Gas Exchange in Animals

All living things respire – release energy from food.

• To respire oxygen has to come in across a membrane, be carried to the cells and carbon dioxide released.

• Gas exchange is movement of gases across a membrane.

• Breathing is the physical movements that help GE.

• Animals need a respiratory surface. All have 3 main requirements:

– Must be moist, gas has to be dissolved in water to pass over membrane

– Large to maximise gas exchange– Thin to allow gas to diffuse across

Different Gas Exchange Surfaces

Diffusion across a membrane• Protozoa, hydra and jellyfish

– It’s cells are bathed in water and as it is only 1 or 2 cells thick the diffusion is efficient and requires no energy.

– Oxygen passes straight into cells where concentration is lower.

Diffusion is an effective means oftransporting gases only when the

distance is < 1 mm

Open circulatory systems greatly increase the efficiency of transport of substances within a body compared

to diffusion

Insects – trachea systems• Throughout the insect is a system of

hollow tubes (trachea) that reach every cell. They open to the outside through spiracles (small holes) in the exoskeleton along the side of the body. Oxygen diffuses in through the trachea. They are held open by chiton bands.

• Some insects also use air sacs, some pump bodies.

Closed circulatory systems are even more efficient than open circulatory

systems

Fig. 42.3

Fish - gills• The respiratory system is external to body.• It is made of thin filaments supported by

bony structures. Each filament is a delicate plate of capillaries.

• Fish continuously pump water through respiratory surface.

• They use the counter current

exchange system.

Fish gillsGill capillaries are the sites of gas exchange with the environment

Counter-current exchange helps maximize the efficiency of gas exchange

Terrestrial vertebrates - lungs• Inside the body cavity so needs an

efficient transport system to connect to gas outside body.

Human Gas Exchange• Air enters through nose passes to

pharynx, then larynx to trachea. Branches down the two bronchi then to bronchioles and finally alveoli. These are covered by capillaries and O2 diffuse in and CO2 out over the thin surface.

Ventilation of lungs• Inhalation: breathing in, diaphragm

lowered, rib muscles contract, making partial vacuum which pulls air in.

• Exhalation: breathing out. Everything relaxes and forces air out.