Gas Exchange and Transport

Gases diffuse down partial pressure gradients. Each gas moves from where its partial pressure (its individual pressure in a mixture) is higher to where it's lower — simple diffusion, no energy required. This principle drives all gas exchange:

• External respiration (at the lungs): the air in the alveoli is high in oxygen and low in carbon dioxide compared with the blood arriving from the body. So oxygen diffuses from alveoli into blood, and carbon dioxide diffuses from blood into alveoli to be exhaled.
• Internal respiration (at the tissues): body cells are using oxygen (so tissue oxygen is low) and producing carbon dioxide (so tissue CO₂ is high). So oxygen diffuses from blood into tissues, and carbon dioxide diffuses from tissues into blood.

The blood thus acts as a shuttle: loading oxygen and unloading CO₂ at the lungs, then unloading oxygen and loading CO₂ at the tissues.

Oxygen transport. Oxygen doesn't dissolve well in blood, so about 98% is carried bound to hemoglobin in red blood cells (recall each hemoglobin carries up to four O₂), with only a small amount dissolved in plasma. Hemoglobin loads oxygen where oxygen is plentiful (lungs) and releases it where oxygen is scarce (active tissues) — and it releases more readily where tissues are warm, acidic, or metabolically active (exactly where oxygen is most needed). Pulse oximetry estimates the percentage of hemoglobin saturated with oxygen.

Carbon dioxide transport. CO₂ is carried in blood three ways:

• As bicarbonate (~70%): most CO₂ is converted to bicarbonate ions (HCO₃⁻) in red blood cells (via carbonic anhydrase), which also links to blood pH (recall the bicarbonate buffer). This is the major transport form.
• Bound to hemoglobin (~20%): carried on hemoglobin (at a different site than oxygen).
• Dissolved in plasma (~10%): a small amount travels dissolved.

Because CO₂ transport is tied to bicarbonate and pH, breathing directly affects acid–base balance: breathing faster removes more CO₂ (raising pH), and breathing slower retains CO₂ (lowering pH) — the respiratory contribution to pH control from A&P I.

How it works

Gas movement, both ends:

Lungs (external respiration): alveoli high O₂/low CO₂ → O₂ INTO blood, CO₂ OUT to alveoli (exhaled)
Tissues (internal respiration): cells low O₂/high CO₂ → O₂ INTO tissues, CO₂ INTO blood
O₂ transport: ~98% on hemoglobin. CO₂ transport: ~70% bicarbonate, ~20% on hemoglobin, ~10% dissolved.

Comparisons

Common confusions

• External vs internal respiration. External = at the lungs; internal = at the tissues.
• Most oxygen rides on hemoglobin, not dissolved in plasma.
• Most CO₂ travels as bicarbonate, not bound to hemoglobin.
• Breathing affects pH via CO₂ — a key acid–base concept.

Memory aids

• "eXternal = eXchange at lungs; Internal = In the tissues."
• O₂ = "on hemoglobin"; CO₂ = "mostly bicarbonate."
• "Blow off CO₂ → raise pH."

Quick review

• Gases diffuse down partial pressure gradients: at the lungs (external respiration) O₂ enters blood and CO₂ leaves; at the tissues (internal respiration) O₂ leaves blood and CO₂ enters.
• Oxygen: ~98% carried on hemoglobin; released more where tissues are warm/active.
• Carbon dioxide: ~70% as bicarbonate, ~20% on hemoglobin, ~10% dissolved — linking breathing to acid–base balance.
• Pulse oximetry, carbon monoxide poisoning, and blood-gas interpretation all rest on these principles.