Gas exchange is a biological process through which (usually two) different gases are transferred in opposite directions across a specialised respiratory surface. Gases are constantly required and produced as a by-product of cellular and metabolic reactions so an efficient system for their exchange is extremely important. It is linked with respiration in animals, and both respiration and photosynthesis in plants.
In respiration, oxygen (O2) is required to enter cells whilst waste carbon dioxide (CO2) must be removed – the opposite is true for photosynthesis, where CO2 enters plants and O2 is released. The exchange of gases essentially occurs as a result of diffusion down a concentration gradient – gas molecules moving from an area of high concentration → low concentration.
- The surface area available
- The distance the gas molecules must diffuse across
- The concentration gradient
In single-celled organisms diffusion can occur straight across the cell membrane; as organisms increase in size so does the distance gases must travel across. Their surface-area-to-volume ratio also decreases. Diffusion alone is not efficient enough and specialised respiratory systems are required. This is the case with humans and fish where circulatory systems have evolved: These are able to transport the gases to and from the respiratory surface and maintain a continuous concentration gradient. 
Both oxygen and carbon dioxide are transported around the body in the blood – through arteries, veins and capillaries. They bind to hemoglobin in red blood cells although this is more effective with oxygen. Carbon dioxide also dissolves in the plasma or combines with water to form bicarbonate ions (HCO−
3). This reaction is catalysed by the carbonic anhydrase enzyme in red blood cells: 
- CO2 + H2O → H2CO3
- H2CO3 → H+ + HCO−
The main respiratory surface in humans are the alveoli. Alveoli are small air sacs branching off from the bronchioles in the lungs. They are one-cell thick and provide a moist and extremely large surface area for gas exchange to occur. Capillaries carrying deoxygenated blood from the pulmonary artery run across the alveoli - they are also extremely thin so the total distance gases must diffuse across is only around 2-cells thick.
Inhaled oxygen is able to diffuse into the capillaries from the alveoli, while carbon dioxide from the blood diffuses in the opposite direction into the alveoli. The waste carbon dioxide can then be exhaled out of the body. Continuous blood flow in the capillaries as well as constant breathing maintains a steep concentration gradient.
During physical exercise excess carbon dioxide is produced as a result of increased respiration: This must be removed and muscles and cells require increased oxygen. The body responds to this change by increasing the breathing rate, therefore maximising the rate of possible gas exchange. 
Air diffuses directly into and out of plants through pores on the underside of leaves known as stomata. The stomata are controlled by guard cells (affected by osmosis and turgor pressure). When the guard cells are turgid stomata are open, when they are flaccid stomata close and gas exchange cannot take place. The main respiratory surface is the spongy mesophyll cells inside the leaf. They have large air spaces and therefore provide a large surface area for gas exchange. Leaves are also very thin so the diffusion distance is small. 
During the day photosynthesis occurs at a faster rate than respiration so there is an overall uptake of CO2 and release of O2. At night (when there is no light available) this is reversed as photosynthesis stops and only respiration can take place.
Fish must extract oxygen dissolved in water, not air, which has led to the evolution of gills and opercula. Gills are specialised organs containing filaments and lamellae – the lamellae contain capillaries and provide a large surface area and short diffusion distance as they are extremely thin. 
Water is drawn in through the mouth and passes over the gills in one direction while blood flows through the lamellae in the opposite direction – this counter current maintains a steep concentration gradient. O2 is able to continually diffuse down its gradient into the blood and CO2 into the water. 
Summary of main systems
|Large surface area||Short diffusion distance||Maintained concentration gradient|
|Human||Total alveoli = 70-100m2 ||Alveolus + Capillary = 2-cells||Constant blood flow in capillaries; breathing|
|Fish||Many lamellae and filaments per gill||Usually 1-cell||Counter-current flow|
|Plant||High density of stomata; air spaces within leaf||1-cell||Constant air flow|
Insects such as crickets do not have an inner skeleton so exchange gases across structures known as trachea and tracheoles: These are tubes that run directly into the body of the insect. Air enters the trachea through valves known as spiracles and diffusion can then occur straight into the respiring tissues.
Amphibians are able to use their skin as a respiratory surface, as well as having lungs and sometimes gills.
- "Gas exchange". Retrieved 19 March 2013.
- Piiper J, Dejours P, Haab P & Rahn H (1971). "Oncepts and basic quantities in gas exchange physiology". Respiration Physiology 13: 292–304.
- Kety SS (1951). "The theory and applications of the exchange of inert gas at the lungs and tissues". Pharmacological Reviews 3: 1–41.
- Raymond H & Swenson E (2000). "The distribution and physiological significance of carbonic anhydrase in vertebrate gas exchange organs". Respiration physiology 121: 1–12.
- "Gas Exchange in humans". Retrieved 19 March 2013.
- Wasserman K, Whipp B, Koyal S & Beaver W (1973). "Anaerobic threshold and respiratory gas exchange during exercise". Journal of Applied Physiology 35.
- Casson S & Gray JE (2008). "Influence of environmental factors on stomatal development". New Phytologist 178: 9–23.
- Newstead James D (1967). "Fine structure of the respiratory lamellae of teleostean gills". Cell and Tissue Research 79: 396–428.
- Hughes GM (1972). "Morphometrics of fish gills". Respiration physiology 14: 1–25.
- Basset J, Crone C, Saumon G (1987). "Significance of active ion transport in transalveolar water absorption: a study on isolated rat lung". The journal of physiology 384: 311–324.
- "Gas Exchange in Insects". Retrieved 19 March 2013.