Biology for AP Courses · The Respiratory System
Transport of Gases in Human Bodily Fluids
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In 30 seconds
Once O₂ and CO₂ cross the respiratory membrane, they must be carried through the bloodstream between the lungs and the tissues. Oxygen rides almost entirely on Hemoglobin Four-subunit RBC protein with 4 heme groups, each binding one O₂ Full entry →: about 98.5% of the O₂ in blood is bound to this iron-containing protein inside red blood cells, with only ~1.5% dissolved in plasma. Carbon dioxide travels mostly as bicarbonate ion (HCO₃⁻): roughly 70% is converted to bicarbonate in the red blood cell, ~20–23% binds to hemoglobin as Carbaminohemoglobin CO₂ bound directly to Hb's globin chains Full entry →, and only ~7% stays dissolved. Two linked phenomena govern how well these gases load and unload: the oxygen–hemoglobin dissociation curve (a sigmoid relationship produced by Cooperative binding Binding of the first O₂ makes subsequent bindings easier (and vice versa) Full entry →, shifted right by the Bohr effect ↓pH, ↑CO₂, ↑temp, ↑BPG shift the curve right (lower affinity) Full entry → when tissues are acidic, hot, or CO₂-rich) and the Haldane effect Deoxygenated Hb binds H⁺ and CO₂ more readily Full entry → (deoxygenated hemoglobin picks up CO₂ and H⁺ more readily, so O₂ unloading and CO₂ loading reinforce each other). This topic completes the respiratory story: diffusion delivers the gases, breathing refreshes the air, and hemoglobin makes the blood a high-capacity shuttle.
Why this matters
The transport system sets the capacity of gas exchange: without hemoglobin, dissolved O₂ alone would supply only a few percent of the body's demand, and no heart could pump enough plasma to compensate. Understanding the dissociation curve explains real clinical patterns — why carbon monoxide is lethal at tiny concentrations (it outcompetes O₂ for the same binding sites), why anemic patients can be hypoxic with normal arterial PO₂, why exercising muscle gets extra O₂ exactly when it needs it (right-shift), and why people acclimatizing to altitude make more red blood cells and more 2,3-BPG RBC metabolite that lowers Hb's O₂ affinity Full entry →. The Bohr and Haldane effects are also textbook examples of how the respiratory and circulatory systems are coupled, and they appear frequently on AP® free-response questions that ask you to interpret a curve.
The college version
Core Concepts
Oxygen transport: dissolved versus bound
O₂ is poorly soluble in water, so plasma alone carries very little — commonly taught as ~1.5% of total blood O₂. The rest binds hemoglobin (Hb), a tetramer of four globin subunits, each with a heme group containing iron that can bind one O₂ molecule — so one Hb molecule carries up to four O₂. The commonly taught carrying capacity is ~1.34 mL O₂ per gram of Hb; with ~15 g Hb per 100 mL of blood, that yields roughly 20 mL O₂ per 100 mL of blood when fully saturated. Cooperative binding is the key behavior: after the first O₂ binds, the protein changes shape and the remaining sites bind O₂ more readily; releasing one O₂ makes the rest release more easily. This cooperativity is what creates the sigmoid (S-shaped) dissociation curve.
The oxygen–hemoglobin dissociation curve
The curve plots % hemoglobin saturation against PO₂. Commonly taught reference points: at alveolar PO₂ ≈ 100 mmHg, Hb is ~97–98% saturated; at tissue PO₂ ≈ 40 mmHg, saturation falls to ~75%; at PO₂ ≈ 20 mmHg it is near ~35%. Two features matter. First, the plateau at high PO₂ means that even if alveolar PO₂ drops somewhat (mild altitude, mild lung disease), saturation stays high — a safety margin. Second, the steep middle region means a small drop in PO₂ (from 40 toward 20 mmHg) releases a large fraction of O₂ — exactly the range where exercising tissues operate. P₅₀ PO₂ at 50% saturation (≈ 26–27 mmHg, commonly taught) Full entry → is the PO₂ at 50% saturation, commonly taught as ~26–27 mmHg; a lower P₅₀ means higher affinity, a higher P₅₀ means lower affinity.
The Bohr effect: shifting the curve right
The curve's position is not fixed. Right shift (lower affinity, more O₂ released at any given PO₂) is caused by increased H⁺ (lower pH), increased CO₂, increased temperature, and increased 2,3-bisphosphoglycerate (BPG), a metabolic intermediate in red blood cells. This is the Bohr effect: actively metabolizing tissue produces CO₂ and acid and heat, all of which tell hemoglobin to hand over more O₂ on the spot. Left shift (higher affinity, less release) occurs with the opposite conditions, and in fetal hemoglobin, which binds O₂ more tightly than adult Hb so it can pull O₂ across the placenta. BPG levels rise with chronic hypoxia — including altitude acclimatization — producing a right shift that improves O₂ unloading in the tissues.
Carbon dioxide transport: three routes
CO₂ is more soluble than O₂, but plasma still carries only ~7% of it dissolved. About 20–23% binds to hemoglobin's globin chains as carbaminohemoglobin (this is what "Haldane" is about — see below). The lion's share, ~70%, is carried as bicarbonate. The conversion happens inside red blood cells, which contain Carbonic anhydrase RBC enzyme catalyzing CO₂ + H₂O ⇌ H₂CO₃ Full entry →, the enzyme that catalyzes:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
The H⁺ produced is buffered largely by deoxygenated hemoglobin (which binds H⁺ better than oxyhemoglobin — that is the Haldane effect), and HCO₃⁻ exits the cell in exchange for Cl⁻ entering — the Chloride shift HCO₃⁻ exits the RBC as Cl⁻ enters, balancing charge Full entry → — preserving electrical neutrality. At the alveolus the reactions reverse: carbonic anhydrase drives H₂CO₃ back to CO₂ and water, CO₂ diffuses into the alveolar airspace, and the chloride shift runs backward.
The Haldane effect: CO₂ rides better on deoxygenated blood
The Haldane effect states that deoxygenated hemoglobin binds H⁺ and CO₂ more readily than oxygenated hemoglobin. The consequences are elegant: at the tissues, O₂ unloads, hemoglobin's affinity for H⁺ and CO₂ rises, and CO₂ loading is enhanced — the same blood that just delivered O₂ is primed to pick up CO₂. At the lungs, O₂ binds, affinity for H⁺ and CO₂ drops, and CO₂ is pushed off into the alveolar air. This is why venous blood carries more CO₂ per unit than arterial blood, and why the Bohr and Haldane effects work as a pair: O₂ delivery and CO₂ pickup are mutually reinforcing.
Clinical and special situations
Carbon monoxide (CO) binds hemoglobin with roughly 200–250 times the affinity of O₂ (commonly taught), displacing O₂ and locking Hb in the high-affinity (left-shifted) state, so even high PO₂ cannot load the blocked sites — the classic treatment context is removing the CO and giving the person high-concentration oxygen, including hyperbaric O₂ in severe cases. Anemia lowers total O₂ content (mL O₂ per 100 mL blood) while saturation at any PO₂ is normal — a low-content, normal-saturation state. Polycythemia (too many red cells, as in altitude acclimatization or some diseases) raises content but thickens blood. Fetal hemoglobin and BPG both fine-tune affinity, and both show up on exams as curve-shift questions.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Bohr effect | Haldane effect | Bohr = O₂ affinity falls with ↓pH/↑CO₂ (affects O₂ loading/unloading). Haldane = deoxygenated Hb carries more CO₂/H⁺ (affects CO₂ loading) |
| O₂ is carried as bicarbonate | CO₂ is carried as bicarbonate | Bicarbonate is a CO₂ transport form; O₂ rides on heme groups |
| Right shift = more O₂ bound | Right shift = less O₂ bound at a given PO₂ | Right shift lowers affinity, so saturation is lower at any PO₂ but unloading at tissues improves |
| CO poisoning lowers arterial PO₂ | CO poisoning leaves PO₂ normal but lowers O₂ content | CO blocks Hb sites and left-shifts the curve; dissolved O₂ in plasma is unaffected |
| Anemia = low saturation | Anemia = normal saturation, low total content | Fewer RBCs/Hb means less O₂ per 100 mL blood even at full saturation |
| The dissolved O₂ in plasma is the main supply | Plasma carries only ~1.5% | Almost all O₂ is hemoglobin-bound; plasma O₂ is what actually diffuses into cells |
| Fetal Hb has lower affinity to steal O₂ | Fetal Hb has higher affinity | Higher affinity pulls O₂ across the placenta from maternal blood |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of hemoglobin as a school bus with four seats for oxygen. At the lungs the bus fills up; at the muscles the kids get off. The bus is also a little greedy: after the first kid gets on, it's easier for the next three — and after one gets off, the rest leave faster. Carbon dioxide mostly doesn't ride the bus at all: the red blood cell shakes it up with water to make "fizzy water" (bicarbonate), and when the bus arrives at the lungs it turns the fizz back into CO₂ gas so you can breathe it out.
Worked example
Follow one O₂ molecule and one CO₂ molecule through a sprint. As the leg muscle contracts hard, it burns ATP, producing CO₂ and H⁺ and heating up. Capillary blood arrives at the tissue with PO₂ ≈ 40 mmHg; the working muscle's PO₂ has fallen lower still, so O₂ diffuses into cells — but the Bohr effect supercharges the delivery: the acidic, hot, CO₂-rich environment shifts the dissociation curve right, so hemoglobin unloads a larger fraction of its O₂ than it would in resting tissue. Meanwhile the same CO₂ diffuses into the red blood cell, carbonic anhydrase converts it to H⁺ and HCO₃⁻, the H⁺ binds to now-deoxygenated hemoglobin (Haldane effect), and HCO₃⁻ leaves through the chloride shift. Back at the lungs, the O₂-starved, CO₂-laden blood reverses course: fresh O₂ binds, shifting the curve left and pushing H⁺ off hemoglobin; carbonic anhydrase runs the bicarbonate reaction backward; CO₂ diffuses into the alveolus; and the cycle is ready for the next sprint. Both effects — Bohr and Haldane — ensure the most active tissues get the most O₂ and give up the most CO₂.
Key takeaways
- O₂: ~98.5% bound to hemoglobin, ~1.5% dissolved. Hb carries up to 4 O₂ (4 heme groups).
- Carrying capacity commonly taught: ~1.34 mL O₂/g Hb → ~20 mL O₂ per 100 mL blood fully saturated.
- Sigmoid dissociation curve from cooperative binding; ~97–98% saturated at PO₂ 100, ~75% at PO₂ 40 (commonly taught).
- Bohr effect (right shift): ↑H⁺/↓pH, ↑CO₂, ↑temperature, ↑BPG → lower O₂ affinity → more unloading at tissues.
- CO₂: ~70% bicarbonate, ~20–23% carbaminohemoglobin, ~7% dissolved.
- Carbonic anhydrase (in RBCs) catalyzes CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻; chloride shift swaps HCO₃⁻ for Cl⁻.
- Haldane effect: deoxygenated Hb binds H⁺ and CO₂ better — O₂ unloading boosts CO₂ loading.
- CO binds Hb ~200–250× stronger than O₂ (commonly taught), shifting the curve left and blocking O₂ transport.
- P₅₀ ≈ 26–27 mmHg (commonly taught); lower P₅₀ = higher affinity (fetal Hb), higher P₅₀ = lower affinity.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What fraction of transported O₂ is dissolved in plasma, and where does the rest ride?
Show answer
Only ~1.5%; the other ~98.5% is bound to hemoglobin inside red blood cells (up to 4 O₂ per Hb molecule).
Sketch in words the O₂–Hb dissociation curve Plot of % saturation vs. PO₂ Full entry → and explain the two physiologically important regions (plateau and steep middle).
Show answer
A sigmoid (S-shaped) curve of % saturation vs. PO₂. The plateau at high PO₂ (≈97–98% at 100 mmHg) means saturation stays high despite moderate drops in PO₂. The steep middle region (≈40 → 20 mmHg) means small PO₂ drops release large amounts of O₂ — the operating range of tissues.
List the four conditions that shift the dissociation curve right (Bohr effect) and state what a right shift does to O₂ delivery at the tissues.
Show answer
Increased H⁺ (lower pH), increased CO₂, increased temperature, and increased 2,3-BPG. A right shift lowers Hb's O₂ affinity, so more O₂ is released to the tissues at any given PO₂.
What are the three routes of CO₂ transport, and which enzyme makes the bicarbonate route fast?
Show answer
Dissolved (~7%), carbaminohemoglobin (~20–23%), and bicarbonate (~70%). Carbonic anhydrase, inside the red blood cell, catalyzes CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻.
State the Haldane effect and explain how it and the Bohr effect reinforce each other in exercising muscle.
Show answer
Deoxygenated hemoglobin binds H⁺ and CO₂ more readily than oxygenated hemoglobin. In exercising muscle, O₂ unloading (favored by the Bohr right shift) makes Hb a better H⁺/CO₂ acceptor, boosting CO₂ loading — and at the lungs, O₂ binding reverses it, releasing CO₂.
Why is carbon monoxide dangerous even when arterial PO₂ is normal?
Show answer
CO binds hemoglobin with ~200–250× the affinity of O₂ (commonly taught), so it blocks O₂-binding sites and shifts the curve left. Arterial PO₂ (dissolved O₂) can be normal while total O₂ content and tissue delivery are dangerously low.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Hemoglobin
- Four-subunit RBC protein with 4 heme groups, each binding one O₂
- Cooperative binding
- Binding of the first O₂ makes subsequent bindings easier (and vice versa)
- O₂–Hb dissociation curve
- Plot of % saturation vs. PO₂
- P₅₀
- PO₂ at 50% saturation (≈ 26–27 mmHg, commonly taught)
- Bohr effect
- ↓pH, ↑CO₂, ↑temp, ↑BPG shift the curve right (lower affinity)
- 2,3-BPG
- RBC metabolite that lowers Hb's O₂ affinity
- Carbonic anhydrase
- RBC enzyme catalyzing CO₂ + H₂O ⇌ H₂CO₃
- Bicarbonate (HCO₃⁻)
- Main transport form of CO₂ in blood
- Chloride shift
- HCO₃⁻ exits the RBC as Cl⁻ enters, balancing charge
- Haldane effect
- Deoxygenated Hb binds H⁺ and CO₂ more readily
- Carbaminohemoglobin
- CO₂ bound directly to Hb's globin chains
Sources & references
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