Functions of the Respiratory System
High-Yield Summary
- Gas exchange happens by diffusion across the one-cell-thick alveolar-capillary membrane — no energy required.
- Pulmonary arteries carry deoxygenated blood to the lungs; pulmonary veins carry oxygenated blood back to the heart — the reverse of the usual artery/vein oxygenation rule.
- Thermoregulation: vasodilation near the surface releases heat; vasoconstriction conserves it.
- Respiratory immune defenses layer from nose to alveoli: vibrissae/mucous membranes → mucociliary escalator → lysozyme → macrophages → IgA → mast cells.
- Blood pH is regulated via the bicarbonate buffer system: increased breathing removes CO₂ and raises pH (corrects acidemia); decreased breathing retains CO₂ and lowers pH (corrects alkalemia).
Common MCAT Trap
- Pulmonary arteries carry DEOXYGENATED blood (heart → lungs); pulmonary veins carry OXYGENATED blood (lungs → heart) — the standard tested exception to "arteries = oxygenated, veins = deoxygenated."
- Gas exchange is passive diffusion, powered by concentration gradients maintained by ventilation + blood flow — not active transport.
Respiratory Immune Defenses
- Vibrissae / mucous membranes
- Nasal hairs and membranes trap larger inhaled particles.
- Mucociliary escalator
- Mucus traps pathogens/debris; cilia beat upward to move it toward the throat.
- Lysozyme
- Enzyme in nasal cavity/saliva that breaks down the peptidoglycan wall of gram-positive bacteria.
- Macrophages
- Patrol the alveoli; engulf/digest pathogens and signal the wider immune system.
- IgA antibodies
- Coat mucosal surfaces, neutralizing pathogens before deeper tissue entry.
- Mast cells
- Carry IgE; release inflammatory chemicals — fights infection but also drives allergic reactions.
Bicarbonate Buffer System
CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺
- CO₂ = Carbon dioxide — acts like an acid in the body
- H₂CO₃ = Carbonic acid
- HCO₃⁻ = Bicarbonate ion
- H⁺ = Hydrogen ion — drives pH down as it accumulates
- More breathing removes CO₂, shifts equilibrium left, raises pH — corrects acidemia.
- Less breathing retains CO₂, shifts equilibrium right, lowers pH — corrects alkalemia.
Acidemia vs. Alkalemia — Respiratory Response
| Acidemia (pH too low) | Alkalemia (pH too high) |
|---|---|
| H⁺ concentration increases | H⁺ concentration decreases (too few H⁺) |
| Chemoreceptors signal medulla to increase breathing rate | Body slows breathing |
| CO₂ removed faster, equilibrium shifts left | CO₂ accumulates, equilibrium shifts right |
| H⁺ falls, pH rises back toward normal | H⁺ rises, pH falls back toward normal |
Quick Recall
Which blood vessels are the exception to "arteries carry oxygenated blood"?
What enzyme in respiratory secretions targets gram-positive bacterial cell walls?
How does increased breathing affect blood pH?