3.1

Created by

annabel jones

Cards (85)

All cells need a supply of oxygen and nutrients to survive.
They also need to remove waste products, so they do not build up and become toxic.
For some organisms, these substances simply diffuse over the surface of the body.
Other organisms require specialised exchange surfaces.
Exchange surface = a specialised area that is adapted to make it easier for molecules to cross from one side of the surface to another.
There are 3 main factors that affect the need for an exchange system:
Size
Surface area to volume ratio
Level of activity
In very small organisms (e.g. single celled), all the cytoplasm is very close to their environment.
Simple diffusion is enough.
Multicellular organisms may have several layers of cells. The bigger the organism, the greater the distance substances need to travel to reach the centre.
This makes the diffusion distance too long.
Diffusion alone would be insufficient to supply the innermost cells.
Small organisms have a small surface area, but they also have a very small volume.
Compared with their volume, their surface area is relatively large.
They have a large surface area to volume ratio.
This means that they have a large enough surface to supply the organism with the resources it needs, and for waste products to diffuse out.
Although large organisms have a larger surface area, their volume is significantly greater.
As size increases, the volume rises more quickly than the surface area.
Therefore, large organisms have a small surface area to volume ratio.
This makes it even harder to absorb enough oxygen for the needs of the body and remove waste products.
Metabolic rate = the amount of energy transferred by that organism within a given period of time.
Metabolic activity requires oxygen to release energy from food in aerobic respiration.
Larger organisms tend to have a higher metabolic rate, so need to exchange lots of materials fast.
Organisms that are very active need to supply their cells with lots of nutrients and oxygen to supply the energy for movement.
This need for energy is increased in animals, such as mammals, that keep themselves warm.
To increase the rate of diffusion, good exchange surfaces have:
A large surface area to overcomes the limitations of the SA:V ratio of larger organisms. This provides more space for molecules to pass through. It is often achieved by folding the walls and membranes involved.
A thin barrier to provide a short diffusion distance. The barrier must be permeable to the substances being exchanged.
A good blood supply and/or ventilation to maintain a steep concentration gradient.
Alveoli 
Large numbers of small, spherical
Increase the surface area
View source
Alveolar walls
Made of squamous epithelium, which consists of flattened cells
One cell thick, which reduces the diffusion distance
View source
Alveoli
Have a good blood supply to maintain the concentration gradient
View source
Alveoli ventilation
1. Constantly supply oxygen
2. Remove carbon dioxide
3. Maintains the concentration gradient
View source
Alveoli 
Warm, so the rate of diffusion stays high
View source
Alveoli
Contain elastic fibres
Allow the alveoli to stretch - increasing the surface area
Allow the alveoli to recoil - helping to force air out, thereby maintaining the concentration gradients
View source
Inner surface of alveoli
Coated in lung surfactant
View source
Lung surfactant
A phospholipid that coats the surfaces of the lungs
Reduces cohesion between water molecules, preventing the alveoli from collapsing
Further increases the surface area
View source
Elastic recoil = the ability to return to original shape and size following stretching.
Air can pass into the lungs through the nose and along the trachea (windpipe), bronchi, and bronchioles.
Finally, it reaches tiny air-filled sacs called alveoli.
These are the surfaces where the exchange of gases takes place – oxygen diffuses into the blood and carbon dioxide diffuses out.
The gaseous exchange system in mammals consists of the lungs and associated airways that carry air into and out of the lungs.
The lungs are a pair of inflatable sacs lying in the chest cavity.
Each of the lungs is enclosed in a double membrane known as the pleural membrane.
The space between the two membranes is called the pleural cavity and is filled with a small amount of pleural fluid.
Pleural fluid lubricates the lungs.
It also adheres to the outer walls of the lungs to the thoracic (chest) cavity by water cohesion, so that the lungs expand with the chest while breathing.
Land animals face a constant battle between gas exchange and retaining water.
The inner surface of the alveoli is covered in a thin layer of a solution of water, salts, and lung surfactant. This surfactant reduces the cohesive forces between the water molecules, making it possible for the alveoli to remain inflated.
Oxygen dissolves in the water, then diffuses into the blood.
However, water can also evaporate into the air of the alveoli and is lost as we breathe out.
Nasal Cavity
Large surface area and good blood supply – this warms the air as it passes into the body.
Hairy lining – hairs trap dust and bacteria in mucus and prevent them from reaching the lungs, which could cause infection.
Moist surfaces – increases the humidity of the incoming air, this reduces the evaporation of water in the lungs.
After passing through the nasal cavity, the air entering the lungs is a similar temperature and humidity to the air already there.
Trachea
The trachea is the airway that leads from the mouth and nose to the bronchi.
It supported by a layer of cartilage that holds the trachea open and prevents it from collapsing.
The rings are incomplete to allow it the bend when food is swallowed down the oesophagus behind.Gaps between the cartilage filled by smooth muscle and elastic fibres.
The cartilage, smooth muscle, and elastic fibres hold the trachea open but allow flexibility during inspiration and expiration.
The trachea is lined with:
Goblet cells, which secrete mucus.
Ciliated epithelial cells, which move the mucus (along with trapped dust and bacteria) away from the lungs.
The bronchi are extensions of the trachea that split into two for the left and right lung
Bronchus
about 20mm in diameter.
Bronchi have a very similar structure to the trachea but smaller.
Cartilage rings hold the pipe open
The bronchus split into much smaller tubes called bronchioles.
These are about 1 mm or less in diameter.
All bronchioles contain smooth muscle, which holds them open.
When this smooth muscle contracts, the bronchioles constrict. This reduces the air flow to the lungs.
Larger bronchioles contain plates of cartilage.
They also have goblet cells and ciliated epithelium to trap dust and microorganisms.
The smaller bronchioles do not have cartilage, goblet cells, or ciliated epithelium.
They have clusters of alveoli at the end.
The bronchioles are lined with a thin layer of epithelial tissue, making some gas exchange possible.
Active inhalation
Diaphragm contracts and flattens.
External intercostal muscles contract, moving the ribcage up and out.
Thoracic volume increases.
Thoracic pressure decreases.
Air flows into lungs (to equalise the pressure difference).
Passive Exhalation
Diaphragm relaxes and curves up.
External intercostal muscles relax, moving the ribcage down and in.
Thoracic volume decreases.
Thoracic pressure increases.
Air flow – out the lungs (to equalise the pressure difference)
Active Exhalation
Internal intercostal muscles contract, pulling the ribcage down and in.
Abdominal muscles contract: moving the diaphragm up.
Thoracic volume decreases hard and fast.
Thoracic pressure increases rapidly.
Air flows out of the lungs.
A peak flow meter is a simple device that measures the rate at which air can be expelled from the lungs – forced exhalation.
These are often used by asthmatics to monitor how well their lungs are working.
A vitalograph is a more sophisticated versions of a peak flow meter.
The patient breathes out as quickly as they can, and a graph is produced.
This volume of air is called the forced expiratory volume in 1 second.