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
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