(22.6) Gas Exchange

Explain Gas Exchange

• Occurs between lungs and blood as well as blood and tissues

• Both process are subjected to 

  1. Basic properties of gases 

  2. Composition of alveolar gas 

Define Dalton’s Law of Partial Pressures

Total pressure exerted by a mixture of gases is the SUM of pressures exerted by each gas in the mixture

• TELLS US WHICH DIRECTION GASES GO (FROM HIGH TO LOWER PRESSURE) 

Define Partial Pressure 

The pressure exerted by a single component of a mixture of gases

• Directly proportional to its percentage in mixture 

Total P_atm = ?

760 mmHg 

How much is Nitrogen in the air?  

78.6% 

Calculate Partial Pressure of Nitrogen (P_N2)

0.786 × 760 mmHg = 597 mmHg N₂ 

How much is Oxygen in the air?  

20.9% 

Calculate Partial Pressure of Oxygen (P_O2)

0.209 × 760 mmHg  = 159 mmHg 

Compare High & Lower Altitudes influence on Partial Pressure

1. HIGH altitudes → Partial pressure DECLINES

2. LOWER altitudes (under water)→ Partial pressure INCREASES significantly

Define Henry’s Law

For gas mixtures in contact with liquids: Each gas will dissolve in the liquid in proportion to its partial pressure of that gas

• At equilibrium → partial pressures in the two phases will be equal

• HOW MUCH OF GAS DISSOLVES INTO BLOOD

Amount of each gas that will dissolve depends on =?

1. Solubility 

   • CO₂ is 20x more soluble in water than O2, and little N2 will dissolve 

2. Temperature 

   • As temperature of liquid rises, solubility decreases 

Compare Approx % Gas Partial Pressure in the Atmosphere & in the Alveoli (for O₂ and CO₂)

Describe how atmospheric and alveolar air differ in composition

Alveoli contain more CO₂ and water vapor than atmospheric air 

Explain why does Alveoli contain more CO₂ and water vapor than Atmospheric air

1. Gas exchanges occurring in the lungs (O₂ diffuses from the alveoli into the pulmonary blood and CO₂ diffuses in the opposite direction)

2. Humidification of air by conducting passages

3. The mixing of alveolar gas that occurs with each breath

Define External Respiration 

Pulmonary Gas Exchange

Involves the exchange of O₂ (uptake) CO₂ (unloading) across respiratory membranes 

List Factors that Influence on External Respiration

1. Partial pressure gradients and gas solubilities 

2. Thickness and surface area of the respiratory membrane

3. Ventilation-perfusion coupling (matching alveolar ventilation with pulmonary blood perfusion)

Explain Partial pressure gradients and gas solubilities 

Influences on External Respiration (1/3)

• A STEEP partial pressure gradient exist between blood in the pulmonary arteries and alveoli and O₂ diffuses rapidly until it reaches equilibrium at P_O2 of 104 mmHg

  • Alveoli → Blood

• CO₂ moves in the opposite direction ALONG a partial pressure that is much LESS steep → reaching equilibrium at 40 mmHg

  • Blood → alveoli 

  • Through gradient is not as steep → CO2 still diffuses in equal amounts with oxygen

  • REASON is that CO2 is 20x MORE soluble in plasma and alveolar fluid than oxygen 

T/F: P_O2 is greater in the alveolus than the blood, so oxygen diffuses into the blood.

→ TRUE

• During pulmonary gas exchange, oxygen will diffuse down its partial pressure gradient from the alveolus into the blood until the partial pressure is equal in both locations.

_______ has a greater partial pressure in the pulmonary capillaries than in the alveoli, so it diffuses into the _______.

CO₂ has a greater partial pressure in the pulmonary capillaries than in the alveoli, so it diffuses into the alveoli

Despite the fact that the partial pressure difference is so much smaller for CO₂, why is there as much CO₂ exchanged between the alveoli and blood as there is O₂?

→ CO₂ is much more soluble in blood than O₂

How would the partial pressures of O₂ and CO_{2 c}hange in an exercising muscle?

→ The partial pressure of O₂ would decrease, and the partial pressure of CO₂would increase

• Cells use O₂ and produce CO₂ during cellular respiration to produce ATP. Exercising muscles need more ATP.

Oxygenation of Blood in the Pulmonary Capillaries @ Rest

• A STEEP partial pressure gradient exist between blood in the pulmonary arteries and alveoli and O₂ diffuses rapidly until it reaches equilibrium at P_O2 of 104 mmHg

  • Alveoli → Blood

• CO₂ moves in the opposite direction ALONG a partial pressure that is much LESS steep → reaching equilibrium at 40 mmHg

  • Blood → alveoli 

Partial pressure gradients promoting gas movements in the body

O2

1. Air enters through the nose or mouth

2. Air travels down the trachea and then enters the bronchi 

3. Air travels down smaller and smaller bronchioles

4. Air reaches small sacs (alveoli)  

CO2

1. CO₂ is released from the mitochondria 

2. CO₂ diffuses into a capillary

3. CO₂ is carried to the lungs

4. CO₂ diffuses into an alveolus

5. Air exits through nose or mouth

Explain Thickness and surface area of the respiratory membrane

Influences on External Respiration (2/3)

• Respiratory membranes are very thin → 0.5-1 um thick 

• Large total surface area of the alveoli is 40x the surface area of the skin 

Explain Ventilation-perfusion coupling

Influences on External Respiration (3/3)

• Ventilation (amount of gas reaching alveoli) and perfusion (amount of blood flowing through pulmonary capillaries) must be coupled for optimal, efficient gas exchange 

  • VENTILATION = how much air reaches the alveoli

  • PERFUSION = how much blood reaches the alveoli

How is Ventilation-perfusion coupling controlled?

• Both controlled by local auto regulatory mechanisms 

  1. Alveolar P_O2 → controls perfusion by changing arteriolar diameter 

  2. Alveolar P_CO2 → controls ventilation by changing bronchiolar diameter

Explain Influence of local P_O2 on Perfusion 

Changes in local alveolar P_O2 cause changes in diameters of local arterioles 

1. If P_O2 is low (from poor ventilation) → arterioles constrict to decrease perfusion 

   • Directs blood to go to well ventilated alveoli → where O2 is high (and CO2 is low), so blood can pick up more oxygen (and remove more CO2) 

2. If P_O2 is high (from good ventilation) → arterioles dilates to increase perfusion 

Opposite mechanism, seen in systemic arterioles that dilate when oxygen is low and constrict with high 

Explain Influence of local P_CO2 on Ventilation  

Poor alveolar ventilation results in low alveolar PO2 (high PCO2) →

1. Pulmonary arterioles constrict → less blood goes to this poorly ventilated alveolus

2. Bronchioles dilate→ to bring more air in and wash out the extra CO₂

   • To improve the match by sending less blood and more air.

Good alveolar ventilation results in high alveolar PO2 (low PCO2)

1. Pulmonary arterioles dilate → more blood is sent to this well-ventilated alveolus

2. Bronchioles constrict→ because not as much extra airflow is needed

   • To match plenty of air with plenty of blood.

What would occur if lung cancer restricts the airflow to a group of alveoli?

→ P_O2 in the affected alveoli would decrease, and their arterioles would vasoconstrict

• If P_O2 is low (from poor ventilation) → arterioles constrict to decrease perfusion

  • Directs blood to go to well ventilated alveoli → where O2 is high (and CO2 is low), so blood can pick up more oxygen (and remove more CO2) 

SUMMARY of Factors that Influence on External Respiration

1. Partial pressure gradients and gas solubilities 

   • A STEEP partial pressure gradient exist between blood in the pulmonary arteries and alveoli and O₂ diffuses rapidly until it reaches equilibrium at P_O2 of 104 mmHg

     • Alveoli → Blood

   • CO₂ moves in the opposite direction ALONG a partial pressure that is much LESS steep → reaching equilibrium at 40 mmHg

     • Blood → alveoli 

2. Thickness and surface area of the respiratory membrane

   • Very thin

   • Huge surface area for efficient gas exchange

3. Ventilation-perfusion coupling

   • Ensures a close match between the amount of gas reaching that alveoli and the blood flow in the pulmonary capillaries

     • VENTILATION = how much air reaches the alveoli

     • PERFUSION = how much blood reaches the alveoli

Explain Cause and Effect of Pneumonia

• CAUSE

  • Effective thickness of respiratory membrane INCREASES dramatically if the lungs become waterlogged and edematous

• EFFECT

  • Pneumonia

Explain Cause, Effect, and Treament of Emphysema 

• CAUSE

  • Reduce the alveolar surface area

• EFFECT

  • Walls of adjacent alveoli break down and the alveolar chambers enlarge

• TREATMENT

  • Administering supplemental oxygen → Increase the P_O2 in the alveoli to increase the diffusion of oxygen across the respiratory membrane

 

Name other examples of Pulmonary Diseases that REDUCE Alveolar Surface Area 

1. Tumors 

2. Mucus 

3. Inflammatory material 

Reduce surface area by blocking gas flow into alveoli

Define Internal Respiration 

Involves capillary gas exchange in body tissues

T/F: Diffusion gradients for O₂ and CO₂ are reversed from those from external respiration and pulmonary gas exchange, the factors promoting has are identical 

→ TRUE 

Compare Partial pressure and diffusion gradients between Internal Respiration vs External Respiration 

Partial pressures and diffusion gradients in Internal Respiration are REVERSED compared to External Respiration

1. Tissue P_O2 is ALWAYS LOWER than in arterial blood P_O2 (40 vs 100 mmHg) → so O₂ moves from blood to tissues

2. Tissue P_CO2 is ALWAYS HIGHER than arterial blood P_CO2 (45 vs 40 mmHg) → so CO₂ moves from tissues into blood

3. Venous blood returning to heart has P_O2 of 40mmHg and P_CO2 of 45 mmHg

Which way would O₂ and CO₂ diffuse during internal respiration?

O₂ would diffuse into the cells, and CO₂ would diffuse into the systemic capillaries.

Relate Dalton’s and Henry’s laws to events of pulmonary and tissue gas exchange.

1. Dalton’s Law → Total pressure exerted by a mixture of gases is the SUM of pressures exerted by EACH gas in the mixture

   • How gas behaves in a mixture of gasses (from higher pressure to lower pressure)

2. Henry’s Law → For gas mixtures in contact with liquids: Each gas will dissolve in the liquid in proportion to its partial pressure of that gas

   • How gases move into and out of solution