Egan's Chapter 12 - Gas Exchange and Transport

Describe how Oxygen (O₂) and Carbon Dioxide (CO₂) move between the atmosphere and tissues.

Diffusion

Identify the conditions that must exist in order for diffusion to occur.

Difference in partial pressures (pressure graduent)

Understand factors contributing to the rate at which gases diffuse, and which gases that would diffuse faster.

• Solubility

• Higher pressure

• Temperature

• pH

. Area thickness

Identify the normal amount of time required for gases to diffuse into the blood, the minimum amount of time for adequate equilibration.

•0.75 seconds

.25 seconds is minimal time

Normal CaO2 concentration

16-20 ml/dl

Gas Exchange is normal when?

Either tissue O2 delivery or CO2 removal is impaired

Identify the adequacy/effectiveness of alveolar ventilation.

Best indicators are pH & PaCO₂

Pressure gradient for CO2

Is 1/10 of the pressure for O2

Identify the effects of barometric pressure on the partial pressures of gases.

Pb does not change % of mixture, but partial pressures are proportional to Pb.

CO2 diffuses how much faster than O2?

Approx. 20x faster across alveloar-capillary membrane

Approximately how much CO2 is normally carried in the blood?

45 to 55 ml/dl of CO2

PaO2 is to be kept above what mmHg?

60mmH

If less than 60, a small decrease in PO2 causes a large decrease in SaO2

Normal SaO2 value

95% to 100% depending on age of patient

Describe the factors that account for the differences between PAO₂ and PaO₂.

Right to left shunts and regional differences in pulmonary ventilation and blood flow V/Q

Pressure, barriers to diffusion, fluid

Regional Variations in Ventilation/Perfusion for a normal lung are mainly caused by_______?

Caused by gravity and are most evident in the upright posture.

Identify the differences between high V/Q and low V/Q

High V/Q- ventilation is greater than normal, perfusion is less than normal

Low V/Q- ventilation is less than normal, perfusion is greater than normal

Identify the largest impact of a V/Q mismatch.

Hypoxemia

Identify the differences between physiologic dead space and alveolar dead space.

Alveolar dead space is the volume of gas in unperfused alveoli. Physiologic dead space is the sum of alveolar dead space and anatomic (150) dead space.

Identify the normal levels of CO₂ production.

• Approximately 200 mL of CO₂/min

• 35-45 mm Hg

Understand the Fick equation, and how it relates to oxygen consumption and cardiac output.

At constant O₂ consumption, if cardiac output increases, C(a-v)O₂ decreases, and vice-versa.

Identify the normal levels of hemoglobin, and their capacity to carry oxygen.

The average Hb content is 15g/dL, and each gram can carry 1.34 mL of O₂

Other causes of hypoxemia

Respiratory diseases and physiologic shunting

Compute total O₂ content.

CaO2 = (0.003 x PaO2) + (1.34 x Hb x SaO2)

Describe the factors affecting O₂ loading and unloading from hemoglobin.

• Blood pH

• Body temperature

• Erythrocyte concentration of organic phospahtes

Identify factors that would cause a shift to the left in the oxygen/hemoglobin dissociation curve.

• ↑ pH alkalosis

• ↑ affinity of Hb and O₂

---

• ↓ demand for O₂

• ↓ 2,3-DPG

• ↓ temperature hypothermic

• ↓ PCO2

. Carboxyhemoglobin

. Methemoglobin

. Fetal

Identify factors that would cause a shift to the right in the oxygen/hemoglobin dissociation curve.

• ↑ demand for O₂

• ↑ 2,3-DPG prmotes O2 unloading

• ↑ temperature-fever

• ↑ PCO₂ hypercapnic

• ↓ pH Acidisis

• ↓ affinity of Hb and O₂

. Sickle

Hypoxemia Levels

80-100 Normal

60-80 Mild

40-60 Moderate

less 40 Severe

Desribe the Bohr effect.

CO₂ loading facilitates O₂ unloading in the tissues. When hemoglobin binds with hydrogen, its affinity for O₂ decreases.

Impact of Changes in pH for Hb affinity for O2

Understand the Hamburger phenomenon, or chloride shift.

Bicarbonate ion is exchanged with chloride ion to maintain electrical neutrality.

Identify the ways that CO₂ is transported, and what percentage of CO₂ is carried in each way.

• Dissolved in physical solution (8%)

• Chemically combined with protein carbanohemoglobin (12%)

• Ionized as bicarbonate (80%)

Identify the ways that O₂ is transported, and the percentage of O₂ that is carried in each way.

• Physically dissolved in blood 1%

• Chemically combined with Hb 99%

Normal respiratory exchange quotient

0.8

What is the significance of increased dead space?

it decreaes alveolar ventilation and hence increases PaCO2

If dead space is increased what must also happen?

Minute ventilation must increase to achieve normal alveolar Ventilation and PaCO2.

How to differentiate between hypoxemia caused by V/Q imbalance or hypoxemia caused by shunting?

If FiO2 is greater than 50% and PaO2 is less than 50mmHG, shunting is present

Identify causes of hypoxia and distinguish the differences between hypoxia, dysoxia, and hypoxemia.

• Hypoxia occurs whenever O₂ delivery falls short of cellular needs.

• Hypoxemia occurs when PaO₂ in the blood is decreased (low PO₂, hypoventilation, impaired diffusion, shunting)

• Dysoxia is a cellular condition (often caused by cyanide) that can not be treated with respiratory therapy

List the 3 barriers to diffusion

1. Capillary Membrane (epithelium) 2. Interstitial space 3. Alveolar Membrane (epithelium)

To pass through RBCs, must pass erythrocyte membrane

What is the main force driving a gas across the alveolar-capillary membrane?

The Partial Pressure of a gas

Hypoxia

abnormal condition in which there is Inadequate delivery of O2 to the tissues to meet metabolic needs

Hypoxemia

Abnormal deficiency of O2 in the blood

Saturation

A ratio of how much blood is saturated with O2, to blood that can be saturated

SaO2=(HbO2\total Hb) x 100

What is the most common cause of low blood O2 in patients with lung disease.

Ventilation Perfusion Imbalance

Pulmonary Fibrosis Patients have this defect that results in low blood O2 levels.

Diffusion Defect

A low blood pressure results in poor tissue O2 delivery and ____________________?

Shock or ischemia

Myocardial Infarction and stroke are an example of _________, a localized reduction in blood flow to tissues that can result in tissue death.

Ischemia

Increased __________ space ventilation may result in increased levels of CO2 in the blood

Dead Space

Inadequate Minute or Alveolar ventilation can be a result of what?

Drug overdose, due to central nervous system depression.

Decreased Vt or respiratory rate

Patients with severe COPD are unable to maintain adequate ventilation as a result of _____________?

Ventilation Perfusion Imbalances

(Hypercapnia)

PACO2 varies inversely with?

Alveolar Ventilation and directly with production of CO2

Ventilation and Perfusion must be ____________ to be effective.

In balance

Blood carries a small amount of O2 (1%) dissolved in the plasma and a large amount (99%) attached to ?

Hemoglobin

What are the 5 causes of a decreased PaO2 or O2?

1. Level in arterial blood are low ambient PO2

2. Hypoventilation

3. impaired diffusion

4. Ventilation-Perfusion Imbalance

5. physiologic shunt

Most of the CO2 in your blood is transported as _________?

Bicarbonate

Hypoxia can occur when?

When Arterial blood O2 content is decreased or if blood flow is decreased.

Abnormal cell function prevents proper uptake of O2

When does alveolar ventilation decrease?

When the minute ventilation is decreased or when dead space ventilation is increased, and V/Q imbalance

What is the classic example of dysoxia?

Cyanide Poisoning interfering with mitochondrial function

When does tissue O2 consumption become dependent on O2 delivery?

(septic shock, ARDS) when oxygen demand exceeds oxygen delivery.

Alveolar Air Equation

PAO2= FiO2 x (PB - PH2O) - (PACO2 ÷ RQ)

PH2O= Water vapor tension. At BTPS 47mmHg

RQ= Respiratory Quotient .8

FiO2= .21

PB= Barometric Pressure, atmospheric pressure 760mmHg

PACO2= Alveolar PCO2, 40mmHg

What can cause an increase in PACO2?

PACO2 increases above level of 40mmHg if CO2 production increases while alveolar ventilation remains constant or if alveolar ventilation decreases while production of CO2 remains constant. Dead Space can also lead to an increase.

In addition to CO2, O2, and water vapor, alveoli normally contain what?

Nitrogen

What is Nitrogens role in gas exchange?

Nitrogen is inert and plays no role in gas exchange; however it occupies space and exerts pressure.

Compute PAN₂

Subtract the pressures exerted by all the other alveolar gases

PAN2= PB - (PAO2 + PACO2 + PH2O

Based on Alveolar Equation, if FiO2 remains constant, PAO2 must vary inversely with PACO2

What 2 factors account for the difference between alveolar and arterial PO2 (5 to 10 mmHg)

1. Right to Left Shunts

2. regional differences in pulmonary ventilation and blood flow.

What 2 Right to Left Shunts exists in normal Humans?

1. bronchial venous drainage

2. thebesian venous drainage

What does a Right to Left Shunt Cause?

causes poorly oxygenated venous blood to move directly into the arterial circulation, reducing the O2 content of arterial blood.

These shunts account for appr. 3/4 of the normal difference between PAO2 and PaO2.

As with O2, CO2 produced by the tissues dissolves in the plasma and erythrocyte intracellular fluid. However, in contrast to O2, dissolved CO2 plays an important role in transport, accounting for approximately 8% of the total released at the lungs: this is because of the higher solubility of CO2, in plasma

Approximately 80% of CO2 in the blood is transported as what?

Bicarbonate

Hydrolysis

Process by which CO2 that dissolves in plasma, the small portion combines chemically with water

Most CO2 undergoes hydrolysis inside the erythrocyte. This reaction is greatly enhanced by an enzyme catalyst called what?

Carbonic anhydrase

Haldane Effect

The influence of oxyhemoglobin saturation on CO2 dissociation.

This phenomenon is a result of changes in the affinity of Hb for CO2, which occur as a result of its buffering of H+ ions. O₂ loading facilitates CO₂ unloading in the lungs. When hemoglobin is saturated with O₂, its affinity for CO₂ decreases.

What lowers PiO2, thus decreasing PAO2 and PaO2?

breathing gases with a low O2 concentration (hypoxia) or at pressures less than atmospheric (high altitude)

What causes mountain sickness?

traveling to high altitudes

Hyperventilation causes?

decreases PACO2 and helps compensate for hypoxemia

Most common cause of Hypoxemia?

V/Q imbalance

Accounting for normal decrease in arterial O2 tension that occurs with aging.

Expected PaO2=100 - (0.323 x Age in years)

Hb deficiencies or anemias can be what?

ABSOLUTE OR RELATIVE

Absolute Hb Deficiency

occurs when the Hb concentration is lower than normal

Relative Hb Deficiency

are caused by either the displacement of O2 from normal HB or the presence of abnormal Hb variants

Low Blood Hb Concentration

may be caused either by a loss of RBCs, as with hemorrhage, or inadequate erythropoiesis

Low Hb Content can seriously impair what?

seriously impair the O2 carrying capacity of the blood even in the presence of a normal supply and adequate diffusion.

What are the 2 types of reduced blood flow?

1. circulatory failure (shock)

2. local reductions in perfusion (ischemia)

Circulatory Failure (shock)

tissue O2 deprivation is widespread. although body tries to compensate for the lack of O2 by directing blood flow to vital organs, this response is limited. Prolonged shock ultimately cause irreversible damage to the central nervous system and eventual cardiovascular collapse.

Local Reductions in Perfusion (ischemia)

even when whole body perfusion is adequate, local reductions in blood flow can cause localized hypoxia. Ischemia can result in anaerobic metabolism, metabolic acidosis, and eventual death of the affected tissue. Myocardial infarction and stroke are examples of ischemic conditions that can cause hypoxia and tissue death.

Dysoxia

is a form of hypoxia, in which the cellular uptake of O2 is abnormally decreased. Cyonide poisoning

What causes a decrease in alveolar ventilation?

1. minute ventilation is inadequate

2. dead space ventilation per minute is increased

3. V/Q imbalance exists

Inadequate Minute Ventilation

caused by decreased tidal volume, or respiratory rate.

Occurs in restrictive conditions such as atelectasis, neuromuscular disorders, or impeded thoracic expansion(kyphoscoliosis)

Increased Dead Space Ventilation

is caused by either 1. decreased tidal volume (rapid, shallow breathing) 2. increased physiologic dead space (various lung diseases)

Alveolar Ventilation

VA= fb(Vt-Vd)

Whats going on in lung to get a V/Q ratio of .01?

High perfusion, low ventilation

What factors increase 2,3 DPG

Acidosis, chronic hypoxemia, anemia

What factors lower 2,3DPG?

Alkalosis

A-a equation

PAO2-PaO2

Normal range 5-10 mmHg

Why cant PAO2 never be equal to PACO2?

Because there are barriers to diffusion

Normal values

PiO2 159-40

PCO2 60