Gas Transport — Concepts, Formulas & Examples

Once oxygen crosses from alveolus to blood, it has to be carried to every cell, and carbon dioxide has to come back. That transport uses haemoglobin and three chemistries. CBSE Class 11 and NEET both test these in the breathing chapter.

The beauty of gas transport is that it is automatic. Haemoglobin loads oxygen where oxygen is plentiful (lungs) and unloads it where oxygen is scarce (active tissues). No nervous signal tells it to do this — the chemistry of oxygen binding and pH does the job. Understanding the why behind each mechanism makes this chapter much easier than rote memorising numbers.

Core Concepts

Oxygen transport

About 97% of oxygen in blood is bound to haemoglobin as oxyhaemoglobin; only 3% is dissolved in plasma. Each Hb molecule has four heme groups, each containing one iron atom ( $\text{Fe}^{2+}$ ) that binds one $\text{O}_2$ . So each Hb can carry up to four $\text{O}_2$ molecules. Saturation depends on the partial pressure of oxygen ( $pO_2$ ).

The binding is cooperative — once the first $\text{O}_2$ binds, it changes the shape of Hb, making it easier for the second, third and fourth to bind. This is why the dissociation curve is sigmoid (S-shaped) rather than a straight line.

Oxygen dissociation curve (ODC)

A sigmoid curve showing Hb saturation (y-axis, 0 to 100%) versus $pO_2$ (x-axis, 0 to about 120 mmHg).

Key points on the curve:

At alveolar $pO_2$ (about 100 mmHg), Hb is nearly 98% saturated — the flat top of the sigmoid
At tissue $pO_2$ (about 40 mmHg), saturation drops to about 75% — releasing roughly 25% of the carried oxygen
During heavy exercise, tissue $pO_2$ can drop to 20 mmHg, and saturation falls to about 35%, releasing much more oxygen

The sigmoid shape is clinically important. On the flat top, even a moderate drop in lung function does not reduce Hb saturation much. On the steep middle part, small changes in $pO_2$ release large amounts of oxygen — exactly what active tissues need.

1 \text{ g Hb} \to 1.34 \text{ mL O}_2

With 15 g Hb per 100 mL blood, maximum $\text{O}_2$ content = $15 \times 1.34 = 20.1$ mL per 100 mL blood.

Bohr effect

Increased $pCO_2$ , lower pH and higher temperature shift the ODC to the right, meaning Hb releases more oxygen at any given $pO_2$ . This is the Bohr effect.

Why does it matter? Active tissues produce $\text{CO}_2$ and heat as metabolic by-products. These by-products automatically shift the curve right, forcing Hb to unload more oxygen precisely where it is needed most. The system is self-regulating.

Conversely, in the lungs where $pCO_2$ is low and pH is slightly higher, the curve shifts left, helping Hb load up with oxygen.

Think of the Bohr effect as a delivery service that reads the local conditions. Hot, acidic, CO $_2$ -rich environment? Must be active tissue — unload more oxygen. Cool, alkaline, low-CO $_2$ environment? Must be the lungs — load up.

2,3-BPG and its role

2,3-bisphosphoglycerate (2,3-BPG) is produced by RBCs during glycolysis. It binds to deoxyhaemoglobin and stabilises the T (tense) state, reducing oxygen affinity. Higher 2,3-BPG shifts the ODC right.

People living at high altitude have elevated 2,3-BPG levels — this helps them extract more oxygen from air that has lower $pO_2$ . After a few weeks at altitude, 2,3-BPG rises and acclimatisation improves.

Fetal haemoglobin (HbF)

Fetal haemoglobin has a higher affinity for $\text{O}_2$ than adult HbA. Its dissociation curve is shifted left compared to adult Hb. This is because HbF does not bind 2,3-BPG as strongly — with less BPG interference, it holds onto $\text{O}_2$ more tightly.

This left shift is critical: in the placenta, fetal blood must extract oxygen from maternal blood. Since HbF has higher affinity, oxygen transfers from maternal HbA to fetal HbF at the same $pO_2$ .

Carbon dioxide transport

$\text{CO}_2$ is transported in three forms:

Form	Percentage	Details
Dissolved in plasma	~7%	Simple physical dissolution
As carbamino-Hb	~23%	$\text{CO}_2$ binds to amino groups on Hb (not to heme iron)
As bicarbonate ( $\text{HCO}_3^-$ )	~70%	Formed in RBCs via carbonic anhydrase

The bicarbonate pathway is the most important:

\text{CO}_2 + \text{H}_2\text{O} \xrightarrow{\text{carbonic anhydrase}} \text{H}_2\text{CO}_3 \to \text{H}^+ + \text{HCO}_3^-

This reaction is fast inside RBCs (where carbonic anhydrase is present) and slow in plasma (where the enzyme is absent). The $\text{HCO}_3^-$ exits the RBC into plasma, and $\text{Cl}^-$ enters to maintain electrical neutrality — this is the chloride shift (Hamburger phenomenon).

Chloride shift in detail

When $\text{HCO}_3^-$ leaves the RBC, the cell would become positively charged if nothing compensated. $\text{Cl}^-$ moves in through a specific anion exchanger (Band 3 protein) to balance the charge. In the lungs, the reverse happens — $\text{CO}_2$ is blown off, and $\text{Cl}^-$ exits back to plasma as $\text{HCO}_3^-$ re-enters the RBC.

Haldane effect

Deoxyhaemoglobin binds $\text{CO}_2$ more readily than oxyhaemoglobin. So at the tissues, where Hb releases $\text{O}_2$ and becomes deoxyHb, it picks up $\text{CO}_2$ more efficiently. At the lungs, where Hb binds $\text{O}_2$ , it releases $\text{CO}_2$ . The Haldane effect complements the Bohr effect — together they ensure efficient gas exchange in both directions.

Worked Examples

HbF has two gamma chains instead of the two beta chains in adult HbA. Gamma chains bind 2,3-BPG less effectively, so HbF holds onto $\text{O}_2$ more tightly (higher affinity). Its dissociation curve is shifted left, allowing the fetus to extract oxygen from maternal blood across the placenta even when maternal Hb is only 75% saturated.

At 15 g/dL Hb with 98% saturation in arteries: bound $\text{O}_2 = 15 \times 1.34 \times 0.98 \approx 19.7$ mL per 100 mL. Dissolved $\text{O}_2 \approx 0.3$ mL. Total arterial $\text{O}_2 \approx 20$ mL per 100 mL.

In veins (75% saturation): bound $\text{O}_2 = 15 \times 1.34 \times 0.75 \approx 15.1$ mL. Dissolved $\approx 0.15$ mL. Total venous $\text{O}_2 \approx 15.3$ mL per 100 mL.

Oxygen delivered = 20 - 15.3 = 4.7 mL per 100 mL blood. With cardiac output of 5 L/min, total $\text{O}_2$ delivery = $4.7 \times 50 = 235$ mL/min at rest.

CO binds to haemoglobin at the same site as $\text{O}_2$ but with about 200 times higher affinity. Even small amounts of CO can occupy a large fraction of Hb binding sites, forming carboxyhaemoglobin (HbCO). This reduces oxygen-carrying capacity AND shifts the ODC left (the remaining oxyHb holds on to its $\text{O}_2$ more tightly), so tissues get starved of oxygen. This is why CO poisoning is lethal even when plenty of oxygen is in the air.

During exercise, tissue $pO_2$ drops (more consumption), $pCO_2$ rises (more production), pH falls (lactic acid) and temperature increases. All four factors shift the ODC right (Bohr effect), causing Hb to release much more oxygen — saturation can drop from 75% to 25%, tripling the oxygen unloaded per pass.

In tissue capillaries: $\text{CO}_2$ enters RBCs, carbonic anhydrase converts it to $\text{H}_2\text{CO}_3$ which dissociates to $\text{H}^+$ and $\text{HCO}_3^-$ . $\text{HCO}_3^-$ exits into plasma via Band 3 protein, $\text{Cl}^-$ enters to maintain charge balance. In lung capillaries: the reverse — $\text{Cl}^-$ exits, $\text{HCO}_3^-$ enters, carbonic anhydrase reforms $\text{CO}_2$ which diffuses into the alveolus and is exhaled.

Common Mistakes

Saying CO $_2$ is carried mainly bound to Hb. It is carried mainly as bicarbonate in plasma (about 70%). Carbamino-Hb accounts for only about 23%.

Confusing oxyhaemoglobin and carboxyhaemoglobin. Oxyhaemoglobin is Hb bound to $\text{O}_2$ (normal and reversible). Carboxyhaemoglobin is Hb bound to CO (toxic and very stable). The names sound similar but the molecules and consequences are entirely different.

Thinking the Bohr effect shifts the curve left. It shifts right — more unloading where tissues need it. Left shift means higher affinity (as in fetal Hb or alkaline conditions).

Saying $\text{CO}_2$ binds to the iron in haemoglobin. $\text{CO}_2$ binds to the amino groups of the globin protein chains (forming carbamino-Hb), not to the heme iron. It is CO (carbon monoxide) that binds to heme iron.

Writing that carbonic anhydrase is in plasma. It is inside RBCs. The reaction is very slow in plasma without the enzyme, which is why $\text{CO}_2$ must enter RBCs first to be efficiently converted to bicarbonate.

Exam Weightage and Strategy

Gas transport is part of the Breathing and Exchange of Gases chapter in CBSE Class 11. NEET typically asks 1-2 questions from this chapter every year, with gas transport being the most frequently tested sub-topic. Expect questions on the oxygen dissociation curve, Bohr effect, CO $_2$ transport percentages, and chloride shift. CBSE boards give 3-5 marks.

The PYQ favourites:

What percentage of $\text{CO}_2$ is transported as bicarbonate? (70%)
What is the Bohr effect? Which direction does the curve shift?
How is fetal Hb different from adult Hb?
What is carbamino-haemoglobin?

Draw the oxygen dissociation curve with alveolar and tissue $pO_2$ marked, then draw the rightward-shifted Bohr curve alongside. That single diagram answers most NEET questions on gas transport. Label the percentages (98% at lungs, 75% at tissues) for quick recall.

Practice Questions

Q1. What is the significance of the sigmoid shape of the oxygen dissociation curve?

The sigmoid shape reflects cooperative binding — once the first $\text{O}_2$ binds to Hb, subsequent binding becomes easier. The flat top means lungs can load Hb efficiently even if $pO_2$ drops somewhat. The steep middle means tissues can extract large amounts of oxygen with small drops in $pO_2$ . This makes Hb an ideal oxygen transporter.

Q2. Why do people at high altitude have more 2,3-BPG in their RBCs?

At high altitude, atmospheric $pO_2$ is lower, so tissue oxygen delivery drops. The body compensates by increasing 2,3-BPG production in RBCs. More 2,3-BPG shifts the ODC right, causing Hb to release more oxygen at tissue $pO_2$ . This acclimatisation takes a few days to weeks.

Q3. Explain the Haldane effect and its physiological significance.

The Haldane effect states that deoxygenated Hb binds $\text{CO}_2$ more readily than oxygenated Hb. At tissues (where Hb releases $\text{O}_2$ and becomes deoxyHb), $\text{CO}_2$ loading is enhanced. At lungs (where Hb picks up $\text{O}_2$ ), $\text{CO}_2$ is released more easily. This ensures efficient $\text{CO}_2$ pickup at tissues and release at lungs without any extra mechanism.

Q4. A patient has normal Hb levels but low oxygen delivery. The blood appears cherry-red. What is the likely cause?

Carbon monoxide poisoning. CO binds to Hb with 200x higher affinity than $\text{O}_2$ , forming carboxyhaemoglobin which gives blood a cherry-red colour. Even though Hb concentration is normal, a large fraction is occupied by CO and unavailable for oxygen transport. Additionally, CO shifts the ODC left, making the remaining oxyHb hold onto its $\text{O}_2$ more tightly.

FAQs

Why is dissolved oxygen in plasma so small (only 3%)?

Oxygen has low solubility in water. At body temperature and normal arterial $pO_2$ , only about 0.3 mL of $\text{O}_2$ dissolves per 100 mL of plasma. Without haemoglobin, the heart would need to pump about 80 L/min to deliver enough oxygen — an impossible task. Hb increases the blood’s oxygen-carrying capacity by about 70 times.

What happens to the $\text{H}^+$ produced during bicarbonate formation?

The $\text{H}^+$ is buffered by deoxyhaemoglobin inside the RBC. DeoxyHb is a better buffer than oxyHb (this is related to the Bohr effect). If the $\text{H}^+$ were not buffered, blood pH would drop dangerously with every pass through the tissues.

Can the oxygen dissociation curve shift left?

Yes. Decreased temperature, decreased $pCO_2$ , increased pH, and decreased 2,3-BPG all shift the curve left (higher affinity). Fetal haemoglobin naturally has a left-shifted curve. In clinical contexts, massive blood transfusion with stored blood (which has low 2,3-BPG) can left-shift the curve, impairing oxygen delivery to tissues.

What is myoglobin and how does its curve differ from haemoglobin?

Myoglobin is a single-subunit oxygen-binding protein in muscles. Its dissociation curve is a rectangular hyperbola (not sigmoid) because it has no cooperative binding. It has higher affinity than Hb at all $pO_2$ values, so it acts as an oxygen reservoir in muscles — it takes oxygen from Hb and releases it only when muscle $pO_2$ drops very low during intense activity.

Gas transport is clever chemistry — haemoglobin tunes itself to load in the lungs and unload in the tissues, without anyone telling it to. The Bohr and Haldane effects are two sides of the same coin, and together they make gas exchange remarkably efficient.