8. Exchange Surfaces

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  • Created by: zoelaad
  • Created on: 28-12-17 21:27

The Need for Specialised Exchange Surfaces

Surface Area to Volume Ratio:

SA:A is the surface area of an organism divided by its volume. It is a key concept as the surface area must be able to provide sufficient oxygen through diffusion from the environment.

As the size of the organism increases-

  • surface area increases
  • volume increases (more than surface area)
  • surface area to volume ratio decreases

The significance of surface area to volume ratio-

Single-celled Organisms are small and have a large SA:V. Their surface area is large enough for sufficient oxygen and nutrients to diffuse into the cell and for waste to diffuse out.

Multicellular Organisms have a small SA:V. Diffusion is too slow for the oxygen and nutrients to diffuse across the whole organism, therefore, a specialised exchange surface is required. 

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Efficient Exchange Surfaces

The Features of Good Gaseous Exchange Surfaces:

Feature: Large surface area

Reason: To provide space for molecules of oxygen and carbon dioxide to pass

In the lungs: Lung epithelium folded to form numerous alveoli

Feature: Thin layer

Reason: To provide a short diffusion pathway

In the lungs: Lung epithelium and capillary endothelium are made from squamous cells

Feature: Steep concentration gradient

Reason: To ensure molecules diffuse rapidly in the correct direction

In the lungs: Good supply of blood on one side and ventilation of the air sacs on the other side

Concentration Gradient- the difference in concentration between two points 

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Gaseous Exchange in Mammals

The Lungs:

Capillaries - Over surface of alveoli - To provide a large surface area for exchange

Cartilage - In walls of bronchi and trachea - To hold the airways open

Ciliated Epithelium - On surface of airways - The cilia move or waft the mucus along

Elastic Fibres - In walls of airways and over alveoli - Recoils to return airway or alveolus to original shape

Goblet Cells - In ciliated epithelium - To produce and release mucus

Smooth Muscle - In walls of airways - Contracts to constrict or narrow the airways

Squamous Endothelium - Capillary walls - To provide a thin barrier for exchange (short diffusion pathway)

Squamous Epithelium - Surface of alveoli - To provide a thin barrier for exchange (short diffusion pathway)

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Ventilation in Mammals

Ventilation:

  • Also known as breathing 
  • Refreshes the air int he alveoli
  • Achieved by the action of the diaphragm and the intercostal muscle

Structure                               Inspiration (inhaling)                                              Expiration (exhaling)

Diaphragm                            Contracts and moves downwards                           Relaxes and is pushed up by the organs

Intercostal Muscle                Contract to raise the rib cage up and out                Relax and allow the rib cage to fall 

Volume change                     Chest cavity increase in volume                             Chest cavity reduces in volume

Pressure change                    Chest increases, atmospheric decrease                   Chest decreases, atmospheric increases

Air movement                      Air pushed into lungs by atmospheric pressure      Air pushed out of lungs by

                                                                                                                                 air pressure in alveoli

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Vital Capacity and Tidal Volume

Vital Capacity:

The maximum volume of air that can be breathed in or out in one breath

Tidal Volume:

The volume of air breathed per breath, usually taken at rest 

Using a Spirometer:

  • Subject wears a nose clip so no oxygen escapes the nose
  • Subject breaths into mouthpiece
  • As subject inhales, oxygen is drawn from the air chamber which therefore descends
  • As the subject exhales, the air chamber rises again
  • Air returns to chamber and passes through soda lime to remove CO2
  • The movement of the chamber is recorded by a data logger or revolving drum 
  • Tidal volume is measured by allowing subject to breathe normally
  • Vital capacity is measured by getting subject to breathe out as deeply as possible
  • Breathing rate is calculated by counting the number of peaks in one min
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Ventilation and Gaseous Exchange in Bony Fish

  • Fish exchange gases with the water they live
  • They use gills to absorb oxygen dissolved in the water and releases carbon dioxide into the water
  • Each gill consists of two rows of gill filaments (primary lamellae) attached to a bony arch
  • The filaments are very thin and their surface is folded into many gill lamellae (gill plates)
  • This gives a large surface area
  • Blood capillaries carry deoxygenated blood close to the surface of the gill plates where exchange takes place
  • The blood flows in the opposite direction to the flow of water (countercurrent flow)
  • Ventilation is achieved by movement of the floor of the mouth (buccal cavity) and operculum (the bony flap over the gills) 
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Ventilation and Gaseous Exchange in Insects

  • Insects do not transport oxygen in blood
  • They have an air-filled tracheal system that supplies air directly to all the respiring tissues
  • Air enters the system via pores called spiracles
  • The air passes through the body in a series of tubes called tracheae
  • These divide into smaller tubes called tracheoles
  • The ends of the tracheoles open into tracheal fluid
  • Gaseous exchange occurs between the air in the tracheole and the tracheal fluid
  • Larger insects can ventilate their tracheal system by movement of the body which squeezes air sacs in the larger trachea 
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