Gas exchange Knowledge-Based Study

Study notes

1. Mechanisms of Gas Exchange

   Gas exchange is the physical process by which oxygen ($O_2$) is absorbed into an organism and carbon dioxide ($CO_2$) is expelled into the environment. In humans, this exchange relies entirely on passive diffusion down partial pressure gradients. No metabolic energy ($ATP$) is directly expended to move these respiratory gases across cell membranes.

   Structural Adaptations of the Alveoli

   The primary site of external gas exchange in the human body is the alveoli, microscopic air sacs clustered at the ends of terminal bronchioles. The lungs contain approximately 300 to 500 million alveoli, creating a total surface area of roughly 70 to 100 square meters. Several key adaptations ensure efficient diffusion according to Fick's Law:

   • Extremely thin barrier: The respiratory membrane consists of the simple squamous epithelium of the alveolar wall (type I pneumocytes), the capillary endothelium, and their fused basement membranes. The total diffusion distance is less than 0.5 to 1.0 micrometers.
   • Moist inner surface: Gases must dissolve in fluid before diffusing across cell membranes. Alveolar walls are lined with a thin layer of fluid, kept stable by pulmonary surfactant secreted by type II pneumocytes to lower surface tension and prevent alveolar collapse.
   • Extensive vascularization: A dense network of pulmonary capillaries envelopes each alveolus, maintaining a continuous blood supply.
   • High concentration gradients: Continuous ventilation (breathing) replenishes alveolar oxygen and removes carbon dioxide, while continuous blood flow delivers deoxygenated blood and carries away oxygenated blood.

   Diffusion Gradients at the Alveolar Surface

   External respiration refers specifically to the exchange of respiratory gases between the air inside the alveoli and the blood flowing through the pulmonary capillaries. This movement is driven strictly by differences in partial pressure (denoted as $P_{O_2}$ and $P_{CO_2}$).

   Partial Pressure Dynamics

   Atmospheric air entering the alveoli mixes with residual air already present in the respiratory tract, resulting in distinct partial pressures compared to outside air:

   • Oxygen Exchange: Deoxygenated blood arriving at the pulmonary capillaries from the right ventricle via the pulmonary arteries has a $P_{O_2}$ of approximately 40 mmHg. The alveolar air maintains a $P_{O_2}$ of roughly 104 mmHg. Driven by this steep gradient of about 64 mmHg, oxygen diffuses rapidly across the respiratory membrane from the alveoli into the blood plasma and immediately enters red blood cells.
   • Carbon Dioxide Exchange: Deoxygenated blood arriving at the alveoli has a $P_{CO_2}$ of approximately 45 to 46 mmHg, while the alveolar air has a $P_{CO_2}$ of about 40 mmHg. Although this concentration gradient (around 5 to 6 mmHg) is much smaller than that for oxygen, carbon dioxide is roughly 20 times more soluble in body fluids and tissues than oxygen, allowing it to diffuse out of the blood and into the alveoli at an equivalent rate.

   By the time blood traverses only one-third of the length of the pulmonary capillary, complete equilibrium is typically reached, leaving the pulmonary capillary blood with a $P_{O_2}$ of ~100 mmHg and a $P_{CO_2}$ of 40 mmHg.

   How Oxygen and Carbon Dioxide Travel

   Once gases cross the respiratory membrane, the circulatory system carries them between the lungs and metabolizing tissues.

   Oxygen Transport

   Oxygen has poor solubility in aqueous solutions such as blood plasma. Consequently, it is transported in two ways:

   • Dissolved in plasma: Approximately 1.5% of total blood oxygen remains physically dissolved in plasma water.
   • Bound to Hemoglobin: About 98.5% is chemically bound to hemoglobin inside erythrocytes. Each hemoglobin molecule contains four iron-bearing heme groups, each capable of binding one molecule of $O_2$ reversibly to form oxyhemoglobin ($Hb(O_2)_4$).

   Carbon Dioxide Transport

   Carbon dioxide produced by cellular metabolism is transported in the blood through three distinct mechanisms:

   1. Dissolved in plasma: Around 7% to 10% is transported directly dissolved in blood plasma.
   2. Carbaminohemoglobin: Roughly 20% to 23% binds reversibly to the globin amino groups of hemoglobin (not to the iron atom), forming carbaminohemoglobin.
   3. Bicarbonate ions ($HCO_3^-$): The majority (approximately 70%) is transported as bicarbonate dissolved in plasma. Inside erythrocytes, the enzyme carbonic anhydrase rapidly catalyzes the reaction between $CO_2$ and water:
      $CO_2 + H_2O ightleftharpoons H_2CO_3 ightleftharpoons H^+ + HCO_3^-$
      Bicarbonate ions exit the erythrocyte in exchange for chloride ions (the chloride shift), preserving electrical neutrality.

   Gas Exchange at the Tissues

   Internal respiration is the exchange of gases between systemic capillaries and the metabolizing cells of the body's tissues. Actively respiring cells continuously consume oxygen during aerobic cellular respiration and produce carbon dioxide as a metabolic byproduct.

   Partial Pressures and Unloading

   At the systemic capillary beds, the partial pressure gradients run in the opposite direction from those in the lungs:

   • Oxygen Delivery: Oxygenated arterial blood arrives at the tissues with a $P_{O_2}$ of ~95 to 100 mmHg. Resting tissue fluid typically has a $P_{O_2}$ of around 40 mmHg (or lower in active muscle). This gradient drives oxygen off hemoglobin and out of systemic capillaries into the interstitial fluid and cells.
   • Carbon Dioxide Removal: Metabolically active cells have a intracellular $P_{CO_2}$ exceeding 45 to 50 mmHg, whereas arterial blood arrives with a $P_{CO_2}$ of 40 mmHg. Carbon dioxide diffuses down its gradient from the cells into the bloodstream.

   Physiological Factors Promoting Oxygen Release

   Metabolically active tissues produce factors that lower hemoglobin's affinity for oxygen, facilitating oxygen unloading where it is most needed:

   • Lower pH / Increased Acidity (Bohr Effect): Lactic acid and carbonic acid lower local pH, shifting the oxygen-hemoglobin dissociation curve to the right.
   • Elevated Temperature: Heat produced by active muscles promotes the dissociation of oxygen from hemoglobin.
   • Increased $P_{CO_2}$: High local $CO_2$ concentrations directly encourage hemoglobin to release oxygen.