Bio Dr P: Exchange surfaces

Define exchange

When all living things need a supply of oxygen and need to remove carbon dioxide

Why can unicellular organisms rely on diffusion alone?

Because they are one cell thick

Why do we need exchange surfaces?

• Size- large organisms have several layers of cells so diffusion pathway is too long and diffusion is too slow to sufficiently supply all cells with oxygen

• Level of activity- cells of organisms with a high metabolism need more oxygen and release more waste as they carry out more aerobic respiration

• SA:V- small organisms have a large SA:V so their SA is large enough to supply all cells but large cells have a small SA:V so they can't

SA:V equation

SA:V = surface area / volume

It should always be written as a:1

Features of a good exchange surface

• Large SA:V to allow more space for molecules to pass through so an increased rate of diffusion

• A thin barrier which is permeable to molecules

• Good blood supply to maintain concentration

Lungs diagram

sing. of alveoli

alveolus

Adaptations of alveoli

• Small but numerous providing a large surface area

• Walls of alveoli are one cell thick providing a short diffusion pathway

• Good blood supply to maintain the conc. gradient

• Well ventilated maintains conc. gradient

• Coated in surfactant which prevents the alveoli from collapsing

General features of airways

Tracheo, bronchi + bronchioles are lined by ciliated epithelium they also contain goblet cells which secrete mucus that traps pathogens. Cilia waft mucus to throat to be swallowed

Features of each tissue

• Alveoli have squamous epithelium and elastic fibres within walls

• Bronchiole walls are made up of smooth muscle and elastic fibres

• Trachea and bronchi are lined by rings of C-shaped cartilage which provides support but allows for flexibility and prevents airways collapsing

The role of smooth muscle and elastic fibres

Smooth muscle contracts to contrict the airways decreasing their diameter and reducing air flow to and from alveoli making it harder to inhale and exhale. Contraction of smooth muscle causes elastic fibres to deform. As the muscles relax fibres return to their original shape dilating airway. Smooth muscle relaxes during exercise

Inspiration

Diaphragm contracts -- externals contract -- volume increases -- pressure in chest cavity falls below atmospheric pressure -- air in

Expiration

Diaphragm relaxes -- externals relax -- volume decreases -- pressure in chest cavity rises above atmospheric pressure -- air out

Forced expiration

Internal intercostal muscles contract forcing the rib cage down and inwards to expel more air from the lungs

Define spirometer

Measures the volume of air in the lungs by measuring movement of air into and out of lungs

How does a spirometer work?

• During inspiration oxygen is removed from the chamber causing the lid to move down

• As the lid moves down it draws a trace on the revolving drum (kymograph)

• During expiration CO2 that enters the apparatus is absorbed by soda lime

• The remaining air is returned to the chamber causing the lid to rise and marking a trace on the revolving drum

Precautions

• Subject breathes through sterilised mouthpiece

• Subject should be healthy and free from asthma

• No air leaks in apparatus as it gives invalid results

Spirometry trace

Define tidal volume

The volume of air moved in and out with each breath at rest (must take 3 readings and calculate a mean)

Define vital capacity

The maximum volume of air that can be moved in one breath (depends on age, gender, fitness)

Define residual volume

Any air that remains in the lungs after a forced expiration

Breathing rate

Calculated by counting the number of peaks over a given time

Total lung capacity equation

TLC = vital capcity + residual volume

Ventilation rate equation

VR = tidal volume x breathing rate

Why is it difficult for fish to carry out gas exchange in water?

Because water has a low oxygen concentration

Gas exchange in bony fish diagram

Structure of the gills

• The gills are covered by an operculum

• Each gill consists of 2 rows of gill filaments attached to a bony arch

• Surface of each filament is folded into lamellae

Adaptations of gills

• Lots of gill fillaments provide large surface area

• Blood capillaries carry blood close to the surface of the lamellae maintaining conc. gradient

• Counter current flow of blood and water maintains conc. gradient

• Lamellae are well ventilated

• Short diffusion distance between blood and water

What is the buccal cavity?

The mouth of a bony fish

Steps for ventilation in fish

• Fish opens mouth lowering floor of buccal cavity increasing the volume and decreasing pressure, water flows in

• Fish closes mouth raising floor and decreasing volume but increasing pressure, water is forced over gills

• Operculum is pushed outwards and opens increasing volume + lowering pressure in opercular cavity, water leaves fish through open operculum

• Fish opens mouth to repeat process

How to display maximum details of fish gas exchange system?

Cut and remove the operculum and place a pencil into the buccal cavity to display the lamellae

Circulatory system in insects

Insects have an open circulatory system

Gas exchange system in insects diagram

Why are tracheae lined with chitin

To support them

Adaptations of tracheoles

• Lots of tracheoles provide large surface area

• Walls of tracheoles are moist to allow gases to dissolve

• Walls are thin and contain no chitin to allow for permeability

How does gas exchange in insects work

• Gas exchange occurs by diffusion across tracheole walls

• At the end of each tracheole there is tracheal fluid and oxygen can dissolve directly into it

How does rhythmic abdominal movement help ventilation in insects?

Rhythmic abdominal movement alters the volume of the thorax and abdomen changing the pressure. This acts to pump air in and out via spiracles

Valves in insects

Some insects (locust) have valves which open + close the spiracles to control volume of air entering and leaving insect

How to display maximum details of insect gas exchange system?

Cut open the exoskeleton and stain the tracheoles with methylene blue