Gas Exchange

outline the respiratory membrane and what it includes

• ~0.5 um thick

• ~300 million alveoli

• outside → in: alveolar epithelium → basement membrane → capillary endothelium → fluid (air-water interface)

diffusion of gas across an exchange surface is proportional to _______

• surface area

• pressure gradient

• solubility of gas in water (CO2 diffusion rate is 20x greater than O2)

diffusion of gas across an exchange surface is inversely proportional to _______

thickness of exchange surface

what is the formula used to calculate the partial pressure of gas x?

P^x = atmospheric pressure x franctional concentration of x in the mixture

summarize Henry’s Law

the concentration of a gas in solution = its partial pressure x its solubility

describe tracheal air

conducting zone: body temp (37 ^oC), saturated w water vapor

describe the composition of alveolar air

P^O2 decreases and P^CO2 increases due to mixing of inspired air w leftover air containing carbon dioxide

in external respiration, what are the values of P(arterial blood) and P(venous blood) for O2?

Pa = 40 mmHg and Pv = 100 mmHg

in external respiration, what are the values of P(arterial blood) and P(venous blood) for CO2?

Pa = 45 mmHg and Pv = 40 mmHg

in internal respiration, what are the values of P(arterial blood) and P(venous blood) for O2?

Pa = 95 mmHg and Pv = 40 mmHg

in external respiration, what are the values of P(arterial blood) and P(venous blood) for CO2?

Pa = 40 mmHg and Pv = 45 mmHg

describe alveolar regulation of respiration

blood flow directed to alveoli with higher P^O2 (capillaries constrict when P^O2 is too low)

describe tissue regulation of respiration

• interstitial P^O2 falls, interstitial P^CO2 rises

• pressure gradient increases btwn tissue and arriving blood

• increases rate of diffusion

outline the three different mechanisms of CO2 transport

1. physically dissolved in plasma (~7%)

2. bound to amino groups on carbaminohemoglobin (~23%)

3. chemically modified in the form of carbonic acid (~70%)

what enzyme present in most cells and abundant in RBCs accelerates the carbonic acid reaction?

carbonic anhydrase (CA)

List the steps of CO2 exchange in tissues

1. Carbon dioxide diffuses into capillary down its partial pressure gradient

2. Absorbed by RBCs

3. Carbaminohemoglobin (23%)

4. Converted to carbonic acid (70%)

5. HCO3- is moved into the plasma in exchange for chloride

List the steps of CO2 exchange in the lungs

1. Carbon dioxide diffuses into alveolus down its partial pressure gradient from plasma

2. RBCs release CO₂ (from 2 sources)

3. Released from hemoglobin

4. HCO3-  moves into RBCs (chloride exchange)

5. Carbonic acid converted back into CO₂

outline O2 that’s physically dissolved in the plasma

• ~2% of O2 in blood

• responsible for PP of O2 in blood

• poor solubility in plasma

outline O2 that’s bound to the heme groups of hemoglobin

• ~90% of oxygen in blood

• oxyhemoglobin carries up to 4 o@ molecules

• doesn’t contribute to PP of O2 in blood

what is cooperative binding?

the process in which the loading of oxygen onto one heme site facilitates loading of oxygen on remaining heme sites 

describe what is happening in the plateau region of the oxyhemoglobin dissociation curve

oxygen loading in the lungs

describe what is happening in the steep region of the oxyhemoglobin dissociation curve

oxygen unloading in the lungs

what is the Bohr effect?

changes in pH shifts the curve

what happens to the curve when pH increases?

it shifts to the left

what happens to the curve when pH increases?

it shifts to the right

what is the effect of an increase in CO2?

• an increase H⁺ and therefore decrease pH which shifts curve

• H⁺ binds to Hb which decreases the affinity of Hb for oxygen

• Increased formation of carbaminohemoglobin which decreases affinity of Hb for oxygen

what effect does a temperature increase have on the curve?

it shifts the curve to right

compare the affinity of carbon monoxide compared to oxygen

CO has a 250 times greater affinity for Hb than O2

what is the effect of carbon monoxide on the Oxy-HB dissociation curve?

it shifts it to the left

describe the impact of CO on heme group affinity?

Heme groups not bound to CO will have an increased affinity for oxygen and will not give up oxygen as easily to the tissues