Gas Exchange

A) External Respiration

Exchange of gases at the alveoli (exterior of body).

• Dalton’s law of partial pressure: each gas in a mixture exerts its own pressure, which is proportional to the volume.

• The partial pressure of the gas is the amount of pressure that each gas exerts in the air, which is symbolized as “P”.

  • Atmospheric pressure = 760 mmHg

  • O₂ is 20.9% in the air → P_O2 = 0.209 × 760 = 159 mmHg

  • CO₂ is 0.04% in the air → P_CO2 = 0.0004 × 760 = 0.3 mmHg

• Gases will diffuse from a place of HIGH PARTIAL PRESSURE to places with LOW PARTIAL PRESSURE.

1) Oxygen

• P_O2 in the pulmonary capillaries is lower than that in the alveoli.
  → O₂ diffuses from the air to the blood.

• O₂ binds with hemoglobin at each heme, forming a new molecule - oxyhemoglobin.

  • Hb + O₂ → HbO₂

• In lungs, oxygen dissolves into the plasma and then forms oxyhemoglobin in RBC’s. This keeps plasma from becoming saturated, so O₂ can keep diffusing into the blood.

  At high altitudes, P_O2 is lower because the total atmospheric pressure is lower. Mountain climbers must use O₂ tanks with pressurized air. Otherwise, the P_O2 of inhaled air will be so low that O₂ will not diffuse rapidly enough into the blood. This is why airplanes have a pressurized cabin.

2) Carbon dioxide

• At the pulmonary capillaries, the gas mixture has more CO₂ than the air, so the P_CO2 is higher at the lungs than in the air.
  → CO₂ diffuses into the alveoli.

• Carbon dioxide leaves the lungs in 3 different forms:

  1. Only about 9% of carbon dioxide diffuses directly out of the plasma into the air

  2. 27% of the carbon dioxide in the body is  carried as carbaminohemoglobin (HbCO₂)
     → at lungs, HbCO₂ → Hb + CO₂ then CO₂ diffuses out to air.

  3. 64% the body’s carbon dioxide is carried as a bicarbonate ion: HCO₃^-
     → at lungs, this ion reforms CO₂ and OH^-, and CO­₂ diffuses out to the lungs.

• At lungs: HHb⁺ dissociates and the H⁺ rejoins the  bicarbonate ion HCO₃^- to form H₂CO₃ which then  breaks down to H₂O and CO₂

  • H⁺ + HCO₃^- → H₂CO₃ → H₂O + CO₂

B) Internal Respiration

Exchange of gas at tissues, again, depends on P_CO2 and P_O2.

1) Oxygen

• P_O2 at tissues is extremely low because cells use up all O­ ₂ for cellular respiration.

• At such low P_O2, causes HbO₂ → Hb + O₂ and O₂ to diffuse from erythrocytes to tissues.

→ Without hemoglobin, O₂ can still dissolve in blood, but only about 0.3 mL / 100 mL of blood. The body requires at least 10 X this much. Hemoglobin increases the blood’s ability to carry oxygen by about 70 X  (up to 20 mL /100 mL of blood!

• Oxyhemoglobin (HbO₂) = hemoglobin is bound to O₂. Thus, normal hemoglobin (Hb) is also called deoxyhemoglobin.

2) Carbon dioxide

• Cells constantly make CO_2, so the P_CO2 of tissues is high compared to the blood

• Thus, carbon dioxide diffuses out of tissues into the blood. 

1. About 9% dissolves into plasma.

2. About 27% binds to Hb in plasma to form HbCO₂ (carbaminohemoglobin)

3. About 64% reacts with water in plasma to form carbonic acid, which then dissociates to  form H­⁺ and HCO₃^– (bicarbonate ion!)

   • CO₂ + H₂O → H₂CO₃ → H­⁺ + HCO₃^–

   • This reaction is catalyzed by the enzyme carbonic anhydrase, inside RBCs, which speeds up the formation of bicarbonate by 250 times

• PROBLEM: H⁺ is acidic and will lower the pH of blood beyond 7.35!

• SOLUTION: Deoxyhemoglobin can act as a buffer  by binding to excess H⁺ and “pulling them out” of the solution

  • H⁺ + Hb → HHb⁺ (reduced hemoglobin!)

C) Binding Capacity of Hb