Pulmonary Physiology Flashcards
Tidal volume
Breath volumes during quiet breathing.
Functional residual capacity
Measured as the balance point where no work is done?? Lowest point of expiration during tidal breathing. Equal to expiratory reserve volume + residual volume.
Harder to measure- use helium dilution
Residual volume
Volume of air that remains in lungs after maximum expiration
Vital capacity
Difference between maximal inspiration and maximal expiration. Measured with gases expired.
Forced expiratory volume
Measured during force expiration test. Gives idea of lung health when plotted vs time. NORMAL EXPIRES MORE THAN 75% IN FIRST 1s.
What is the pattern of obstructive disease in forced expiration tests?
FEV1/FVC is reduced (can still exhale but takes longer so FEV1 is reduced)
What the the pattern of restrictive disease in forced expiration test?
- FVC is reduced
- FEV1/FVC is not reduced?
- May be due to decreased lung capacity OR increased residual volume
Diffusion capacity
How well gas diffuses across a membrane. Dictated by: 1. Pressure difference of gas 2. Surface area of membrane 3. Thickness of membrane 4. Also related to pulmonary blood volume (need hemoglobin to take up CO) Measured using carbon monoxide
What can cause reduced diffusion capacity?
- Decreased area, decreased hemoglobin, interstitial lung disease, pulmonary vascular disease
COPD (reduced area)
lung resection
Pus in airspace (pneumonia)
Swelling or fibrosis (increased thickness)
Decrease in pulmonary blood volume (emboli or HTN)
Total lung capacity
Volume of air in lungs at the end of a maximal inspiration. Equal to sum of tidal volume, reserve volumes, residual volume.
What is the major determinant of hemoglobin oxygen saturation (SaO2)?
The partial pressure of oyxgen dissolved in the blood (PaO2).
How is total oxygen content calculated?
Sum of hemoglobin saturation (SaO2) and free oxygen dissolved in plasma (PaO2).
On an oxygen dissociation curve, when will hemoglobin be maximally saturated? 75% saturated? 50% saturated?
> 60= over 90% saturated
40=75% saturated
27.5=50% saturated
In an oxygen dissociated curve, what are the advantages of:
- the flat part of the curve
- The steep part of the curve
- This means that even if the PO2 (atmospheric) drops, actual body O2 won’t really be affected (Ie. moving to a higher altitude). There will still be a large pressure difference between the alveoli and blood, driving diffusion.
- Peripheral tissues can get a lot of O2 with only a small change in the pressure of oxygen in the capillaries. Ie. this allows us to offload oxygen to tissues very quickly when running away from something.
How long does diffusion in the alveoli take and why is the physiologically advantageous?
Diffusion is pretty much finished by the time a RBC has gone 1/3 of the distance across the capillary.
This gives wiggle room- if blood is flowing rapidly (like in exercise), there is still enough time for diffusion to occur.
What factors affect the affinity of hemoglobin for oxygen?
- temperature (alters structure)
- pH (drives the binding equilibrium backwards)
- Partial pressure of CO2 (will compete for binding spots)
- Concentration of 2,3-diphosphglycerate in cells (metabolic enzyme that responds to low 02)
What is a “right shift” in the oxygen dissociation curve and what causes it?
A right shift means oxygen is unloaded more easily from hemoglobin at any capillary PO2
Caused by: heat, decreased pH (Acidic), increased PCO2 (ie. hot muscles that need more oxygen)
What is a “left shift” in the oxygen dissociation curve and what causes it?
A left shift means that oxygen is unloaded less easily. Also, a lower PO2 is needed to get Hb fully saturated- LOADING is easier.
How does carbon monoxide interfere with normal oxygen saturation?
- CO binds to hemoglobin with a much higher affinity that oxygen
- BUT it is measured similarly by a pulse oximeter
- SO the oxygen delivery will be decreased
- PLUS CO decreases tissue unloading of O2= BAD
How is carbon dioxide transported in the blood?
- As bicarbonate ions (HCO3-)
- Bound to hemoglobin (carbaminohemoglobin)
- Dissolved as a gas (CO2)
What is the A-a gradient and how is it useful clinically?
- A-a is the difference between the pressure of oxygen in the alveoli (PAO2) and the pressure in the arteries (PaO2). ASSESSMENT OF 02 TRANSFER EFFICIENCY**
- Normal A-a gradient is 5-10mmHg (increases with age)
- Can be used to differentiate between causes of hypoxemia (low PaO2)
What increases the A-a gradient?
- Diffusion defect (ie interstitial lung disease)
- V/Q mistmatch (ie emphysema, PE, pneumonia)
- R-L shunt- blood is bypassing ventilation
What is shunt?
Perfusion of of parts of the lung that are not ventilated- ie airway obstruction (or shunt in heart)
Low V/Q
This is difficult to correct with supplemental oxygen.
What is dead space?
Ventilation of lungs that are not perfused- ie PE
High V/Q
What is V/Q mistmatch?
When ventilated and perfusion are are not matching up!
This can occur a little bit physiologically (base of lungs more perfused)
Generally mismatch lowers the arterial PaO2= bad
What are the 5 mechanisms of hypoxemia?
- Low inspired oxygen (ie at altitude, low atmospheric pressure)
- Hypoventilation
- Diffusion limitation
- V/Q mistmatch
- Shunt
Define hypoxemia
Partial pressure of oxygen lower than 60mmHg
What is the alveolar gas equation?
PAO2 (alveolar pressure oxygen)= PIO2 (inspired pressure O2)-PAC02/R +F (alveolar pressure CO2 divided by a exchange ratio)
What are the muscles involved in breathing?
- Diaphragm- increases chest cavity (presses down during inspiration)
- External intercostals- pull ribs up and out (creates space during inspiration
- Internal intercostals- pull ribs down and in (expiration)
- Abdominals- expiration
Scalene, sternomastoids- accessory muscles
Which parts of breathing are active or passive?
Inspiration= always active Expiration= passive during rest, active during activity
What forces cause the lung to passively expand?
The increase in volume of the chest cavity creates negative pressure, which pulls the lung open.
What is compliance?
Volume change of the lung per pressure change
Fibrosis will decrease compliance
Emphysema will increase compliance
What forces cause the lung to contract?
- Alveolar recoil- snapping back into position after being expanded
What is the advantage of having a surfactant in the lung?
- Reduces surface tension with in the alveoli, making it easier to expand them during inspiration
- Makes alveoli less likely to collapse
- Reduces fluid drive into alveoli (keeps them from filling up!)
What forces are acting when the lungs are at the functional residual capacity?
- Lungs want to spring inwards
- Chest wall wants to spring outwards
- But forces are exactly balanced!
What does airway resistance depend on?
- Length of tube (longer=higher)
- Gas viscosity
- Radius of tube (smaller= higher)
Where is airway resistance greatest? What happens to it during breathing?
- Greatest in medium bronchi
2. Drops during inspiration as small airways are pulled open
What is the difference between anatomical and physiological dead space?
- Anatomical dead space are the places where gas exchange doesn’t occur- ie in the conducting airways. About 150 mL (of the 500 mL in each breath) is left behind here.
- Physiological dead space= volume of gas that does not eliminate CO2
1 and 2 should be the same in most healthy people.
What is minute ventilation?
Tidal volume x respiration rate/minute
How is CO2 pressure related to alveolar ventilation?
Volume of alveolar gas (ventilation)= inversely proportional to the PaCO2.
Ie. Decrease in ventilation will increase pressure of CO2
How do alveolar and extraalveolar vessels differ?
- Alveolar- exposed to alveolar pressure, compress when that increases
- Extraalveolar- pulled open by traction of lung parenchyma
What are the determinants of pulmonary blood flow?
- Gravity: more blood flow at base of lungs
- Hypoxic vasoconstriction: smooth muscle contracts to decrease blood flow to poorly ventilated areas
- Lung Volume
