3.1. Exchange Surfaces

Question 1

Why is simple diffusion alone sufficient to supply nutrients to unicellular organisms?

Answer 1

• Unicellular organisms like Amoeba have a very high surface area to volume ratio
• This means there is a large area over which diffusion can occur, which will produce a high rate of diffusion
• It also means the metabolic requirements of the organism will be low, so nutrients like glucose and oxygen are not needed in large quantities
• The distance from the centre of the organism to the edge is small, so diffusion can readily take place across this distance and reach every part of the organism

Question 2

Why do multicellular organisms need specialised exchange surfaces and transport systems?

Answer 2

• Multicellular organisms have relatively low surface area to volume ratios
• This means that, relative to the surface area over which diffusion can take place, the metabolic demands of multicellular organisms are high
• The diffusion distance from the outside of the organism to where nutrients are needed, e.g. respiring tissue, is high, so simple diffusion would be too slow
• Therefore, systems that efficiently absorb nutrients and transport them around the organism are essential for survival

Question 3

What are the equations for the volume and surface area of a sphere?

Answer 3

• Volume: 4/3 x π x r²
• Surface area: 4 x π x r²

Question 4

What are the typical characteristics of specialised exchange surfaces?

Answer 4

• Very high surface area - overcomes the SA/V problem in multicellular organisms; often achieved through extensions like villi or root hairs
• Very thin - minimises diffusion distance, increasing speed and efficiency as particles lose energy due to collisions over time
• Rich blood supply/vascular connections - to maintain a steep concentration gradient
• Ventilation (for gases) - to maintain a steep concentration gradient

Question 5

What do mammalian gas exchange systems need to balance?

Answer 5

• The need to intake oxygen and expel carbon dioxide with the need to conserve water
• Thus, there are many adaptations designed to limit water loss while facilitating the exchange of respiratory gases

Question 6

What is the structure of the pleural membrane in mammals and what is its function?

Answer 6

• The pleural membrane is a double-layered membrane encasing the lungs with a gap called the pleural cavity between each membrane that contains pleural fluid
• They help reduce friction as the lungs expand and contract and maintain a pressure gradient between the atmosphere and the thoractic cavity

Question 7

What are the adaptations of the nasal cavity in mammals?

Answer 7

• Large surface area with large blood supply to warm air as it enters the respiratory tract
• Moist surfaces to moisten air before it enters the lungs
• Hairy ciliated epithelium lining with goblet cells to secrete mucus that traps debris and pathogens

Question 8

What is the structure of the trachea in mammals?

Answer 8

• The trachea is a wide tube supported by C-shaped rings of strong, flexible cartilage to prevent it from collapsing
• It is lined with ciliated epithelium containing goblet cells to produce and waft mucus, trapping pathogens and debris that escaped the nasal lining
• It contains small amounts of smooth muscle and elastic fibres

Question 9

What is the structure of the bronchi in mammals?

Answer 9

• The bronchi are two wide tubes leading to either lung with a similar structure to the trachea
• C-shaped rings of cartilage prevent collapse
• Ciliated epithelium and goblet cells trap debris and pathogens
• They contain small amounts of smooth muscle and elastic fibres

Question 10

What is the structure of the bronchioles in mammals?

Answer 10

• The bronchioles are small tubes leading to the alveoli
• They do not contain cartilage, so rely on smooth muscle to prevent collapse, which also helps them adjust their lumen diameter to alter pressure
• They are surrounded by an epithelium lining to facilitate some gas exchange
• They contain some elastic fibres

Some of the larger bronchioles may contain some cartilage and ciliated epithelium but the terminal ones do not

Question 11

What is the structure of the alveoli?

Answer 11

• The alveoli are tiny air sacs where most gas exchange occurs in mammals, connected to bronchioles via aveolar ducts
• They are surrounded by one-cell thick flattened epithelium tissue (alveolar walls)
• This tissue is rich in elastic fibres (composed of elastin) to allow alveoli to stretch and recoil as air enters and leaves them, but has no cartilage or smooth muscle

Question 12

How are alveoli well suited to gas exchange?

Answer 12

• The hundreds of millions of alveoli produce a large surface area for gas exchange
• The epithelial tissue is one-cell thick, enabling rapid diffusion
• Each alveolus has a dense capillary network around it with a rich blood supply to maintain a steep concentration gradient of oxygen and carbon dioxide
• Alveoli are well ventilated by constant breathing to maintain a steep concentration gradient
• They have a moist inner surface to dissolve oxygen before it diffuses across the alveolar epithelium
• The moist inner surface has surfactants to prevent collapse during exhalation by reducing surface tension

Question 13

What is the process of inspiration in mammals?

Answer 13

• The diaphragm and external intercostal muscles contract
• The internal intercostal muscles relax
• The ribs are pulled upwards and outwards
• The volume of the thorax increases
• The pressure of the thorax decreases
• The resultant pressure gradient forces air into the lungs from the atmosphere until equilibrium is reached

Question 14

What is the process of expiration in mammals?

Answer 14

• The diaphragm and external intercostal muscles relax
• In passive exhalation, gravity pushes the ribs downwards and inwards and the alveoli, bronchioles and bronchi elastically recoil without requiring energy
• In active exhalation, the internal intercostal muscles contract to force the ribs downwards and inwards and the abdominals contract to quickly force the diaphragm to return to its dome shape
• The volume of the thorax decreases and the pressure increases
• The resultant pressure gradient draws air out of the lungs into the atmosphere until equilibrium is reached

Question 15

What are the definitions of tidal volume, vital capacity, inspiratory reserve volume and expiratory reserve volume?

Answer 15

• Tidal volume - the average volume of air inhaled or exhaled per breath
• Vital capacity - the highest volume of air that can be breathed in in a single breath; to measure, a maximally deep inhalation must be followed by a maximally deep exhalation
• Inspiratory reserve volume - the maximum volume of air able to be breathed in beyond a regular inhalation
• Expiratory reserve volume - the maximum volume of air able to exhaled beyond regular expiration

• Regular inspiration and expiration refers to the inhalation/exhalation of the tidal volume
• Therefore, expiratory and inspiratory reserve volume decrease with exercise as the tidal volume is larger

Question 16

What are the definitions of residual volume and total lung capacity?

Answer 16

• Residual volume - the volume of air left in your lungs after a maximally forceful expiration
• Total lung capacity - the sum of vital capacity and residual volume

Question 17

What is minute ventilation?

Answer 17

Tidal volume x breaths per minute

Question 18

How can ventilation metrics be measured?

Answer 18

• With peak flow metres or vitalographs, which measure vital capacity
• With spirometers, which draw spirometry traces that show breathing rate, vital capacity or tidal volume

Question 19

What are spiracles in insects?

Answer 19

• Small openings along the outer surface of the thorax and the abdomen of an insect
• They provide a connection between the inside of the insect as the exterior
• They are essential for the exchange of oyxgen and carbon dioxide as the rest of the outer surface is a thick exoskeleton

Question 20

What are sphincters in insects and what role do they play in gas exchange?

Answer 20

• Muscular valves that are able to open and close spiracles
• They regulate the passage of oxygen into and carbon dioxide out of the insect
• They close when metabolic activity is low or when they are in environments conducive to evaporation to limit water loss
• They open when internal carbon dioxide levels are high (and internal oxygen levels are low) in times of great metabolic activity to facilitate respiration

Question 21

What are tracheae in insects?

Answer 21

• Small tubes leading away from the spiracles
• They are lined by spirals of chitin to prevent collapse, though this means no gas exchange takes place in the tracheae
• Insects rely on simple diffusion to move oxygen and carbon dioxide through the tracheae

Question 22

What are tracheoles in insects and how do they facilitate gas exchange?

Answer 22

• Tracheoles are smaller tubes that branch from tracheae
• They are not lined with chitin, so gas exchange can occur freely across their walls
• They are spread throughout respiring tissue and run between cells, relying on simple diffusion to provide oxygen and extract carbon dioxide
• There is a small amount of tracheal fluid at the end of tracheoles, which oxygen will dissolve into before diffusing into cells

When metabolic activity is high, e.g. during flight, some tracheal fluid can be withdrawn to increase the surface area of fluid that oxygen gas can dissolve into

Question 23

Why are insects unable to grow to large sizes?

Answer 23

• Their respiratory systems are too inefficient for a combination of reasons
• In insects, gases diffuse through the air over relatively long distances, instead of simply flowing through the organism dissolved in a specialised transport fluid
• The surface area over which gas exchange can occur, while high due to the large number of tracheoles, is low compared to what alveoli provide

Question 24

How can insects mechanically increase the rate of gas exchange?

Answer 24

• Muscles in the thorax and abdomen contract and relax
• This alters the volume of the insect’s body and thus the pressure
• The resultant pressure gradients increase the rate of air movement in or out of the insect
• Moreover, some non-chitinous sections of tracheae can be enlarged or collapsed to act as air sacs/reservoirs, helping push air through the insect