7.4. VENTILATION AND GAS EXCHANGE IN OTHER ORGANISMS

Question 1

why can’t insects have the same gaseous exchange system as mammals?

Answer 1

• have a tough exoskeleton through which little or no gaseous exchange can occur
• don’t usually have blood pigments that can carry oxygen

Question 2

how does gas exchange take place in insects?

Answer 2

• along the thorax and abdomen of most insects are small openings known as spiracles
• air enters and leaves the system through the spiracles, but water is also lost
• leading away from the spiracles is the tracheae, which run both into and along the body of the insect
• the tracheae branch to form tracheoles
• at the end of the tracheoles is the tracheal fluid which then allows air to enter the tissues

Question 3

how is water loss reduced in an insects gas exchange system?

Answer 3

• in many insects, the spiracles can be opened or closed by sphincters
• the spiracle sphincters are kept closed as much as possible to minimise water loss

Question 4

how does an insects activity affect the spiracles?

Answer 4

• when it is inactive and oxygen demands are very low, the spiracles will all be closed most of the time
• when the oxygen demand is raised or the carbon dioxide levels build up, more of the spiracles open

Question 5

what are the tracheae and what are they made of?

Answer 5

• largest tubes of the insects respiratory volume, up to 1mm in diameter, and they carry air into the body
• lined with chitin, which keeps them open if they are bet or pressed
• this is the material that makes up the cuticle
• it is relatively impermeable to gases, so little gaseous exchange can take place in the trachea

Question 6

what are tracheoles?

Answer 6

• minute tubes of diameter 0.6-0.8μm
• each tracheole is a single, greatly elongated cell with no chitin lining so are freely permeable to gases
• due to their small size, they spread through the tissues of the insect, running between individual cells
• this is where most of the gaseous exchange takes place between the air and the respiring cells

Question 7

in most insects, how does air move along the tracheae and tracheoles?

Answer 7

• by diffusion alone, reaching all the tissues
• the vast number of tracheoles give a very large surface area for gas exchange
• oxygen dissolves in moisture on the walls of the tracheoles and diffuses into surrounding tissues

Question 8

what is tracheal fluid?

Answer 8

• located near the end of the tracheoles
• limits penetration of air for diffusion
• however, when oxygen demands build up (i.e. when flying) a lactic acid build up in the tissues results in water moving out of the tracheoles by osmosis
• this exposes more surface area for gas exchange

Question 9

what is the insect gas exchange system called?

Answer 9

• tracheal system

Question 10

how do larger insects (i.e. locusts) supply extra oxygen needed to cope with very high energy demands?

Answer 10

• mechanical ventilation of the tracheal system
• collapsible enlarged tracheae or air sacs

Question 11

what does mechanical ventilation of the tracheal system entail?

Answer 11

• air being actively pumped into the system by muscular pumping movements of the thorax and/or the abdomen
• these movements change the volume of the body and this changes the pressure in the tracheae and tracheoles
• air is drawn into the tracheae and tracheoles, or forced out, as the pressure changes

Question 12

how does having collapsible enlarged trachea/ air sacs help supple oxygen to insects?

Answer 12

• act as air reservoirs
• used to increase the amount of air moved through the gas exchange system
• usually inflated and deflated by the ventilating movements of the thorax and abdomen

Question 13

what difficulties do bony fish need to overcome to have effective respiratory systems?

Answer 13

• water is 1000 times denser than air and 100 times more viscous and a much lower oxygen content
• to cope with the viscosity of water and the slow rate of oxygen diffusion, fish have evolved very specialized respiratory systems that are different from those of land dwelling animals
• would use too much energy to move dense, viscous water in and out of lung-like respiratory organs
• moving water in one direction only is much simpler and more economical in energy terms

Question 14

how have bony fish adapted their ventilation system to take oxygen from the water in one direction?

Answer 14

• gills (organs of gas exchange)
• they have a large surface area, good blood supply and thin layers needed for successful gas exchange

Question 15

where are gills contained in bony fish?

Answer 15

• contained in a gill cavity
• covered by protective operculum (a bony flap)
• this is also active in maintaining a flow of water over the gills

Question 16

what are the main two features of gills?

Answer 16

• gill lamellae
• gill filaments (occur in large stacks called gill plates)

Question 17

what is ram ventilation?

Answer 17

• more primitive cartilaginous fish such as sharks and rays rely on continual movement to ventilate the gills
• they ‘ram’ the water past the gills

Question 18

how does the ventilation system work for fish who don’t use ram ventilation?

Answer 18

• mouth opens and the floor of the buccal cavity is lowered
• increases volume of buccal cavity, and the pressure drops allowing water to move into the buccal cavity
• at the same time, opercular valve is shut and opercular cavity containing the gills expands
• lowers pressure in opercular cavity containing the gills
• floor of buccal cavity starts to move up, increasing the pressure so water moves from the buccal cavity over the gills
• mouth closes, operculum opens and the sides of the opercular cavity move inwards
• this all increases the pressure in the opercular cavity and forces water over the gills and out of the operculum
• floor of the buccal cavity is steadily moved up, maintaining a flow of water over the gills

Question 19

what are two extra adaptions gills have that help ensure the most effective possible gaseous exchange occurs in the water?

Answer 19

• tips of the adjacent gill filaments overlap
• water moving over the gills and the blood in the gill filament flow in different directions

Question 20

how does having overlapping adjacent gill tips help effective gas exchange?

Answer 20

• increases resistance to the flow of water over the gill surfaces and slows down the movement of the water
• more time for gas exchange to occur

Question 21

how does having water over the gills and blood in the gill filaments flowing in different directions help to ensure effective gas exchange?

Answer 21

• creates a countercurrent exchange system
• ensures steeper concentration gradients are maintained and a much higher level of oxygen saturation in the blood is achieved
• more gas exchange can take place
• removes roughly 80% of oxygen from the water flowing over them

Question 22

what system does cartilaginous fish have?

Answer 22

• parallel systems
• blood in gill filaments and water over gills flows in the same direction
• extracts 50% of oxygen from the water flowing over them
  gives an initial steep concentration gradient, then diffusion happens until in equilibrium, now there is no net movement of oxygen into the blood