Comparing Structures for Gas Exchange

Compare: / Insects / Fish / Mammals
Environment / Terrestrial / Aquatic / Terrestrial
Oxygen available / 21% of the air / 1% dissolved oxygen in water / 21% of the air
Percentage of oxygen extracted from that taken in / <25% / 80% / 25%
Tube Strengthening Bands / Chitin Rings – heavy, around Trachea / None / Cartilage Rings – light, around Trachea, Bronchi and bronchioles
Association with transport system / None / Close / Close
Haemoglobin / None / Yes / Yes
Ventilation – to increase volume of gases across membranes during active periods and high cellular respiration needs. / Air sacs / Abdominal contraction / Gulping water / Expand/contract the chest cavity using the intercostals muscles and diaphragm.
Poikilothermic OR Homeothermic? / Poikilothermic / Poikilothermic / Homeothermic
SA:Vol / Highest / Lower / Lowest

(1) On your own paper, write statements that give reasons for the similarities and differences describe in the table above:

e.g. In insects a specialised gas exchange system is necessary because of increased size and the lowering SA/vol ratio. Tracheal systems or gills have high SA and maintain overall SA needed for gas exchange. In fish a specialised gas exchange system is necessary because of increased size and the lowering SA/vol ratio. Gills have high SA and maintain overall SA needed for efficient gas exchange. In mammals a specialised gas exchange system is necessary because of increased size and the lowering SA/vol ratio. Lungs have high SA and maintain overall SA needed for efficient gas exchange.

All groups need to ventilate – fish gulp water, insects pump their abdomen by muscular contractions and mammals expand/contract the chest cavity using the intercostals muscles as well as those of the diaphragm. These movements ensure that the gases are brought to/taken from the gas exchange surfaces rapidly – this is especially important in regions of low 02 e.g. fish in stagnant water, mammals and insects at high altitudes and during times of high physical exercise. This is because the energy demands of exercise require increased 02 for cellular respiration and need to remove rapidly the CO2 produced in respiration or the blood becomes to acid.

Insects and mammals both have internal gas exchange surfaces supplied by tubes e.g. trachea, bronchi, so these need to be kept open constantly or the animal will suffocate. This is done by bands of chitin in insects and cartilage in mammals. As fish have an external system (the gills float freely and are bathed in water) there is no need for this.

Systems in fish and mammals are all closely associated with the transport system for rapid transport of CO2 and O2 from and to the gas exchange surfaces to maximise the concentration gradient for efficient exchange. The transport system then transports gases to and from every cell. In insects there is no association with the transport system as the air tubes supply and remove gases directly to the cells.

In fish and mammals the gases are transported in the blood. The red blood cells have the pigment haemoglobin which increases greatly the oxygen carrying capacity of the blood. The blood of insects does not have such a pigment as it does not carry oxygen. Lack of an O2 carrying pigment and/or close association between the transport and gas exchange systems is less efficient than other animal groups and imposes size limitations.

Fish obtain O2 from water where it is in lower supply than air, hence the adaptation of the counter current system to enhance the concentration gradient of O2 and CO2 between water and blood. Insects and Mammals have much larger supply of oxygen available to them and as a result their gas exchange systems are nearly as efficient as extracting oxygen as that of the fish.

Insects and Fish are poikilothermic and so require less oxygen for respiration. Compared with mammals that are homeothermic and require more oxygen for respiration.

(2) On your own paper, make a statement relating to the animals way of life. Insects has been done for you:

The Insect Gas Exchange System permits a high rate of metabolism because the oxygen is taken directly to the body tissues but is not suitable for large animals hence most insects are small.

The Fish Gas Exchange System is restricted to water as gills collapse when not supported by water. Gills internal and this provides them with protection

The Mammal Lungs are internal for physical protection and to prevent dehydration. Transport system returns blood to heart before moving to body tissues and this increases efficiency of oxygen delivery to body cells. Total reliance on gas exchange via internal lungs. (dry skin) allows exploitation of a wider variety of habitats, wet – dry, aquatic – terrestrial etc.

(3) On your own paper, give reasons why each group could not survive with the gas exchange system of another

e.g. Insects could not survive with a gill system or lung system as a gill system would dry out quickly as it would not be constantly bathed in water and a lung system requires a close association with the transport system to allow transport of gases to all body cells.

Fish could not survive with a tracheal system or lung system as a tracheal system would be too heavy and lungs would require huge amounts of water to extract sufficient oxygen.

Mammals could not survive with a tracheal system or a gill system as a tracheal system would not be able to diffuse gases quickly enough because mammals have a smaller surface to volume ratio compared that of insects and a gill system would dry out quickly as it would not be constantly bathed in water.