Anatomy and Physiology 2e · The Respiratory System
Transport of Gases
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In 30 seconds
Gas exchange puts oxygen into the blood at the lungs and carbon dioxide into the blood at the tissues, but blood must then carry those gases between the two sites. Very little oxygen rides dissolved in plasma — if that were the only mechanism, the heart would need to pump impossibly large volumes of blood. Instead, about 98% of the oxygen carried is bound to hemoglobin, the iron-containing protein inside red blood cells. Carbon dioxide takes a different route: a small amount dissolves, some binds to hemoglobin as carbamino compounds, and most (about 70%) is converted to bicarbonate ions — a reaction catalyzed by Carbonic anhydrase Enzyme in red blood cells that speeds CO₂ ⇌ bicarbonate conversion Full entry → inside red blood cells. These two cargo systems are not independent: loading and unloading oxygen changes how well the blood carries carbon dioxide, and vice versa. The famous oxygen–hemoglobin Dissociation curve Graph of hemoglobin O₂ saturation vs. PO₂ (sigmoid) Full entry → summarizes how hemoglobin loads oxygen in the lungs, unloads it at the tissues, and adjusts to temperature, pH, and the red-blood-cell metabolite 2,3-BPG Metabolite in red blood cells that lowers hemoglobin's oxygen affinity Full entry →.
Why this matters
The transport systems determine how much oxygen reaches tissues and how efficiently CO₂ leaves. The dissociation curve explains everyday clinical facts: why a small drop in arterial PO₂ (100 → 60 mmHg) barely reduces saturation yet a further drop causes a sudden, dangerous fall in delivery; why fever, acidosis, and exercise release more oxygen to active tissues (right shift); and why carbon monoxide is so deadly — it binds hemoglobin far more tightly than oxygen. People with chronic lung disease compensate for chronically low oxygen by producing more 2,3-BPG, shifting the curve so oxygen is released more readily. Blood gas measurements (PO₂, PCO₂, pH, calculated saturation) are interpreted every day with exactly these relationships — the reasoning behind oxygen therapy and the management of respiratory failure.
The college version
Core Concepts
Oxygen transport: dissolved versus hemoglobin-bound
At a normal arterial PO₂ of about 100 mmHg, only about 0.3 mL of oxygen dissolves in each 100 mL of plasma — roughly 1.5% of the total. The rest is carried by Hemoglobin (Hb) Iron-containing protein in red blood cells that binds O₂ (up to 4 molecules) Full entry →: four polypeptide chains, each holding a heme group with an iron atom that binds one O₂ molecule, so one hemoglobin can carry up to four oxygens (forming Oxyhemoglobin Hemoglobin with oxygen bound Full entry →). The amount of oxygen bound depends on PO₂: at the lungs' high PO₂, hemoglobin loads to near-saturation; at the tissues' low PO₂, it unloads. The maximum oxygen-carrying capacity of fully saturated blood is roughly 20 mL O₂ per 100 mL blood — a commonly taught textbook figure that depends on hemoglobin concentration and should be verified against current references.
The oxygen–hemoglobin dissociation curve
Plotting hemoglobin saturation (%) against PO₂ gives a sigmoid (S-shaped) curve. The shape comes from cooperativity: binding the first oxygen molecule makes it easier for the next ones to bind, and releasing the first makes the rest easier to release. Two regions of the curve matter clinically:
- The flat upper portion (PO₂ ~60–100 mmHg): saturation stays high (about 90–98%). This is the safety margin — arterial PO₂ can fall substantially before oxygen delivery drops much, and it is why people tolerate moderate altitude exposure or mild lung disease.
- The steep middle portion (PO₂ ~20–60 mmHg): small changes in PO₂ cause large changes in saturation. Tissues normally sit on this part of the curve, so active cells pulling PO₂ down get a big release of oxygen. It also means a severe drop in arterial PO₂ causes a rapid, dangerous fall in oxygen delivery.
Shifting the curve: the Bohr effect and its friends
The curve's position shifts with conditions in the blood:
- Right shift (lower affinity, more unloading at tissues): increased temperature, increased PCO₂, decreased pH (more acidic), and increased 2,3-BPG. The curve moves down/right, meaning hemoglobin holds oxygen less tightly and releases it more readily. This is the Bohr effect Lower pH / higher PCO₂ shift the curve right (lower O₂ affinity) Full entry →: actively metabolizing tissues produce CO₂ and acid, and the resulting lower pH and higher PCO₂ help hemoglobin hand over oxygen exactly where it is needed. Fever and exercise shift right for the same reason.
- Left shift (higher affinity, more loading at lungs, less release at tissues): decreased temperature, decreased PCO₂, increased pH, and decreased 2,3-BPG. Hemoglobin clings to oxygen more tightly.
2,3-BPG, produced in red blood cells during glycolysis, binds hemoglobin and lowers its oxygen affinity. Chronic hypoxia (high altitude, anemia, chronic lung disease) raises 2,3-BPG, shifting the curve right and improving tissue oxygen delivery.
Carbon dioxide transport: three routes
CO₂ travels from tissues to lungs in three forms: (1) dissolved in plasma (about 7–10%); (2) bound to hemoglobin as Carbaminohemoglobin CO₂ bound to amino groups of hemoglobin's globin chains Full entry → (about 20–23%), attaching to amino groups of the globin chains rather than the heme iron — deoxygenated hemoglobin binds CO₂ better than oxygenated hemoglobin; and (3) as Bicarbonate (HCO₃⁻) The dominant transport form of CO₂ in blood Full entry →, the dominant route (~70%). The bicarbonate reaction is the centerpiece: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. Inside red blood cells, carbonic anhydrase speeds this reaction thousands of times. As bicarbonate accumulates inside the cell, it exits through a membrane exchanger that brings a chloride ion in — the Chloride shift Cl⁻ enters red blood cells as HCO₃⁻ exits Full entry → — which keeps the cell electrically balanced and lets bicarbonate ride in the plasma for the trip to the lungs.
The Haldane effect: how oxygen and CO₂ transport couple
The Haldane effect Deoxygenated hemoglobin carries more CO₂ and H⁺ than oxygenated hemoglobin Full entry → is the CO₂ side of the Bohr effect: deoxygenated hemoglobin carries more CO₂ (as carbaminohemoglobin) and buffers more H⁺ than oxygenated hemoglobin. In the tissues, the same deoxygenation that delivers oxygen therefore picks up CO₂; in the lungs, oxygen binding lowers hemoglobin's affinity for CO₂ and H⁺, pushing CO₂ off to be exhaled. This is why venous blood carries more CO₂ than arterial blood even at the same PCO₂. The Bohr and Haldane effects are two sides of the same elegant design: oxygen delivery and carbon dioxide removal reinforce each other at both ends of the circulation.
From lungs to tissues: the full circuit
Trace the cycle: at the pulmonary capillaries, hemoglobin binds oxygen (high PO₂), and the Bohr/Haldane effects favor CO₂ release — bicarbonate re-enters the red blood cell, reacts back to CO₂, and diffuses into the alveoli. At the tissues, PO₂ is low and PCO₂ is high; oxygen unloads (steep part of the curve, boosted by right-shifting conditions), CO₂ diffuses in, carbonic anhydrase converts it to bicarbonate, the chloride shift balances charges, and the cycle returns. Every step is driven by the partial-pressure gradients from the gas-exchange topic.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Saturation (%) | Oxygen content (mL O₂/100 mL blood) | Saturation is the fraction of hemoglobin binding sites occupied; content also depends on how much hemoglobin exists. A patient can be 100% saturated yet anemic (low content). |
| Right shift | Left shift | Right shift = hemoglobin holds O₂ less tightly (more tissue delivery, lower saturation at any PO₂); left shift = holds it more tightly (more lung loading, less delivery). |
| Bohr effect | Haldane effect | Bohr = effect of CO₂/pH on oxygen affinity; Haldane = effect of oxygenation on CO₂ carrying. Two directions of the same coupling. |
| Dissolved O₂ | Bound O₂ | Only ~1.5% of O₂ is dissolved; the rest is bound to hemoglobin. The dissolved fraction is what sets PO₂, which drives diffusion. |
| Carbaminohemoglobin | Carboxyhemoglobin (CO-Hb) | Carbamino = CO₂ bound to globin (normal transport); carboxy = carbon monoxide bound to heme (pathological, blocks O₂). Sound similar, totally different. |
| 2,3-BPG rise (chronic hypoxia) | Immediate oxygen therapy | Adaptation takes days (more 2,3-BPG); giving oxygen is the acute intervention. Both aim to improve tissue oxygen delivery. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your blood carries oxygen the way a delivery truck carries packages — and the trucks are the red blood cells, packed with a special protein called hemoglobin that grabs oxygen. Most of the oxygen you breathe in gets loaded onto these trucks in your lungs, and when the trucks reach your tired muscles, they hand the oxygen over. The trucks also pick up the "trash" gas, carbon dioxide, and carry most of it back to your lungs dissolved in fizzy-drink form (bicarbonate) — where it gets dumped into the air you breathe out. When your muscles work hard and get hot and acidic, the trucks get better at dropping off oxygen right where it's needed.
Worked example
Imagine a person jogging on a warm day. Muscle cells burn fuel faster, consuming oxygen and producing CO₂, heat, and lactic acid. Interstitial PO₂ around the working muscles falls, so hemoglobin unloads more oxygen down the steep part of the dissociation curve. At the same time, the local temperature rises and the blood in the capillaries becomes slightly more acidic with more CO₂ — the Bohr effect shifts the curve right, so hemoglobin's affinity for oxygen drops and it releases even more oxygen to the working cells. Back at the lungs, the reverse logic applies: alveolar PO₂ is high, and as hemoglobin binds fresh oxygen it becomes less willing to hold CO₂ and H⁺ — the Haldane effect — so CO₂ is converted back to gas and exhaled. The same jogger's red blood cells, adapted to regular training or altitude, may carry slightly more 2,3-BPG, nudging the curve further right. One exercise session demonstrates every major concept in this topic: gradient-driven loading and unloading, the Bohr effect, the Haldane effect, and the metabolic signals that fine-tune gas delivery.
Key takeaways
- ~98% of O₂ is carried bound to hemoglobin; only ~1.5% dissolves in plasma. Hemoglobin binds up to 4 O₂ (one per heme iron).
- The dissociation curve is sigmoid due to cooperative binding. Flat on top (PO₂ 60–100 mmHg, saturation 90–98%), steep in the middle (PO₂ 20–60 mmHg) — the operating range for tissue unloading.
- Right shift = lower affinity = more unloading: increased temperature, PCO₂, H⁺ (lower pH), and 2,3-BPG (the Bohr effect). Left shift = higher affinity = more loading, less delivery.
- CO₂ rides three ways: ~70% as bicarbonate (via carbonic anhydrase + chloride shift), ~20% as carbaminohemoglobin, ~7–10% dissolved. (Percentages are commonly taught textbook approximations.)
- The Haldane effect: deoxygenated hemoglobin carries more CO₂ and buffers more H⁺ — oxygen unloading at the tissues boosts CO₂ pickup; oxygen loading at the lungs boosts CO₂ release.
- Carbon monoxide binds hemoglobin ~200+ times more tightly than O₂ (commonly taught figure — verify), blocking oxygen transport and shifting the curve left.
- Anemia and low hemoglobin reduce total oxygen-carrying capacity even when saturation is normal.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What fraction of oxygen is carried dissolved in plasma, and what carries the rest? How many O₂ molecules can one hemoglobin bind?
Show answer
Only about 1.5% of oxygen is dissolved in plasma; roughly 98% is bound to hemoglobin, which can carry up to four O₂ molecules (one per heme iron).
Why is the oxygen–hemoglobin dissociation curve S-shaped, and why is the flat top clinically useful?
Show answer
The S-shape reflects cooperative binding — each O₂ bound makes the next easier. The flat top (PO₂ 60–100 mmHg) keeps saturation near 90–98% over a wide range, so moderate drops in arterial PO₂ barely reduce oxygen delivery — a built-in safety margin.
List four factors that shift the dissociation curve right and explain what a right shift does for tissues.
Show answer
Increased temperature, increased PCO₂, decreased pH, and increased 2,3-BPG. A right shift lowers hemoglobin's oxygen affinity, so it releases more oxygen to tissues at any given tissue PO₂ (at the cost of lower saturation at any given arterial PO₂).
Name the three forms in which CO₂ is transported, with the approximate contribution of each.
Show answer
Dissolved in plasma (~7–10%), as carbaminohemoglobin bound to globin (~20–23%), and as bicarbonate (~70%) — commonly taught textbook approximations.
Describe the chloride shift and why it happens.
Show answer
Carbonic anhydrase converts CO₂ to bicarbonate inside red blood cells; as HCO₃⁻ accumulates it exits via a membrane exchanger that imports Cl⁻ (the chloride shift), preserving electrical neutrality and allowing bicarbonate to travel in plasma.
State the Haldane effect and explain how it couples oxygen and carbon dioxide transport at the tissues.
Show answer
Deoxygenated hemoglobin carries more CO₂ (carbaminohemoglobin) and buffers more H⁺ than oxygenated hemoglobin. At the tissues, unloading O₂ therefore increases CO₂ pickup; at the lungs, binding O₂ drives CO₂ off — oxygen delivery and CO₂ removal reinforce each other.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Hemoglobin (Hb)
- Iron-containing protein in red blood cells that binds O₂ (up to 4 molecules)
- Oxyhemoglobin
- Hemoglobin with oxygen bound
- Dissociation curve
- Graph of hemoglobin O₂ saturation vs. PO₂ (sigmoid)
- Cooperative binding
- Each O₂ bound makes the next bind more easily; each released makes the next release more easily
- Bohr effect
- Lower pH / higher PCO₂ shift the curve right (lower O₂ affinity)
- 2,3-BPG
- Metabolite in red blood cells that lowers hemoglobin's oxygen affinity
- Carbonic anhydrase
- Enzyme in red blood cells that speeds CO₂ ⇌ bicarbonate conversion
- Bicarbonate (HCO₃⁻)
- The dominant transport form of CO₂ in blood
- Chloride shift
- Cl⁻ enters red blood cells as HCO₃⁻ exits
- Haldane effect
- Deoxygenated hemoglobin carries more CO₂ and H⁺ than oxygenated hemoglobin
- Carbaminohemoglobin
- CO₂ bound to amino groups of hemoglobin's globin chains
Sources & references
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