Chapter 7 - Exchange Surfaces and Breathing

Why do organisms need specialised exchange surfaces?

• metabolic activity of single cell organisms is low so demands are low

• larger organisms are made up of tissues and organs

• metabolic activity is much higher

• SA:V of large organisms is low

Surface area : volume ratio

• the smaller the surface area : volume ratio, the larger the diffusion distances

Features of specialised exchange surfaces

• increased surface area

• thin layer

  • decreases diffusion distances

  • makes process fast and efficient

• good blood supply

  • steeper the concentration gradient, the faster diffusion takes place

  • maintains a steep concentration gradient

• ventilation to maintain diffusion gradient

  • maintains concentration gradient

Human gaseous exchange system pathway

nasal cavity → larynx → trachea → bronchus → bronchioles → alveoli

Human gaseous exchange system structure

Nasal cavity

• large surface area

• good blood supply - warms air

• lined with hairs & secrets mucus

  • traps dust and bacteria

• moist

  • increases humidity

Trachea

• wide tube

• incomplete rings of flexible cartilage

  • so food can move easily down oesophagus

• ciliated epithelium

  • waft the mucus away from the lungs

• goblet cells

  • secrete mucus

  • trap dust/microorganisms

Bronchus

• trachea splits in 2

• similar structure to trachea

• smaller

Bronchioles

• bronchi divide

• no cartilage

• walls contain smooth muscle

  • contract - bronchioles constrict

  • relax - bronchioles dilate

• thin layer of flattened epithelium

Alveoli

• air sacs

• main gas exchange surfaces

• diameter of around 200-300µm

• layer of thin, flattened epithelium cells

• collagen

• elastic fibres (elastin)

  • stretch as air is drawn in

  • squeeze air out when returning to resting size

  • known as elastic recoil

Features of alveoli

• large surface area

  • 300-500 million per lung

• thin layers

  • walls are only 1 epithelial cell thick

  • short diffusion distances

• good blood supply

  • surrounded by a network of capillaries

  • maintains a steep concentration gradient

• good ventilation

  • breathing moves air in and out

  • maintains steep diffusion gradients for oxygen and carbon dioxide

Inspiration

• energy using process

• diaphragm contracts (flattened and lowered)

• external intercostal muscles contract

• moves ribs upwards and outwards

• volume of thorax increases so pressure is reduced

• air is drawn into the lungs

Expiration

• passive process

• diaphragm relaxes

• external intercostal muscles relax

• moves ribs inwards and downwards

• elastic fibres return to normal length

• decreases the volume of thorax so pressure increases

• forces air out of the lungs

Forceful expiration

• uses energy

• internal intercostal muscles contract

• pulls ribs down hard and fast

• forces diaphragm up

Peak flow meter

• measures rate at which air can be expelled from the lungs

Vitalographs

• more sophisticated versions of peak flow meter

• patient breathes out as quickly as possible

Spirometer

• measured different aspects of lung volume

• investigates breathing patterns

Components of lung volume

• tidal volume

• vital capacity

• inspiratory reserve volume

• expiratory reserve volume

• residual volume

• total lung capacity

Tidal volume

• volume of air that moves into and out of the lungs with each resting breath

Vital capacity

• volume of air that can be exhaled when the deepest possible intake of breath is followed by the strongest possible exhalation

IRV

• maximum volume of air you can breathe in over and above a normal inhalation

ERV

• extra amount of air you can force out of your lungs over and above the normal tidal volume of air you breathe out

Residual volume

• volume of air left in your lungs when you have exhaled as hard as possible

Total lung capacity

• sum of the vital capacity and residual volume

ventilation rate

tidal volume x breathing rate (per minute)

Spirometer graph

Why can’t insects

• have a tough exoskeleton

  • little/no gaseous exchange can take place

• no blood pigments to carry oxygen

How does gaseous exchange take place in insects

• spiracles along the thorax and abdomen

  • air/enters or leaves

  • water is lost

  • can be opened/closed by sphincters

  • kept closed as much as possible to minimise water loss

• tracheae

  • carry air into the body

  • lined with spirals of chitin

  • keep them open

  • relatively impermeable to gases

• tracheoles

  • single elongated cells

  • no chitin lining

  • freely permeable to gases

  • lots give a large SA for gaseous exchange

  • oxygen dissolves in moisture and diffuses into surrounding cells

• tracheal fluid

  • limits penetration of air for diffusion

How do insects cope with increased oxygen demand?

• lactic acid build up means water moves out of tracheoles by osmosis

  • exposes more SA for gaseous exchange

• larger insects

  • mechanical ventilation

    • air is pumped into the system by thorax/abdomen

  • collapsible enlarged tracheae/air sacs

    • air reservoirs

Gills

• flow of water over in one direction

• large surface area

• good blood supply

• thin layers

• contained in gill cavity

• covered by operculum (flap of bone that maintains water flow)

• gill arch

  • supports structure of the gills

• gill lamellae

  • main site of gaseous exchange

• gill filaments

  • large stacks (gill plates)

  • flow of water to keep them apart

  • exposes large SA

• afferent blood vessel

  • brings blood in

• efferent blood vessel

  • takes blood out in opposite direction to water

  • maintains steep concentration gradient

How does water flow over the gills?

• open mouth and operculum to keep flow of water when swimming

• ram ventilation

  • ram water past the gills

  • only some fish

• most bony fish don’t rely on this

Stages of fish respiration

• mouth is opened and floor of buccal cavity is lowered

• increases volume of buccal cavity

• opercular valve is shut and opercular cavity expands

• lowers pressure in opercular cavity

• floor of buccal cavity moves up - increases pressure

• forces water into opercular cavity over the gills

• mouth closes and operculum opens

• sides of opercular cavity move inwards

• increase pressure in opercular cavity

• force water over the gills and out the operculum

How is fish respiration so effective?

• tips of gill filaments overlap

  • increases the resistance to flow of water

  • slows down movement of water

• water over the gills and blood in gill filaments flow in different directions

  • steep concentration gradient

  • countercurrent exchange system

  • more gaseous exchange can take place