Ch 18: Gas Exchange and

Transport

Dissolve CO2 & O2 for transport

Transport O2 – role of hemoglobin

Transport CO2

Regulate ventilation

Running Problem: High Altitude

Developed by

John Gallagher, MS, DVM

Diffusion and Solubility of Gases

• Diffusion most rapid over short

distances

– At alveolar and systemic

capillaries

• Concentration Gradient expressed

as Partial Pressure

Diffusion

rate

Surface area x conc. gradient x membr. permeability

Membrane thickness

Fick’s law furthers the principles of diffusion:

Fig 18-1

Partial Pressure

1. = P O2 = 100 mmHg at sea level 2. Since gases can diffuse/dissolve into

liquids, partial pressure allows comparison between the two media.

1. Determines concentration gradient

3. Solubility of gas depends on

1. solubility of molecule in particular liquid

2. pressure gradient

3. temperature

4. Equilibrium not necessarily the same concentration

1. CO2 is 20x more soluble than O2, explains the need for Hb

Dalton’s Law (p 565): the total pressure

exerted by a mixture of gases is equal to

the sum of the pressures exerted by the

individual gases.

78 % N2

PN2=______

mm Hg

21% O2

PO2= _______

mmHg

Total atmospheric

pressure at sea level =

760 mmHg

Air is:

Gas Exchange in Lungs

Fig 18-3 1o factor: Partial Pressure Gradient

Alveolar PO2 = 100mm Hg

Venous PO2 = 40 mm Hg

Simple diffusion drives the transfer

PCO2 has the opposite

Diffusion from capillary to alveoli

Gas Exchange in Lungs

Influence of altitude on PO2 : Mt. Everest: atmospheric P ~ 250 mmHg PO2 = ? (Running Problem)

Alveolar hypoventilation affecting gas exchange airway resistance (?) lung compliance (?)

Resp. membrane changes affecting gas exchange membrane thickness (?) surface area (?)

Hypoxia and hypercapnea

CD Animation Respiratory System: Gas Exchange

Fig 18-4

Oxygen Transport in Blood

• > 98% carried by Hb

• Rest dissolved in plasma

– O2 poorly soluble in plasma

• O2-Hb dissociation curve demonstrates relationship between PO2 and Hb binding of O2

• Other factors affecting O2-Hb dissociation curve

Fig 18-9

Fig 18-10

Hemoglobin (Hb)

• Four protein fractions

• Four heme groups with Fe – 70% of Fe in the body is in heme

• Binds reversibly to O2

– HbO2 or oxyhemoglobin

– 100% binding = saturation

– Pulse Oximeter

• Binding increased by many different conditions:

– ↑ Plasma PO2 • Alveolar PO2 determines plasma PO2

– ↑ pH

– ↓Temperature

– ↑ CO2

– ↓ 2,3-DPG • ↑ by hypoxia, e.g., high altitude

• ↓ in stored blood

– HbF

O2 - Hb DissociationCurve

• Binding is expressed as a %

• Amount of O2 that is delivered is

dependent on available Hb

• Range 70-98%

• Easily measured with pulse

oximeter

pH, Temp and PCO2

• (2,3-DPG also has a role)

Fig 18-13

CO2 Transport in Blood

1. 7% directly dissolved in plasma

2. 70% transported as HCO- dissolved in plasma (acts as a buffer)

1. CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-

2. Carbonic Anhydrase in RBC

3. 23% bound to Hb Carbaminohemoglobin

Excess CO2 in blood = Hypercapnia Leads to acidosis, CNS depression & coma

At the alveoli, CO2 removed via PP gradients

Fig 18-14

Regulation of Ventilation

• Respiratory centers in brain stem integrate input from cortex,

limbic & both central and peripheral chemoreceptors

– Carotid & aortic chemoreceptors for O2, CO2 & H+

– Medullary chemoreceptor for CO2

• Phrenic and intercostal nerves inspiratory muscles

• When neurons cease firing muscles relax expiration

• Low [O2], high [CO2] & high [H+] ventilation

– CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-

Fig 18-20