Pulmonary Physiology 2 (9/25a) [Biomedical Sciences 1] Flashcards
Partial Pressure
Proportional to concentration in a gas mixture
The pressure a gas would exert if it occupied the entire volume of the mixture
The driving force for diffusion of gases
PA= alveolar pressure Pa= arterial blood pressure Pv= venous blood pressure
Pressure units are millimeters of mercury (mmHg)
Partial Pressure Equation
Px = Pb * F
Px = partial pressure Pb = barometric pressure F = fractional concentration
Fractional concentration of oxygen in air is 21% (0.21)
PO2= (Pb - Pwater vapor)*0.21
Why is PO2 of dry air 160?
PO2 = 760 mmHg * 0.21 = 160 mmHg
Why is PO2 of tracheal air 150?
Pressure of water vapor is 47 mmHg
PO2= (760-47) * 0.21 = 150 mmHg
Why is PAO2 100 mmHg?
Reflects balance between O2 entry to and exit from alveoli
Why is PACO2 40 mmHg?
CO2 diffuses into alveoli from venous blood
Why does PAO2 = PaO2 and PACO2 = PaCO2?
Arterial blood equilibrates with alveolar air under normal circumstances
Diffusion Rate
Vx = DA (ΔP/Δx)
Vx = flow of gases per unit time D = diffusion coefficient A = surface area ΔP = pressure gradient Δx = thickness of the membrane
As pressure gradient increases, diffusion will ___
increase
As surface area increases, diffusion will ___
increase
As membrane thickness increases, diffusion will ___
decrease
Blood equilibrates within the first ____ of the length of the capillary
⅓
You could cut the length in half and the blood would still fully equilibrate
Normally, O2 equilibrates quickly, so PaO2 = PAO2
In fibrosis, thickening of ___-___ ___ slows O2 exchange so PaO2 < PAO2
blood-gas barrier
Can have diffusion limitation
How much oxygen is dissolved in plasma
Oxygen dissolved in the plasma is a really small component
- 2% of total O2 content
- Measured by PO2
If PaO2 = 100 mmHg, [O2]a = 0.3 mL O2 / 100 mL blood
- If CO=5 L/min, 15 mL O2/min delivered to tissues
- If resting metabolic rate= 250 mL O2/min, we have 4 sec before we are anoxic
How much oxygen is bound to hemoglobin
98% of total O2 content
4 binding sites on each Hb molecule
Measured by % saturation (SaO2)
Blood O2 Content
O2 Content = (Constant * [Hb] * % Saturation) + (Solubility * PO2)
O2 content = 20.3 mL O2 / 100 mL blood
Oxy-Hemoglobin Dissociation Curve
relates SaO2 and PaO2
In areas of high PaO2, large changes in PaO2 correspond to small changes in SaO2 (EX: in the lungs)
-Allows better “loading” of O2 in the lungs
In areas of low PaO2, small changes in PaO2 correspond to large changes in SaO2 (EX: in the tissues)
-Allows better “unloading” of O2 in the tissues
Oxy-Hb Curve - Affinity and P50
Decreased affinity = easier unloading of O2 to tissues
P50 defines affinity of Hb for O2, the PO2 at 50% Hb saturation
Normal P50 = 25 mmHg
-50% of Hb binding sites are occupied when PaO2 = 25 mmHg
Facilitates unloading of O2 to exercising tissues → increased oxygen release to muscle tissues
Oxy-Hb Curve - Decreasing Affinity
Increased temperature and Bohr effect → decreases affinity of Hb for O2, shifting the curve to the right
Bohr effect → increased PCO2 and decreased pH can cause right shift
Increased P50 actually means decreased affinity, Hb will let go of O2 much more easily
EX: if P50=37 mmHg→ 50% of Hb binding sites are occupied when PaO2 = 37 mmHg, Need higher PaO2 to fill 50% of Hb binding sites, so Hb is less sticky to O2
3 Modes of carbon dioxide transport
Dissolved in plasma
- 5% of total
Bound to hemoglobin
- Binds at a different site than O2
- 3% of total
Chemically modified form
- Protons (H+) and bicarbonate (HCO3-)
- 92% of total
Transport of CO2
Where PCO2 is high (EX: tissues), reaction goes to the right
Where PCO2 is low (EX: lungs), reaction goes to the left
When bicarbonate levels rise, chloride shift occurs
Hypoxemia
PaO2 < 80 mmHg
Sensed by peripheral chemoreceptors in carotid and aortic bodies
Less sensitive than central chemoreceptors
Causes
- Breathing hypoxic gas
- Hypoventilation
- Diffusion limitation (fibrosis)
- Ventilation/perfusion is not matched (most common and important in lung disease)
Hypoventilation
PaO2 and PAO2 fall, PaCO2 rises
Decreased ventilatory drive (brain damage, drugs)
Paralysis/weakness of ventilatory muscles
Damage to chest wall
Hypercapnia
PaCO2 > 45 mmHg
Sensed by central chemoreceptors in brain stem
Very sensitive to pCO2 and pH of cerebrospinal fluid
Most important regulator of ventilation
Abnormalities in blood gases are processed by
respiratory centers in medulla and pons
Increased ventilatory drive in response to abnormal blood gases
Exhalation of more CO2 → PaCO2 returns to normal
Inhalation of more O2 → PaO2 returns to normal
Hypoxemia vs Anemia
If normal O2 content is 20 mL / 100 mL blood and normal [Hb]=15
Hypoxemia → PaO2=60 mmHg, SaO2 = 90% → O2 = 19 mL O2 / 100 mL blood
Anemia → [Hb]=10 → 13.9 mL O2 / 100 mL blood
- O2 content is much more affected by [Hb]
- Anemia can have a huge effect on oxygen content even with normal PaO2 and SaO2
Ventilation and Perfusion
Ventilation (V) → air in L/min
Perfusion (Q) → cardiac output in L/min
V/Q defines the relationship between air flow and cardiac output
-Normal V/Q is 4/5 → 0.8
Ventilation and perfusion gradients due to gravity
Ventilation: base→ apex
Blood flow: base→ apex
Shunt (airway obstruction)
Perfusion is normal, but ventilation is zero
Blood goes through lung region, but no gas exchange → it is just like venous blood
Dead space (pulmonary embolism)
Ventilation is normal, but perfusion is zero
Distribution of Perfusion
Pulmonary blood flow affected by:
1) Gravity
- Flow at bases > flow at apex
2) Pressure in
- Pulmonary artery (Pa), Pulmonary vein (Pv), Alveoli (PA)
Capillaries in apex are underperfused/overventilated, while those in base are overperfused/underventilated
Distribution of Ventilation
Ventilation is also affected by gravity, but not as much as perfusion is
Alveoli at apex are pulled open at rest → very small increase in volume during inspiration
Alveoli at base → much larger increase in volume during inspiration
Flow of blood through zones
Mixed venous blood→ base→ zone 3→ zone 2→ zone 1→ apex→ inspired air
When V/Q is high
ventilation is greater than flow
blood coming out of lung looks more like atmospheric/inspired air
When V/Q is low
ventilation is less than flow
blood coming out of lung looks more like venous blood
Zone 1 (V/Q)
(closer to apex)
V lower Q lowest V/Q highest (3) PaO2 130 PaCO2 28
Zone 2 (V/Q)
V mid Q mid V/Q mid (0.8) PaO2 100 PaCO2 40
Zone 3 (V/Q)
(closer to base)
V higher Q highest V/Q lowest (0.6) PaO2 89 PaCO2 42
Pulmonary system exchanges oxygen and carbon dioxide between ___ and ___
cells and environment
Increased minute ventilation to account for needed increase in O2 and CO2 during exercise
REST
- VO2 250 mL/min
- VCO2 240 mL/min
- Minute ventilation 5 L/min
EXERCISE
- VO2 5000 mL/min
- VCO2 3000 mL/min
- Minute ventilation 100 L/min
Ventilation and moderate exercise
Phase 1: fast
-Probably neural in origin
Phase 2: primary
Phase 3: steady state
Ventilation increased in proportion to VO2 and VCO2 → maintains constant PaCO2 → Well matched to oxygen consumption and carbon dioxide production
In pathological conditions, defects in V/Q are a major cause of ___
hypoxemia
