Exchange surfaces and gas exchange definitions

Surface area to volume ratio: why size limits diffusion

Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random motion. It is passive and requires no metabolic energy.

Exchange surface

A surface across which substances move between an organism and its environment, or between two compartments within it.

Diffusion distance

The thickness of the barrier a substance has to cross, sometimes called the length of the diffusion pathway.

Concentration gradient

The difference in concentration between two points, divided by the distance between them.

Gas exchange in mammals: lungs and ventilation

Cartilage

A firm, flexible supporting tissue. In the trachea it forms incomplete rings, and in the bronchi irregular blocks, holding the airway open against the pressure drop of inspiration.

Ciliated epithelium

An epithelium whose cells carry cilia that beat in a co-ordinated wave, sweeping mucus and trapped particles up towards the throat.

Goblet cell

A cell in the airway epithelium that secretes mucus, which traps dust, pollen and bacteria.

Smooth muscle

Involuntary muscle in the walls of hollow organs and vessels. In the airway wall its contraction narrows the lumen, and it is the tissue that contracts inappropriately during an asthma attack.

Elastic fibres

Fibres that stretch as the lungs inflate and recoil as the muscles relax, providing much of the force for quiet expiration.

Squamous epithelium

An epithelium of very flattened cells, giving a wall a fraction of a micrometre thick; the lining of an alveolus.

Tidal volume

The volume of air moved in or out in one normal breath at rest, about 0.5 dm³ in an adult.

Ventilation rate

The number of breaths taken per minute, about 12 to 15 at rest. Also called breathing rate.

Vital capacity

The largest volume that can be breathed out after the deepest possible breath in; roughly 4.6 dm³ in a young adult man and 3.1 dm³ in a young adult woman.

Residual volume

The volume left in the lungs after the hardest possible breath out, about 1.2 dm³. It cannot be measured with a spirometer, because it never leaves the lungs. Vital capacity plus residual volume is the total lung capacity.

Forced vital capacity (FVC)

The total volume of air breathed out in one forced breath after the deepest possible breath in. In healthy lungs it is close to the vital capacity.

Forced expiratory volume in one second (FEV₁)

The volume of air breathed out in the first second of a forced breath out. It falls when the airways are narrowed, because air cannot leave them quickly.

FEV₁ ÷ FVC ratio

The fraction of the forced vital capacity breathed out in the first second. In a healthy adult it is usually above 0.7, often around 0.8; a value below 0.7 is the threshold commonly used to indicate airway obstruction.

Obstructive lung disease

A disease that narrows the airways and raises the resistance to air flow, so FEV₁ falls more than FVC and the ratio drops. Asthma and emphysema are examples.

Restrictive lung disease

A disease that stops the lungs expanding fully, so FVC and FEV₁ fall together and the ratio stays normal or rises. Pulmonary fibrosis is an example.

Risk factor

Anything that increases the probability that a person will develop a disease, such as smoking for lung cancer and emphysema.

Incidence

The number of new cases of a disease in a population over a stated period, usually given per 100 000 people per year so that populations of different sizes can be compared.

Respiratory minute ventilation

The volume of air breathed in or out per minute: tidal volume × breathing rate, in dm³ min⁻¹. Also called pulmonary ventilation rate.

Oxygen consumption

The volume of oxygen the body takes up per minute. On a closed-circuit spirometer trace with soda lime present, it is the fall per minute of a line through the troughs.

Gas exchange in insects, fish and plants

Spiracle

A pore in the body wall of an insect, guarded by valves, through which air enters and leaves the tracheal system.

Tracheole

One of the finest branches of an insect's tracheal system, under a micrometre across, running between and into the cells and delivering oxygen to them directly.

Gill filament

One of the many thin projections from a gill arch, sometimes called a primary lamella.

Lamella

One of the thin plates set at right angles along a gill filament, covered by an epithelium one cell thick with blood flowing just beneath it; the actual exchange surface.

Counter-current flow

An arrangement in which two fluids move past each other in opposite directions, so a concentration gradient is maintained along the whole length of the exchange surface.

Operculum

The bony flap covering the gills of a bony fish, acting as a valve and part of the pump that maintains the flow of water.

Guard cell

One of the pair of cells around a stoma. The pair bow apart and open the pore when they become turgid, and close it when they become flaccid.

Xerophyte

A plant whose leaves and stems have features that reduce water loss by transpiration, found where water is scarce.

Dorsal

The back or upper surface of an animal, opposite the ventral surface.