4 - Gas Exchange

General gas law

PV = nRT

Boyle’s law

pressure and volume of a gas are inversely proportional

Partial pressure

pressure of a single type of gas in a mixture of gases

How to find the partial pressure of a dry gas

Px = Pb (total pressure) + F (fractional concentration of dry gas)

Fick’s law of diffusion

the movement of particles from an area of higher concentration to an area of lower concentration

Determinants of volume of gas transferred

↑diffusion coefficient, ↑volume of gas transferred

↑partial pressure gradient, ↑volume of gas transferred

↑membrane thickness, ↓volume of gas transferred

How does the diffusion coefficient of CO₂ compare to that of O₂?

it is 20x higher

Henry’s law

at equilibrium, the partial pressure of a gas in the liquid phase equals the partial pressure in the gas phase

How to find the concentration of a dissolved gas

Cx = Px X solubility

How does exercise affect lung diffusing capacity?

additional capillaries are perfused with blood, causing DL to increase

How does emphysema affect lung diffusing capacity?

Alveoli are destroyed, reducing surface area for gas exchange, reducing DL

How does fibrosis affect lung diffusing capacity?

membrane thickens, increasing diffusion distance, decreasing DL

How does pulmonary edema affect lung diffusing capacity?

interstitial volume increases, increasing diffusion distance, increasing DL

How does anemia affect lung diffusing capacity?

Decreases hemoglobin, decreasing transfer of oxygen, decreasing DL

In what forms can gasses be found?

• dissolved

• bound to a protein

• chemically modified

  • HCO₃⁻

Which form of gas does not contribute to partial pressure?

bound gas

How is the partial pressure of dry inspired air found?

Px = Pb (total pressure) + F (fractional concentration of dry gas)

How is the partial pressure of air found in the conducting airway?

(Pb - Px)*F

What is the alveolar pressure of oxygen compared to the pressure in the veins?

much higher, causing it to diffuse to the lower pressure area - the veins

What is the alveolar pressure of CO2 compared to the veins?

much lower, causing venous CO2 to diffuse to the alveolus

What proportion of blood O2 is dissolved?

2%, produces partial pressure in the blood

What proportion of blood O2 is bound?

98% of the total

What is the significance of hemoglobin structure?

• Each hemoglobin molecule has 4 heme groups, each capable of binding one O₂ molecule.

• Binding is reversible, allowing loading in the lungs and unloading in tissues

What is the O2-binding capacity of blood?

20.1 mL O2/100 mL blood

How to calculate oxygen content in the blood?

O2 content = (O2 binding capacity * %saturation) + dissolved O2

What does tissue oxygenation depend on?

cardiac output, oxygen content (dissolved + bound)

What does the sigmoidal shape of the oxygen-hemoglobin curve describe?

binding of one O2 increases Hb affinity for the next O2, allowing efficient loading and unloading of O2

P50 value of O2-hemoglobin curve

PO2 at which hemoglobin is 50% saturated

↑P50, ↓affinity

↓P50, ↑affinity

What can cause a right shift in the O2-hemoglobin curve?

decrease in affinity due to:

• ↑ PCO₂

• ↓ pH (Bohr effect)

• ↑ Temperature

• ↑ 2,3-DPG

What can cause a left shift in the O2-hemoglobin curve?

increased affinity due to:

• ↓ PCO₂

• ↑ pH

• ↓ Temperature

• ↓ 2,3-DPG

Carbon monoxide poisoing

impairs oxygen delivery by interfering with hemoglobin’s ability to bind and release oxygen; binds to hemoglobin to form carboxyhemoglobin and reduce number of site available for oxygen binding

CO poisoning affect on O2-hemoglobin curve

left shift

Carbaminohemoglobin

CO2 binds directly to globin portion of hemoglobin

Pulmonary blood flow relationship to pressure gradient and resistance

↑pressure gradient, ↑blood flow

↑resistance, ↓blood flow

What factors regulate pulmonary blood flow?

• hypoxic vasoconstriction

• vasoactive substances regulate resistance

• lung volume

What is the average V/G ration?

0.8

Physiologic shunt

PaO2 < PAO2

a small fraction of pulmonary blood flow is not arterialized

right-to-left shunts

shunting of blood from right heart to left heart due to a defect of the wall between the left and right ventricle, causing hypoxemia

left-to-right shunts

do not cause hypoxemia

What conditions may a right-to-left shunt be seen in?

• Airway obstruction

• Pneumonia

• Atelectasis

Chronic hypoxia

stimulates the production of EPO in kidneys, increasing red blood cell production and oxygen-carrying capacity

How is blood flow affected from the top of the lung to the bottom?

higher at the bottom, lowest at the top

How is V/P ratio affected from the top of the lung to the bottom?

highest at the top, lowest at the bottom

How would a pulmonary embolus effect V/Q?

infinitely high; ventilation but no perfusion

How would a large alveolus or small capillary size effect V/Q?

high V/Q due to a high concentration of O2 and a low concentration of CO2

How would a small alveolus or a large capillary size effect V/Q?

low V/Q due to a low concentration of oxygen and high concentration of CO2