Why does oxygen move from your lungs into your blood? Why does carbon dioxide move the opposite direction? The answer lies in partial pressures.

Dalton's Law

The air you breathe is a mixture of gases: mostly nitrogen (78%), oxygen (21%), and tiny amounts of other gases. According to Dalton's Law, the total pressure of a gas mixture equals the sum of the pressures of each individual gas. Each gas contributes a "partial pressure" proportional to its concentration.

At sea level, atmospheric pressure is 760 mmHg. Since oxygen is 21% of air, the partial pressure of oxygen (PO₂) in atmospheric air is about 160 mmHg (760 × 0.21).

Why Partial Pressures Matter

Gases diffuse from areas of higher partial pressure to lower partial pressure. It's like rolling a ball down a hill — it naturally moves from high to low. This principle governs all gas exchange in your body.

The Key Partial Pressures

In alveolar air: PO₂ = 104 mmHg, PCO₂ = 40 mmHg

In venous blood arriving at lungs: PO₂ = 40 mmHg, PCO₂ = 46 mmHg

See the gradients? Oxygen in the alveoli (104) is higher than in venous blood (40), so O₂ diffuses into the blood. Carbon dioxide in venous blood (46) is higher than in alveoli (40), so CO₂ diffuses into the alveoli to be exhaled. These gradients make gas exchange automatic!

Gas exchange happens in two locations, with opposite directions of oxygen and carbon dioxide movement.

External Respiration — In the Lungs

This is gas exchange between alveolar air and pulmonary capillary blood. When venous blood arrives at the lungs, it's low in oxygen (PO₂ = 40 mmHg) and high in carbon dioxide (PCO₂ = 46 mmHg).

As blood passes through pulmonary capillaries wrapped around alveoli:

• Oxygen diffuses FROM alveoli (PO₂ = 104) INTO blood (PO₂ = 40)

• Carbon dioxide diffuses FROM blood (PCO₂ = 46) INTO alveoli (PCO₂ = 40)

By the time blood leaves the lungs, it's now oxygenated arterial blood with PO₂ ≈ 100 mmHg and PCO₂ ≈ 40 mmHg. This entire exchange happens in just 0.25 seconds — blood moves quickly through the lungs!

Internal Respiration — In the Tissues

This is gas exchange between systemic capillary blood and body cells. Your cells are constantly using oxygen and producing carbon dioxide through metabolism.

When arterial blood reaches your tissues:

• Oxygen diffuses FROM blood (PO₂ = 100) INTO tissues (PO₂ ≈ 40 or lower)

• Carbon dioxide diffuses FROM tissues (PCO₂ ≈ 46) INTO blood (PCO₂ = 40)

The result: blood leaves the tissues as venous blood, now low in oxygen and high in carbon dioxide, ready to return to the lungs for replenishment.

Once oxygen enters your blood, how does it get to your cells? Here's the challenge: oxygen doesn't dissolve well in water (including blood plasma). If oxygen only traveled dissolved in plasma, your blood couldn't carry enough to sustain life.

Hemoglobin — The Oxygen Carrier

Enter hemoglobin — a remarkable protein inside red blood cells that transports 98.5% of the oxygen in your blood. Each hemoglobin molecule is like a taxi with four seats: it can carry up to four oxygen molecules.

Hemoglobin consists of four protein chains (two alpha, two beta), each containing a heme group with an iron atom at its center. The iron binds oxygen — that's why you need iron in your diet for healthy blood!

Loading and Unloading

In the lungs (high PO₂): Oxygen binds to hemoglobin, forming oxyhemoglobin (bright red color)

In the tissues (low PO₂): Oxygen detaches from hemoglobin and diffuses into cells

This relationship is shown by the oxygen-hemoglobin dissociation curve. At high PO₂ (lungs), hemoglobin is nearly 100% saturated. At lower PO₂ (tissues), it releases oxygen more readily.

The Bohr Effect — Delivering Oxygen Where It's Needed Most

Active tissues produce more CO₂, which lowers pH (more acidic). The Bohr Effect describes how hemoglobin releases oxygen more readily in these conditions. This is brilliant: tissues that are working hardest (and need oxygen most) get preferential oxygen delivery!

Factors that promote oxygen unloading: decreased pH, increased CO₂, increased temperature, increased 2,3-DPG (a metabolite)

While oxygen has one main transport method (hemoglobin), carbon dioxide uses three different strategies to get from your tissues to your lungs.

Method 1: Dissolved in Plasma (7%)

A small amount of CO₂ simply dissolves in the plasma, just like carbon dioxide in a soda. This is the smallest fraction but includes the CO₂ that drives breathing — your brain monitors dissolved CO₂ levels to regulate your breathing rate.

Method 2: Bound to Hemoglobin (23%)

CO₂ can bind to hemoglobin at sites different from the oxygen binding sites, forming carbaminohemoglobin. Interestingly, deoxygenated hemoglobin binds CO₂ more readily — so hemoglobin that just released its oxygen in tissues is ready to pick up CO₂ for the return trip.

Method 3: As Bicarbonate (70%) — The Major Route

Most CO₂ is converted to bicarbonate ions (HCO₃⁻) inside red blood cells through this reaction:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

This reaction is catalyzed by the enzyme carbonic anhydrase, making it extremely fast.

The Chloride Shift

Here's the problem: as bicarbonate builds up inside red blood cells, it would create an electrical imbalance. To maintain neutrality, bicarbonate ions move OUT of the cell into the plasma, while chloride ions (Cl⁻) move INTO the cell. This exchange is called the chloride shift (or Hamburger phenomenon).

In the Lungs: Everything Reverses

When blood reaches the lungs, the process reverses: bicarbonate re-enters red blood cells, chloride exits, carbonic acid reforms, and CO₂ is released from the blood into the alveoli to be exhaled. The entire system is elegantly reversible!