Pulmonary: Physiology Review

Question 1

Definition of Respiration

Answer 1

Question 2

Partial Pressure Abbreviations and Definitions

Answer 2

Question 3

Other Respiratory Definitions

Answer 3

Question 4

What are the funcitons of the upper respiratory tracts?

Answer 4

Air conditioning: humidification and warming (mouth, nose, nasal cavity, pharynx)

Filtration: nose, nasal cavity, pharynx

Olfaction

Phonation

Question 5

What are the functional characteristics of the lower respiratory system?

Answer 5

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.

Question 6

What is the driving force for gas exchange in the lungs?

Answer 6

The difference in gas partial pressures

Question 7

How do we determine the partial pressure of a gas in the lungs?

Answer 7

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: P_x = P_b * 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:

P_x=(P_{B -} P_H2)) * F

Question 8

What is the fraction of Oxygen in the air?

What is the barometric pressure?

Answer 8

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

Question 9

What is the point of measuring and calculating partial pressures?

Answer 9

When a patient is experiencing dyspnea,

knowing the partial pressures helps figure out where the problem is.

Question 10

What are the two ways air flows in the respiratory system and where do they happen?

Answer 10

Convection (bulk flow) in the airways

(May be turbulent or laminar/smooth)

Diffusion in the alveoli

Question 11

Since there is not mucus or cilia in the alveoli,

how are they protected from pathogens?

Answer 11

Macrophages phagocytize pathogens

Surfactant opsonizes them and modulates T-cells.

Question 12

What seven barriers must gases move through for gas exchange?

Answer 12

Alveolar fluid layer, epithelium, basement membrane

Interstitial space

Capillary basement membrane and endothelium

Question 13

What affects the concentration of gas in the body fluids?

Answer 13

The concentration of gas in the body fluids

is proportional to its partial pressure P_x and its solubility

Henry’s Law:

C_x =P_x * solubility

Question 14

Compare the solubility of oxygen versus carbon dioxide.

Answer 14

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.

Question 15

What determines how easily a gas diffuses across cell membranes?

Answer 15

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)

Question 16

What are the limiting factors for gas movement in the body?

Answer 16

Diffusion through tissue water:

Thickness (inversely proportional)

Surfact area (proportional)

Note that both of these can be affected by disease states.

Question 17

What is the diffusion coefficient? (D)

Answer 17

The relative diffusion coefficient is based on oxygen (D=1).

Carbon Dioxide (D=20.3) likes to move through fluid

Question 18

How does surface area change as air moves down the airways?

Answer 18

While the surface area of individual airways

decreases as you go deeper into the lungs,

the total surface area increases dramatically.

Question 19

What is the total surface area available for gas exchange?

Answer 19

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.

Question 20

What is the V/Q ratio?

Answer 20

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)

Question 21

What is the diffusing lung capacity?

How is it used?

Answer 21

Diffusing Lung Capacity D_L combines the factors that affect diffusion: the diffusion coefficient (D), the surface area (A) and the membrane thickness (Δx).

D_L = D · A/Δx

So Fick’s Law becomes:

V (flux) = D_L · (P1 – P2)

This accounts for the time it takes for gas (oxygen)

to combine with blood proteins (Hb)

Question 22

How do we measure lung diffusion capacity and what affects it?

Answer 22

We use carbon monoxide to measure it: D_LCO

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

Question 23

How is oxygen transported in the blood?

Answer 23

1. 5% dissolved in plasma
2. 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

Question 24

What factors decrease Hb saturation?

(What causes the Hb to unload Oxygen?)

Answer 24

Increased temperature

Increased P_CO2

Increased 2,3 DPG

Increased acidity (decreased pH)

Question 25

How is Carbon Dioxide transported in the blood?

Answer 25

7% dissolved in plasma

23% bound to Hb in the RBCs

70% in bicarb buffer system in RBCs

Question 26

What muscles assist with ventilation during rest?

During exerise or disease?

Answer 26

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.

Question 27

What is the net driving pressure for air movement during ventilation?

Answer 27

Driving pressure for ventilation:

The force of muscle contraction

Lung compliance (ΔP/ΔV)

Airway resistance

Question 28

What is the general gas law and how does it apply to ventilation?

Answer 28

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.

Question 29

Which volumes and capacities cannot be measured by spirometry?

Answer 29

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!!!

Question 30

What happens to the pressure in the lungs when the volune in the lungs increases?

Answer 30

It decreases. See attached.

Question 31

What is physiologic dead space?

Answer 31

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.

Question 32

What is the difference between pulmonary ventilation (breathing)

and alveolar ventilation?

Minute ventilation (amount of air expired in a minute) = Tidal Volume * Respiratory Rate

(V_E) = V_T (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

(V_A) = (V_T –V_D) x RR

Question 33

What is happening to the volume of air when the lungs are at rest?

Answer 33

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.

Question 34

What are compliance and elastance and how are they related?

Answer 34

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.

Question 35

What affects compliance and elastance?

Answer 35

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

Question 36

What role does Surfactant play in pulmonary compliance?

Answer 36

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).

Question 37

How does airway resistance affect the flow of air?

Answer 37

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.

Question 38

What is the key factor that affects airway resistance?

Answer 38

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!

Question 39

How does lung volume affect resistance?

Answer 39

Increased volume decreases resistance

(Inversely proportional)

Question 40

How does the automomic system regulate airway resistance?

Answer 40

Sympathetic: bronchodilation decreases resistance

(via circulating catecholamines on bronchial smooth muscle)

Parasympathetic: bronchoconstriction increases resistance

(via Ach from vagus nerve on bronchial smooth muscle)

Question 41

How do we estimate physiologic dead space?

Answer 41

It is the tidal volume times the proportion of air

that does not participate in gas exchange:

VD = VT x (PaCO2 – PECO2 )/ PaCO2

Question 42

How do we measure P_aCO2?

(alveolar partial pressure of CO2)

Answer 42

We cannot meausure it directly

so we used P_ACO2 (arterial partial pressure of CO2)(via blood gas)

as a surrogate because they are closely related.

Question 43

How are P_ACO2 and P_AO2 related?

Answer 43

They are inversely proportional.

So when one increases, the other decreases.

Question 44

What are the regional differences in the V/Q ratio?

Answer 44

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)

Question 45

What is shunt

and what types of shunts are there?

Answer 45

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

Question 46

What are two causes of a V/Q mismatch?

Answer 46

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

Question 47

What parts of the brain control ventilation?

Answer 47

Medulla: rhythm generation

Pons: regulates medulla

Cortex: conscious and emotional response

See attached for details.

Question 48

How does respiratory information from the periphery get to the brain?

Answer 48

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.

Question 49

How do central chemoreceptors communicate with the brain?

Answer 49

Directly sense decreased pH in the CSF

Indirectly sense increased P_aCO2 in the blood

Question 50

Which chemoreceptor is the most important during normal breathing?

Answer 50

The central chemoreceptor.

Question 51

How do we assess respiratory function in a patient with dyspnea?

(What tests can we use?)

Answer 51

Spirometry to assess lung volumes, capacities re obstructive/restrictive disease

Pulse oximetry to indirectly assess PaCO2

Arterial blood gas (ABG) to assess gas exchange

Question 52

What is the alveolar-arterial gradient?

(A-a gradient)

Answer 52

The A-a gradient tells us how much inspired oxygen

is actually getting into the blood stream.

A-a gradient = [P_IO2 - (P_ACO2 / R)] - P_aO2

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.

Question 53

What affects the A-a gradient?

Answer 53

A-a gradient = [P_IO2 - (PA_CO2 / R)] - P_aO2

Alveolar P_CO2 is affected by: respiratory rate, tidal volume and dead space

Arterial P_O2 is affected by changes in inspired oxygen, poor diffusion across the respiratory membrane, R-L shunting and V/Q defects.