Why You Can Never Breathe Every Bit of Air Out of Your Lungs

Cells need oxygen to release energy from food and must get rid of carbon dioxide. Tiny animals manage this across their body surface, but larger, more active animals need organs with a large, moist surface to exchange gases fast enough.

This part covers respiratory organs across animals, the human respiratory tract, the mechanism of breathing, and respiratory volumes.

The respiratory organ an animal uses depends on its habitat and level of organisation: simple animals exchange gases over the body surface, insects use tracheal tubes, most aquatic animals use gills, and land vertebrates use lungs.

Across groups:

- Body surface, by simple diffusion — sponges, coelenterates and flatworms
- Moist cuticle — earthworms
- Tracheal tubes — insects; a network of tubes carries air directly to the tissues
- Gills (branchial respiration) — most aquatic arthropods and molluscs, and fishes
- Lungs (pulmonary respiration) — amphibians, reptiles, birds and mammals
- Moist skin (cutaneous respiration) — frogs, alongside their lungs

What they share. Every respiratory surface is thin, moist and large compared with the animal's needs, and most lie close to a blood supply.

An everyday example. A fish gasping at the surface of an aquarium when the air pump stops shows how much its gills depend on oxygen dissolved in the water.

The substance. Insect tracheal tubes deliver oxygen straight to cells, so insect blood plays little part in carrying oxygen.

Air passes through the external nostrils, nasal chamber, pharynx, larynx, trachea, bronchi and bronchioles to the alveoli; everything up to the terminal bronchioles is the conducting part, which filters, warms and moistens the air.

The pathway:

- External nostrils $\to$ nasal passage $\to$ nasal chamber
- Pharynx — a common passage for food and air
- Larynx (sound box) — a cartilaginous box; during swallowing, the epiglottis covers the glottis so food does not enter
- Trachea — divides at the level of the fifth thoracic vertebra into right and left primary bronchi
- Secondary and tertiary bronchi $\to$ bronchioles $\to$ terminal bronchioles
- Alveoli — very thin, irregular-walled, vascularised bag-like structures

The trachea, bronchi and initial bronchioles are held open by incomplete cartilaginous rings.

Two parts:

- Conducting part — nostrils to terminal bronchioles; transports air, clears foreign particles, humidifies it and brings it to body temperature
- Exchange part — alveoli and their ducts; where O${}_2$ and CO${}_2$ actually diffuse

An everyday example. Choking on a sip of water while laughing happens when the epiglottis does not close the glottis in time.

The substance. The rings are incomplete at the back, which keeps the trachea open while letting the food pipe behind it expand during swallowing.

Breathing in happens when the diaphragm and external intercostal muscles contract, enlarging the chest and lowering lung pressure below atmospheric pressure; breathing out happens when they relax, the chest shrinks, and lung pressure rises above atmospheric pressure.

Inspiration:

- Diaphragm contracts, increasing chest volume front to back
- External intercostal muscles contract, lifting the ribs and sternum, increasing volume side to side
- Lung volume rises, so intra-pulmonary pressure falls below atmospheric pressure
- Air flows in

Expiration:

- Diaphragm and intercostal muscles relax
- Chest and lung volumes decrease
- Intra-pulmonary pressure rises above atmospheric pressure
- Air is expelled

Forceful breathing. Abdominal muscles add strength to both inspiration and expiration.

Worked example — predict the flow. Lung pressure is 758 mm Hg and outside air is 760 mm Hg. Air moves into the lungs, from higher to lower pressure.

An everyday example. Blowing up a balloon needs a hard push from the abdominal muscles, far more than quiet breathing.

The substance. The lungs never pull air in themselves — they are passive and follow the pressure changes made by the muscles.

Respiratory volumes are separate amounts of air breathed in or out, and capacities are sums of them; vital capacity is tidal volume plus both reserve volumes, and total lung capacity adds the residual volume that always stays behind.

Volumes in a healthy adult:

- Tidal volume (TV) — one normal breath, about 500 mL
- Inspiratory reserve volume (IRV) — extra air forcibly inhaled, about 2500 to 3000 mL
- Expiratory reserve volume (ERV) — extra air forcibly exhaled, about 1000 to 1100 mL
- Residual volume (RV) — air left after forcible expiration, about 1100 to 1200 mL

Capacities:

- Inspiratory capacity = TV + IRV
- Expiratory capacity = TV + ERV
- Functional residual capacity = ERV + RV
- Vital capacity = ERV + TV + IRV
- Total lung capacity = RV + ERV + TV + IRV

Worked example — one person's lungs. TV = 500 mL, IRV = 2500 mL, ERV = 1000 mL, RV = 1200 mL.

$$\text{VC}=1000+500+2500=4000\text{ mL}\text{TLC}=4000+1200=5200\text{ mL}$$

At 12 breaths a minute, air moved is $500\times 12=6000$ mL per minute.

An everyday example. Shehnai and flute players train to take very deep breaths, drawing on their inspiratory reserve volume to hold long notes.

The substance. A spirometer cannot measure residual volume, because that air never leaves the lungs.