve on bronchial smooth muscle)
How do we estimate physiologic dead space?
It is the tidal volume times the proportion of air
that does not participate in gas exchange:
VD = VT x (PaCO2 – PECO2 )/ PaCO2
How do we measure PaCO2?
(alveolar partial pressure of CO2)
We cannot meausure it directly
so we used PACO2 (arterial partial pressure of CO2)(via blood gas)
as a surrogate because they are closely related.
How are PACO2 and PAO2 related?
They are inversely proportional.
So when one increases, the other decreases.
What are the regional differences in the V/Q ratio?
At the apex of the lung, the alveoli are full but the capillaries are thin so the Ventilation to Perfusion (V/Q) ration is high (3.0)
At the base of the lungs, the alveoli are compressed by gravity but the capillaries are more full (gravity causes the blood to pool there), so the V/Q ratio is low (.6)
What is shunt
and what types of shunts are there?
A shunt is when the blood bypasses the respiratory membrane
and therefore does not participate in gas exchange.
Examples:
Physiologic shunts: Bronchial perfusion and thebesian channels
Nonphysiologic shunts: developmental anomalies such as foramen ovale
Local shunts: when blood is diverted away from a faulty alveolus
What are two causes of a V/Q mismatch?
Airway obstruction, such as a blockage or a shunt,
Q is normal, but V is zero: V/Q =0
Perfused but not ventilated
Pulmonary embolus or dead space
V is normal but Q is zero
V/Q is infinite
Ventilated but not perfused
What parts of the brain control ventilation?
Medulla: rhythm generation
Pons: regulates medulla
Cortex: conscious and emotional response
See attached for details.
How does respiratory information from the periphery get to the brain?
Chemoreceptors sense CO2 and O2
Mechanoreceptors in lung and muscle sense stretch and joint movement
Irritant receptors in the airway epithelial cells sense toxins/irritants
J receptors in the alveolar walls capillaries sense engorged capillaries during edema
See attached.
How do central chemoreceptors communicate with the brain?
Directly sense decreased pH in the CSF
Indirectly sense increased PaCO2 in the blood
Which chemoreceptor is the most important during normal breathing?
The central chemoreceptor.
How do we assess respiratory function in a patient with dyspnea?
(What tests can we use?)
Spirometry to assess lung volumes, capacities re obstructive/restrictive disease
Pulse oximetry to indirectly assess PaCO2
Arterial blood gas (ABG) to assess gas exchange
What is the alveolar-arterial gradient?
(A-a gradient)
The A-a gradient tells us how much inspired oxygen
is actually getting into the blood stream.
A-a gradient = [PIO2 - (PACO2 / R)] - PaO2
Normal A-a gradient: Age/4 + 4
If the gradient is high then O2 in the alveolus is not getting into the arterial blood effectively.
What affects the A-a gradient?
A-a gradient = [PIO2 - (PACO2 / R)] - PaO2
Alveolar PCO2 is affected by: respiratory rate, tidal volume and dead space
Arterial PO2 is affected by changes in inspired oxygen, poor diffusion across the respiratory membrane, R-L shunting and V/Q defects.
