# Gas exchange and transport in humans

> IB Biology SL · Theme B: Form and Function
> Source: https://www.owlsprep.com/study/ib-biology-sl-u2-gas-exchange-and-transport-in/

This module covers the structure of the human respiratory system, the mechanism of breathing, and how oxygen and carbon dioxide are transported between lungs and respiring tissues. You will learn key adaptations for efficient gas exchange.

**Prerequisites:** [Understanding of aerobic cell respiration](https://www.owlsprep.com/study/ib-biology-sl-u1-cell-respiration/); Basic cell membrane structure and diffusion

## Learning objectives

- Describe the structure of the human gas exchange system
- Explain the mechanism of resting ventilation
- Outline transport of oxygen and carbon dioxide in blood
- Explain the effect of the Bohr shift on oxygen delivery

## Structure of the Gas Exchange System

Air travels through a series of conducting structures before reaching the site of gas exchange: nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. Conducting structures warm, filter, and moisten air but do not exchange gas themselves.

**Alveoli** — Tiny, thin-walled air sacs that are the exclusive site of gas exchange between air and blood in the lungs.

*Example:* Human lungs have ~480 million alveoli, providing ~70 m² of surface area for gas exchange.

**Worked example:** Identify three adaptations of alveoli that make them efficient for gas exchange, and explain each adaptation.

1. 1. Large total surface area
2. The combined surface area of all alveoli maximizes the total rate of diffusion of respiratory gases.
3. 2. Walls made of a single layer of squamous epithelium
4. This creates a very short diffusion distance between air in the alveolus and blood in surrounding capillaries, increasing diffusion rate.
5. 3. Dense capillary network
6. Constant flow of blood maintains a steep concentration gradient: deoxygenated blood continuously arrives to pick up oxygen, and oxygenated blood is removed, keeping diffusion fast.

## Mechanism of Ventilation

Ventilation is the movement of air in and out of the lungs, required to refresh alveolar air and maintain the concentration gradient needed for gas exchange. It follows Boyle's Law: for a fixed amount of gas, pressure and volume are inversely related.

**Ventilation** — Bulk flow of air driven by pressure changes in the thoracic cavity, consisting of inspiration (inhalation) and expiration (exhalation).

**Worked example:** Explain how resting inspiration occurs in humans.

1. 1. The external intercostal muscles and diaphragm contract
2. Contraction flattens the diaphragm downwards and pulls the rib cage up and out, increasing the volume of the thoracic cavity.
3. 2. Pressure in the alveoli drops below atmospheric pressure
4. By Boyle's Law, increasing volume decreases pressure, creating a pressure gradient between the atmosphere and the lungs.
5. 3. Air flows into the lungs
6. Air moves down the pressure gradient until alveolar pressure equals atmospheric pressure, stopping inflow.

- **Resting expiration**: Passive process: inspiratory muscles relax, thoracic volume decreases, pressure rises above atmospheric pressure, air flows out
- **Active expiration (exercise)**: Requires contraction of abdominal muscles to push the diaphragm up further, increasing air outflow

## Transport of Respiratory Gases

After oxygen diffuses into blood from alveoli, it is transported to respiring tissues, and waste carbon dioxide is transported back to the lungs for exhalation. Gases are transported in different forms depending on their solubility and reactivity.

**Hemoglobin** — A quaternary protein in red blood cells that can bind up to four oxygen molecules, greatly increasing the oxygen carrying capacity of blood.

*Notation:* Hb

**Worked example:** State and describe the three forms of carbon dioxide transport in human blood, including approximate percentages for each.

1. 1. Dissolved in blood plasma (≈7-10%)
2. A small fraction of carbon dioxide is directly soluble in plasma and carried in this form.
3. 2. Bound to hemoglobin as carbaminohemoglobin (≈20-23%)
4. Carbon dioxide binds to the amino groups on the globin (protein) chains of hemoglobin (not the heme group that binds oxygen), forming carbaminohemoglobin for transport.
5. 3. As bicarbonate ions (≈70%)
6. Most carbon dioxide diffuses into red blood cells, reacts with water catalyzed by the enzyme carbonic anhydrase to form carbonic acid, which dissociates into hydrogen ions and bicarbonate ions. Bicarbonate then diffuses out into plasma for transport.

## Oxygen Dissociation Curve and Bohr Shift

Hemoglobin's affinity for oxygen changes with partial pressure of oxygen and pH. This is shown by the oxygen dissociation curve, which plots percentage of hemoglobin saturated with oxygen against partial pressure of oxygen.

**Worked example:** Explain how the Bohr shift adapts oxygen delivery to the needs of actively respiring tissues.

1. 1. Active tissues produce more carbon dioxide
2. High rates of aerobic respiration produce more CO₂, which lowers blood pH by forming carbonic acid.
3. 2. Lower pH reduces hemoglobin's affinity for oxygen
4. This causes the entire oxygen dissociation curve to shift to the right, an effect called the Bohr shift.
5. 3. More oxygen is unloaded in active tissues
6. At the partial pressure of oxygen found in active tissues, a right-shifted curve means hemoglobin holds less oxygen, so more oxygen is released to meet the high demand of respiring cells.

## Common pitfalls

- **Wrong:** Confusing ventilation with gas exchange
  - Why it fails: Ventilation is bulk movement of air, gas exchange is diffusion of O₂/CO₂ across cell membranes
  - Correct: Ventilation maintains the concentration gradient required for gas exchange; they are separate processes
- **Wrong:** Claiming most oxygen is transported dissolved in plasma
  - Why it fails: Only ~1.5% of oxygen is soluble in plasma
  - Correct: ~98.5% of oxygen is bound to hemoglobin in red blood cells
- **Wrong:** Stating expiration requires muscle contraction at rest
  - Why it fails: Resting expiration is a passive process that does not require energy
  - Correct: Only active expiration during exercise requires muscle contraction; resting expiration occurs when inspiratory muscles relax
- **Wrong:** Claiming carbon dioxide binds to the heme group of hemoglobin
  - Why it fails: Oxygen binds the heme group, carbon dioxide binds a different site
  - Correct: Carbon dioxide binds to the globin (protein) chains of hemoglobin to form carbaminohemoglobin
- **Wrong:** Thinking the Bohr shift increases hemoglobin's affinity for oxygen
  - Why it fails: The shift right corresponds to lower affinity, not higher
  - Correct: Lower pH from high CO₂ reduces affinity, allowing more oxygen to be released to active tissues

## Cheatsheet

| Component/Process | Key Exam Fact |
| --- | --- |
| Alveoli adaptations | Large SA, 1-cell thick wall, dense capillaries, moist surface |
| Resting inspiration | Diaphragm/external intercostals contract → volume up → pressure down → air in |
| Resting expiration | Inspiratory muscles relax → volume down → pressure up → air out |
| Oxygen transport | ~98.5% bound to Hb, ~1.5% dissolved in plasma |
| Carbon dioxide transport | ~70% HCO₃⁻, ~20% carbaminoHb, ~10% dissolved |
| Bohr shift | Low pH shifts curve right → more O₂ released to active tissues |

## What's next

This sub-topic forms the foundation of human respiratory physiology, explaining how the body supplies oxygen to cells for aerobic respiration and removes waste carbon dioxide. This knowledge connects to understanding how the body adapts to different conditions like exercise and high altitude, and how diseases such as emphysema and asthma impair gas exchange. It also integrates closely with other topics in IB Biology SL, including the structure and function of the circulatory system, cell respiration, and whole-body homeostasis. Explore the related links below to build a complete understanding of how human organ systems work together.

- [Digestion and absorption in humans](https://www.owlsprep.com/study/ib-biology-sl-u2-digestion-and-absorption-in-humans/)
- [Hormones, homeostasis and reproduction](https://www.owlsprep.com/study/ib-biology-sl-u2-hormones-homeostasis-and-reproduction/)

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