Chapter 22Respiratory System

Lecture 8

Part 2: O2 and CO2 Transport

Marieb’s Human

Anatomy and Physiology

Ninth Edition

Marieb Hoehn

3

Diffusion Across Respiratory Membrane

Figure from: Hole’s Human A&P, 12th

edition, 2010

4

Diffusion Through Respiratory MembraneThe driving force for the exchange of gases between alveolar air and capillary blood is the difference in partial pressure between the gases. (PP gradients of different gases are independent)

At a given temperature, the amount of a particular gas in solution is directly proportional to its partial pressure outside the solution (Henry’s Law)

Figure from: Hole’s Human A&P, 12th

edition, 2010

6

Composition of Inspired and Alveolar Air

From: Saladin, Anatomy & Physiology, McGraw Hill, 2007

7

Factors Affecting O2 and CO2 Transport

• O2 and CO2 have limited solubility in plasma; so a more efficient way to carry these gases is needed...

• This problem is solved by RBCs– Bind O2 to hemoglobin

– Use CO2 to make soluble compounds

– Reactions in RBCs are• Temporary• Completely reversible

8

Oxygen Transport

• Most oxygen binds to hemoglobin to form oxyhemoglobin (HbO2)• Oxyhemoglobin releases oxygen in the regions of body cells• Much oxygen is still bound to hemoglobin in the venous blood

But what special properties of the Hb molecule allow it to reversibly bind O2?

Lungs

Tissues

Figure from: Hole’s Human A&P, 12th

edition, 2010

PO2 ≈ 40 mm Hg PO2 ≈ 100 mm Hg

9

Review of Hemoglobin’s Structure

Figure From: Martini, Anatomy & Physiology, Prentice Hall, 2001

10

The O2-Hb Dissociation Curve

Recall that Hb can bind up to 4 molecules of O2 = 100% saturation

At 75% saturation, Hb binds 3 molecules of O2 on average

Sigmoidal (S) shape of curve indicates that the binding of one O2 makes it easier to bind the next O2

This curve tells us what the percent saturation of Hb will be at various partial pressures of O2

Figure from: Hole’s Human A&P, 12th

edition, 2010

11

Oxygen ReleaseAmount of oxygen released from oxyhemoglobin increases as

• partial pressure of carbon dioxide increases• the blood pH decreases and [H+] increases (Bohr Effect; shown below)• blood temperature increases (not shown)• concentration of 2,3 bisphosphoglycerate (BPG) increases (not shown)

Figure from: Hole’s Human A&P, 12th

edition, 2010

12

Carbon Dioxide Transport in Tissues

• dissolved in plasma (7%)• combined with hemoglobin as carbaminohemoglobin(15-25%)• in the form of bicarbonate ions (68-78%)

CO2 + H2O ↔ H2CO3

H2CO3 ↔ H+ + HCO3-

CO2 exchange in TISSUES

Figure from: Hole’s Human A&P, 12th

edition, 2010

13

14

Carbon Dioxide Transport in Tissues

• dissolved in plasma (7%)• combined with hemoglobin as carbaminohemoglobin(15-25%)• in the form of bicarbonate ions (68-78%)

CO2 + H2O ↔ H2CO3

H2CO3 ↔ H+ + HCO3-

CO2 exchange in TISSUES

Figure from: Hole’s Human A&P, 12th

edition, 2010

15

Chloride Shift• bicarbonate ions diffuse out RBCs• chloride ions from plasma diffuse into RBCs• electrical balance is maintained

Figure from: Hole’s Human A&P, 12th

edition, 2010

16

Carbon Dioxide Transport in Lungs

CO2 exchange in LUNGS

Figure from: Hole’s Human A&P, 12th

edition, 2010

17

Review

• Oxygen travels in the blood bound to Hb– Four O2 molecules can be bound to 1 Hb

– O2 bound to Hb - oxyhemoglobin

– Uptake and release of O2 is dependent upon PO2 in alveoli and tissues

– Several factors can increase the release of O2 from Hb

• Increased PCO2

• Increased [H+] (decreased pH)

• Increased temperature of blood

18

Review

• Carbon dioxide can travel in several ways– Dissolved in plasma (7%)– As carbaminohemoglobin (15-25%)– As HCO3

- ion (70%)• Recall: H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3

-

• Carbonic anhydrase in RBCs accelerates interconversion between CO2 and HCO3

-

• H+ combines with or dissociates from Hb• HCO3

- diffuses into plasma or into RBCs • Cl- diffuses into RBC (chloride shift) as HCO3

- exits

• Diffusion of CO2 is related to PCO2 in alveoli and tissues