Section 5

Question 1

Define gas exchange.

Answer 1

Gas Exchange: The diffusion of oxygen from the alveoli into the blood, and carbon dioxide from the blood to the alveoli

Question 2

What factors contribute to the pressure gradient in the context of gas exchange in the alveoli and pulmonary artery?

Answer 2

The pressure gradient is based on the partial pressures of the gas in the alveoli (PA) and the pulmonary artery (PV), along with the resistance to diffusion.

Question 3

What does the resistance to diffusion depend on in the context of gas exchange?

Answer 3

The resistance to diffusion is dependent upon the surface area of the membrane (A), its thickness (T), and the diffusibility (D) of the gas.

Question 4

What does the constant “D” represent in the context of gas exchange and diffusion?

Answer 4

“D” represents diffusibility, which is a constant. However, it is often ignored in calculations due to its constant nature.

Question 5

How is the partial pressure of a substance within the pulmonary artery denoted, and what kind of blood does this artery contain?

Answer 5

The partial pressure in the pulmonary artery is denoted as PV. The pulmonary artery contains mixed venous blood, as it delivers blood from all systemic tissues to the lung.

Question 6

According to Dalton’s law, how is the pressure of a gas in a mixture related to the percentage of that gas in the mixture?

Answer 6

The pressure exerted by a gas in a mixture is directly proportional to the percentage of that gas in the mixture, as per Dalton’s law.

Question 7

If air at sea level has an atmospheric pressure of 760 mmHg, calculate the partial pressure of nitrogen and oxygen based on their percentages in air.

Answer 7

The partial pressure of nitrogen is 600 mmHg (0.79 x 760 mmHg), and the partial pressure of oxygen is 160 mmHg (0.21 x 760 mmHg).

Question 8

What are the two key factors determining the amount of gas that can dissolve in a liquid?

Answer 8

1) Partial pressure in air: The greater the partial pressure, the more gas will be driven into the liquid.

2) Solubility in the liquid: The more soluble a gas is in a liquid, the more will dissolve.

Question 9

Define Partial Pressure.

Answer 9

Partial Pressure is the pressure that would be exerted by one of the gases in a mixture of gases if it occupied the same volume on its own.

Question 10

How does the composition of alveolar air differ from inspired air?

Answer 10

Alveolar air differs as it becomes saturated with water vapor immediately upon inhalation, impacting the partial pressures of other gases.

Question 11

What is the contribution of water vapor to the gas mixture in alveolar air, and how does it affect the partial pressures of other gases?

Answer 11

Water vapor, with a partial pressure (PH2O) of 47 mmHg at body temperature, contributes to the gas mixture in alveolar air. The remaining gases, excluding water vapor, account for 713 mmHg, affecting the partial pressures of other gases.

Question 12

Calculate the partial pressure of inspired nitrogen and oxygen in alveolar air, considering the saturation with water vapor.

Answer 12

The partial pressure of inspired nitrogen is 563 mmHg (79% of 713 mmHg), and the partial pressure of inspired oxygen is 150 mmHg (21% of 713 mmHg).

Question 13

What complicates the composition of alveolar air further, and how does it affect gas exchange?

Answer 13

The concept of anatomical dead space complicates alveolar air composition. This anatomical dead space influences the effective ventilation and distribution of gases during respiration.

Question 14

What percentage of alveolar gas is considered “fresh” air at the end of inspiration, and how does this impact the partial pressure of oxygen (PA O2)?

Answer 14

At the end of inspiration, only about 15% of alveolar gas is “fresh” air, leading to a further drop in the partial pressure of oxygen (PA O2).

Question 15

What is the actual PA O2, and how is it calculated using the alveolar gas equation?

Answer 15

The actual PA O2 is around 100 mmHg, calculated using the alveolar gas equation: PI O2 - PA CO2/R = PA O2. Where PI O2 is the partial pressure of inspired oxygen (150 mmHg), PA CO2 is the partial pressure of alveolar carbon dioxide (about 40 mmHg), and R is the respiratory quotient, roughly 0.8 in healthy people.

Question 16

Despite inhalation and exhalation, why do PA O2 and PA CO2 remain essentially constant in the alveoli?

Answer 16

PA O2 and PA CO2 remain constant due to the low amount of fresh air reaching the alveoli at the end of inspiration and the fast rate of diffusion of oxygen from the alveoli into the blood. Any increase in PA O2 or CO2 is rapidly equalized by enhanced gas exchange.

Question 17

How does the rate of gas exchange respond to changes in PA O2 and PA CO2 in the alveoli?

Answer 17

An increase in PA O2 leads to increased gas exchange to equalize it. Similarly, more expired CO2 enhances the partial pressure gradient, causing more CO2 to leave the blood for the alveoli, keeping PA CO2 constant.

Question 18

Describe the direction of movement of carbon dioxide and oxygen during gas exchange at the pulmonary capillaries.

Answer 18

Carbon dioxide moves from the blood to the alveoli, while oxygen moves from the alveoli to the blood during gas exchange at the pulmonary capillaries.

Question 19

What drives the movement of gases during gas exchange at the pulmonary capillaries, and what process facilitates this movement?

Answer 19

The movement of gases is driven by partial pressure gradients, and diffusion facilitates this process during gas exchange at the pulmonary capillaries.

Question 20

How is the constant replenishment of alveolar oxygen and removal of carbon dioxide achieved during ventilation?

Answer 20

The process of ventilation constantly replenishes alveolar oxygen and removes carbon dioxide.

Question 21

Given the partial pressures of oxygen and carbon dioxide in the alveoli (100 and 40 mmHg, respectively) and the mixed venous blood entering the lungs (PVO2 = 40 mmHg, PVC O2 = 46 mmHg), what will occur due to these gradients during gas exchange, and why?

Answer 21

Oxygen will move from the alveoli into the blood, and carbon dioxide will move from the blood into the alveoli until the partial pressures are equalized. Ventilation, maintaining alveolar oxygen at 100 mmHg and carbon dioxide at 40 mmHg, ensures that the blood leaving the lungs contains these gases at these partial pressures.

Question 22

How does the solubility of gases affect the actual concentration of dissolved gases in the blood?

Answer 22

The actual concentration of dissolved gases in the blood is affected by both the partial pressure and the solubility of a gas. For example, carbon dioxide is 20 times more soluble in the blood than oxygen, meaning even if their partial pressures were the same, the actual concentration of dissolved carbon dioxide would be greater.

Question 23

Explain the difference in dissolved CO2 concentration in blood leaving the lungs (40 mmHg) and returning to the lungs (46 mmHg). Why is there not a dramatic change in dissolved CO2?

Answer 23

Blood leaving the lungs has a CO2 partial pressure of 40 mmHg, equivalent to about 480 ml CO2/L. Blood returning to the lungs has a partial pressure of 46 mmHg, roughly 520 ml CO2/L. The lack of a dramatic change in dissolved CO2 is due to the essential role of CO2 (and carbonic acid) in acid-base balance, which will be discussed further in Module 05.

Question 24

Describe the difference in oxygen partial pressure in blood leaving the lungs (100 mmHg) and returning to the lungs (40 mmHg). What does this difference indicate?

Answer 24

Blood leaving the lungs has an oxygen partial pressure of 100 mmHg, approximately 200 ml O2/L. Blood returning to the lungs has a partial pressure of 40 mmHg, around 150 ml O2/L. This difference indicates that, at rest, approximately 50 ml O2/L was removed by the tissues, while the remaining 150 ml O2/L represents the functional reserve for increased tissue demand.

Question 25

What are the main driving forces behind gas exchange, and what additional factors contribute to this process?

Answer 25

The main driving forces behind gas exchange are partial pressure gradients of dissolved gases. Additional factors include surface area, capillary transit time, and membrane thickness.

Question 26

How does surface area affect gas exchange, and what happens to the surface area during exercise and in lung diseases like emphysema?

Answer 26

The greater the surface area, the greater the gas exchange. During exercise, increased pulmonary blood pressure opens closed capillaries, effectively increasing surface area. In lung diseases like emphysema, alveolar damage reduces surface area for gas exchange.

Question 27

Explain the concept of capillary transit time and its influence on gas exchange. How does it change during exercise, and what impact does it have under physiological conditions?

Answer 27

Capillary transit time is the duration of exposure of capillary blood to alveolar gas. At rest, blood remains in pulmonary capillaries for about 0.75 seconds, three times longer than necessary for exchange. Even during exercise, capillary transit time decreases only to 0.4 seconds. Under physiological conditions, capillary transit time is not a limitation to gas exchange.

Question 28

How does membrane thickness influence gas exchange, and what happens if it is increased due to inflammation?

Answer 28

Increased membrane thickness, often due to inflammation, decreases gas exchange. In diseases like asthma, where mucous secretion occurs, the membrane thickens. If the thickness is significant, capillary transit time may not be sufficient for complete gas equalization.

Question 29

Define Capillary Transit Time.

Answer 29

Capillary Transit Time is the duration of exposure of capillary blood to alveolar gas.

Question 30

How do the fundamental principles of gas exchange across systemic capillaries compare to those in pulmonary capillaries?

Answer 30

The fundamental principles of gas exchange across systemic capillaries are essentially identical to those in pulmonary capillaries.

Question 31

What are the average cellular PO2 and PCO2, and how do they compare to arterial PO2 and PCO2?

Answer 31

The average cellular PO2 is around 40 mmHg, and the cellular PCO2 is around 46 mmHg. Arterial PO2 is around 100 mmHg, and arterial PCO2 is around 40 mmHg. This difference indicates that oxygen moves into the tissues, while carbon dioxide moves into the blood.

Question 32

How do the partial pressures of oxygen and carbon dioxide in the blood leaving systemic capillaries compare to the tissue values?

Answer 32

By the time the blood leaves the systemic capillaries, its partial pressures for oxygen and carbon dioxide have equilibrated to the tissue values of 40 mmHg and 46 mmHg, respectively.

Question 33

How does an increase in tissue metabolism, such as during exercise, affect the pressure gradients for gas exchange and the partial pressure of oxygen returning to the lungs?

Answer 33

Increased tissue metabolism during exercise leads to a decrease in tissue PO2 and an increase in tissue PCO2, further enhancing the pressure gradients driving exchange. This results in a decrease in the partial pressure of oxygen returning to the lungs, prompting more oxygen to move from the alveoli to the blood.

Question 34

Describe the characteristics of venous blood entering the lungs and explain why it has low O2 and high CO2 levels.

Answer 34

Venous blood entering the lungs has low O2 (40 mmHg, decreasing during exercise) and high CO2 (46 mmHg) levels due to the consumption of O2 and production of CO2 by the tissues.

Question 35

Why do alveolar PO2 and PCO2 remain high (100 mmHg and 40 mmHg, respectively) during the breathing cycle?

Answer 35

Alveolar PO2 remains high (100 mmHg) and alveolar PCO2 remains low (40 mmHg) because only a portion of the alveolar air is replaced with fresh atmospheric air during each breath.

Question 36

Explain the role of partial pressure gradients in gas exchange between the alveoli and pulmonary capillary blood.

Answer 36

Partial pressure gradients for O2 (100 - 40 = 60 mmHg) and CO2 (46 - 40 = 6 mmHg) between the alveoli and pulmonary capillary blood cause O2 to diffuse into the blood, and CO2 to diffuse into the alveoli. Diffusion continues until the partial pressures in the blood and alveoli become equal.

Question 37

How do the partial pressures and contents of O2 and CO2 in the blood leaving the lungs compare to those delivered to the tissues?

Answer 37

Blood leaving the lungs has a high partial pressure and content of O2 and a low partial pressure and content of CO2, which are identical to those delivered to the tissues.

Question 38

Compare the partial pressures of O2 and CO2 in the tissue cells to those in arterial blood.

Answer 38

The partial pressures of O2 and CO2 in tissue cells are lower and higher, respectively, compared to those in arterial blood.

Question 39

Describe the process of gas exchange at the cellular level, including the diffusion of O2 and CO2.

Answer 39

O2 diffuses from arterial blood into cells to support their metabolic requirements, and metabolically produced CO2 diffuses into the blood.

Question 40

Explain the characteristics of blood leaving the tissues and its subsequent journey back to the lungs.

Answer 40

Blood leaving the tissues is relatively low in O2 and high in CO2. It then returns to the lungs to replenish on O2 and release CO2, restarting the cycle of gas exchange.