3.1.1 Exchange Surfaces

Question 1

What are the three main factors that affect the need for an exchange system?

Answer 1

• size
• surface area to volume ratio
• level of activity

Question 2

How does size affect the need for an exchange system?

Answer 2

• in very small organisms, all the cytoplasm is very close to the environment in which they live
• diffusion supplies enough oxygen and nutrients to keep the cells alive and active
• however, multicellular organisms may have several layers of cells
• any oxygen or nutrients diffusing in from the outside have a longer diffusion pathway
• diffusion is too slow to enable a sufficient supply t the innermost cells

Question 3

How does surface area to volume ratio affect the need for an exchange system?

Answer 3

• small organisms have a small surface area but they also have a small volume
• their surface area is relatively large compared to their volume
• therefore, they have a large surface area to volume ratio
• so their surface area is large enough to supply all their cells with sufficient oxygen
• larger organisms have a larger surface area, but also a larger volume
• as size increases, the volume rises more quickly than the surface area
• therefore, they have a small surface area to volume ratio
• some organisms increase their surface area by adopting a different shape to give a larger surface area to volume ratio
• larger organisms need a range of tissues to give the body support and strength so their surface area to volume ratio remains relatively small

Question 4

How does level of activity affect the need for an exchange system?

Answer 4

• some organisms are more active than others
• metabolic activity uses energy from food and requires oxygen to release energy in aerobic respiration
• the cells of an active organism need good supplies of nutrients and oxygen to supply energy for movement
• this need for energy is increased in animals the need to keep themselves warm

Question 5

What are features of a good exchange surface?

Answer 5

• a large surface area to provide more space for molecules to pass through
• often achieved by folding the walls and membranes involved
• e.g. root hairs in plants
• a thin barrier to reduce the diffusion distance
• the barrier must be permeable to the substances being exchanged
• e.g. alveoli in lungs
• good blood supply
• it can bring fresh supplies of molecules to one side, keeping the concentration high
• or it may remove molecules from the demand side to keep concentration low
• this is important to maintain a steep concentration gradient so that diffusion can occur rapidly
• e.g. gills in fish

Question 6

Label the mammalian gas exchange diagram

Answer 6

Question 7

Briefly explain how gaseous exchange in the lungs occurs?

Answer 7

• gases pass by diffusion through the thin walls of the alveoli
• oxygen passes from the air in the alveoli to the blood in the capillaries
• CO₂ passes from the blood to the air in the alveoli
• the lungs must maintain a steep concentration gradient in each direction in order to ensure that diffusion can continue

Question 8

Describe how the lungs have a large surface area for molecules to pass through?

Answer 8

• there are so many alveoli (each about 100-300µm across) that the total surface area of the lungs is much larger than our skin’s
• alveoli are lined by a thin layer of moisture, which evaporates and is lost as we breathe out
• the lungs must produce a surfactant that coats the internal surface of the alveoli to reduce the cohesive forces between the water molecules, as these forces tend to make alveoli collapse

Question 9

Describe how the barrier to exchange is permeable to carbon dioxide and oxygen in the lungs

Answer 9

• the barrier to exchange is comprised of the wall of the alveolus and the wall of the blood capillary
• the cells and their plasma membranes readily allow the diffusion of oxygen and carbon dioxide, as the molecules are small and non-polar

Question 10

Describe how the lungs have a thin barrier that reduce diffusion distance

Answer 10

• the alveolus wall is one cell thick
• capillary wall is one cell thick
• both walls consist of squamous cells, which are flattened or thin
• capillaries are in close contact with the alveolus walls
• capillaries are so narrow that the red blood cells are squeezed against the capillary wall
• this makes them closer to the air in the alveoli and reduces their rate of flow
• the total barrier to diffusion is only two flattened cells
• it is less than 1µm thick

Question 11

Describe how the lungs have a good blood supply

Answer 11

• the blood supply helps to maintain a steep concentration gradient, so that the gases continue to diffuse
• the blood system transports carbon dioxide from the tissues to the lungs
• this ensures that the concentration of carbon dioxide in the blood is higher than that in the air of the alveoli
• therefore, carbon dioxide diffuses into the alveoli
• the blood also transports oxygen away from the lungs
• this ensures that the concentration of oxygen in the blood is kept lower than that in the alveoli
• so oxygen diffuses into the blood

Question 12

What is the point of ventilation in the lungs?

Answer 12

• the concentration of oxygen in the air of the alveolus remains higher than that in the blood
• the concentration of carbon dioxide in the alveoli remains lower than that in the blood
• therefore, the concentration gradient necessary for diffusion is maintained

Question 13

Describe inspiration

Answer 13

• the diaphragm contracts to move down and become flatter
• this displaces the digestive organs downwards
• the external intercostal muscles contract to raise the ribs
• the volume of the chest cavity is increase
• the pressure in the chest cavity drops below the atmospheric pressure
• air is moved into the lungs

Question 14

Describe expiration

Answer 14

• the diaphragm relaxes and is pushed up by the displaced organs underneath
• the external intercostal muscles relax and the ribs fall
• the internal intercostal muscles can contract to help push air out more forcefully
• this usually happens only during exercising or coughing and sneezing
• the volume of the chest cavity is decreased
• the pressure in the lungs increases and rises above the pressure in the surrounding atmosphere
• air is moved out of the lungs

Question 15

Describe the structure of alveoli

Answer 15

• comprised of squamous epithelium
• surrounded by blood capillaries, so that distance that gases diffuse is very short
• alveolus walls contain elastic fibres that stretch during inspiration but then recoil to help push air out during expiration
• the alveolus walls are so thin that it may not be possible to distinguish separate cells under light microscope

Question 16

What requirements do airways have to meet to be effective?

Answer 16

• be large enough to allow sufficient air to flow without obstruction
• be supported to prevent clla[se when the air pressure inside is low during inspiration
• be flexible in order to allow movement

Question 17

What lines the airways?

Answer 17

• ciliated epithelium
• goblet cells in the epithelium release mucus, which traps pathogens
• the cilia then move the mucus up to the top of the airway, where it is swallowed
• the glandular tissue in the loose tissue also produces mucus

Question 18

Describe the structure of the trachea and the bronchi

Answer 18

• bronchi are narrower than the trachea
• the airways are supported by rings of cartilage which prevent collapse during inspiration
• the rings of cartilage in the trachea are C-shaped rather than a complete ring
• this allows flexibility and space for food to pass down the oesophagus

Question 19

Describe the structure of the bronchioles

Answer 19

• much narrower than the bronchi
• larger bronchioles may have some cartilage, but smaller have none
• the wall is comprised mostly of smooth muscle and elastic fibres
• the smallest bronchioles end in cluster of alveoli

Question 20

Describe the contraction smooth muscle and elastic tissue

Answer 20

• smooth muscle can contract
• the action of smooth muscle will constrict the airway
• this makes the lumen of the airway narrower
• constriction of the lumen can restrict the flow of air to and from the alveoli
• controlling the flow of air to the alveoli might be important if there are harmful substances in the air
• contraction of the smooth muscle and control of air flow is not a voluntary act and may occur due to allergic reaction
• once smooth muscle has contracted it cannot reverse this effect on its own
• the smooth muscle is elongated again by the elastic fibres
• when the muscle contracts, it deforms the elastic fibres
• as the muscles relax, the elastic fibres recoil to their original size and shape
• this act dilates the airway

Question 21

Briefly describe the spirometer

Answer 21

• a float-chamber spirometer consists of a chamber of air or medical-grade oxygen floating on a tank of water
• during inspiration, air is drawn from the chamber so that the lid moves down
• during expiration, the air returns to the chamber, raising the lid
• these movements may be recorded on a datalogger
• the carbon-dioxide-rich air is exhaled and is passed through a chamber of soda lime, which absorbs the carbon dioxide
• this allows the measurement of oxygen consumption

Question 22

What are some precautioons for using the spirometer?

Answer 22

• the subject must be healthy and free from asthma
• the soda lime should be fresh and functioning
• there should be no leaks in the apparatus, as this would five invalid or inaccurate results
• the mouthpiece should be sterilised
• the water chamber must not be overfilled

Question 23

What is vital capacity?

Answer 23

• the maximum volume of air that can be moved by the lungs in one breath
• this is measured by taking a deep breath and expiring all the air possible from the lungs
• it depends on factors such as age, gender, size, exercise level
• usually in the region of 2.5-5.0 dm³

Question 24

What is residual volume?

Answer 24

• the volume of air that remains in the lungs even after forced expiration
• this air remains in the airways and alveoli
• approximately 1.5 dm³

Question 25

What is the tidal volume?

Answer 25

• the volume of air moved in and out with each breath
• normally measured at rest
• a typical tidal volume might be 0.5 dm³
• this is usually sufficient to supply all of the oxygen required in the body at rest

Question 26

Describe the spirometer diagram

Answer 26

Question 27

Describe how oxygen intake can be measured using a spirometer

Answer 27

• as a person breathes from the spirometer, oxygen is absorbed by the blood and replaced by carbon dioxide
• this carbon dioxide is absorbed by soda lime in the spirometer, so that the volume of air in the chamber decreases
• this decrease can be observed and measured on the spirometer trace
• we can assume the volume of carbon dioxide released and absorbed the soda lime equals the volume of oxygen absorbed by the blood
• therefore, measuring the gradient of the decrease in volume enables us to calculate the rate of oxygen uptake

Question 28

How to calculate oxygen uptake from a spirometer trace

Answer 28

• draw a line from point A down to the horizontal axis and another line from point B to the horizontal axis
• measure the length of time between these points
• measure the difference in volume between points A and B
• divide by the time taken for this decrease
• the unit will be dm³ s^-1

Question 29

How d you measure the breathing rate from a spirometer?

Answer 29

• count the number of peaks in each minute

Question 30

What increases the oxygen intake?

Answer 30

• increased breathing rate
• deeper breaths

Question 31

Describe the gas exchange system of bony fish

Answer 31

• they use gills to absorb oxygen dissolved in water and release carbon dioxide into the water
• oxygen concentration will be typically much lower than is found in air
• most bony fish have five pairs of gills, which are covered by a bony plate called the operculum
• each gill consists of two rows of gill filaments attached to a bony arch
• the filaments are very thin and their surface is folded into many secondary lamellae (or gill plates)
• this provides a very large surface area
• blood capillaries carry deoxygenated blood close to the surface of the secondary lamellae where the exchange takes place

Question 32

Describe countercurrent flow

Answer 32

• blood flows along the gill arch and out along the filaments to the secondary lamellae
• the blood then flows through capillaries in the opposite direction to the flow of water over the lamellae
• this arrangement creates a countercurrent flow that absorbs the maximum amount of oxygen from the water

Question 33

Describe the ventilation in bony fish

Answer 33

• bony fish can keep water flowing over the gills by using a buccal-opercular pump
• the buccal cavity (mouth) can change the volume
• the floor of the mouth moves downwards, drawing water into the buccal cavity
• the mouth closes and the floor is raised again pushing water through the gills
• movements of the operculum are coordinated with the movements of the buccal cavity
• as water is pushed from the buccal cavity, the operculum moves outwards
• this movement reduces the pressure in the opercular cavity (the space under the operculum) helping water to move through the gills

Question 34

Briefly describe the exchange system in insects

Answer 34

• they do not transport oxygen in the blood
• they have an open circulatory system in which the body fluid acts as both blood and tissue fluid
• circulation is slow and can be affected by body movements
• insects possess an air-filled tracheal system, which supplies air directly to all the respiring tissues
• air enters the system via a pore in each segment called a spiracle
• the air is transported into the body through a series of tubes called tracheae
• these divide into smaller and smaller tubes called tracheoles
• the ends of tracheoles are open and filled with a fluid called tracheal fluid
• gas exchange occurs between the air in the tracheole and tracheal fluid
• some exchange can also occur across the thin walls of tracheoles

Question 35

Desscribe what happens when insects are active

Answer 35

• when tissues are active, the tracheal fluid can be withdrawn into the body fluid in order to increase the surface area of the tracheole wall exposed to air
• this means that more oxygen can be absorbed when the insect is active

Question 36

How can larger insects ventilate their tracheal system by movement?

Answer 36

• sections of the tracheal system are expanded and have flexible walls
• these act as air sacs which can be squeezed by the action of the flight muscles
• repetitive expansion and contraction of these sacs ventilate the tracheal system
• in some insects, movements of the wings alter the volume of the thorax
• as the thorax volume decreases, air in the tracheal system is put under pressure and is pushed out of tracheal system
• when the thorax increases in volume, the pressure inside drops and air is pushed into the tracheal system from outside
• some insects, such as locusts, are able to alter the volume of their abdomen by specialised breathing movements
• these are coordinated with opening and closing valves in the spiracles
• as the abdomen expands, the spiracles at the front end of the body open and air enter the tracheal system
• as the abdomen reduces in volume, the spiracles at the rear end f the body open and air can leave the tracheal system