Transport of Oxygen and Carbon Dioxide in Blood

Transport of oxygen and carbon dioxide in blood is how the circulatory system carries respiratory gases between the lungs and every cell of the body. Blood picks up oxygen in the lungs, delivers it to tissues, collects carbon dioxide, and returns it to the lungs for removal.

This transport matters because diffusion alone can move gases only a few millimetres. Without haemoglobin and plasma chemistry, your toes and brain could never receive enough oxygen, and acidic carbon dioxide would poison your cells within minutes.

Key Concepts

Haemoglobin is the red protein inside red blood cells, each molecule carrying up to four oxygen molecules as oxyhaemoglobin. About 97 percent of oxygen travels this way; only about 3 percent dissolves in plasma. Carbon dioxide travels in three forms: about 70 percent as bicarbonate ions in plasma, about 20 to 25 percent bound to haemoglobin as carbaminohaemoglobin, and about 7 percent dissolved. The enzyme carbonic anhydrase inside red cells speeds bicarbonate formation. Oxygen release is favoured by high carbon dioxide, acidity and warmth in active tissues, known as the Bohr effect.

• Arterial blood: bright red, oxygen-rich, pO2 about 95 mmHg.
• Venous blood: darker, carbon dioxide-rich, pO2 about 40 mmHg.
• Oxygen dissociation curve: S-shaped graph showing haemoglobin saturation versus pO2.

Formulas

Loading in lungs: haemoglobin + 4 oxygen → oxyhaemoglobin, written Hb + 4O2 → Hb(O2)4, favoured by high pO2 and low pCO2. Unloading in tissues is the exact reverse. For carbon dioxide, the key reaction inside red cells is carbon dioxide + water → carbonic acid → hydrogen ion + bicarbonate ion, written CO2 + H2O → H2CO3 → H+ + HCO3−, catalysed by carbonic anhydrase. In the lungs, low pCO2 drives this reaction backward, regenerating CO2 for exhalation. Each 100 mL of arterial blood carries about 19 to 20 mL of oxygen, of which tissues normally extract about 5 mL at rest.

Worked Example

Assume haemoglobin is 15 grams per 100 mL of blood and each gram carries 1.34 mL of oxygen when fully saturated. Maximum capacity is 15 multiplied by 1.34, which equals about 20.1 mL oxygen per 100 mL blood. Arterial blood at 97 percent saturation carries about 19.5 mL. Venous blood at rest is still about 75 percent saturated and carries about 15 mL. The difference, about 4.5 to 5 mL per 100 mL, is what tissues consumed. With a cardiac output of 5 litres per minute, oxygen delivery is roughly 5,000 mL blood multiplied by 0.195, near 975 mL per minute, comfortably above resting use of about 250 mL per minute.

Common Misconceptions

• Misconception: Most carbon dioxide is carried by haemoglobin. Correction: The majority, about 70 percent, travels as bicarbonate in plasma.
• Misconception: Venous blood contains no oxygen. Correction: It still holds about three-quarters of its oxygen at rest as a reserve.
• Misconception: Oxygen is carried dissolved in plasma. Correction: Dissolved oxygen is only about 3 percent; haemoglobin is essential.

Real-World Applications

Anaemia reduces haemoglobin and therefore oxygen delivery, causing fatigue. Carbon monoxide poisoning is dangerous because it binds haemoglobin 200 times more strongly than oxygen. Blood gas analysis guides ventilator settings in intensive care. Athletes train at altitude to raise haemoglobin and improve oxygen transport.

Oxygen rides mainly on haemoglobin and is released where tissues are active and acidic, while carbon dioxide rides mainly as bicarbonate. This elegant two-way transport keeps cells supplied and the blood pH stable.