EXCHANGE SURFACES AND BREATHING

Diffusion alone is enough to supply the needs of single-celled organisms as...

• metabolic activity is usually low so oxygen demands and carbon dioxide production of the cell are relatively low 
• SA to V ratio is large

As organisms get larger they can be made up of millions of billions of cells arranged in tissues, organs and organ systems.

Their metabolic activity is higher

Oxygen demands of muscle cells deep in the body will be high and produce lots of carbon dioxide.

The distance between the cells where the oxygen is needed and the supply of oxygen is too far.

Smaller SA:V ration

Increased surface area - provides the area needed for exchange and overcomes the limitations of the SA:V ratio (e.g root hair cells and villi)

Thin Layers - distances that substances have to diffuse are short so the process is fast and effective (e.g alveoli and villi)

Good blood supply - steeper the concentration gradient the faster diffusion takes place. Ensures substances are constantly delivered to and removed from the exchange surface. (e.g alveoli, gills and villi)

Ventilation to maintain diffusion gradient - for gases it helps maintain a steep concentration gradient and makes the process more efficient (e.g alveoli, gills)

Mammals have a small SA:V ratio and a large volume of cells.

High metabolic rate because they are active and maintain their body temperature independent of the environment so needs lots of oxygen for cellular respiration and produces carbon dioxide which needs to be removed.

Large SA with good blood supply which warms air to body temperature

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 

Main airway carrying clean, warm, moist air from the nose down into the chest.

Wide supported by incomplete rings of strong, flexible cartilage to stop it collapsing.

The rings are incomplete so that food can move easily down the oesophagus behind the trachea.

Trachea and its branches are lined with ciliated epithelium and goblet cells.

Goblet cells secrete mucus onto the lining of the trachea to trap dust and microorganisms.

Cilia beat and move the mucus

Smoking stops cilia beating

Trachea divides into the left bronchiole and right bronchiole.

Similar in structure to trachea but are smaller

Bronchi divide to form smaller bronchioles.

Have no cartilage 

Contain smooth muscle which contracts to constrict the bronchiole.

Changes the amount of air reaching the lungs.

Lined with a thin layer of flattened epithelium 

Tiny air sacs which are the main gas exchange surfaces of the body.

Unique to mammalian lungs

Consist of a thin layer of thin, flattened epithelial cells with some collagen and elastic fibres.

Elastic fibres allow the alveoli to stretch as air is drawn in and squeeze air out when return to resting state.

Known as elastic recoil

Air is moved in and out of the lungs as a result of pressure changes in the thorax.

The rib cage provides a semi-rigid cage within which pressure can be lowered with respect to the air outside it.

The diaphragm is a broad, domed sheet of muscle.

The external intercostal muscles are found between the ribs.

The thorax is lined by the pleural membranes.

Space between them is usually filled with a thin layer of lubricating fluid.

Diaphragm contracts, flattening and lowering.

External intercoastal muscles contract, moving the ribs upwards and outwards.

Volume of the thorax increases so the pressure is reduced and is now lower than the pressure of the atmospheric air so air is drawn through the nasal passages.

Passive process

Muscles of diaphragm relax so it moves up

External intercostal muscles relax so ribs move down

Elastic fibres in the alveoli of the lungs return to their normal length 

Decrease the volume of the thorax so the pressure is greater than that of the atmospheric air.

Peak flow meter measures the rate at which air can be expelled from the lungs

Vitalographs are sophisticated versions of the peak flow meter. Patient breaths out as quickly as they can through a mouthpiece and the instrument produces a graph of the amount of air they can breathe out and how quickly

Spirometer is used to measure different aspects of the lung volume

Tidal Volume - volume of air moves into and out the lungs with each resting breath

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

Inspiratory reserve volume - maximum volume of air you can breathe in and above a normal inhalation

Expiratory reserve volume - extra amount of air you can force out of your lungs

Residual volume - volume of air that is left in your lungs when you have exhaled as hard as possible

Total lung capacity - sum of the vital capacity and the residual volume

Breathing rate is the number of breaths taken per minute.

Ventilation rate is the total volume of air inhaled in one minute.

When the oxygen demands of the body increase the tidal volume of air moved in and out of lungs with each breath can increase from 15% to 50%

Insects are very active mainly land-dwelling animals with relatively high oxygen requirements however they have a tough exoskeleton.

Little or no gaseous exchange can take place.

They do not usually have blood pigments that can carry oxygen

Along the thorax and abdomen of the insects are small opening called spiracles.

Air enters and leaves the system in the spiracles but water is also lost. 

Spiracles can be opened or closed by sphincters and are kept closed as much as possible to minimise water loss.

Leading away from the spiracles are the trachea. The tubes are lined with chitin which keep them open if they are bent or pressed.

Chitin is relatively impermeable to gases so not gaseous exchange takes place.

Trachea branch to form narrower tubes until they divide into the tracheoles.

Tracheoles have no chitin so are freely permeable to gases which spread throughout the tissues of the insect.

Towards the end of the tracheoles there is tracheal fluid which limits the penetration of air for diffusion

They do not try and prevent water loss.

Water is 1000 times denser than air and 100 times more viscous with a much lower oxygen content 

SA:V ratio means diffusion would not be enough to supply their inner cells with the oxygen they need.

Their scaly outer covering does not allow gaseous exchange.

They maintain a flow of water in one direction over the gills which are their organs of gaseous exchange

Gills have a large SA, good blood supply and thin layers.

Contained in a gill cavity and covered by a protective operculum.

To allow efficient gas exchange at all times, fish need to maintain a continous flow of water over the gills

When fish are swimming they can keep a current of water flowing over their gills by opening their mouth and operculum. 

Primitive cartilaginous fish such as sharks just ram the water past the gills but bony fish have evolved a sophisticated system

Tips of adjacent gill filaments overlap increasing resistance to the flow of water over the gill surfaces and slows down the movement of the water

Water moving over the gills and the blood in the gill filaments flow in different directions.

A countercurrent exchange system is set up ensuring a steeper concentration gradient is maintained.