Functions of the Respiratory System

Moving air in and out of the lungs is only the mechanical half of the respiratory system's job. This page covers what that airflow actually accomplishes: gas exchange, thermoregulation, immune defense, and blood pH control.

Key Takeaways

• Gas exchange happens by diffusion across the one-cell-thick alveolar-capillary membrane; 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 works through vasodilation (releases heat) and vasoconstriction (conserves heat) in surface blood vessels.

• Respiratory immune defenses include vibrissae/mucous membranes, the mucociliary escalator, lysozyme (targets gram-positive peptidoglycan), macrophages (phagocytosis and signaling), IgA (mucosal neutralization), and mast cells (IgE-triggered inflammatory response).

• Blood pH is regulated through the bicarbonate buffer system (CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺): increased breathing removes CO₂ and raises pH (corrects acidemia); decreased breathing retains CO₂ and lowers pH (corrects alkalemia).

Gas Exchange

Gas exchange takes place in the alveoli, the small, balloon-like sacs at the very end of the respiratory tree. Surrounded by a dense network of capillaries, the alveoli provide a massive surface area for gas movement.

Deoxygenated blood — low in oxygen, high in carbon dioxide — arrives at the lungs via the pulmonary arteries, which carry blood from the right side of the heart directly to the capillaries surrounding the alveoli.

MCAT Callout — Exception to the Rule: arteries are usually described as carrying oxygenated blood, but the pulmonary arteries are the standard tested exception — they carry deoxygenated blood from the heart to the lungs. The pulmonary veins carry the exception the other way, returning oxygenated blood to the heart.

The alveoli themselves are lined by a single layer of thin epithelial cells; the capillaries right next to them are lined by a single layer of endothelial cells. Together, the alveolar wall and capillary wall are just one cell thick, minimizing the distance gases have to diffuse.

This setup allows efficient gas exchange by diffusion — a process that requires no energy. Gases move from areas of high concentration to areas of low concentration, so oxygen flows into the blood and carbon dioxide flows out into the alveolus to be exhaled. This movement is driven by steep concentration gradients, maintained by both ventilation (bringing fresh air into the alveoli) and blood flow (constantly delivering CO₂-rich blood and carrying away oxygenated blood).

Once oxygen has diffused into the bloodstream, the newly oxygenated blood returns to the heart through the pulmonary veins, which deliver it to the left atrium — from there, it's pumped out to the rest of the body. In short: pulmonary arteries bring in CO₂-rich, oxygen-poor blood; gases exchange across one thin cell layer; pulmonary veins carry the oxygenated blood back to the heart — all of it passive, powered by diffusion and pressure gradients.

Thermoregulation

The respiratory system also contributes to thermoregulation — how the body maintains a stable internal temperature. The body releases or conserves heat by adjusting blood flow to the skin and respiratory surfaces, especially in the lungs.

Vasodilation and Vasoconstriction

When the body needs to cool down, it triggers vasodilation: capillaries near the body's surface expand, allowing more blood to flow through them. With more blood circulating near the surface, more thermal energy is lost to the environment.

When the body needs to conserve heat — like in cold conditions — it triggers vasoconstriction: capillaries narrow, so less blood flows near the surface, helping the body retain thermal energy.

Immune Defense

Every inhalation brings in more than air — dust, pollen, bacteria, and other potential pathogens come along with it. The respiratory tract is equipped with several layers of defense:

pH Control and the Bicarbonate Buffer System

The body maintains a very narrow range of blood pH — around 7.35 to 7.45 — and the respiratory system helps by regulating carbon dioxide levels, which are directly tied to how acidic or basic the blood becomes.

This happens through the bicarbonate buffer system, a reversible chemical equilibrium:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺

Because this reaction produces hydrogen ions, an increase in carbon dioxide results in a lower pH — carbon dioxide functions like an acid in the body.

Acidemia and Alkalemia

In acidemia (blood pH too low), hydrogen ion concentration increases. Chemoreceptors detect the rise in H⁺ and signal the medulla oblongata to increase respiratory rate. Breathing faster removes CO₂ from the blood; as CO₂ falls, the equilibrium shifts left, reducing H⁺ and raising pH back toward normal.

In alkalemia (blood pH too high), there are too few hydrogen ions. To compensate, the body slows breathing, allowing CO₂ to accumulate. This shifts the equilibrium right, generating more H⁺ and bringing pH back down toward normal.

MCAT Callout — Which Way Does It Shift?: more breathing removes CO₂ and shifts the equilibrium left (raises pH, treats acidemia). Less breathing retains CO₂ and shifts the equilibrium right (lowers pH, treats alkalemia).

Overall, the lungs play a powerful role in acid-base balance by adjusting how much CO₂ is retained or exhaled — directly influencing the bicarbonate equilibrium.

Common MCAT Mistakes

• Assuming all arteries carry oxygenated blood. The pulmonary arteries are the tested exception — they carry deoxygenated blood from the heart to the lungs; the pulmonary veins carry oxygenated blood back.

• Forgetting why gas exchange is so efficient. The alveolar-capillary membrane is only one cell thick on each side, which minimizes diffusion distance and lets gas exchange happen passively, with no energy input.

• Mixing up vasodilation and vasoconstriction. Vasodilation expands surface capillaries to release heat (cooling); vasoconstriction narrows them to conserve heat (warming).

• Confusing acidemia and alkalemia responses. Acidemia (low pH, excess H⁺) triggers faster breathing to blow off CO₂ and shift the bicarbonate equilibrium left; alkalemia (high pH, too few H⁺) triggers slower breathing to retain CO₂ and shift the equilibrium right.

MCAT-Style Concept Check

Question: A patient's blood pH is measured at 7.30 (acidemia). Which of the following correctly predicts the change in respiratory rate and its effect on the bicarbonate buffer equilibrium (CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺)?

• A) Respiratory rate decreases, shifting the equilibrium right and lowering pH further

• B) Respiratory rate increases, shifting the equilibrium left, which consumes H⁺ and raises pH back toward normal

• C) Respiratory rate increases, shifting the equilibrium right, which lowers pH further

• D) Respiratory rate stays unchanged, since pH is regulated only by the kidneys

Answer: B

Explanation: Chemoreceptors detect the elevated H⁺ concentration associated with acidemia and signal the medulla oblongata to increase respiratory rate. Because CO₂ behaves like an acid in the bicarbonate equilibrium, removing it by exhaling faster pulls the reaction leftward — consistent with the acidemia/alkalemia relationship already established above, where more breathing shifts the equilibrium left and less breathing shifts it right.

FAQ

Why do the pulmonary arteries carry deoxygenated blood when arteries are usually described as carrying oxygenated blood?

The pulmonary arteries are the standard tested exception to that rule — they carry deoxygenated blood from the right side of the heart to the lungs, where it picks up oxygen. The pulmonary veins carry the opposite exception, returning oxygenated blood to the heart.

Why is the alveolar-capillary membrane only one cell thick?

Both the alveolar wall (epithelial cells) and the capillary wall (endothelial cells) are single-cell layers, so together the membrane gases cross is just one cell thick on each side — this minimizes diffusion distance and allows efficient, passive gas exchange.

What's the difference between vasodilation and vasoconstriction in thermoregulation?

Vasodilation expands surface capillaries, increasing blood flow near the skin so more heat is lost to the environment (cooling the body). Vasoconstriction narrows those capillaries, reducing surface blood flow so more heat is retained (warming the body).

How does the respiratory system help control blood pH?

Through the bicarbonate buffer system. Because CO₂ behaves like an acid, the body adjusts breathing rate to control CO₂ levels: faster breathing removes CO₂ and raises pH (correcting acidemia), while slower breathing retains CO₂ and lowers pH (correcting alkalemia).