Gas Exchange and Transport

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Chapter 18

• Gas Exchange and Transport

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Gas Exchange

• Diffusion between alveoli and the blood
• Rate of diffusion is directly proportional to the
  partial pressure (concentration gradient)
• Rate of diffusion is directly proportional to
  surface area
• Rate of diffusion is inversely proportional to
  the thickness of the membrane
• Diffusion is most rapid over short distances

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Pulmonary Circulation

• High Flow, Low Pressure
• Pulmonary artery pressure 25/8 mm Hg

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Daltons Law of Partial Pressures

• Total pressure of mixed gases is the sum of the
  pressures of the individual gasses
• Atmospheric pressure 760 mm Hg
• Partial pressure of an atmospheric gas
• P atm x gas in atmosphere
• 21 oxygen in atmosphere
• PO2 760 mmHg x .21 160 mm Hg

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Boyles Law

• P1V1 P2V2
• If volume is reduced by ½, pressure doubles
• If volume doubles, pressure is reduced by ½

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Movement of Gases

• High pressure to low pressure

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Solubility of Gases

• 1. Pressure gradient
• 2. Solubility in a given liquid
• 3. Temperature

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Solubility

• PO2 in water and gas is the same at equilibrium
• However, CONCENTRATION of oxygen is not the same,
  due to solubility
• 5.2 mmoles O2/L in air at PO2 100 mmHg
• 0.15 mmoles O2 /L in water
• Oxygen is not very soluble in aqueous solns

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CO2 Solubility

• CO2 is 20X more soluble than O2 in aqueous soln
• PCO2 of 100 mmHg
• 5.2 mmole CO2/L air
• 3.0 mmole CO2/L water

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Partial Pressure

• Alveolar PO2 100 mmHg
• Systemic Venous Blood (Pulmonary Artery) PO2 40
  mmHg
• Alveolar PCO2 40 mmHg
• Systemic Venous Blood (Pulmonary Artery) PCO2
  46 mmHg
• Systemic Arterial Blood (Pulmonary Vein) PO2
  100 mmHg
• Systemic Arterial Blood (Pulmonary Vein) PCO2
  40 mmHg

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Differences in Pressures

• Atmospheric vs. Alveolar

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Gas exchange requires pressure gradients

• Any factor that decreases alveolar PO2 decreases
  the pressure gradient and result in less oxygen
  entering the blood.
• Conditions that result in low alveolar PO2
• 1. Changes in the composition of inspired air
  (altitude/pressure changes)
• 2. Disturbances in Alveolar Ventilation
• (Lung Disorders)

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Low alveolar PO2

• 1.) Composition of inspired air
• High Altitudes Mountain sickness
• Deep Sea Diving Nitrogen Narcosis

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Low alveolar PO2

• 2. Alveolar ventilation
• If composition of inspired air is normal, but
  alveolar PO2 is low, then the cause is due to a
  decrease in alveolar ventilation
  hypoventilation
• Causes Increased resistance (asthma), decreased
  compliance (fibrosis), overdose of drugs or
  alcohol that depress CNS and slow rate of
  ventilation

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Normal alveolar PO2, Low Blood PO2

• Problem with Gas exchange
• 1. Decrease in alveolar surface area
• Ex. Emphysema
• 2. Increase in diffusion distance between
  alveoli and blood
• Ex. Pulmonary edema (excess interstitial fluid
  in the lungs)

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Gas Exchange In Tissues

• Cellular respiration O2 consumption, CO2
  production
• PO2 40 mmHg, PCO2 46 mmHg
• Arterial blood PO2 100 mmHg, PCO2 40 mmHg

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Gas Transport in Blood

• Hemoglobin Oxygen binding protein in red blood
  cells
• Total blood oxygen content amount of O2
  dissolved in plasma amount bound to hemoglobin

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Gas Transport in the Blood

• Total blood oxygen content 3 mL plasma O2 197
  mL HbO2 200 mL O2/L blood
• Amount of O2 that binds to hemoglobin depends on
  2 factors
• 1. PO2 of the plasma surround the red blood
  cells
• 2. Number of potential O2 binding sites
  available within RBCs

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Available Hb Binding Sites

• PO2 of plasma is the primary factor determining
  how many of the available hemoglobin binding
  sites are occupied by oxygen.
• Composition of inspired air
• Alveolar inspiration rate
• Efficiency of gas exchange between lung and blood

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Hb

• Total of binding sites depends on the number of
  hemoglobin molecules in the blood

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Hb

• One Hb molecule has four globin subunits, each
  centered around a heme group whose central iron
  atom binds reversibly with oxygen.
• One hemoglobin molecule can bind 4 oxygen
  molecules

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Affinity of Hemoglobin for Oxygen

• Oxyhemoglobin HbO2
• Changes in pH, PCO2, and temperature of blood can
  alter the oxygen-binding capacity of hemoglobin.

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2,3 diphosphoglycerate (2,3-DPG)

• Produced in erythrocytes from an intermediate of
  glycolysis
• If oxyhemoglobin levels are low, which occurs
  when O2 supply is limited, 2,3-DPG synthesis
  occurs
• 2,3-DPG lowers the binding affinity of hemoglobin
  for oxygen, enhancing unloading of oxygen at
  tissues
• Anemia and high altitudes increase 2,3-DPG
  production

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Carbon Dioxide

• Removal of CO2 is physiologically important
  elevated PCO2 causes pH disruption acidosis
• Abnormally high PCO2 depresses CNS function,
  causes confusing, coma, or death

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Carbon Dioxide

• 7 of CO2 produced as a result of cellular
  respiration is carried in the venous blood as
  dissolved CO2
• 93 diffuses into RBCs 70 is converted to
  bicarbonate ion, and 23 binds to hemoglobin
  (Hb-CO2).

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Bicarbonate

• 1. Additional means of CO2 transport to lungs
• 2. Available to buffer
• Carbonic anhydrase, an enzyme in RBCs catalyzes
  the conversion of CO2 to carbonic acid

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Carbonic anhydrase

• CO2 H2O ? H2CO3 ? H HCO3-

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Removal of HCO3-

• Antiport Exchange of one HCO3- for one Cl -
• Transfers HCO3- into the plasma
• Chloride shift

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Removal of Free H

• Hb acts as a buffer and binds H
• Hb-H
• Prevents large changes in the bodys pH

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Carbaminohemoglobin

• CO2 Hb ? Hb-CO2

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CO2 removal at the lungs

• Venous blood in the lungs reverse of the process
  that took place in systemic capillaries. PCO2 of
  alveoli is lower than venous blood. CO2 diffuses
  out of the plasma into the alveoli, and plasma
  PCO2 falls.
• Decrease in plasma PCO2 allows dissolved CO2 to
  diffuse out of RBCs.

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Figure 18-13
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Control of Respiration

• Central Pattern Generator Group of neurons in
  the medulla oblongata that form a network with
  intrinsic rhythmic activity

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Control of Respiration

• 1. Respiratory neurons in the medulla control
  inspiration and expiration
• 2. Neurons in the pons influence the rate and
  depth of ventilation
• 3. Rhythmic pattern of breathing arises from a
  network of spontaneously discharging neurons
• 4. Ventilation is subject to modulation by
  chemical factors and by higher brain centers.

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Reflex Control of Ventilation
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Chemoreceptors

• Sensory input from chemoreceptors modifies the
  rhythmicity of the central pattern generator
• CARBON DIOXIDE!!!!
• Oxygen and pH play lesser roles

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Peripheral chemoreceptors

• Carotid and aortic bodies
• Respond to a decrease in P02, increases in plasma
  H, and PCO2
• Any condition that decreases pH or P02 or
  increases PCO2 stimulates ventilation through the
  peripheral chemoreceptors

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Figure 18-18
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Central Chemoreceptors

• Chemoreceptors located in the medulla
• Receptors set respiratory pace

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Figure 18-20