ASEN 5016 Lecture 4: Respiration and the Oxygen Cascade

OBJECTIVES

1. Define Standard Atmosphere Pressure and Composition

2. Describe Respiration Factors Associated with Increasing Altitude / Decreasing Atmospheric Pressure

3. Identify Critical Parameters for Normoxic Conditions

4. Describe the Oxygen Cascade and Complicating Factors

5. Explain Engineering Trade Factors wrt Cabin Pressure

6. Discuss Current Research into Effects of Space Flight on Pulmonary Function

1. Standard Atmosphere Pressure and Composition

Normoxic Conditions

· 3.1 psia ppO₂ (160 torr)

· 21% of 1 standard atm (14.7 psia or 760 torr)

"Shirt Sleeve" Environment

Dalton’s Law of Partial Pressure

Partial pressure of gas in a mixture = total pressure if it was the only gas in same volume

2. Factors Associated with Increasing Altitude

Exponential Pressure Decay

· 14.7 psia at sea level

· ~ 10 psia at 10,000 feet MSL

· ~ 6.7 psia at 20,320 feet MSL

· At ~15 km (P = 87 torr) à 100% oxygen needed, since combined water vapor and CO2 in the lung = total barometric pressure
Above this point, gas exchange becomes physiologically impossible (~transition from hypoxia to anoxia)

· Above 19.2 km (P = 47 torr) à body fluids boil at 37° C (Armstrong limit)

Discuss Earth’s equatorial ‘bulge’ and ‘Physiological Altitude’ equivalence

3. Critical Parameters for Normoxic Conditions

· Alveolar Oxygen Pressure (PaO₂)

The PaO2 relationship given below defines the partial pressure of oxygen in the alveoli as a function of various environmental parameters.

PaO2 = [FiO2 (Pb – PH2O)] – PaCO2 [FiO2 + (1 - FiO2)]

R

where:

PaO2 = mean alveolar oxygen pressure

FiO2 = fraction of oxygen in the inspired air

Pb = ambient barometric pressure

PH2O = water vapor pressure at 37 C

PaCO2 = mean alveolar pressure of CO2

R = respiratory exchange ratio

When PaO2 reaches 60 mmHg, the blood is only 87% saturated with oxygen and hypoxia sets in.

CO2

· CO2 levels on Earth

~350 - 380 ppm now

Was ~280 ppm pre-industrial revolution era

· Effects on respiration

CO2 Diffuses ~20x faster than O₂

Too much = blood becomes acidic – no O₂ – fainting

Too little = alkaline – hyperventilation – fainting

· CO2 limits in buildings

ASHRAE standard 62-1989 recommends a maximum CO2 level of 1000 ppm. The proposed new OSHA standard (29CFR Part 1910.1033) calls for investigation if the CO2 level exceeds 800 ppm of CO2.

· CO₂ runs in the 2000-3000 ppm range in the orbiter with 10,000 (1%) = safety limit

4. The Oxygen Cascade

· O2 Cycle

· Acquisition – diffusion (breathing)

· Exchange with CO2

· Transportation – blood flow

· Delivery – RBC (takes ~0.75 sec to traverse capillary and ~0.5 sec for diffusion exchange)

· Utilization – Mitochondria

· Byproduct – CO₂

O2 Cascade

· Atmosphere / Lung / Alveoli / Capillaries / Arteries / Capillaries / Tissue (mitochondria) / Veins

· ppO2 is highest in the air, lower in the lungs where it is diluted with CO2 and H2O, and continues to drop in pp down through the cascade

– diagram corresponding delta P across each

Lungs

Tidal volume ~500 ml in/out with each normal breath
Only about 350 ml of new air reaches the alveoli with each breath
Expired air is still about 16% O2 (sea level)
Inspiratory reserve ~ 3000 ml – max extra volume that can be inspired over / above tidal
Inspiratory capacity ~3500 ml total
Expiratory reserve ~ 1100 ml – max extra vol that can be expired after normal breath
Residual volume ~ 1200 ml – remainder after max forceful expiration
Vital capacity = inspiratory reserve (3000) + tidal volume (500) + expiratory reserve (1100) = (~4600 ml) = max out after max in
Total lung capacity = ~5800 ml (vital + residual) max volume that lungs can be expanded
Additional (anatomic) dead space ~ 150 ml (everything outside the aveoli)
Physiological dead space – some alveoli non-functional in pulmonary disease (fluid or foreign material - pulmonary edema, pneumonia, etc.)

(All volumes are ~20-25% less for women and greater in large and athletic individuals)

Normal respiration rate = 12 breaths / min
Normal respiratory volume ~ 6 liters / min (~2 gal/min)
Max resp rate = 40-50 breaths / min or > 200 liters / min (~53 gal/min)

Diffusion area of the lungs ~750 Sq ft (~100x larger than the chest volume alone)

Lungs literally ‘float’ in the thoracic cavity, surrounded by pleural fluid that acts as a lubricant as the chest expands and contracts

Air rises to within ~ 1 deg F of normal body temp and within 2-3% of H2O saturation in the trachea

· TLC = RV + ERV + TV + IRV

– Total Lung Capacity

– Residual Volume

– Expiratory Reserve Volume

– Tidal Volume

– Inspiratory Reserve Volume

Discuss transport phenomena of O2 and CO2 in the blood…

Factors Affecting O2 Cascade

In the lungs à ppO₂ too low to diffuse across lung (or total pressure below 87 torr)
Lungs to blood à fluid or foreign material in alveoli
In the blood à too few or malfunctioning RBC’s
Capillaries à ppO₂ too low to diffuse into cells
In the cells à mitochondria not functioning (disease)

Too little O2 = hypoxia

Low light vision degraded
Physical performance ability decreased
Mental task capacity diminished
Motor skills impaired
Long term effects ?????

Too much O2 = toxicity

· Exposure time and pressure dependent

· 100% O₂ at SL for 6-24 hours can become toxic (chronic issues)

· Apollo astronauts lived for 2 weeks at 5 psia 100% O₂

· Saturation / deep diver concerns

· 4 atm O2 leads to acute toxicity, coma likely within 30-40 minutes

Causes of reduced O2 uptake, some potentially relevant to space flight conditions:

ppO2

Deviation from “normoxic” standard atmosphere, sea level conditions (see above)

Presence of other gasses

CO2 - (see above)

CO - Forms a bond 200x stronger than O2 with hemoglobin

Symptoms include headache, nausea, confusion, sleepiness, dizziness, or longer term, memory loss, neurological or psychological problems

Usually reversible if caught in time, dependent on how much and how long

Even low levels of CO can lead to permanent damage to heart, brain and CNS, however

When people are said to be ‘overcome by smoke’ in a fire, the culprit is CO, which can range from 10k-50k ppm

just a few breaths can lead to 100% blood saturation and quickly result in unconsciousness or death

(cigarette smoking exposes lungs to 1k-2k ppm CO, smokers typically have CO blood levels just below concentration considered poisonous)

At altitude (reduced total pressure) CO has additional advantages over O2

Binding rate elevated and effects more pronounced of same CO concentration at sea level due to lack of available O2

Transport

Too few RBC’s (anemia)

Reduced blood flow

Obstruction / disease in the lungs / alveoli

Fibrosis or emphysema

Pneumonia

Asthma

H.A.P.E.

Asbestosis

Mechanical aspects

Constriction of chest cavity (reducing tidal volume of lungs)

Diaphragm malfunction

Physiological effects

Environmental effects (aerosols)

Gravity?

‘Train low / sleep high’ athletic performance enhancement concept

5. Engineering Trade Factors related to Oxygen (and Total Pressure) Levels

Engineering (and Physiological) Trade Variables

· Maintain normoxic (i.e., neither hyperoxic nor hypoxic) conditions

· Minimize flammability (< 30% O2)

· Sufficient density for convective cooling

· Minimize risk of DCS (caisson disease, the bends) during transition to EVA

· Ensure sufficient pressure shell structural strength

· Compensate for cabin leakage

· Remove CO₂ and humidity

Shuttle Atmosphere

“An average of 1.76 pounds of oxygen is used per flight crew member per day. Up to 7.7 pounds of nitrogen and 9 pounds of oxygen are expected to be used per day for normal loss of crew cabin gas to space and metabolic usage.”

Critical relationship between cabin pressure, physiology, vehicle structure and material, launch mass, ECLSS performance, space suit ops, etc…

6. Research into Effects of Space Flight on Pulmonary Function

Issues primarily related to cephalic fluid shift / altered blood circulation / lung geometry

J Appl Physiol 2000 Sep; 89(3):1239-48 Glenny RW, Lamm WJ, Bernard SL, An D, Chornuk M, Pool SL, Wagner WW Jr, Hlastala MP, Robertson HT.

…both gravity and the geometry of the pulmonary vascular tree influence regional pulmonary blood flow - Selected contribution: redistribution of pulmonary perfusion during weightlessness and increased gravity.

Microgravity and the lung. Prisk GK. Appl Physiol 2000 Jul; 89(1):385-96J

Diffusing capacity increases markedly, due to increases in both pulmonary capillary blood volume and membrane diffusing capacity, likely due to more uniform pulmonary perfusion. Both ventilation and perfusion become more uniform throughout the lung, although much residual inhomogeneity remains. Despite the improvement in the distribution of both ventilation and perfusion, the range of the ventilation-to-perfusion ratio seen during a normal breath remains unaltered, possibly because of a spatial mismatch between ventilation and perfusion on a small scale. There are unexpected changes in the mixing of gas in the periphery of the lung, and evidence suggests that the intrinsic inhomogeneity of the lung exists at a scale of an acinus or a few acini. In addition, aerosol deposition in the alveolar region is unexpectedly high compared with existing models.

J Appl Physiol 89:1 379-384, 2000, Historical Perspectives: Physiology in microgravity, John B. West

Pulmonary function is greatly altered but apparently not seriously impaired.

The lung is exquisitely sensitive to gravity, which normally causes regional differences in ventilation, blood flow, gas exchange,alveolar size, intrapleural pressure, and parenchymal stress.Therefore, it is not surprising that a variety of changes in pulmonaryfunction have now been described. The distributions of ventilationand blood flow become more uniform, although not entirely so,and there are reductions in some lung volumes, including functionalresidual capacity and residual volume. However, overall, gas exchangeis little affected. One of the reasons for monitoring pulmonaryfunction during spaceflight is because of the vulnerability ofthe lung to changes in the atmosphere of the spacecraft. A dramaticexample was a fire that occurred on Mir, which, fortunately, wasrapidly brought under control. Aerosol contamination of the spacecraftis another potentialhazard.