3.1 - Exchange Surfaces

Why have specialised exchange surfaces (SA:V)

• Organisms need to exchange materials

  • Urea, carbon dioxide and heat out of body

  • Oxygen, glucose into

  • Occurs across plasma membrane

• High SA:V

  • Diffusion of substances is fast

  • Generally for smaller organisms

• Low SA:V

  • Diffusion is slower

  • Generally for larger organisms

• Calculating

  • Calculate SA

  • Calculate V

  • SA:V with V = 1

Why specialised exchange surfaces in multicellular organisms

• Require specialised surfaces unlike single celled organisms

• Due to:

  • Cells are not in direct contact with external environment

  • Diffusion distances between cells and their environment are large

  • Larger organisms have higher metabolic rates so they need more oxygen and glucose

Key features of specialised exchange surfaces

• Large surface area

  • Larger area across which substances can be exchanged

  • More substances can travel across per area

  • Root hair cells

• Thin walls

  • Minimises diffusion distance

  • Alveoli

• Good blood supply

  • Maintains steep gradient

  • Gills

• Being surrounded by partially permeable membranes

  • Control what substances are exchanged

Lungs

• Allow oxygen to enter the blood and carbon dioxide to leave

• Uses exchange surfaces called alveoli

• Is inside the body

  • Air is not dense enough to support and protect these delicate structures

  • The body would otherwise lose water and dry out

Pathway of air

• Air enters the trachea

• Travels into the two bronchi, with one bronchus going to each lung

• Travels into smaller airways called bronchioles

• Air travels into clusters of air sacs called alveoli

Airway tissue

• Ciliated epithelium

  • Contains goblet cells and ciliated epithelial cells

  • Goblet cells produce and secrete mucus to trap dust and microbes

  • Cilia waft the mucus upwards to the mouth

Trachea

• Large tube that carries air from throat to lungs

  • Rings of cartilage keep the airway open

  • Smooth muscle can contract or relax to open/close the airway and change airflow

  • Elastic tissue allow stretching and recoiling

  • Lined with ciliated epithelium

Bronchi

• Two main branches extending from the trachea that carry air into each lung

  • Reinforced with cartilage to keep the airway open

  • Smooth muscle to contract/relax and change airflow

  • Elastic tissue allows stretching and recoiling

  • Lined with ciliated epithelium

Bronchioles

• Two smaller airways branching from the bronchi

  • No cartilage to change shape

  • Smooth muscle to contract/relax and change airflow

  • Elastic tissue allows stretching and recoiling

  • Squamous epithelium

Alveoli

• Gas exchange

  • Oxygen diffuses from alveoli into the pulmonary capillaries where it binds to haemoglobin

  • Carbon dioxide dissociated from haemoglobin and diffuses into the alveoli

Adaptations of alveoli

• One layer of squamous epithelial cells

• Large SA

• Partially permeable

• Surrounded by dense network of capillaries

  • Brings blood close to air for gas exchange

• Ventilation of air

  • Maintains steep diffusion gradient

• Elastic fibres

  • Allow stretching and recoiling

• Collagen fibres

  • Prevents overstretching and bursting

• Moist inner layer

  • Gases to dissolve

Ventilation

• Is the constant movement of air into and out of the lungs

• Consists of expiration and inspiration

Muscles involved in ventilation

• Diaphragm

  • Sheet of muscles that moves the ribcage up and out when it contracts

• External intercostal muscles

  • Found between the ribs and pull the ribcage up and out when they contract

• Internal intercostal muscles

  • Found between the ribs and pull the ribcage down and in when they contract

Inspiration

• External intercostal muscles contract while the internal intercostal muscles relax

  • Ribcage moves up and out

• Volume of the thoracic cavity increases

• Diaphragm contracts and flattens

  • Increases the volume of the TC

• Lung pressure decreases below atmospheric pressure

• Air flows into the lungs down the pressure gradient

Expiration

• Normally a passive process however forced expiration can occur when playing wind instrument or after exercise

• EI relax

  • Ribcage moves down and in

• Volume of thoracic cavity decreases

• Diaphragm relaxes and unflattens

  • Decreases the volume of TC

• Lung pressure increases above atmospheric pressure

• Air if forced out of lungs

• Elastic fibres in alveoli also shrink when pressure decreases

Measuring ventilation

• Ways to measure data on lung function, volume and capacity

  • Peak flow meter

  • Vitalograph

  • Spirometer

Measuring lung volume

Definitions of: breathing rate, tidal volume, vital capacity

• Breathing rate

  • Number of breaths taken per minute

  • Measured by counting the number of peaks in a minute

• Tidal volume

  • Volume of air breathed in or out in an average breath during rest

• Vital capacity

  • Maximum volume of air that can be inhaled or exhaled in one deep breath

  • Measured from max peak height

Definitions: inspiratory reserve volume, expiratory reserve volume, residual volume

• Inspiratory reserve volume

  • Maximum volume of air that can be inhaled above normal inhalation

• Expiratory reserve volume

  • Maximum volume of air that can be inhaled above a normal inhalation

• Residual volume

  • Volume of air that remains in lungs after the largest possible exhalation

Calculating oxygen consumption

Slope of the spirometer trace

Ventilation rate equation

Ventilation rate = tidal volume x breathing rate

Why insects need gas exchange

• Have chitin exoskeleton which prevents gas exchange

  • Covered in a waterproof cuticle to prevent water loss

• To deliver oxygen to cells

  • Allows aerobic respiration to occur to release energy by hydrolysis of ATP

• To remove carbon dioxide

  • Reduced pH which can denature enzymes

Tracheae

• Air filled tubes branching through the body

• Adaptations

  • Reinforced with spirals of chitin to prevent collapsing

  • There are multiple to increase SA

Tracheoles

• Fine branches of tracheae that deliver gases to cells

• Adaptations

  • Penetrate directly into tissues to reduce gas diffusion distance

  • Thin walls

  • High branched to maximise SA

  • Not reinforced with chitin to allow gas exchange

  • Fluid at ends (tracheal fluid) allows oxygen to dissolve to aid diffusion and reduce water loss

Spiracles

• External opening of the tracheal system on exoskeleton along abdomen and thorax

• Can be opened or closed to control gas exchange and minimise water loss

Process of gas exchange in insects

• Air enters the tracheal system through open spiracles

• Air moves into larger tracheae and diffuses into smaller tracheoles

• Tracheoles branch out throughout body

• Oxygen dissolves in tracheal fluid and diffuses down concentration gradient from tracheoles into body cells

• Carbon dioxide diffuses out of cells into tracheoles

• Air is then carried back to spiracles and released

How is concentration gradient maintained in insects

• Cells using up oxygen for respiration

  • Keeps concentration low in cells

• CO2 production in cells to keep concentration high

• Continuous ventilation

  • Fresh air is supplied to tracheal system via spiracles

6 additional insect ventilation mechanisms

• More spiracles open

  • Allows more oxygen to enter the tracheal system

• Mechanical active ventilation

  • When muscles around tracheae contract and relax changing the volume and pressure on the abdomen and pumps air in and out the spiracles

• Movement of tracheal fluid out of tissue

  • Increases diffusion rate and SA for gas exchange

• Collapsable tracheae, accessory sac and air reservoirs

  • Inflate or deflate to ventilate and can increase the volume of air moved through the system

• Movement of wing muscles connected to sacs

  • Pump air to ventilate tracheal system

• Vibration of thoracic muscles

  • Pumps air to ventilate tracheal system

Lactic acid accumulation in insects

• Can affect rate of gas exchange

  • Reduces the water potential in tracheal fluid at the end of tracheoles

  • Water leaves the tracheoles via osmosis

  • Higher SA for gas exchange

Respiratory system in bony fish

• Have high oxygen needs

  • Live under water which is denser than air so slower diffusion of oxygen

  • Has lower oxygen concentration

  • Very active so high oxygen demands

Structure of gills

• Covered by an operculum flap

• Consists of stacked filaments containing gill lamellae

• Gill lamellae are surrounded by extensive blood vessels

Adaptations of gills

• Lamellae provide large SA

• Lamellae membranes are thin to minimise diffusion distance

• Gills have a rich blood supply to maintain steep diffusion gradients

• Countercurrent flow of blood and water creates even steeper gradient

• Overlapping filament tips increase resistance so water flow over gills more slowly

Counter-current

• Blood and water flow over each lamellae in opposite directions

• Means that oxygen rich blood meets water that is at its most oxygen rich when it first moves across gills

  • Maximising diffusion of oxygen

• Oxygen poor blood returning from body tissue meets oxygen reduced water

  • Still allows diffusion of oxygen

• Maintains a steep concentration gradient across the entire gill

Parallel flow

• An equilibrium would be reached meaning less oxygen would diffuse

• Less effective and efficient than counter-current

Ventilation: closed mouth

• Floor of the mouth is raised so pressure increases and volume decreases

• Means water is pushed over the gills and into the gill cavity

• Oxygen is transferred into the blood

Ventilation: open mouth

• When a fish opens its mouth, water enter buccal cavity

  • Floor of the mouth is lowered so volume increases and pressure decreases

  • Means water travels down the pressure gradient in the buccal cavity