Res applied science 105 wk 7

Primary function of the respiratory system

Continuous absorption of O2 and excretion of CO2

External respiration

Exchange of gases between the external environment and the 'distributing system': between the alveoli of the lungs and the blood

Internal respiration

Exchange of gases between the cells of the body and the blood (at the tissue level)

Respiration vs ventilation

Respiration = gas exchange (as above); ventilation = air in/air out

Alveolar-capillary (AC) membrane

A large but very thin blood-gas barrier created by the pulmonary circulation and alveoli; allows gas exchange by simple diffusion

Surface area of the AC membrane (notes)

Massive: about 80 square meters in an adult (plus about 70 m of vascular lining)

Barriers to diffusion at the AC membrane

Alveolar epithelium, interstitial space, and capillary endothelium

Fick's First Law of Diffusion

Describes bulk movement of gas through a biologic membrane; the greater the surface area, the greater the diffusion

Respiration includes

Both gas exchange AND gas transport

Ways disease can impair respiration

Impair gas exchange (e.g., alveoli filled with secretions), impair gas transport (e.g., blood clot), or both

Diffusion (in gas exchange)

Movement of gases in response to a pressure gradient

Gradient

A change in the value of a quantity (concentration, pressure, or temperature) with a change in another variable, usually distance

Pressure gradient

A change in pressure over a distance

No gradient means…

No movement of gas

Partial pressure

In a mixture of gases, the pressure each gas contributes (its OWN pressure) to the total pressure of the system

Gas pressure vs gas tension

They are the same thing

Atmospheric CO2 (notes)

About 0.04-0.05%

Normal inspired PO2 (PiO2) at sea level

159 mmHg (from 21% O2)

O2 pressure cascade

PiO2 159 mmHg, to alveoli about 100 mmHg, to capillaries 40 mmHg or less, to intracellular about 5 mmHg

Intracellular PO2

About 5 mmHg (the final gradient for O2 diffusion into the cell)

CO2 diffusion gradient vs O2

Opposite of the O2 gradient

PCO2 in cells vs room air

Highest in cells (about 60 mmHg); lowest in room air (about 1 mmHg)

What does the reverse CO2 cascade do?

Moves CO2 from tissues into venous blood, transported to the lungs, and out to the atmosphere with ventilation

Alveolar PCO2 (PACO2): what does it vary with?

Directly with CO2 production (more production = higher PACO2) and inversely with alveolar ventilation (more ventilation = lower PACO2)

Alveolar CO2 equation

PACO2 = (VCO2 x 0.863) / VA

PACO2 equation: VCO2

Rate of CO2 production (mL/min at STPD)

PACO2 equation: VA

Alveolar ventilation (L/min at BTPS)

PACO2 equation: what is 0.863?

Factor to correct from dry gas at 0°C and 760 mmHg to saturated gas at body temperature and ambient pressure

0.863 vs 863

Use 0.863 when VA is in L/min; use 863 when VA is in mL/min (keep units consistent)

Example: VCO2 200 mL/min, VA 4315 mL/min; PACO2?

PACO2 = (863 x 200) / 4315 = 40 mmHg

Capital A vs lowercase a (notes)

Big A = alveoli (PAO2, PACO2); little a = arterial (PaO2, PaCO2)

PACO2 will increase if

CO2 production increases or alveolar ventilation decreases

PACO2 will decrease if

CO2 production decreases or alveolar ventilation increases

Normal range kept by respiratory control mechanisms (PACO2)

35 to 45 mmHg

Effect of CO2 on pH (notes)

CO2 up = pH down (more acidic); CO2 down = pH up (more alkalotic)

Sedation effect on CO2 (notes)

Sedation can decrease the respiratory rate, raising CO2

COPD and CO2 compensation (notes)

COPD patients can't compensate by breathing fast, so the body holds bicarbonate to balance CO2

PAO2

Alveolar partial pressure of O2

PAO2 is determined by

PiO2 (pressure of inspired O2), and water vapor and CO2 in the lungs

Why calculate PAO2?

PaO2 should be close to PAO2 if the lungs are doing their job properly

Alveolar air equation

PAO2 = [FiO2 x (PB - 47)] - (PACO2 / 0.8)

Alveolar air equation: FiO2

Fraction of inspired O2 (as a decimal)

Alveolar air equation: PB

Barometric pressure (mmHg)

Alveolar air equation: 47

Water vapor tension (mmHg) at 37°C

Alveolar air equation: 0.8

Normal respiratory exchange ratio (R)

Respiratory exchange ratio (R)

Ratio of CO2 excretion to O2 uptake; normally 0.8; dividing by 0.8 corrects for the difference between O2 and CO2 movement in and out of the alveoli

Example: FiO2 0.21, PB 760, PaCO2 40; PAO2?

[0.21 x (760 - 47)] - (40 / 0.8) = 149.73 - 50 = 99.73 mmHg

Simplified alveolar air equation

PAO2 = FiO2 (PB - 47) - PaCO2; the R correction can be dropped if the patient is on 60% O2 or more

Hypothermia and the 47 mmHg value (notes)

If a patient is hypothermic or intentionally cooled, water vapor tension would not be 47 mmHg

What gases are in the alveoli?

CO2, O2, water vapor, and nitrogen

Role of nitrogen in the alveoli

Inert, no role in gas exchange, but it takes up space and exerts pressure

Which alveolar partial pressures change?

Only O2 and CO2 (water vapor and nitrogen stay generally constant)

Conclusion 1 (alveolar gases)

If FiO2 is constant, PAO2 varies inversely with PACO2

Rule of thumb on room air

PAO2 + PACO2 = about 140 mmHg (100 + 40)

Example: room air, PACO2 rises 40 to 60 mmHg

PAO2 falls by about 20 mmHg (to keep the sum at 140), assuming R is constant

Conclusion 2 (alveolar gases)

PACO2 varies inversely with alveolar ventilation: less ventilation raises PACO2 and lowers PAO2; more ventilation lowers PACO2 and raises PAO2 (assuming constant CO2 production)

Limits on increasing ventilation

Neural control mechanisms and work of breathing; PACO2 does not drop much below 15 to 20 mmHg

Maximum PaO2 on room air during hyperventilation

No more than about 120 mmHg at sea level; higher values require supplemental O2

Pulmonary diffusion gradients

O2 moves from alveoli to capillaries; CO2 moves from capillaries to alveoli; a pressure difference is required

What does ventilation maintain?

Mean alveolar pressures at about 100 mmHg for O2 and about 40 mmHg for CO2

Arterialized blood

Blood that has equilibrated with alveolar pressures (gives up CO2 and loads O2 until pressures are equal)

Normal alveolar PO2 and PCO2

PAO2 about 100 mmHg; PACO2 about 40 mmHg

Normal venous blood PO2 and PCO2

PvO2 about 40 mmHg; PvCO2 about 46 mmHg

Pressure gradient for O2 across the AC membrane

About 60 mmHg (100 - 40)

Pressure gradient for CO2 across the AC membrane

About 6 mmHg (46 - 40); about 1/10th of the O2 gradient, yet CO2 moves easily into alveoli

Why does CO2 move easily despite a small gradient?

CO2 diffuses very readily (much more soluble than O2)

Time limits to diffusion

Blood must spend enough time in contact with the alveoli to be fully oxygenated; diffusion time depends on pulmonary blood flow

What happens to diffusion time when pulmonary blood flow increases?

Less time for O2 to move; healthy lungs still reach equilibrium, but diseased lungs may not (e.g., exercise, fever, septic shock)

Systemic (tissue) diffusion gradients

Opposite of lung gradients: metabolism uses O2 so O2 diffuses from blood to cell; metabolism produces CO2 so CO2 diffuses from cell to blood

Tissue and blood values at the systemic level

Blood PO2 about 100 mmHg, cell PO2 less than 40 mmHg; cell PCO2 more than 46 mmHg, blood PCO2 about 40 mmHg

Blood leaving the tissues

PO2 about 40 mmHg and PCO2 about 46 mmHg (venous blood)

What do arterial and venous blood reflect?

Arterial blood reflects pulmonary gas exchange; venous blood reflects events occurring in the tissues