exchange surfaces

• size 

single-celled organisms= cytoplasm is very close to environment=diffusion of oxygen and nutrients is enough.

multicellular organisms= have several layers of cells=diffusion pathway is longer and slower to reach innermost cells.

• SA:Vol ratio

small organisms= large ratio= SA is large enough to supply all their cells with sufficient oxygen (some organisms can adopt a diff. shape to increase SA)

large organisms=small ratio=SA is realtively small compared with their volume= not enought to supply cells with sufficient oxygen

• Level of activity

mammals need to thermoregulate, animals that respire(aerobic) needs more oxygen= needs exchange system.

• large surface area=more space for molecules to pass through(folding the walls and membranes e.g. root hairs)
• a thin barrier=reduce diffusion distance=must be permeableto the subs. that need to be exchanged (e.g. alveoli)
• good blood supply=bring fresh supplies to one side=keeping concentration gradient high or may remove wastes to keep concentration low.=important to maintain steep concentration gradient= diffusion can occur rapidly.

• consists of lungs,trachea, bronchi, bronchioles and alvoli (tiny air-filled sacs).
• lungs are protected by ribcage. ribs are held together by intercostal muscles.
• action of intercostal muscles and diaphragm helps to produce breathing movements (ventilation)

• gases pass by diffusion
• oxygen diffuse from alveoli to capillaries
• CO2 passes from blood to air in alveoli
• lungs must maintain steep concentration gradient in each direction= ensure that diffusion can continue.

• individual alveoli are very small but so numerous=for molecules to pass through
• alveoli are lined by a thin layer of moisture=evaporates as we breathe out=lungs must produce a surfactant= reduce the cohesive forces between water molecules that can make alveoli to collapse

• barrier to exchange is permeable to oxygen and carbon dioxide.
• plasma membrane only= allows small,non-polar molecules to pass through.

Adaptations to reduce diffusion distance

• alveolus wall is one cell thick
• capillary wall is one cell thick
• both walls consists of squamous cells-flattened or very thin
• capillaries are in close contact with alveolus walls
• capillaries are so narrow= RBC are squeezed=closer to the air in alveoli and reducing their rate of flow

A good blood supply

• blood system transports CO2 from tissues to lungs=conc. of CO2 in blood is higher than air in alveoli= CO2 can diffuse into the alveoli
• blood transports O2 away from lungs= conc. of O2 in the blood is kept lower than in alveoli=O2 diffuses into the blood

• replaces used air with fresh air= bring more oxygen and remove carbon dioxide
• ensures that concentration of O2 in the alveolus remains higher than in the blood
• ensures that concentration of CO2 in the alveoli remains lower than that in the blood.

INSPIRATION

• diaphragm contracts=move down and become flatter= pushes digestive organs downwards
• external intercostal muscles contract= raise ribs
• volume of chest cavity increased
• pressure in the chest cavity drops below atm.pressure
• air is moved into the lungs

EXPIRATION

• diaphragm relaxes= pushed up by digestive organs 
• external intercostal muscles relax= ribs fall
• internal intercostal muscles contracts=push air out more forcefully (exercise/coughin/sneezing)
• volume of chest cavity decreased
• pressure in the chest cavity rises above atm.pressure
• air is moved iout of the lungs

Alveolus

• air-filled sacs=sites of gas exchange
• comprised of squamous epithelium and are surrounded by capillaries
• walls have elastic fibres=stretch during inspiration and recoil to push air out during expiration
• walls are so thin

For effective airways

• must be large enough to allow sufficient air to flow without obstruction
• be supported so it will not collapse when air peressure is low (inspiration)
• be flexible to allow movement
• lined by ciliated epithelium= keep lungs healthy
• have goblet cells that secrete mucus=traps pathogens- cilia move mucus up to the top of airway= swallowed

Trachea and Bronchi

• have similar structure (bronchi are narrower)
• supported by C-shaped rings of cartilages in trachea(allow flexibility and space for food to pass down the oesophagus) irregular shaped cartilages in bronchi

Bronchioles

• much narrower than bronchi
• smaller ones have no cartilages
• mostly smooth muscle(constrict airway) and elastic fibres
• smallest bronchioles end in clusters of alveoli

Smooth muscle and Elastic fibres

• can contract=constrict airway=lumen is narrower=control flow of airto and from alveoli= imporatnt if there are harmful substances in the air=not voluntary (allergic reaction)
• elastic fibres recoil to their original size and shape(dilate airway)

Spirometer= a device that can measure the movement of air into and out of the lungs

• during inspiration=air drawn from the chamber-lid moves down
• during expiration=air returns to the chamber- lid rises
• movements are recoreded on a datalogger (trace)
• chamber is filled with medical-grade oxygen
• has a chamber of soda-lime where CO2-rich air passes- absorbs CO2= allows measurement of oxygen consumption

Precautions

• subject should be healthy (free from asthma)
• soda lime should be fresh and functioning
• ensure that there are no aoir leaks in the apparatus=give invalid+inaccurate results
• mouthpiece should be sterilised or disposable
• water chamber must not be overfilled= water may enter air tubes

Total lung volume= vital capacity(can be measured)+ residual volume(can't be measured)

• vital capacity=maximum volume of air that can be expelled from the lungs after taking the deepest possible breath. depends on size/height of a person, age and gender, level of regular exercise (2.5-5.0 dm3)
• residual volume=volume of air that remains in the lungs even after forced expiration=remain in airways and alveoli (1.5 dm3)
• tidal volume=volume of air inhaled or exhaled in one breath,usually measured at rest (0.5 dm3)--this is usually sufficient to supply all the oxygen required in the body at rest

Oxygen uptake

• CO2 is absorbed in the soda lime so volume of air in the chamber decreases.
• raate of oxygen uptake= gradient of the decrease in volume
• may depend on demande.g. during exercise
• increase when breathing rate increases and deeper breaths

breathing rate= number of breaths per minute (12-14 breaths per minute)

• have 5 pairs of gills which are covered by a bony plate called operculum
• each gill consists of 2rows of gill filaments(primary lamellae) attached to a bony arch= they are very thin  and surfaces are folded into many secondary lamellae=provides large surface area
• surface of secondary lamellae=site of gas exchange

Countercurrent flow

• where 2 fluids flow in opposite directions (blood)
• absorbs the maximum amount of oxygen from the water

Ventilation

• buccal-opercular pump=keep water flowing over the gills
• buccal cavity=can change volume--open=more water drawn in--close=push water through the gills
• as water is pushed from the buccal cavity, the operculum moves outwards=reduces the pressure in the opercular cavity=help water to flow through the gills

• oxygen is not transported by blood
• have air-filled tracheal system=supplies air directly to all the respiring tissues
• air enters via a pore in each segment(spiracle)--air is transported into the body through a series of tubes called tracheae--divide into smaller tubes called tracheoles--ends of tracheoles are open and filled with fluid called tracheal fluid.
• gaseous exchange occurs between air in the tracheole and the tracheal fluid. some can also occur acroos the thin walls of the tracheoles.
• active insects=tracheal fluid can be withdrawn into the body fluid=increase SAof the tracheole wall exposed to air--more O2 can be absorbed when insect is active

Ventilation

• sections of the tracheal system are expanded and have flexible walls-act as air sacs-can be squeezed by action of flight muscles-repetitive expansion and contraction of sacs-ventilate tracheal system (TS)
• movement of wings alter volume of thorax-when decreased-air in TS is put under pressure and is pushed outof TS-when increased-pressure drops-air pished into TS from outside
• locusts can alter volume of their abdomen by specialised breathing movements-coordinated by opening and closing valves in the spiracle. Abdomen expands-spiracles(front end) open-air enters TS. Abdomen reduces in volume-spiracles(rear end)open-air can leave TS.