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Human Physiology II · Systems Physiology

Gas Transport in Blood

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Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Almost all oxygen in blood is carried reversibly bound to hemoglobin inside red blood cells; only about 1.5% dissolves in plasma. Hemoglobin binds oxygen cooperatively, giving the oxygen-hemoglobin dissociation curve its sigmoid shape and a of about 27 mmHg — a shape that loads O₂ fully in the lungs and unloads it readily in tissues. The curve shifts right when tissues are acidic, hot, or rich in , easing O₂ release (the ). Carbon dioxide travels as dissolved gas, as , and mostly as , and the lets deoxygenated blood carry more CO₂.

Why this matters

The dissociation curve explains everyday clinical observations. A right shift (fever, acidosis) makes oxygen easier to unload — helpful in exercising muscle, harmful if loading is already compromised. Pulse oximetry reports hemoglobin saturation, not content, so an anemic person can read 100% saturated yet carry too little total oxygen. Reference ranges and interpretation vary by institution and jurisdiction; these notes support education only and do not replace clinical instruction or supervision.

The college version

1. Oxygen transport: dissolved versus hemoglobin-bound

Oxygen is poorly soluble, so at an arterial PO₂ of 100 mmHg only ~0.3 mL dissolves per 100 mL of blood. The other ~98.5% binds to the iron atoms of hemoglobin, whose four heme groups each carry one O₂ — about 20 mL O₂ per 100 mL, roughly 60-fold above what plasma alone could hold.

2. The oxygen-hemoglobin dissociation curve

Plotting saturation against PO₂ gives a sigmoid (S-shaped) curve. — each bound O₂ increases affinity for the next — produces the shape. The P50 is the PO₂ at 50% saturation (~27 mmHg). The plateau at high PO₂ means loading in the lungs (PO₂ ≈ 100 mmHg) stays near-total even if alveolar PO₂ drops somewhat. The steep region (~10–60 mmHg) means a small drop in tissue PO₂ releases a large amount of O₂.

3. Curve-shifting factors and the Bohr effect

Anything that decreases hemoglobin's O₂ affinity shifts the curve right (higher P50, easier unloading): increased H⁺ (lower pH), increased temperature, and increased 2,3-BPG (a red-cell glycolytic metabolite, boosted by hypoxia and altitude). The Bohr effect is the pH part: active tissues make CO₂ and H⁺, shifting the curve right so more O₂ is released there; in the lungs CO₂ is exhaled, pH rises, and the curve shifts left to promote loading.

4. Carbon dioxide transport and the Haldane effect

CO₂ travels three ways: ~7–10% dissolved in plasma; ~20–23% as carbaminohemoglobin (on hemoglobin's amino groups); and ~70% as bicarbonate (HCO₃⁻), made by carbonic anhydrase and exchanged for Cl⁻ (the chloride shift). The Haldane effect mirrors the Bohr effect: deoxygenated hemoglobin binds CO₂ and H⁺ more readily, so tissues pick up extra CO₂ and the lungs shed it.

How it works

  1. In the lungs, high PO₂ saturates hemoglobin (~98%) on the curve's plateau.
  2. Arterial blood carries ~20 mL O₂/dL, overwhelmingly on hemoglobin.
  3. In tissues, low PO₂, H⁺, CO₂, and heat shift the curve right, releasing O₂ where it is needed.
  4. Tissue CO₂ enters red cells and becomes bicarbonate (carbonic anhydrase), with the chloride shift preserving charge balance.
  5. Deoxygenated hemoglobin buffers the H⁺ and binds CO₂ as carbaminohemoglobin (Haldane effect); in the lungs, O₂ binding reverses these reactions and releases CO₂.

Common confusions

Do not confuseWithDifference
Oxygen contentOxygen saturationContent = total O₂ carried (depends on Hb amount); saturation = % of heme sites filled
P50PO₂P50 is the PO₂ at half-saturation; PO₂ is the current partial pressure
Bohr effectHaldane effectBohr = H⁺/CO₂ reduce O₂ affinity; Haldane = deoxy-Hb binds more CO₂/H⁺
CarbaminohemoglobinCarboxyhemoglobinCarbamino = CO₂ on amino groups (normal transport); carboxy = CO on heme (toxic)
Right shiftLeft shiftRight = lower affinity, easier unloading; left = higher affinity, harder unloading

Memory aids

"CADET faces Right" for factors that decrease affinity (shift the curve right): CO₂, Acid (H⁺/pH), DPG (2,3-BPG), Exercise, Temperature. And "Most CO₂ rides as Bi-Car-B" — Bicarbonate (~70%) > Carbamino (~20%) > Barely dissolved (~7–10%).

Quick review

Topic Recap

Hemoglobin carries almost all blood oxygen, and its cooperative binding produces a sigmoid dissociation curve (P50 ≈ 27 mmHg) that loads O₂ on the plateau and unloads it on the steep region. pH (Bohr effect), temperature, and 2,3-BPG tune affinity to tissue demand. Carbon dioxide travels as bicarbonate, carbaminohemoglobin, and dissolved gas, with the Haldane effect coordinating CO₂ carriage with O₂ release.

Knowledge Check

  1. Roughly what fraction of blood oxygen is carried bound to hemoglobin?
  2. What does the P50 represent, and what is its approximate normal value?
  3. Name three factors that shift the dissociation curve to the right.
  4. What is the most abundant form of CO₂ transport, and what enzyme makes it?
  5. How does the Haldane effect help CO₂ leave the blood in the lungs?

Answers and Rationales

  1. About 98.5% — only ~1.5% dissolves in plasma because oxygen is poorly soluble.
  2. The PO₂ at which hemoglobin is 50% saturated; about 27 mmHg.
  3. Increased H⁺ (low pH), increased temperature, and increased 2,3-BPG (also increased CO₂).
  4. Bicarbonate (~70%), made from CO₂ and water by carbonic anhydrase inside red blood cells.
  5. As hemoglobin binds O₂ in the lungs, it releases the H⁺ and CO₂ it was holding, driving the bicarbonate reaction backward and freeing CO₂ to diffuse into the alveolus.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of hemoglobin as a four-seat taxi carrying up to four oxygen passengers. Once the first passenger boards, the taxi opens its remaining doors wider so the next hop in faster; when one gets out, the others are nudged to leave too. That "help each other in, help each other out" behavior is cooperativity.

Carbon dioxide is three kinds of cargo: some rides loose (dissolved), some is strapped onto the taxi (carbaminohemoglobin), and most is repackaged as bicarbonate until swapped back at the lungs.

Where it stops being exact: the taxi's doors don't always open at the same spot. In hot, acidic, working tissues (warm, high 2,3-BPG) they open earlier so passengers get off faster; in the cool, alkaline lungs they stay shut so passengers load up. And a real taxi's seats don't change their grip based on other cargo — but hemoglobin does, which is exactly the Bohr and Haldane effects.

Simple Example

Squeeze a nearly full bus: it gets harder to cram in the last few — a curve flattening off. Hemoglobin does the opposite at first, so its curve climbs steeply in the middle and flattens only when nearly full — the sigmoid "S" shape.

Worked example

  1. Oxygen loading (lungs). PO₂ ≈ 100 mmHg sits on the plateau, so hemoglobin is ~98% saturated despite small fluctuations; high pH and low temperature shift the curve left, reinforcing loading.
  2. Oxygen unloading (tissues). PO₂ falls toward 40 mmHg; on the steep part of the curve, saturation drops sharply. H⁺, CO₂, heat, and 2,3-BPG shift the curve right, so active tissues receive more O₂ at the same PO₂ — the Bohr effect.
  3. CO₂ pickup (tissues). CO₂ diffuses into red cells, where carbonic anhydrase catalyzes CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- (carbonic acid dissociating into a proton and bicarbonate). Bicarbonate exits via the chloride shift; the H⁺ is buffered by deoxygenated hemoglobin (Haldane effect).
  4. CO₂ release (lungs). The reactions reverse: HCO₃⁻ re-enters red cells, re-forms CO₂, and CO₂ diffuses into the alveolus. Oxygen binding releases the H⁺ and CO₂ hemoglobin was holding, driving the reversal (Haldane effect).

Key takeaways

  • High yield: ~98.5% of O₂ is hemoglobin-bound; dissolved O₂ is negligible for transport but sets PO₂.
  • High yield: The dissociation curve is sigmoid because of cooperativity; P50 ≈ 27 mmHg.
  • High yield: Right shift (↓ affinity) = easier unloading: low pH (Bohr), ↑ temperature, ↑ 2,3-BPG.
  • High yield: CO₂ is carried ~70% as bicarbonate, ~20% as carbaminohemoglobin, ~7–10% dissolved.
  • High yield: Haldane effect: deoxygenated blood carries more CO₂; the Bohr effect is its O₂-side mirror.

Keep learning

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Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Compare the tiny amount of oxygen carried dissolved in plasma with the large amount bound to hemoglobin, and explain why hemoglobin is essential.
  • Interpret the oxygen-hemoglobin dissociation curve, including cooperativity, the P50, and the plateau and steep regions.
  • Explain how pH (the Bohr effect), temperature, and 2,3-BPG shift the curve and what that means for O₂ loading and unloading.
  • Describe the three forms in which carbon dioxide is transported and explain the Haldane effect.

Key vocabulary

Dissolved O₂
O₂ free in plasma (~1.5%)
Hemoglobin-bound O₂
O₂ on the four heme irons (~98.5%)
Cooperativity
One bound O₂ raises affinity for the next
P50
PO₂ at 50% saturation (~27 mmHg)
Bohr effect
H⁺/CO₂ (low pH) shifts curve right
2,3-BPG
Red-cell glycolytic metabolite
Carbaminohemoglobin
CO₂ bound to hemoglobin amino groups
Bicarbonate
HCO₃⁻ made by carbonic anhydrase
Haldane effect
Deoxy-hemoglobin binds more CO₂/H⁺

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