Summary
Biology OCR A level Exchange surfaces Summary Notes
Biology OCR A level Exchange surfaces Summary Notes
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1 of 6B7- Exchange surfaces and breathing
3.1.1 Exchange surfaces
(a) The need for specialised exchange surfaces.
Key words:
Exchange surfaces- surfaces over which materials are exchanged from one to another.
• Cells need to take in substances and excrete waste products.
Reasons for exchange systems (unicellular)
• Metabolic activity is low, so oxygen demands and carbon dioxide production are low.
• SA:V is large, as the distance to the cell needing oxygen is short.
Reasons for exchange systems (multicellular)
• Have a small SA:V ratio.
• Cells in the centre of the organism would not receive any materials if they survived on diffusion
alone.
• High metabolic rate, high demand for oxygen and high production of carbon dioxide.
Ratio= SA / V
SA:V calculations
Cuboids Cylinders Sphere
Volume LxWxH π r2 4/3 π r 3
Surface area 4LH+2HW 2πr x H x 2π r2 4π r2
(b) The feature of an efficient exchange surface.
Increased surface area
• Large surface area provides a large area over which the exchange of materials can occur.
• Includes root hair cells in plants and villi in small intestine of mammals.
Thin layer
• Creates a short diffusion path for gases, increasing the speed of exchange.
• Includes alveoli as they are made from a single layer of thin, flat cells.
Good blood supply/ventilation to maintain gradient- gills/alveolus
• Good, constant blood supply maintains a large diffusion gradient and ensures the exchange
substances are constantly moving to the area needed.
• Ventilation maintains the diffusion gradient, making the process faster and more efficient.
• Includes alveoli, as they are surrounded by a large capillary network, giving each its own blood
supply, maintaining its constant concentration gradient.
• Includes gills, contains a large network of capillaries well supplied with blood and well
ventilated- fresh water constantly flows over them, help to maintain a concentration gradient.
(c) The structures and functions of the components of the mammalian gaseous exchange
system.
Key words:
Nasal cavity-
Trachea- the main airway, supported by incomplete rings of cartilage, which carries warm moist
air down from the nasal cavity into the chest.
Cartilage- strong, flexible connective tissue found in many areas of the bodies of humans and
other animals.
Ciliated epithelium- thin tissue that has hair-like structures on it. The cilia move back and forth to
move particles out of the body, found in the respiratory tract.
Goblet cells- differentiated cells specialised to secrete mucus.
Bronchus- one of the two tubes that connect the lungs with the trachea.
Bronchioles- airways in the lungs that lead from the bronchi to the alveoli.
Alveoli- tiny sacs of lung tissue specialised for the movement of gases between air and blood.
, 2 of 6
Elastic recoil- the ability to return to original shape and size following stretching. Particularly of
the alveoli of the lungs and of the arteries.
Lung surfactant- chemical mixture containing phospholipids and both hydrophilic and
hydrophobic proteins, which coats the surfaces of the alveoli and prevents them collapsing after
every breath.
Inspiration- breathing in.
Expiration- breathing out.
Distribution and functions of cartilage
Ciliated epithelium
Goblet cells
Smooth muscle and elastic fibres in the trachea
Bronchi
Bronchioles
Alveoli
Nasal cavity
• Large SA, with a good blood supply, which warms air to body temperature.
• Hairy lining, which secretes mucus to trap dust and bacteria, protecting delicate lung tissue
from irritation and infection.
• Moist surfaces, which increase the humidity of the incoming air, reducing evaporation from the
exchange surface.
Trachea
• Main airway carrying clean, warm, moist air away from the nose down into the chest.
• Is a wide tube supported by incomplete rings of strong, flexible cartilage, stops trachea from
collapsing.
• Rings are incomplete so food can move easily down the oesophagus.
• Trachea and its branches are lined with ciliated epithelium with goblet cells between and below
the epithelial cells.
• Goblet cells secrete mucus on the lining of the trachea, to trap dust and microorganisms.
• Cilia beat and move the mucus, along with any trapped dirt and microorganisms, away from the
lungs.
• Most of it goes into the throat and is swallowed and digested.
• Cigarette smoke stops cilia beating.
Bronchus
• Trachea divides to form the left and right bronchus.
• Similar in structure to the trachea with the same supporting rings of cartilage, but smaller.
Bronchioles
• The bronchi divide to form many small bronchioles.
• Smaller bronchioles (diameter 1mm<) have no cartilage rings.
• Walls contain smooth muscle. When the muscles contract, the bronchioles constrict, when it
relaxes they dilate.
• Changes the amount go air reaching the lungs.
• Lined with thin layer of flattened epithelium.
Alveoli
• Tiny air sacs, which are the main gas exchange surface of the body.
• Unique to mammalian lungs.
• Diameter of around 200-300μm and consists of a layer go thin flattened epithelial cells, along
with some collagen and elastic fibres.
• Elastic tissues allow the alveoli to stretch as air is drawn in. When they return to their resting size
they help squeeze the air out, elastic recoil.
• Large SA- if lungs were simple balloon like structure, SA wouldn’t be big enough for oxygen to
diffuse.
• Thin layers- only a single epithelial cell thick, so diffusion distances are short.
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