Chapter 18a

Gas Exchange and Transport

About this Chapter

• Diffusion and solubility of gases

• Gas exchange in lungs and tissues

• Gas transport in the blood

• Regulation of ventilation

Figure 18-1

Overview

• Oxygen and carbon dioxide move into and out of the blood at pulmonary and systemic capillaries • Internal respiration

6

5

4 3

CO2 O2

Alveoli of lungs

Airways

CO2

CO2

O2

O2

Pulmonarycirculation

CO2 O2

Cellularrespiration

ATPNutrientsCells

Systemiccirculation

CO2 O2

Oxygen exchangeat cells

Oxygen transport

CO2 exchangeat alveolar-capillaryinterface

Oxygen exchangeat alveolar-capillaryinterface

CO2 exchangeat cells

CO2 transport

2

1

Diffusion and Solubility

• Constants influencing diffusion in the lungs • Surface area• Contact between air and blood

• Membrane thickness• Alveoli and endothelium

• Diffusion distance• Distance between blood and air

• Concentration gradient • Most important factor as others usually are

constant

Movement of Gases

• Pressure gradient• Partial pressure change

• Solubility• Gas into liquid

• Temperature• Higher faster

Figure 18-2a–c

Gases in Solution

(a)

PO2 = 100 mm Hg

PO2 = 0 mm Hg

(b) (c)

PO2 =

100 mm Hg[O2] =5.20 mmol/L

PO2 =

100 mm Hg[O2] =0.15 mmol/L

Figure 18-2c–d

Gases in Solution

(c)

100 mm Hg[O2] =5.20 mmol/L

[O2] =0.15 mmol/L

(d)

100 mm Hg[CO2] =5.20 mmol/L

[CO2] =3.00 mmol/L

PO2=

100 mm HgPO2

=

PCO2=

100 mm HgPCO2

=

Solubility difference between O2 and CO2

Figure 18-3

Gas Diffusion

PO2 = 100 mm Hg

Alveoli

Peripheral tissue

Alveoli

Peripheral tissue

(b) CO2 diffusion(a) Oxygen diffusion

Circulatorysystem

Circulatorysystem

PO2 = 40 PO2

= 100

PO2 = 40 PO2

= 100

PCO2 = 46 PCO2

= 40

PCO2 = 46 PCO2

= 40

PO2 < 40 mm Hg– PCO2

> 46 mm Hg–

PCO2 = 40 mm Hg

Gas Exchange

PLAY Interactive Physiology® Animation: Respiratory System: Gas Exchange

Table 18-1

Partial Pressures

Table 18-2

Gas Exchange

Causes of Low Alveolar PO2

1. Inspired air has abnormally low oxygen content• Altitude

2. Alveolar ventilation is inadequate• Decreased lung compliance• Increased airway resistance• Overdose of drugs• What types??

Figure 18-4a

Alveolar Ventilation

• Pathological conditions that reduce alveolar ventilation and gas exchange

Figure 18-4b

Alveolar Ventilation

Fewer alveoli

Figure 18-4c

Alveolar Ventilation

Low compliance

Figure 18-4d

Alveolar Ventilation

Figure 18-4e

Alveolar Ventilation

Figure 18-5

Gas Exchange

• Oxygen diffuses across alveolar epithelial cells and capillary endothelial cells to enter the plasma – respiratory membrane

Fused basementmembranes

Alveolarair space

Plasma

Alveolarepithelium

0.1–1.5 m

Surfactant

Nucleus ofendothelial

cell

Capillarylumen

RBC

O2

O2

Gas Exchange

• Pathological changes• Decrease in amount of alveolar surface area• emphysema

• Increase in thickness of alveolar membrane• fibrosis

• Increase in diffusion distance between alveoli and blood• pneumonia

Figure 18-6

Oxygen Transport

O2

O2

O2 dissolved in plasma (~PO2) < 2%

O2 dissolved in plasma

O2 + Hb Hb•O2

> 98%

Hb + O2Hb•O2

ARTERIAL BLOOD

Alveolarmembrane

Alveolus

Capillaryendothelium

Transportto cells

Red blood cell

Cells

Used incellular

respiration

Figure 18-7a

Oxygen Transport

• Hemoglobin increases oxygen transport by blood

O

(a) Oxygen transport in blood withouthemoglobin. Alveolar PO2

= arterial PO2

PO2 = 100 mm Hg

PO2 = 100 mm Hg

Alveoli O2 molecule

Arterialplasma

Oxygen dissolves in plasma.

Total O2 carryingcapacity

O2 content of plasma = 3 mL O2/L bloodO2 content of redblood cells = 0

3 mL O2/L blood

Red blood cells with hemoglobin are carry-ing 98% of their maximum load of oxygen.

(b) Oxygen transport at normalPO2

in blood with hemoglobin

PO2 = 100 mm Hg

PO2 = 100 mm Hg

Total O2 carryingcapacity

O2 content of plasma = 003 mL O2/L bloodO2 content of redblood cells = 197 mL O2/L blood

200 mL O2/L blood

Figure 18-7b

Oxygen Transport

Red blood cells carrying 50% of theirmaximum load of oxygen.

(c) Oxygen transport at reduced PO2

in blood with hemoglobinPO2

= 28 mm Hg

PO2 = 28 mm Hg

Total O2 carryingcapacity

O2 content of plasma = 000.8 mL O2/L bloodO2 content of redblood cells = 099.5 mL O2/L blood

100.3 mL O2/L blood

Figure 18-7c

Oxygen Transport

Figure 18-8

The Hemoglobin Molecule

• Hemoglobin consists of 4 subunits, each centered around Fe2+ (a)

Chain

Hemegroup

(b) R = additional C, H, O groups

Porphyrinring

Chain

Figure 18-9

Oxygen-Hemoglobin Dissociation Curve

Figure 18-10a

Oxygen Binding• Physical factors alter

hemoglobin’s affinity for oxygen shift curve right or left

• pH• Temperature• pCO2

• BPG• RBCs produce during

hypoxia• Hb type• Fetal HbF

Figure 18-10b

Oxygen Binding

Figure 18-10c

Oxygen Binding

Figure 18-11

Oxygen Binding

• 2,3-DPG decreases hemoglobin’s affinity for oxygen

Figure 18-12

Oxygen Binding

• Maternal and fetal hemoglobin have different oxygen-binding properties

Figure 18-13

Oxygen Binding

• The total oxygen content of arterial blood depends on the amount of oxygen dissolved in plasma and bound to hemoglobin

TOTAL ARTERIALO2 CONTENT

Oxygen dissolved inplasma (PO2 of plasma)

Oxygenbound to Hb

Total number ofbinding sites

% Saturationof Hb

Adequateperfusionof alveoli

Oxygen diffusionbetween alveoli

and blood

Diffusiondistance

Surfacearea

Lungcompliance

Amount ofinterstitial fluid

Membranethickness

Composition ofinspired air

Numberof RBCs

x

xAirwayresistance

Alveolarventilation

pHPCO2

affected by

helpsdetermineis influenced by

Rate anddepth of

breathing2,3–DPG Hb content

per RBCTemperature

Carbon Dioxide Transport

• Dissolved: 7%• Converted to bicarbonate ion: 70%• Bound to hemoglobin: 23% • Hemoglobin also binds H+

• Hb and CO2: carbaminohemoglobin

H20 + CO2 H2CO3 H+ + HCO3-

Figure 18-14

Carbon Dioxide Transport in the Blood

CO2

CO2

Dissolved CO2

(7%)

Dissolved CO2 Dissolved CO2

CO2 + Hb

Hb + CO2

Hb•CO2 (23%)

Hb•CO2

CO2 + H2O

H2O + CO2

H2CO3

H2CO3

HCO3–

HCO3–

HCO3– in

plasma (70%)

HCO3–

inplasma

H+ + Hb Hb•H

H+ + HbHb•H

Cl–

Cellularrespiration

inperipheral

tissues

VENOUS BLOOD

Alveoli

Transportto lungs

Red blood cell

CA

CA

Capillaryendothelium

Cell membrane

Cl–

Systemiccirculation

Cells

O2CO2

Venous blood

CO2 transportHCO3

– = 70%Hb•CO2 = 23%

Dissolved CO2 = 7%Pulmonarycirculation

Arterial blood

O2 transportHb•O2 > 98%

Dissolved O2 < 2%

(~PO2)

Dry air = 760 mm HgPO2

= 160 mm HgPCO2

= 0.25 mm Hg

Alveoli

O2CO2

PO2 = 100 mm Hg

PCO2 = 40 mm Hg

PO2 = 100 mm Hg

PCO2 = 40 mm Hg

PO2 = 40 mm Hg

PCO2 = 46 mm Hg

PO2 < 40 mm Hg–

PCO2 > 46 mm Hg–

Figure 18-15

Summary of O2 and CO2 Exchange and Transport

Gas Transport

PLAY Interactive Physiology® Animation: Respiratory System: Gas Transport