Every cell in your body runs cellular respiration, using oxygen to release energy from glucose and producing carbon dioxide as waste. A cell deep inside your thigh muscle has no direct contact with air, so the body needs a delivery system. The respiratory system brings air close to the blood and lets oxygen in and carbon dioxide out; the circulatory system carries the gases the rest of the way. Keep three words separate: ventilation (breathing: moving air in and out of the lungs), gas exchange (oxygen and carbon dioxide diffusing between air, blood and cells), and cellular respiration (the chemical reactions inside mitochondria that use the oxygen).
The airway: from nose to alveoli
| Structure | Role |
|---|---|
| Nasal cavity | Warms and moistens incoming air; hairs and sticky mucus trap dust and microbes. |
| Pharynx (throat) | Shared passage for air and food. |
| Larynx (voice box) | Contains the vocal cords. The epiglottis, a flap above it, closes over the airway during swallowing so food goes down the esophagus. |
| Trachea (windpipe) | Held open by C-shaped rings of cartilage. Its lining has mucus and cilia that sweep trapped particles up toward the throat. |
| Bronchi (singular bronchus) | Two branches of the trachea, one to each lung. |
| Bronchioles | Smaller and smaller branches with smooth muscle in their walls that can narrow or widen the airway. |
| Alveoli (singular alveolus) | Millions of tiny air sacs at the ends of the bronchioles, wrapped in capillaries; this is where gas exchange happens. |
| Diaphragm and intercostal muscles | The dome-shaped diaphragm under the lungs and the muscles between the ribs change the volume of the chest cavity. |
| Pleural membranes | Two thin, moist membranes around each lung that let the lungs slide smoothly as the chest moves. |
Ventilation: how breathing moves air
Lungs have no muscle of their own that pulls in air. Instead, breathing depends on a pressure difference. For a fixed amount of gas, a larger volume means lower pressure (this is Boyle's law, from chemistry). Air always flows from higher pressure to lower pressure. So when the chest cavity gets bigger, the pressure inside the lungs falls slightly below the air pressure outside, and air flows in. When the chest cavity gets smaller, pressure inside rises above outside pressure, and air flows out.
- Diaphragm contracts, flattens and moves down
- External intercostal muscles contract, lifting the ribs up and out
- Chest cavity volume increases
- Pressure in the lungs drops below atmospheric pressure
- Air flows in; this is an active process that uses muscle energy
- Diaphragm relaxes and domes upward
- Intercostal muscles relax; ribs move down and in
- Chest cavity volume decreases
- Pressure in the lungs rises above atmospheric pressure
- Air flows out; at rest this is mostly passive, as elastic lung tissue recoils
Gas exchange in the alveoli
Gases cross the alveolar wall by diffusion: movement from an area of higher concentration to an area of lower concentration, with no energy spent by the body. The difference in concentration is the diffusion gradient. Air in the alveoli has more oxygen than the blood arriving from the body, so oxygen diffuses into the blood. That blood carries more carbon dioxide than alveolar air, so carbon dioxide diffuses out. Physiologists often express this with partial pressures: oxygen is typically around 100 mmHg in alveolar air but around 40 mmHg in the blood returning from the body. Several features make the lungs very efficient at exchange: a huge total surface area from millions of alveoli; walls only one cell thick, pressed against capillary walls that are also one cell thick, so the distance is tiny; a moist lining, so gases can dissolve before diffusing; and constant blood flow and breathing, which keep the gradients steep.
| Gas | Inhaled air (approx.) | Exhaled air (approx.) |
|---|---|---|
| Nitrogen | 78% | 78% |
| Oxygen | 21% | 16% |
| Carbon dioxide | 0.04% | 4% |
| Water vapour | Varies (often low) | Saturated (high) |
Hemoglobin: carrying oxygen to the cells
Oxygen does not dissolve well in plasma, so almost all of it is carried by hemoglobin, a protein packed inside red blood cells. Each hemoglobin molecule has four heme groups, each holding an iron atom, so one hemoglobin molecule can carry up to four molecules. In the lungs, where oxygen is plentiful, hemoglobin loads oxygen to become oxyhemoglobin. In body tissues, where cells have used up oxygen, it unloads it. Hemoglobin also releases oxygen more readily where carbon dioxide is high, the pH is lower and the temperature is higher, which is exactly the situation in a hard-working muscle. The effect of carbon dioxide and pH on oxygen release is called the Bohr effect. Carbon dioxide travels back mostly as bicarbonate ions dissolved in plasma, with smaller amounts bound to hemoglobin or dissolved directly. Carbon monoxide is dangerous because it binds to hemoglobin far more tightly than oxygen does, taking up the sites oxygen needs.
Control of breathing and the response to exercise
You do not have to remember to breathe. A breathing centre in the medulla oblongata at the base of the brain sets the rhythm. The main signal it responds to is a rise in carbon dioxide in the blood, detected as a drop in pH; sensors in the aorta and carotid arteries also detect low oxygen. For healthy adults at rest, a typical breathing rate is about 12 to 20 breaths per minute, with about 500 mL of air moved per breath (the tidal volume). During exercise, muscles produce more carbon dioxide, so both the rate and depth of breathing increase and much more air is moved each minute. Heart rate rises at the same time to move the extra oxygen. After exercise stops, breathing stays faster for a while as the body recovers.