3.1 - Exchange Surfaces Flashcards
Why don’t single-celled organisms need an exchange surface?
They can rely on diffusion alone. This is because:
- the metabolic activity of single-celled organisms is usually low, so the oxygen demands and CO2 production of the cells are relatively low
- the SA:V ratio of the organism is large
Why do multicellular organisms need specialised exchange surfaces?
- the larger the organism, the larger the SA:V ratio
- their metabolic activity is usually much higher than most single-celled organisms
This means diffusion alone isn’t enough for the oxygen demands and CO2 production of larger organisms
How do you work out SA:V ratio?
SA:V = surface area ÷ volume
What are the features that make an effective exchange surface?
- increased surface area
- thin layers
- good blood supply/good ventilation to maintain concentration gradient
How does increased surface area increase effectiveness of exchange surfaces?
- provides the area needed for exchange surfaces
- overcomes the limitations of SA:V ratio of larger organisms
- eg root hair cells in plants, villi in small intestines
How do thin layers increase effectiveness of exchange surfaces?
- the distances that substances have to diffuse across are short, making process fast and efficient
- eg alveoli in lungs, villi of small intestine
How does a good blood supply increase effectiveness of exchange surfaces?
- the steeper the concentration gradient, the faster diffusion takes place.
- a good blood supply ensures substances are constantly delivered to and removed from the exchange surface, therefore maintaining a steep concentration gradient
- eg capillaries in alveoli or lamellae
How does ventilation increase effectiveness of exchange surfaces?
- for gases, a ventilation system helps maintain concentration gradients and makes the process more efficient
- eg alveoli, gills of a fish (ventilation = flow of water carrying dissolved gases)
What makes root hair cells efficient exchange surfaces?
- very thin extensions of cells making up the epidermis of a root
- there may be thousands on each branch of a root, providing a very large surface area
- the cell walls are thin and freely permeable
- water potential is lower in the cell than in water, so water enters cell via osmosis
- inorganic mineral ions are absorbed from soil into cells via facilitated diffusion
- if ions are at a lower concentration in the soil than in the cell, the ions are taken up by active transport, through carrier proteins, using ATP
What are the key structures in the mammalian gaseous exchange system?
- nasal cavity
- trachea
- bronchus
- bronchioles
- alveoli
What is the nasal cavity?
- it has a large surface area with good blood supply, which warms 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 incoming air, reducing evaporation from exchange surfaces
What is the trachea?
- the main airway carrying clean, warm, moist air from the nose down to the chest
- it is a wide tube supported by incomplete rings of strong, flexible cartilage, which stops the trachea from collapsing.
- the rings are incomplete so that food can pass down easily behind the trachea
- it is lined with ciliated epithelium and goblet cells
What is a bronchus?
- plural bronchi
- two main branches from the trachea
- left bronchus leads to left lung, right bronchus leads to right lung
- similar in structure to the trachea, but are smaller
What are bronchioles?
- bronchi divide into many bronchioles in the lungs
- have no cartilage rings
- walls of bronchioles contain smooth muscle. when it contracts, bronchioles constrict. when smooth muscle relaxes, bronchioles dilate. this changes the amount of air reaching the lungs
- bronchioles are lined with a thin layer of flattened epithelium, making some gas exchange possible.
What are alveoli?
- tiny air sacs, which are the main gas exchange surfaces of the body
- unique to mammalian lungs
- each alveolus had a diameter of around 200-300 micrometers
- consist of a layer of thin, flattened epithelium cells, some collagen and elastic fibres.
- the elastic fibres allow the alveoli to stretch as air is drawn in, and recoil to help squeeze air out and return the alveoli to its original size (elastic recoil)
How are alveoli adapted to their function?
Large SA:
- 300-500 million alveoli per adult lung
- numerous alveoli increases SA for gaseous exchange in lungs
Thin layers:
- both alveoli and capillaries that surround them have walls only 1 squamous epithelial cell thick, so diffusion distance between air in alveolus and blood in capillaries is very short.
Good blood supply:
- alveoli are surrounded by capillaries supplying a constant flow of blood.
- the blood brings CO2 to lungs and leaves with oxygen
- this maintains a steep concentration gradient as blood is moved away from the exchange surface
Good ventilation:
- breathing moves air in and out of the alveoli
- this helps maintain a steep concentration gradient for both oxygen and carbon dioxide between blood and lungs
What is cartilage?
- prevents collapse during times of low pressure
- found in trachea and bronchi
What is smooth muscle?
- has elasticity for stretching (eg it relaxes in bronchioles during exercise to allow them to increase diameter (dilate))
- can also contract to constrict bronchioles, controlling airflow and preventing harmful substances entering lung
- not under conscious control
- found in trachea, bronchi + bronchioles
What are elastic fibres?
- stretch during inhalation
- recoil during exhalation for more rapid expulsion from alveoli
- after asthmatic muscles contraction they ensure recoil
- found in trachea, bronchi, bronchioles, alveoli (everything)
What are goblet cells and glandular tissue?
- goblet cells and glands secrete mucus which lines airways
- mucus traps microbes and particulates
- glandular tissue found in trachea and bronchi
- goblet cells found in trachea, bronchi and bronchioles
What is ciliated epithelium?
- cilia waft mucus produced from goblet cells out of airways, taking dirt and microbes with it
- mucus is then swallowed and stomach acid kills the microbes
- found in trachea, bronchi and bronchioles
What are lung surfactants?
- the inner surface of the alveoli is covered in a thin layer of a solution of water, salts and lung surfactants
- the surfactant makes it possible for alveoli to remain inflated
- oxygen dissolves in the water before diffusing into the blood, but water can also evaporate into the air in alveoli
Describe the process of inspiration
1) external intercostal muscles contract so that the ribs move up and out
2) the diaphragm contracts, meaning it flattens and moves down
3) this increases the volume of the thorax, decreasing the pressure below atmospheric pressure
4) air moves into the lungs down a pressure gradient
Describe the process of exhalation
1) external intercostal muscles relax, and ribs fall in and down
2) diaphragm relaxes and moves upwards
3) this reduces the volume of the thorax, increasing the pressure above atmospheric pressure
4) air moves out of the lungs, down the pressure gradient
What is tidal volume?
the volume of air breathed in or out during one normal relaxed breath
What is vital capacity?
the volume of air that can be exhaled when the deepest possible intake of breath is followed by the strongest possible exhalation
What is residual volume?
the volume of air that is left in your lungs when you have exhaled as hard as possible
cannot be measured directly
What is total lung capacity
the sum of vital capacity and residual volume
Why do insects need a specialised gas exchange system?
- even though they are small organisms, they tend to be highly active
- they have a waxy exoskeleton which does not allow for effective gas exchange
- they have evolved a gas exchanges system called the tracheal system, which delivers oxygen directly to every tissue in the body
What are the key structures of the tracheal system?
- spiracles and sphincters
- trachea
- tracheoles
- tracheal fluid
What are spiracles?
- small openings in along the thorax and abdomen of insects
- air enters and leaves the spiracles, but water is also lost
What are sphincters?
- can be used to open and close spiracles
- kept closed as much as possible to reduce water loss
- when an insect is inactive, oxygen demands are low and spiracles are closed most of the time
- when oxygen demand is raised/CO2 levels build up, more spiracles open
What are the tracheae in insects?
- largest tubes of the tracheal system
- lead away from spiracles and carry air into body
- lined with chitin which keep them open if bent/pressed. Chitin is relatively impermeable so little gaseous exchange occurs in the trachea
What are tracheoles in insects?
- tracheae branch into smaller tracheoles
- each tracheole is a single, elongated cell with no chitin lining, so are freely permeable to gases
- can spread throughout tissues of the insect, running between individual cells
- where most of the gaseous exchange takes place between air and respiring cells
What is tracheal fluid?
- found towards the end of tracheoles
- at times of high oxygen demand, lactic acid builds up in cells which draws tracheal fluid out of tracheole ends via osmosis
- this increases the area over which gas exchange can take place
How can larger insects supply their extra oxygen demand?
Mechanical ventilation of tracheal system:
- air is actively pumped into the system by muscular pumping movements of thorax/abdomen
- these movements change the volume of the body, forcing air in or out
Collapsible enlarged tracheae/air sacs:
- act as air reservoirs
- used to increase the amount of air moves through the gas exchange system
- usually inflated/deflated by the ventilating movements of the thorax and abdomen
Why do fish need a specialised exchange system?
- they have a relatively small SA:V ratio
- they are very active
- scales don’t allow for gaseous exchange
What are the key features of the exchange system of a fish?
- buccal cavity
- operculum
- gill arch
- filaments
- lamellae
How do fish ventilate the gills?
- buccal cavity can change volume
- mouth opens and buccal cavity floor is lowered. opercular valve is closed.
- this increases volume, decreasing pressure and forcing water into mouth.
- mouth closed and buccal floor is raised. opercular valve opens
- this decreases volume in mouth, increasing pressure and forcing water over gills and out of the opercular valve.
How do fish achieve a large surface area?
- most bony fish have 4 pairs of gills, covered with operculum for protection
- each gill consists of 2 rows of gill filaments attached to a bony arch
- filaments are very thin and the surface is folded into many lamellae, which are filled with blood
- the folding of lamellae increases SA for oxygen to diffuse across
How do fish achieve a short diffusion distance?
Blood capillaries carry deoxygenated blood close to the surface if the lamellae where exchanges take place
Lamellae are very thing for rapid diffusion between oxygen in water and gases in capillaries
How to fish maintain a steep concentration gradient ?
- good ventilation of water
- blood and water flow over the gill lamellae in opposite directions - this is a counter current
- this means water always has a higher concentration of oxygen, so concentration gradient is maintained
- as a result, oxygen can diffuse from the water into the blood all the way along the lamellae
