Gas Exchange

  • Created by: charley
  • Created on: 22-08-18 12:17

Gas Exchange in Humans

Ventilation of the lungs: Inhilation (reverse for exhalation)

  • External intercostal muscles contract, internal intercostal muscles relax
  • Ribs are pulled upwards and outwards while diaphragm contracts and flattens
  • Thorax volume increases
  • Pressure in the lungs decreases
  • Atmospheric air pressure is greater than the pressure in the lungs so air is forced in

The main cause for air being forced out is the elastic recoil of the lungs. The surfaces of the aveoli are coated in a surfactant with a low surface tension made of moist secretions which prevents the aveoli from collapsing during exhalation and allows gases to dissolve before they diffuse.

Aveoli are efficient at gas exchange because they're moist, have a large surface area, have a short diffusion pathway and are surrounded by an extensive capillary network.

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Gas Exchange in Animals

Unicellular organisms exchange gases across their cell membrane. Flatworms and earthworms exchange gases across their surface. Flatworms are more efficient than earthworms.

In insects, gas exchange occurs through holes (spiracles) which lead to tracheae which branch into tracheoles. Tracheoles are fluid filled. Oxygen diffuses directly into the muscles (theres no circulation or respiratory pigment). Spiracles can close to reduce water loss. During rest, insects rely on diffusion but during activity movements of the abdomen ventilate the tracheae.

Amphibians exchange gases through their lungs and external surface. Reptiles exchange gases in the lungs. Birds exchange gases in the lungs, their ribs and flight muscles venture their lungs producing large volumes of oxygen.

Cartilaginous fish must keep swimming to maintain ventilation. They have parallel flow (blood and water travel in the same direction) so bloods oxygen conc. is limited to 50%. Gas exchange only takes place along part of the lamella. Carbon dioxide diffuses from the blood to the water. They have gill pouches and slits. 

Bony fish have counter-current flow (blood and water travel in opposite directions) so gas exchange takes place along the entire length of the lamellae. Oxygen conc. is around 80%. Gills are covered by the operculum. They have 4 gills each with gill filaments which have gill lamellae, supported by a gill arch. Carbon dioxide diffuses out along the entire length of the lamellae. To take in water; the mouth opens, the operculum closes, the floor of the mouth is lowered, increasing the volume and decreasing the pressure inside the mouth cavity. Water flows in as external pressure is higher than the pressure inside the mouth. To force water out the process is reversed.

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Gas Exchange in Plants

Gases diffuse through the stomata down a concentration gradient. Gases then diffuse through the intercellular spaces between the spongy mesophyll cells and into cells. 

The direction of diffusion depends on the concentration of gases in the atmosphere and the reations in the plant cells.

Leaves are thin, have a large surface area for many stomata, have air spaces in the spongy mesophyll and stomatal pores, making it an efficient gas exchange surface.

Stomata are bound by two guard cells.

Water enters the guard cells, they become turgid and the pore opens. The reverse happens for the pore to close. The chloroplasts in the guard cells photosynthesise producing ATP. ATP provides energy for active transport of potassium ions into the guard cells from the epidermal cells. The potassium and malate ions lower the water potential so water flows in by osmosis.

Stomata close at night, in bright light and if theres excessive water loss.

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