lect 8 objs

• Partial pressure


  The pressure exerted by a specific gas in a mixture of gases
• Fractional concentration


  The fraction or percentage of a specific gas in a mixture of gases
• Normal fractional concentrations and sea level partial pressures

  • O2: 21%, 160 mmHg
  • CO2: 0.03%, 0.24 mmHg
  • N2: 79%, 600 mmHg
• Anatomic dead space


  Volume of air in the conducting airways incapable of gas exchange with blood
• Alveolar dead space


  Volume of air in the alveoli incapable of gas exchange with blood
• Physiological dead space


  Sum of anatomic and alveolar dead space
• Total minute ventilation


  Amount of total gas ventilated in one minute
• Alveolar minute ventilation


  Amount of air reaching the alveoli per minute
• Calculating total minute ventilation

  VE = VT x f (Minute ventilation = Tidal volume x Breathing frequency)
• Calculating alveolar ventilation


  VA = (VT - VD) x f (Alveolar ventilation = (Tidal volume - Dead space volume) x Breathing frequency)
• Breathing patterns


  Breathing deeply and slowly: VE stays the same but VA increases<br>Breathing shallowly and rapidly: VE stays the same but VA decreases (even to zero)
• Alveolar ventilation and PCO2

  There is an inverse relationship between PACO2 and VA
• The sum of the partial pressures of a gas must be equal to the total pressure
• The partial pressure of a gas is equal to the fraction of gas in the gas mixture times the total pressure
• By the time inspired gas reaches the trachea, it is fully saturated with water vapor, which exerts a pressure of 47 mm Hg at body temp and dilutes the partial pressures of N2 and O2
• The conducting airways do not participate in gas exchange, and therefore the partial pressures of O2, N2 and H2O vapor remain unchanged in the airways until the gas reaches the alveolus
• Minute ventilation (VE) = VT (ml/breath) x f, respiratory rate (breaths/min)
• At rest, VE = 6,000 ml = 500 ml x 12 breaths/min (exercise can increase 25-fold, to 150 L/min)
• VT is more important than respiratory rate when minute ventilation increases
• Anatomical dead space = 150 ml
• At rest, VA = 4,200 ml = (500 ml - 150 ml) x 12 breaths/min
• Due to its high diffusibility, PACO2 = PaCO2
• In healthy individual, PAO2 is very close to PaO2; difference is the A-a gradient
• Pulmonary system


  • Low resistance, low pressure system
  • Receives a very large flow compared to individual systemic circulations
  • Virtually all of the cardiac output is transported through the lungs for exchange of gases amongst its many functions
• Pulmonary arteries


  • Less elastin and smooth muscle in its walls compared to the aorta
  • Pulmonary arterioles are thin-walled and contain less smooth muscle, which means that they have less ability to constrict compared to arterioles in the systemic circulation
• Pulmonary capillary beds


  • Form a mesh-like network making blood flow as a sheet
• The entire cardiac output is perfused through the lungs despite only a 10 mmHg pressure gradient
• Pulmonary vasculature resistance (PVR)


  About 1/10th of that of the systemic circulation
• Increases in Cardiac output


  Decrease PVR
• Decreases in cardiac output, such as heart failure


  Result in increases in PVR
• Increases in cardiac output, which occur during exercise


  Decrease PVR
• How increases in cardiac output decrease PVR

  1. Increases in pulmonary artery pressure lead to decreased PVR and increased blood flow due to:

  2. Recruitment of more capillaries (adding in parallel)

  3. Distention of capillaries (increasing the diameter of the vessels)
• These changes in PVR with cardiac output are mediated by pressures, and NOT VASODILATION
• Why PVR increases at high and low lung volumes


  1. At high lung volumes extra-alveolar vessels are pulled open, but the expanding alveoli collapse the alveolar vessels, increasing PVR

  2. At low lung volumes, alveolar vessels widen due to less alveolar expansion, but extra-alveolar vessels are compressed due to pleural pressures
• PVR is lowest at functional residual capacity
• Zone 1


  Established when alveolar pressure is greater than arterial pressure, thus collapsing the blood vessels and allowing no blood flow
• Zone 2


  Occurs when arterial pressure is greater than alveolar pressure, blood flow is determined by the difference between arterial and alveolar pressure
• Zone 3


  Venous pressure becomes greater than alveolar pressure and blood flow is dictated by the difference between arterial and venous pressure
• Pulmonary artery and venous pressures decrease as you go from base to apex
• The blood vessels get more constricted as you go from base to apex due to the relationships of the 3 pressures