Gas Exchange

Question 1

Normal values for flow (uptake)

Answer 1

Resting:

Vdot CO2 = 0.2 L/min

Vdot O2 = 0.25 L/min

R = 0.8

Maximal Exercise:

Vdot CO2 = 5.6 L/min (28X)

V dot O2 = 4 L/min (16X)

R = 1.2

Question 2

Gas Laws

Answer 2

Boyle’s Law: P1V1 = P2V2

Charles’ Law: V1/T1 = V2/T2

Universal gas law: PV = nRT

Dalton’s law: total pressure of a gas is sum of partial pressures of each gas

Question 3

Normal partial pressures of gas in atmosphere

Answer 3

N2 = 79% = mmHg

O2 = 21% = 100 mmHg

Question 4

Normal partial pressure of gas in human lung

Answer 4

N2 = 79% = 573 mmHg

CO2 = 5% = 40 mmHg

O2 = 16% = 100 mmHg

H2O = 47 mmHg

Question 5

ATPS, BTPS and STPD

Answer 5

ATPS: ambient temperature and pressure, saturated; saturation with water vapor

BTPS: body temperature and pressure, saturated; saturation with water vapor (used for lungs!)

STPD: standard temperature and pressure, dry; no water vapor

Question 6

Equation to get flow (Vdot) from volume

Answer 6

Vdot = Volume x f_R

Vdot = Volume x rate (breaths per minute)

(Normal total minute ventilation Vdot E = 8-12 L/min)

Question 7

What is anatomical dead space?

Answer 7

Volume of conducting airways (gas exchange does not occur here)

(trachea, bronchi, etc)

Question 8

What is alveolar dead space?

Answer 8

When you have an alveolus that is not perfused, so doesn’t contribute to gas exchange (has a capillary next to it but the capillary has no blood)

Question 9

What is physiological dead space?

Answer 9

Anatomical dead space + alveolar dead space

Total volume of lungs that does not participate in gas exhcange

Question 10

What happens to alveolar ventilation if you have rapid, shallow breathing?

Answer 10

At given level of total minute ventilation, you’ll have less alveolar ventilation if you have rapid shallow breathing

This is because your dead space ventilation increases

Air stays in your alveoli and you’re “ventilating” the trachea, bronchi, terminal bronchioles, places where gas exchange can’t occur –> more wasted ventilation

Question 11

Respiratory exchange ratio (R)

Answer 11

R = VCO2/VO2 = 200/250 = 0.8

Note: more O2 uptake than CO2 blown off

Question 12

Alveolar ventilation (V_A dot)

Answer 12

V_A dot = V_E dot - V_D dot

Alveolar minute ventilation: volume of air per minute moving in and out of the alveolar compartment

Question 13

Equation for physiological dead space

Answer 13

V_{D(physiological)} = V_T x (P_aCO2 - P_EbarCO2)/P_aCO2

CO2 concentration breathed out into bag is less than partial pressure of CO2 in arterial circulation due to the dilution of exhaled air by the physiological dead space air

Dead space volume is 25-33% of tidal volume at rest, 10-20% during exercise and 45-50% in obstructive diesease or pulmonary vascular occlusive disease

Question 14

Equation for partial pressure of O2 in alveoli using alveolar ventilation

Answer 14

P_AO2 = P_IO2 - (863 x VdotO2)/VdotA

Question 15

Normal alveolar ventilation

Answer 15

VdotA = 4 L/min

Question 16

Hyperpnea and hypopnea

Answer 16

Increase or decrease in ventilation

Question 17

Apnea, tachypnea and bradypnea

Answer 17

No breathing, rapid (often shallow), slow breathing

Question 18

Hyperventilation and hypoventilation

Answer 18

Hyperventilation: Increase in ventilation out of proportion to any increase in metabolic CO2 production/flow, and therefore causes low arterial PCO2

Hypoventilation: Decrease in ventilation out of proportion to any decrease in metabolic CO2 production/flow, and therefore causes high arterial PCO2

Question 19

Alveolar hypoventilation

Answer 19

Leads to hypoxemia and hypercapnia (too much CO2 in blood)

Seen in diseases that affect medullary respiratory center, respiratory neuromuscular function, severe obstructive or restrictive pulmonary disease

Question 22

3 things that govern diffusion according to Fick’s law

Answer 22

1) Partial pressure gradient
2) Diffusion coefficient
3) Geometry of the interface (amount of surface area)

Question 23

Diffusion of O2 and CO2 within the alveolus and across the alveolar capillary membrane

Answer 23

Within alveolus: O2 diffuses 18% faster than CO2 because 2 has smaller MW (diffusion is inversely proportional fo square root of MW = Graham’s Law)

Across alveolar-capillary membrane: for getting into alveoli from blood, CO2 is 19 times more diffusible than O2 in liquid phase because CO2 is way more soluble (Henry’s Law: VO2/Vol Plasma = PO2 x alpha)

Question 23

Normal gas tensions

Answer 23

Mixed venous blood (coming into lung): PO2 = 40; PCO2 = 46

Alveolar = arterial: PO2 = 100; PCO2 = 40

Question 24

Time for complete diffusion of O2 and CO2 across alveolar-capillary membrane

Answer 24

CO2 goes from pulm capillary to alveoli and takes <0.1 sec for Alveolar CO2 to equal arterial CO2 (as a result can set PaCO2 = PACO2)

O2 goes from alveoli to pulmonary capillaries and takes 0.33 sec for arterial O2 to approximately equal Alveolar O2 (but Alveolar O2 always 1 mmHg higher than arterial O2 due to diffusion, and 9 mmHg higher because of venous admixture)

Question 24

Factors causing diffusion impairment

Answer 24

1) Decreased pulmonary capillary transit time (exercise)
2) Increased diffusion path length (alveolar proteinosis, pulmonary edema, interstitial inflammation or fibrosis, pneumocystis carinii pneumonia)
3) Reduction of functioning pulmonary capillary bed (pulmonary embolism, emphysema, pulmonary vascular disease)
4) Reduced alveolar PO2 (high altitude)

Question 25

What can cause reduced PO2?

Answer 25

High altitude

Hypoxic gas mixture given to patient

Alveolar hypoventilation

Question 26

Can diffusion impairment affect PCO2?

Answer 26

NO, mostly just O2!

Because any change in PaCO2 is corrected by ventilatory control mechanisms

Question 27

Hypercapnia

Answer 27

CO2 retention

PaCO2 greater than normal (greater than 40 mmHg)

Question 28

Hypoxemia

Answer 28

PaO2 less than normal (less than 90-100 mmHg)

Question 29

Oxygen content

Answer 29

Total amount of oxygen carried in the blood

1) Dissolved in solution (very little)
2) Attached to hemoglobin (a lot)

C_aO2 or C_vbarO2

Same thing for CO2 content

Question 30

How do you measure diffusing capacity of the lung?

Answer 30

Hard to measure PO2, so we use CO.

Inhale 0.3% CO, hold for 10 sec –> CO diffuses into capillaries –> measure concentration of CO inhaled then exhaled to get “diffusing capacity of lung for CO”

Question 31

Factors that increase or decrease diffusing capacity of the lung

Answer 31

Increase: body size (SA, Vc), lung volume (SA), exercise, supine (Vc)

Decrease: age, anemia (theta, Vc), diffusion impairment, resection (SA)

Question 32

O2 saturation (SO2)

Answer 32

Percentage of Hb-O2 capacity occupied with O2

Normal arterial blood: 97%

Hypoxic arterial blood: 90%

Mixed venous blood: 75%

Question 33

Right shift of Hb-O2 dissociation curve

Answer 33

Increase delivery of O2 to tissue; let go of O2 easier

1) Increased temperature (exercise)
2) Increased 2,3-DPG
3) Increased PCO2 and increased [H+] (more acidic)

Question 34

Left shift of Hb-O2 dissociation curve

Answer 34

Decreased delivery of O2 to tissure; hold on to O2 better

1) Decreased temperature
2) Decreased 2,3-DPG
3) Decreased PCO2 and decreased [H+] (more basic)

Question 35

Normal hemoglobin concentration in blood

Answer 35

[Hb] = 15 g/100mL

Note: 15 g/100mL x 1/34 mL O2/g Hb = 201 mL O2/L = O2 capacity at normal hemoglobin concentration

Question 36

How do anemia and carbon monoxide poisoning affect O2 content of the blood?

Answer 36

Anemia: lower [Hb] means less O2 content in blood

CO poisoning: CO binds to Hb molecule and actually increases affinity of Hb for O2 so it binds O2 but doesn’t let it go so no O2 gets to tissues (left shift); have normal PO2 but very low O2 content of blood (bc CO binds Hb instead of O2)

Question 37

Where is CO2 during transport?

Answer 37

5% dissolved CO2 in plasma

90% HCO3- in plasma

5% bound to amino groups of hemoglobin

Question 38

What is the Cl- shift (Hamburger shift)?

Answer 38

HCO3- is produced in the RBC and has to get back out to plasma. HCO3- is exchanged for bringing Cl- into the cell (to maintain electroneutrality)

Question 39

When does the amino group of hemoglobin bind CO2 best? (Haldane effect)

Answer 39

Amino group of Hb binds CO2 best when O2 is NOT bound

Deoxygenated hemoglobin binds CO2 best, and so deoxygenated blood can carry more CO2 than oxygenated blood

Haldane effect

Facilitates loading and unloading of CO2

Question 40

Bohr effect

Answer 40

Effect of PCO2 and pH on the Hb-O2 dissociation curve

Low PCO2 and low [H+] mean higher affinity for O2 = arterial blood coming from lungs

High PCO2 and high [H+] mean lower affinity for O2 = mixed venous blood in tissues (tissue have higher [H+] because of metabolism)

Ability to give up O2 in tissues and bind O2 avidly in the lungs

Question 41

Alveolar Gas Equation

Answer 41

P_AO2 = P_IO2 - P_ACO2/R

Question 42

How does alveolar ventilation (Vdot A) depend on metabolic rate of O2 and CO2 (Vdot O2 and Vdot CO2) and partial pressures in alveoli (PAO2 and PACO2)?

Answer 42

Vdot A directly proportional to metabolic rate: if CO2 produced or O2 needed more, will increase ventilation

Vdot A inversely proportional to alveolar partial pressure of O2 and CO2: if alveolar pressure of CO2 is high, won’t be able to push as much out (but explanation for O2..?)

Question 43

Alveolar ventilation equation

Answer 43

PaCO2 = 863 x (VdotCO2/VdotA)