Gas Exchange Flashcards
Why can single-celled organisms such as amoeba and very small multicellular organisms exchange substances with the environment by diffusion alone?
Because:
• the diffusion distances from the outside to the innermost areas are very small
• there is a large SA:V ratio so there is a relatively big surface area over which substances can diffuse into or out of organisms
• the metabolic demands are low - the organisms do not regulate their own temperature and the cells do not use much oxygen and food or produce much carbon dioxide
How do you work out the surface area and volume of a cube and a sphere?
- SA cube: area of side x number of sides
- volume of cube: length of a side cubed
- SA sphere: 4pi r squared
- volume sphere: 4/3 x pi r cubed
Why do larger organisms have a higher metabolic rate?
- mammals and birds control their body temperature and are very active
- mammals have higher demands in every cell for oxygen and food and so produce more waste products than small organisms
Where does gas exchange take place in humans, fish, insects and plants?
- humans: lungs
- fish: gills
- insects: tracheal system
- plants: leaves
What are gas exchange systems specialised for and how are the gases exchanged?
- they are specialised for the exchange of oxygen and carbon dioxide between the body of the organism and the environment
- the gases are exchanged by simple diffusion
What factors is the rate of diffusion across a membrane controlled by?
- the surface area: the bigger the surface area the more particles can be exchanged at the same time
- the concentration gradient of the particles diffusing: particles diffuse faster from an area of high concentration to low concentration
- the distance over which diffusion is taking place
What features do effective gas exchange systems have in common?
- large surface area - it has to compensate for the small SA:V ratio of the whole organism
- thin layers to minimise the diffusion distance from one side to the other
- in animals a rich blood supply to the respiratory gases. It helps maintain a steep concentration gradient
- moist surfaces as diffusion takes places with gases in solution
- permeable surfaces that will allow free passage of the respiratory gases
What is the gas exchange system linked to the outside world through?
The mouth and nose
How do the passages of the nasal cavity prepare the air before entering the rest of the system?
- the lining excretes mucus and is covered in hair. The hairs and the mucus filter out and clean up much of the dust, small particles and pathogens such as bacteria that you breathe in
- they have moist surfaces which increase the level of water vapour in the air
- they have a rich blood supply which raises the temperature of the air if necessary
What does the Epiglottis do?
It is a flap of tissue that closes over the glottis in a reflective action when food is swallowed. This prevents food from entering the gas exchange system
What does the Larynx do?
The voice box: uses the flow of air across it to produce sound
Trachea
It’s the major airway to the bronchi, lined with cells including mucus - secreting cells. Cilia on the surface move mucus and any trapped microorganisms and dust away from the lungs
What do the incomplete rings of cartilage do?
They prevent the trachea and bronchi from collapsing but allow food to be swallowed and moved down the oesophagus
What do the left and right bronchus do?
They are tubes leading to the lungs which are similar to the trachea but narrower and divide to form bronchioles
What do the lungs do?
The organ where gas exchange takes place
What do the bronchioles do?
Small tubes that spread through the lungs and end in alveoli. For tubes with diameter 1mm or less there is no cartilage and they collapse quite easily. Their main function is still as an airway but a little gas exchange may occur
What do the alveoli do?
They are the main site of gas exchange in the lungs
What do the ribs do?
They are a protective bony cage around the gas exchange system
What are the intercostal muscles and what do they do?
They are found between the ribs and are important in breathing
What do the pleural membranes do?
Surround the lungs and line the chest cavity
What does the pleural cavity do?
It’s a space between the pleural membranes, usually filler with a thin layer of lubricating fluid that allows the membrane to slide easily with breathing movements
What does the diaphragm do?
It’s a broad sheet of tissue that forms the floor of the chest cavity, also important in breathing movements
What is an alveolus made up of?
A single layer of flattened epithelial cells
What is the elastic recoil of the lungs?
Between the capillaries and alveolar us a layer of elastic connective tissue holding everything together. The elastic tissue helps to force air out of the lungs which are stretched as you breathe in
The alveoli have a natural tendency to collapse. How is this prevented?
It is prevented by a special phospholipid known as lung surfactant that coats the alveoli and makes breathing easier
What does the gaseous exchange in humans occur by a process of?
A process of simple diffusion between the alveolar air and the deoxygenated blood in the capillaries the blood has a relatively low oxygen content and correspondingly high carbon dioxide content
How do the alveoli provide a huge surface area for the gas exchange in the body?
- Each alveolus is folded to provide a large surface area
* there are about 500 million alveoli in your lungs which gives a surface area of 40-75 m squared
How is a short diffusion distance in the alveoli maintained?
Both the walls of the alveoli and the capillaries are only one cell thick so the distance that diffusing gases have to travel between them is very short (0.5-1.5 micrometers)
How is a steep concentration gradient maintained in the lungs?
- Blood is continuously flowing through the capillaries past the alveoli exchanging gases. This maintains a concentration gradient as the blood will always have low oxygen levels
- the air within the alveoli is constantly being refreshed with air from outside but breathing
What are the features of an alveolus?
- alveolur duct
- thin squamous epithelium lining the outside
- macrophages that engulf bacteria keeping the alveoli free of bacteria
- liquid surfactant on inner surface
What are the two parts of the process of breathing and what does breathing involve?
- inhalation: taking air into the chest
- exhalation: breathing air out again
- breathing involves a series of pressure changes in the chest cavity that brings about the movement of air
There is only one way for air to move out of the chest cavity. Where is this?
Through the trachea
What happens in inhalation?
- it is an energy using process
- the muscles around the diaphragm contract and as a result it is lowered and flattened
- the intercostal muscles between the ribs contract raising the rib cage upwards and outwards
- these movements result in the volume of the chest cavity increasing which reduces the pressure in the cavity. It is now lower than the pressure of the atmospheric air outside so air moves in through the trachea, bronchi and bronchioles into the lungs to equalise the pressure inside and out
What happens in normal exhalation?
- it is a passive process
- the muscles surrounding the diaphragm relax so that it moves up into its resting domed shape
- the intercostal muscles also relax so that the ribs move down and in and the elastic fibres of the alveoli of the lungs return to their normal length
- as a result the volume of the chest cavity decreases causing an increase in pressure. The pressure in the chest cavity is now greater than that of the outside air so air moves out of the lungs, through the bronchioles, bronchi and trachea to the outside air
If you need to force air out of your lungs more quickly than passive exhalation allows you can use forced exhalation. How does this work?
- the intercostal muscles contract pulling the ribs down and in and the abdominal muscles contract forcing the diaphragm upwards
- this increases pressure in the chest cavity causing exhalation
When is forced exhalation used?
- singers use it to achieve a powerful voice and to maintain long notes
- free divers do it before a dive so they can fill their lungs with as much air as possible
- coughing is an exaggerated form of forces exhalation which is used to force mucus out from the respiratory system
How does the gas exchange system try to protect your lungs?
- the air you breathe contains many pathogens
- your respiratory system produces lots of mucus that lines your airway and traps the tiny particles and organisms
- the mucus is runny so it is easily moved up airways but cilia that sweep upwards to the back of your throat. The majority of your mucus is swallowed here.
- the acid in your stomach and your digestive enzymes digest the mucus and everything carried with it
What does the respiratory system of insects do?
It delivers oxygen directly to cells and removes carbon dioxide in the same way
What do spiracles in insects do?
They are found along the thorax and abdomen of most insects. They are the site of the entry and exit of the respiratory gases. In many insects the spiracles can be opened and closed by sphincters, which is of great value in the control of water loss
What do the tracheae (singular trachea) do in insects?
- the largest tubes of the insect respiratory system
- carry air directly into the body for gas exchange with the cells. They run both into the body of the insect and along it
- the tubes are supported by spirals of chitin. The chitin spirals hold the trachea open if they are squashed or deformed as the insect moves like rings of cartilage
- the chitin makes the tracheae relatively impermeable to gases so little gas exchange takes place here
- the tracheae divide to form narrower and narrower tubes until they branch out into the tracheoles
What do the tracheoles do in insects?
- minute tubes of diameter 0.6- 0.8 ųm
- each one is a single elongated cell and they have no chitin lining so are freely permeable to gases.
- the tracheoles spread throughout the tissues of the insect. They are so small they run between and even penetrate into individual cells.
- it is in the tracheoles that most of the gas exchange takes place in insects
What is a major site of water loss for insects and how is this minimised?
Spiracles are a major site of water loss and so they are kept closed as much as possible
How does air move along the tracheae
• they move by diffusion alone
How do the tracheoles make gas exchange easier?
They have a very large surface area
Why do tracheoles contain water towards the end of their length?
- it limits the penetration of the gases for diffusion
- however when the insect is very active and needs more oxygen, lactic acid builds up in the muscle tissues. This affects the osmotic concentration of the cells and so water moves out of the tracheoles into the cells for osmosis. This exposes additional surface area in the tracheoles for gaseous exchange
How is 25% of carbon dioxide produced by the cells lost?
Directly through the cuticle
What controls the rate of gas exchange in insects?
The opening and closing of spiracles
What is the opening and closing of spiracles coordinated by?
Respiratory centres in the nervous system which are stimulated by increasing carbon dioxide levels and by the lactic acid that builds up in active tissues when there is a lack of oxygen.
How have insects with very active lifestyles evolved ways of ventilating their gas exchange system?
- mechanical ventilation: air is actively pumped into the tracheal system. The spiracles open and the insect makes muscular pumping movements of the thorax, abdomen or both. These ventilating movements change the volume and therefore the pressure inside the body, drawing air in and out of the trachea and tracheoles
- Collapsible trachaea or air sacs that act as air reservoirs: these increase the volume of air moved through the respiratory system. The ventilating movements of the thorax and abdomen inflate and deflate them. In some insects they can be ventilated by the general body movements
How are gills well adapted for successful gas exchange?
- they have a large surface area for diffusion
- a good blood supply to maintain concentration gradients
- thin walls giving short diffusion distances
What are the gills of a bony fish contained in and covered by?
They are contained in a gill cavity and covered by a protective bony flap called the operculum
What is the structure of the gills?
- they have fragile gill filaments which occur in large stacks. They need water to keep them apart and so to expose the large surface area needed for gas exchange
- the gill lamellae are the main site of gas exchange. They have a rich blood supply and give the gill filaments their large surface area
- blood leaving the gills flows in the opposite direction to the incoming water (countercurrent exchange) ensuring the most effective possible exchange of gases
Diffusion in water tends to be slow so what do fish require to get the oxygen they need?
They need a constant flow of water over the gills and the most efficient possible exchange system between the blood and the water
Why cant fish survive out of water?
Because the gill filaments stick together and the remaining exposed surface area is not big enough for effective gas exchange to take place
How does the operculum mean that bony fish can ventilate their gills even when they’re not moving?
Their operculum is open when the mouth is closed which ensures their is a continuous flow of water over the gills when the fish is not moving
What are the two extra adaptations that enables fish to be so active in water?
- the countercurrent exchange system: the blood in the gill filaments and the water moving over the gills flow in different directions. This maintains a steeper concentration gradient so more gas exchange can take place. Bony fish extract about 80% of the oxygen from the water flowing over their gills
- overlapping gill filaments: diffusion in water tends to be slow so if the water passes over the gills too quickly it limits the amount of oxygen and carbon dioxide that can be exchanged. The tips of adjacent gill filaments overlap, increasing the resistance to the flow of water. This slows down the flow of water over the gill surfaces giving more time for the exchange of gases to take place
The cartilaginous fish do not have an operculum. How do they keep the water flowing over their gills?
They have to constantly keep their mouth open
Why does the net movement of gases in plant tissues vary?
- during the day the photosynthesising tissues need to take more carbon dioxide into their cells than they produce by respiration. They also make more oxygen than needed so release some into the surrounding air
- at night the photosynthesisng tissues simply take in oxygen and release carbon dioxide in respiration
- the non-photosynthesising parts of the plant take in oxygen and release carbon dioxide for respiration all the time
How does a leaf maintain a large surface area?
- they have a large external surface area
* the gas exchange surfaces of the spongy mesophyll cells have irregular shapes increasing their surface area
What happens at the spongy mesophyll cells in a leaf and how do they make this easier?
- gas exchange occurs freely between the cells of the leaf and the air spaces by diffusion
- the cells have irregular shapes increasing their surface area
- they are arranged with large air spaces between them
- the surfaces of the spongy mesophyll cells are moist
- during the day carbon dioxide moves by diffusion into the cells and oxygen diffuses out
- water also passes by evaporation from the cells into the air spaces
What does the impermeable waxy cuticle do?
It acts as a barrier to the diffusion of gases and particularly the evaporation of water.
It also protects the leaf
Gases move in and out of the cells by diffusion. What do they diffuse through?
The stomata (stoma) - Specialised pores found mainly in the epidermis on the underside of the leaf
What are guard cells and what are their features?
- they border the stomata opening and closing it
- they are sausage shaped
- contain a sap vacuole
- contain chloroplasts
- unevenly distributed cellulose in their cell walls
How is the opening and closing of the stomata controlled?
- it is a turgid driven process.
- the guard cells respond to carbon dioxide levels in the leaf
- when turgor in the guard cells is low the asymmetric thickening of the cellulose on the cell walls closes the pore
- when conditions are favourable for photosynthesis solutes, particularly potassium ions are moved into the guard cells by active transport. Water then moves into the guard cells from the surrounding epidermal cells by osmosis and the guard cells swell so the turgor pressure increases. The stomata pores open again because of the uneven bending resulting from the arrangement of cellulose in the cell walk
- when conditions are less favourable for photosynthesis the active pumping of potassium ions into the cell stopsinto the potassium ions are excreted. As a result water leaves the cell by osmosis, turgor is reduces and the guard cells become flaccid closing the stomatal pore
How do we know the stomatal opening involves active transport?
Because metabolic poisons stop both stomatal opening and the accumulation of potassium ions
Apart from the underside of leaves where else are stomata present?
In stems to allow gas exchange to take place
When layers of impermeable cork or bark form on a tree how to the tissues underneath take in oxygen and remove carbon dioxide?
Special spingy areas called lenticels develop which are made up of loosely arranged cells with many air spaces. They link the inner tissues of the trunk or woody stem with the outside world so gas exchange can take place.
They also form on roots so they can exchange gases with the air in the soil
