8. Exchange Surfaces
- 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.
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
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)
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
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
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)
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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