3.7 - Exchange surfaces and breathing

Question 1

Why do unicellular organisms not need specialised exchange surfaces

Answer 1

• the metabolic activity of a single-celled organism is usually low, so the demands and waste production are usually low
• the surface area to volume ratio of the organism is large

Question 2

Why do multicellular organisms need specialised exchange surfaces

Answer 2

• surface area to volume ration is low because there are many cells within the organism
• high metabolic activity as there is high movement, and they are often homeothermic
• there are many cells, so not all cells are exposed to the substances they require

Question 3

Characteristics of an effective exchange surface

Answer 3

• increased surface area
• thin layers
• good blood supply
• ventilation to maintain diffusion gradient

Question 4

Key structures of the mammalian gaseous exchange system

Answer 4

• nasal cavity
• trachea
• bronchi
• bronchioles
• alveoli

Question 5

Features of the nasal cavity

Answer 5

• a large surface area with a good blood supply that warms the air to body temperature
• a hairy lining, which secretes mucus to trap dust and bacteria, protecting delicate lung tissue from irritation and infection
• moist surfaces, which increase the humidity of the incoming air, reducing evaporation from the exchange surfaces

Question 6

Features of the trachea

Answer 6

• incomplete rings of strong, flexible cartilage which stops the trachea from collapsing (incomplete so it does not squash oesophagus)
• lined with ciliated epithelium tissue
• goblet cells secrete mucus to trap pathogens and particulates, ciliated epithelial cells sweep mucus away from lungs with a beating motion

Question 7

Bronchi

Answer 7

Trachea divides to form the left bronchus and the right bronchus, leading to the left and right lung. Similar in structure to the trachea but smaller

Question 8

Bronchioles

Answer 8

In the lungs, the bronchi divide into many bronchioles.
- smaller bronchioles have no cartilage
- walls contain smooth muscle and can relax and contract, changing the amount of air that can reach the lungs
- lined with a thin layer or flattened epithelium, making some gaseous exchange possible

Question 9

Alveoli

Answer 9

• 200-300µm
• squamous epithelium cells decrease diffusion distance
• some collagen and elastic fibres allow the alveoli to stretch as air is drawn in. When they return to their resting size, air is squeezed out (elastic recoil)
• large surface area (300-500 million in adult lung)
• thin layers of alveoli and capillaries
• good blood supply, many capillaries are wrapped around each alveoli
• good ventilation, breathing moving air in and out of the alveoli maintains concentration of oxygen and carbon dioxide between the blood and the lungs

Question 10

Inspiration/inhalation

Answer 10

• requires energy
• diaphragm contracts, flattening and moving down
• external intercostal muscles (between ribs) contract, moving ribs upwards and outwards
• causes volume of thorax to increase, decreasing the pressure inside the thorax
• causes air to move down the pressure gradient from a higher pressure (the atmosphere) to a lower pressure (the lungs)

Question 11

Expiration/exhalation

Answer 11

• passive (does not require energy)
• diaphragm relaxes, returning to dome shape
• external intercostal muscles (between ribs) relax, moving the ribs downwards and inwards due to gravity
• elastic fibres in the alveoli return to their normal length
• decreases the volume of the thorax, causing the pressure to increase
• causes air to move down the pressure gradient from ana rea of higher pressure (the lungs) to an area of lower pressure (the atmosphere)

Question 12

Ways of measuring lung volume/capacity`

Answer 12

• peak flow meter
• vitalograph
• spirometer

Question 13

Components of lung volume

Answer 13

• tidal volume
• vital capacity
• inspiratory reserve volume
• expiratory reserve volume
• residual volume
• total lung capacity

Question 14

Tidal volume

Answer 14

The volume of air that moves into and out of the lungs with each resting breath. It is around 500cm³, 15% of the vital capacity

Question 15

Vital capacity

Answer 15

The volume of air that can be breathed in when the strongest possible exhalation is followed by the deepest possible intake of breath

Question 16

Inspiratory reserve volume

Answer 16

The maximum additional volume of air that can be breathed in after normal inhalation

Question 17

Expiratory reserve volume

Answer 17

The maximum additional amount of air that can be breathed out after exhalation

Question 18

Residual volume

Answer 18

The volume of air that is left in your lungs when you have exhaled as hard as possible. Cannot be measured directly

Question 19

Total lung capacity

Answer 19

The sum of the vital capacity and residual volume

Question 20

How does a spirometer work?

Answer 20

• The person being examined breathes in and out through the spirometer
• Carbon dioxide is absorbed from the exhaled air by soda lime in order to stop the concentration of carbon dioxide in the re-breathed air from getting too high, as this can cause respiratory distress
• As the subject breathes through the spirometer, a trace is drawn on a rotating drum of paper or a graph is formed digitally, which can be viewed on a computer
  From this trace, the subject’s vital capacity, tidal volume and breathing rate can all be calculated
• Oxygen uptake can also be calculated using a spirometer
• Carbon dioxide is removed from the exhaled air, meaning that the total volume of air available in the spirometer gradually decreases, as oxygen is extracted from it by the subject’s breathing
• This change in volume is used as a measure of oxygen uptake

Question 21

ventilation rate equation

Answer 21

ventilation rate = tidal volume * breathing rate (pm)

Question 22

Breathing rater

Answer 22

Number of breaths taken per minute

Question 23

Ventilation rate

Answer 23

Total volume of air inhaled in one minute

Question 24

Why do insects have a different gaseous exchange system to mammals

Answer 24

All insects possess a rigid exoskeleton with a waxy coating that is impermeable to gases (chitin), and do not usually have blood pigments that carry oxygen

Question 25

Spiracles

Answer 25

• small openings along the thorax and abdomen of insects
• air enters and leaves through the spiracles, but water vapour is also lost
• sphincters surround spiracles so they can open and close and therefore reduce water loss
• sphincters are normally closed, but open when there is oxygen demand or carbon dioxide build up

Question 26

An insect’s trachea

Answer 26

• largest tubes of the insect respiratory system (<1mm diameter)
• carry air into the body, running along and into the body of the insect
• lined with spirals chitin to keep them open if they are bent or pressed
• relatively impermeable to gases, so little gaseous exchange takes place in the trachea

Question 27

Tracheoles

Answer 27

• each tracheole is a single greatly elongated cell
• no chitin
• freely permeable to gases
• large network
• diffusion of oxygen down the concentration gradient into haemocoel

Question 28

Haemocoel

Answer 28

the primary body cavity of most invertebrates, containing circulatory fluid

Question 29

How does the gaseous exchange system differ in larger or more active insects?

Answer 29

• mechanical ventilation = muscle contraction moves fluid around inside the haemocoel, maintaining concentration gradient
• collapsible trachea = to alter the pressure difference and move air in and out of spaces

Question 30

Why do fish have a different gaseous exchange system to mammals

Answer 30

• do not need to try and prevent water loss from their gaseous exchange surfaces
• water is denser and more viscous than air, and has a lower oxygen content
• because of the low oxygen content, moving water in one direction is much more economical in energy terms than moving water in and out of lungs

Question 31

Discontinuous gas exchange cycles in insects

Answer 31

DGCs have been found to be relatively common in many species of insects. In DGC, spiracles have three states:
- Closed, no gases move in or out of the insect. Oxygen moves into cells by diffusion from the tracheae and carbon dioxide diffuses into the body fluids of the insect, where it is held in a process called buffering
- Fluttering, spiracles open and close rapidly. Moves fresh air into the trachea to renew the supply of oxygen, while minimising water loss
- Open, when carbon dioxide levels build up really high, the spiracles open widely

Question 32

Gill structure in bony fish

Answer 32

• gills on each side of the head
• contained in a gill cavity and covered by a protective bony operculum
• gill arch supports two stacks of filaments
• on the surface of each filament, there are rows of lamellae
• the lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries

Question 33

Structure of gill filaments

Answer 33

• occur in large stacks (gill plates)
• need a flow of water to keep the stacks apart and expose the large surface area needed to gas exchange
• they have rows of lamellae, which have a rich blood supply and are the main site of gaseous exchange in fish

Question 34

How gills maintain effective gaseous exchange

Answer 34

• The blood flow within the capillary system is flowing the opposite way to the water flowing across the gills
• this creates a counter-current system, ensuring the concentration gradient is maintained across the whole length of the capillary
• the water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood
• the tips of adjacent gill filaments overlap, increasing the resistance to the flow of water over the gills, slowing down the water so there is more time for gaseous exchange to take place

Question 35

How do bony fish maintain water flow over the gills when not moving

Answer 35

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