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Biochemistry > Control of Carbon Dioxide and Oxygen (B2: W7) > Flashcards

Control of Carbon Dioxide and Oxygen (B2: W7) Flashcards

1
Q

Which sensors in the blood control CO2 levels?

A
  • Central chemoreceptor in the medulla
    • An increase in PaCO2 flowing to the medulla causes an increase in ventilation
  • Peripheral chemoreceptor in carotid and aortic bodies
    • Drives breathing when PaO2 falls below 60 mmHg
    • Has little effect on the breathing of a normal person resting at sea level
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2
Q

What equation relates arterial CO2 to ventilation?

A

PaCO2 = (VCO2 • 0.86) / VA

VCO2 = Metabolic production of CO2

VA = Alveolar ventilation

  • Buildup of CO2 in the blood is due to failure of some component of the respiratory system
    • Not an increase in metabolic CO2 production
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3
Q

What is the Henderson Hasselbalch acid-base equation for relating arterial CO2 to pH?

A

pH = 6.1 + log [HCO3-] / (0.03)(PaCO2)

  • A buildup of CO2 in the blood causes the pH to fall
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4
Q

What is the relationship between alveolar CO2 and arterial CO2?

A

PACO2 = PaCO2

  • Both controlled by rate of alveolar ventilation (breathing) and rate of CO2 production (metabolism)
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5
Q

How does an increase in CO2 influence breathing?

A

Increases breathing

  • Breathing maintains the PaCO2
  • Blow off CO2 at a higher rate
    • Higher rate of alveolar ventilation (VA)
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6
Q

What is the arterial CO2 in the event of hypercapnia, and what is the state ventilation?

A

Hypercapnia: PaCO2 > 45 mm Hg

  • Caused by hypoventilation
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7
Q

What is the arterial CO2 in the event of eucapnia, and what is the state of ventilation?

A

Eucapnia: PaCO2 = 35-45 mm Hg

  • Seen during normal ventilation
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8
Q

What is the arterial CO2 in the event of hypocapnia, and what is the state of ventilation?

A

Hypocapnia: PaCO2 < 35 mm Hg

  • Seen in hyperventilation
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9
Q

How do the patient’s respiratory rate, depth of breathing, or breathing effort influence hyer- and hypoventilation?

A

These terms are unrelated to respiratory rate, depth, effort

  • All dependent on PaCO2
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10
Q

How is a patient’s state of alveolar ventilation measured?

A

Can only be measured by arterial blood gas

  • Measure PaCO2
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11
Q

What is hypercapnia?

A

Elevated PaCO2

  • Failure of some component of the respiratory system
  • The only physiologic reason for elevated PaCO2 is a level of alveolar ventilation inadequate for the amount of CO2 produced and deliverd to the lungs
  • Sign of advanced organ system impairment
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12
Q

Why is hypercapnia potentially dangerous?

A
  • As the PaCO2 increses, pH falls (unless compensated)
  • The higher the PaCO2, the less defended the patient is against any further decline in alveolar ventilation (VA)
  • As PaCO2 increases, PAO2 and PaO2 fall, unless inspired O2 is supplemented
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13
Q

How can the rate of alveolar ventilation be defined?

A

VA refers only to ventilation rate of ALIVE volume of lung

  • Dead space does not count
  • VA = VE - VD = total - dead
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14
Q

How is the total ventilation rate (VE) calculated?

A

VE (L/min) = respiratory rate • tidal volume

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15
Q

What is dead space ventilation (VD)?

A

Space that is ventilated but not perfused

  • Gas entering and leaving
  • No blood flow
  • No gas exchange because it is not perfused
  • Ex: trachea
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16
Q

What components are required for gas exchange in the lungs?

A
  1. Gas entering and leaving
  2. Blood flow = perfusion
  3. Diffusion of gase across capillary membrane
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17
Q

What is the difference between physiological and anatomical dead space?

A
  • Anatomic dead space
    • All the airways that are ventilated but not perfused
    • Can never take part in gas exchange because of normal anatomy
    • ~150 mL
  • Physiologic dead space
    • Anatomic + all other dead space
    • Dead alveoli receive air but do not exchange gas
    • No blood flow and no perfusion to these alveoli
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18
Q

When will a patient become hypercapnic?

A
  • Inadequate total ventilation (VE)
    • Anything that limits the rate or depth of breathing
      • Massive obesity
      • Respiratory muscle weakness
      • Severe pulmonary fibrosis
      • Central nervous system depression
  • Increase in VD - alveoli lose perfusion
  • Both of the above
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19
Q

What is the requirement for minute ventilation?

A
  • The level of total ventilation (VE) needed to keep PaCO2 constant
    • Tries to maintain CO2 homeostasis in the blood
    • Body does this normally
  • When dead space (VD) increases, minute ventilation requirement increases
    • A rise in VE is needed to keep PaCO2 constant
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20
Q

What is the normal partial pressure for CO2 in the alveoli?

A

PACO2 = 40 mm Hg

PACO2 = PaCO2

Therefore,

PaCO2 = 40 mmHg

21
Q

What is the normal partial pressure of CO2 in the veins?

A

PvCO2 = 45 mm Hg

  • CO2 picked up by capillaries from tissues and delivered to veins
22
Q

If PACO2 is equal to PaCO2, is the same true for O2?

A

PAO2 ≠ PaO2

  • In a normal lung, these are not the same
23
Q

What is the percent of O2 in the atmosphere, and how does that affect PO2?

A
  • Atmosphere on earth contains 21% O2
  • Inspired air is 21% O2
  • PB = 760 mm Hg
  • 0.21 • 760 mm Hg = 159 mm Hg = PO2 at sea level
24
Q

What is the difference between PO2, inspired air (PIO2), and terminal respiratory units (PAO2)?

A
  • Atmospheric air: PO2 = 159 mm Hg
  • Inspired air with H2O: PIO2 = 150 mm Hg
  • Terminal respiratory units: PAO2 = 100 mm Hg
25
Q

What is the definition of a terminal respiratory unit?

A
  • Physician’s working “alveolus” (not a single alveolus)
    • Region in which gas phase diffusion is so rapid that the PAO2 and PACO2 are uniform throughout the unit
    • All alveolar ducts and their alveoli from proximal (first) respiratory bronchiole
26
Q

What is an acinus?

A

10-12 respiratory units

  • Anatomic unit used by pathologists
27
Q

Why is it good to know the PAO2 if we already know the PaO2 from an ABG?

A
  • At the level of the lung, terminal respiratory unit, or patient, PAO2 ≠ PaO2
    • PAO2 calculated from the alveolar gas equation is not the same as measured PaO2 from an ABG, even in a healthy patient with no gas exchange problems
  • Prediction of PAO2 is important to assess if the measured PaO2 is within its normal limits for the patient
28
Q

How is the PAO2 calculated from the PaCO2?

A

PAO2 = PIO2 - (PaCO2 • 1.2)

PIO2 = inspired O2

29
Q

How is inspired partial pressure of O2 (PIO2) calculated to be used in the alveolar gas equation?

A
  • Fraction of O2 in the air is always 0.21 at all altitudes
  • PB = barometric pressure = 760 mm Hg at sea level
  • Water vapor pressure in the trachea = 47 mm Hg

PIO2 = 0.21 • (760 - 47 mm Hg) = 150 mm Hg at sea level

30
Q

What is the PAO2 in a healthy lung with no gas exchange problems?

A

PAO2 = 100 mm Hg

  • Alveolar gas equation

PAO2 = PIO2 - 1.2(PaCO2)

PAO2 = 150 mmHg - 1.2(40 mmHg) ≈ 100 mm Hg

31
Q

What is PAO2 dependent upon?

A

Depends on environment

  • Depends on PIO2
    • Barometric pressure
    • Oxygen percent in air
  • This value is close to what you would expect a normal patient to have with no gas exchange defect
  • This value is what you expect if the lungs are properly transferring O2 into the blood
32
Q

What is a normal P(A - a)O2 range?

A

Between 5 and 20 mm Hg

  • PaO2 will always be less than PAO2
    • Unless there is a gas exchange problem
33
Q

How is normal PaO2 for a patient calculated based on age?

A

Normal PaO2 = 100 - (0.4 x age)

  • One definition of hypoxemia: partial pressure of oxygen in the blood is less than normal for the patient’s age
  • Normal PaO2 decreases with age
34
Q

How does age affect P(A - a)O2?

A

Normal P(A - a)O2 difference = [Patient age / 4] + 4

  • A high P(A - a)O2 indicates hypoxemia
35
Q

What are the two definitions of hypoxemia?

A
  • Partial pressure of oxygen in the blood is less than normal for the subject’s age
  • Oxygen content in the blood is less than normal for the subject’s age
36
Q

What is hypoxia?

A

Decreased oxygen supply to organs and tissues

  • Oxygen delivery to cells is insufficient for the demand
  • Does not require that the PO2 in the blood is low
    • But hypoxia can be caused by hypoxemia (hypoxic hypoxemia)
37
Q

What are some causes of hypoxia?

A
  • Respiratory causes - leads to hypoxemia
    • Chronic obstructive pulmonary disease
    • Coking, asphyxiation, smoke inhalation, suffocation, carbon monoxide poisoning
  • Circulatory causes
    • Content of the blood
      • Cyanize toxicity
      • Accumulation of abnormal hemoglobin in the blood
      • Anemia
    • Poor blood circulation
      • Very low blood pressure
      • Ischemic heart disease
      • Heart failure
      • Shock
38
Q

What are the three stages of diffusion of gases?

A
  1. In the aireways
  2. Across the alveolar-capillary membrane and RBC membrane
  3. RBC interior to Hb
39
Q

What equation is used for categorizing all the things that affect diffusion rate across a barrier?

A

Fick’s law

Gass diffusion rate = Area/Thickness • Solubility/√mol wt • (P1 - P2)

  • Property of barrier: area/thickness
    • Rate is slower with thick barrier
  • Property of gas: solubility/√mol wt
    • Gas has to be soluble in a barrier before going through
    • Gases with higher masses take longer
  • Driving force: P1 - P2
    • Partial pressure gradient
40
Q

When using Fick’s law, most of the variables in a particular situation are fixed. Which is not?

A

Partial pressure gradient (P1 - P2) changes with time

  • Initially as a gas reaches the alveolus the pressure difference will be the greatest
  • With time, the pressures start to equalize and the net gas diffusion will eventually goe to zero
    • P1 = P2
41
Q

Using Fick’s law; does CO2 or O2 have a faster barrier diffusion rate?

A

CO2 is 20 times more soluble than O2

42
Q

How quick is the RBC capillary transit time for O2?

A

About 0.75 seconds

43
Q

What are capillary transit time and capillary reserve time?

A

Capillary transit time: The time during which an RBC is transiting a capillary - surrounding an alveolus

  • Typically 0.75 sec
    • Is shorter as the blood moves faster, e.g. exercise
  • RBC loads all the O2 possible during that time
    • Process of O2 loading is about 0.25 sec
    • The rest is the capillary reserve time
      • Needs this reserve time during exercise
  • Abnormal transit time
    • Gas exchange problem - exaggerated with exercise
    • Potentially leading to hypoxemia
44
Q

What is the limiting factor for net O2 transfer?

A

Net gas exchange is perfusion limited

  • Diffusion is over in plenty of time
    • If perusion was slower, then it would even have more PAO2 to be reached
    • Diffusion is not rate-limiting
  • Normal conditions: O2 diffusion from alveolar air into pulmonary capillary blood is perfusion limited
  • Abnormal conditions: if the rate of perrusion starts getting faster (exercise) and at the same time diffusion is slower
    • Possible that diffusion becomes rate limiting
45
Q

When is the NET O2 transfer rate diffusion-limited versus perfusion-limited?

A
  • Under normal conditions: Net O2 transfer rate from alveolar air into pulmonary capillary blood is perfusion-limited
  • Diffusion-limited net O2 transfer can occur
    • Pathological conditions: emphysema, pulmonary fibrosis, edema
    • Sever exercise - capillary transit time is shortened sufficiently
    • Pressure differences across membrane are lower (high altitude)
    • Combination of all
46
Q

Why is net O2 transfer so important?

A

Anything that decreases net O2 transfer will decrease PaO2

  • Hypoxemia
47
Q

What is the capillary transit time for CO2?

A

0.75 seconds

  • Gets shorter as blood moves faster (e.g. exercise)
  • During the 0.75 sec, the RBC (which has picked up CO2 from the tissueshas PvCO2 = 45 mm Hg
  • Objective is to unload all the CO2 possible during the time transiting the capillary
    • P2 - P1 = 45 - 40 = 5 mmHg
    • Unloading takes about 0.45 sec
48
Q

Will PACO2 (and thus PaCO2) increase with exercise?

A

PaCO2 will not increase with exercise

  • Ventilation rate will increase to maintain homeostasis and keep PaCO2 normal
  • If there is a gas exchange problem
    • It will take quite a while to get the full unloading of CO2
    • In exercise there may not be full unloading - potentially leading to hypercapnia
49
Q

Is CO2 net gas transfer diffusion-limited or perfusion limited under normal conditions?

A

Like O2, it is perfusion limited

Biochemistry (3 decks)

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