Respiratory System: Gas Exchange

Respiratory System: Gas Exchange and Transport

• Gas exchange in the lungs occurs at the alveolar-capillary interface, where air and blood exchange gases.
• Oxygen (O₂) enters the blood from the alveoli.
• Carbon dioxide (CO₂) leaves the blood and enters the alveoli.
• Oxygenated blood is pumped to the rest of the body.
• Deoxygenated blood returns to the lungs.
• Gas exchange is driven by pressure differences (pressure gradient).
• Gases move from higher pressure areas to lower pressure areas.

Gas Exchange

• Gas exchange across alveolar-capillary and capillary-tissue interfaces occurs through simple diffusion.
• The pressure gradient dictates the movement of gases.

Rate of Gas Diffusion

• Fick's Law of Diffusion describes the rate: it's proportional to surface area, membrane permeability, and the concentration gradient, inversely proportional to membrane thickness.
• Diffusion rate is primarily determined by the concentration gradient.

Gas Solubility

• Gas solubility affects gas movement between air and blood (plasma).
• Highly soluble gases dissolve easily at low pressures.
• Less soluble gases require higher pressures to dissolve.
• CO₂ is much more soluble than O₂ in water. This is critical for CO₂ transport.

Gas Exchange: Gas Solubility (cont.)

• Oxygen binding proteins (e.g., hemoglobin) are crucial for O₂ transport because O2's low solubility in water makes it inefficient to dissolve enough O2 into plasma to meet bodily needs.

Gas Exchange: Oxygen Binding

• Hemoglobin (Hb) in red blood cells binds most O₂.
• Each Hb molecule can bind up to four O₂ molecules.
• The amount of O₂ bound to Hb depends on the partial pressure of O₂ (P02).
• The percentage of O₂ bound to Hb is known as the percentage saturation.

Oxygen-Hemoglobin Dissociation Curve

• This curve shows the relationship between P02 and the percentage of Hb saturated with O₂.
• The curve's shape reflects how O₂ binding affinity changes with different P02 levels.

Factors Affecting O₂ Binding Affinity of Hb

• pH: Lower pH (more acidic) reduces O₂ binding affinity (Bohr effect). Higher pH increases O₂ binding.
• Temperature: Higher temperature reduces O₂ binding affinity. Lower temperature increases it.
• 2,3-DPG: A metabolic by-product that reduces O₂ binding affinity. Higher concentrations are linked to lower tissue O₂-carrying capacity, while lower concentrations yield higher O₂-carrying capacity.
• PCO2 : Higher PCO2 reduces O₂ binding affinity. Lower PCO2 increases it.

Physiologic Significance of the Bohr Effect

• Increased tissue metabolic activity releases H⁺ into the blood, decreasing pH, and causing Hb to release more O₂ to meet tissue needs
• Blood cools in the lungs, increasing pH and causing Hb to bind to more O₂.

Gas Transport: Transport of CO₂ in the Blood

• CO₂ is a byproduct of cellular respiration.
• CO₂ travel in the blood in three forms:
  • Dissolved in plasma (7%)
  • Bound to hemoglobin (23%)
  • As bicarbonate ions (HCO₃⁻) (70%)
• Carbonic anhydrase facilitates CO₂ conversion to bicarbonate.

Regulation of Lung Ventilation

• Breathing is rhythmic and controlled by neural centers in the central nervous system (CNS).
• Normal quiet breathing is automatic and subconscious.
• Rate and depth of breathing are adjusted by chemoreceptors responding to changes in blood gas levels (e.g., CO₂, O₂, and pH)
• Higher brain centers can also influence respiratory rate and depth (e.g., emotions or voluntary control).
• Chemoreceptors in the blood and cerebrospinal fluid monitor pH and blood gas levels. Changes in these levels lead to changes in respiratory rate.