OCR A level Biology (A)
Module 3 — Exchange and Transport Notes
1. Exchange Surfaces
1.1 Why organisms need specialised exchange surfaces
All living cells need to exchange substances — oxygen, carbon dioxide, nutrients and waste products — with their
environment, and in the simplest organisms this can happen by simple diffusion across the cell surface. A single-
celled organism such as Euglena can rely on diffusion alone because it is small, has a large surface area to volume
(SA:V) ratio, and therefore a short diffusion pathway, and has a relatively low level of metabolic activity, so its
demand for oxygen and nutrients is modest. As organisms become larger and more complex, however, their SA:V
ratio decreases — surface area increases with the square of a linear dimension while volume increases with the
cube, so a bigger organism has proportionally less surface for the amount of tissue it needs to supply. (For
reference, the volume of a sphere is 4/3 × π × r³ and its surface area is 4 × π × r².) Organisms with more than about
two layers of cells cannot rely on diffusion alone across their outer surface, because the diffusion pathway into
the centre of the body becomes too long and the demand of the tissues too great. This is why larger, more
metabolically active animals have evolved specialised exchange surfaces and, alongside them, mass transport
systems to move substances quickly between the exchange surface and the rest of the body.
1.2 Features of an efficient exchange surface
Whatever the organism, effective exchange surfaces share the same three features: a large surface area, which
maximises the amount of exchange that can happen at any one time; a short diffusion distance, usually achieved
by the surface being very thin (often just one cell thick); and a rich blood supply combined with ventilation of
some kind, which keeps a steep concentration gradient across the surface by constantly removing the substance
that has diffused in and replenishing the substance that is diffusing out. These three principles — surface area,
diffusion distance and concentration gradient — recur throughout this topic, whether the exchange surface in
question is an alveolus, a gill lamella or a tracheole.
2. The Mammalian Gas Exchange System
2.1 Structure of the airways
Air passes from the trachea into the two bronchi and then into progressively smaller bronchioles before reaching
the alveoli, where gas exchange actually takes place. The trachea and bronchi share several structural features.
Both are lined with a ciliated epithelium interspersed with goblet cells; the goblet cells produce and secrete
mucus, which traps pathogens and dust particles, while the cilia beat to move that mucus up and out of the airway
so that it can be swallowed rather than reaching the lungs. Both are also supported by rings of cartilage, which
are C-shaped rather than complete circles — this keeps the airway held open while still allowing enough flexibility
for the oesophagus, which sits directly behind the trachea, to expand as food passes down it. Smooth muscle in
the walls of the airways can contract involuntarily in response to harmful substances; when it does so, the lumen
, of the bronchioles narrows and this restricts airflow to the alveoli beyond. Cartilage becomes progressively sparser
moving down the airways and is generally absent from the smallest bronchioles, which instead rely on elastic
fibres to stay open; the smallest bronchioles can be identified under the microscope by the absence of goblet cells
and ciliated epithelium, which are only found in the trachea and bronchi.
A bronchiole in cross-section — note the absence of goblet cells and cartilage seen further up the airway.
Airway histology: ciliated epithelium with goblet cells (top) and smooth muscle (bottom).
2.2 The alveoli
The alveoli are the tiny air sacs where gas exchange occurs, and they are extremely well adapted to the three
requirements of an efficient exchange surface. Collectively they provide an enormous surface area. The diffusion
distance between the air in an alveolus and the blood in the surrounding capillary is less than a micrometre,
because both the alveolar wall and the capillary wall are only one cell thick and are made of flattened (squamous)
Module 3 — Exchange and Transport Notes
1. Exchange Surfaces
1.1 Why organisms need specialised exchange surfaces
All living cells need to exchange substances — oxygen, carbon dioxide, nutrients and waste products — with their
environment, and in the simplest organisms this can happen by simple diffusion across the cell surface. A single-
celled organism such as Euglena can rely on diffusion alone because it is small, has a large surface area to volume
(SA:V) ratio, and therefore a short diffusion pathway, and has a relatively low level of metabolic activity, so its
demand for oxygen and nutrients is modest. As organisms become larger and more complex, however, their SA:V
ratio decreases — surface area increases with the square of a linear dimension while volume increases with the
cube, so a bigger organism has proportionally less surface for the amount of tissue it needs to supply. (For
reference, the volume of a sphere is 4/3 × π × r³ and its surface area is 4 × π × r².) Organisms with more than about
two layers of cells cannot rely on diffusion alone across their outer surface, because the diffusion pathway into
the centre of the body becomes too long and the demand of the tissues too great. This is why larger, more
metabolically active animals have evolved specialised exchange surfaces and, alongside them, mass transport
systems to move substances quickly between the exchange surface and the rest of the body.
1.2 Features of an efficient exchange surface
Whatever the organism, effective exchange surfaces share the same three features: a large surface area, which
maximises the amount of exchange that can happen at any one time; a short diffusion distance, usually achieved
by the surface being very thin (often just one cell thick); and a rich blood supply combined with ventilation of
some kind, which keeps a steep concentration gradient across the surface by constantly removing the substance
that has diffused in and replenishing the substance that is diffusing out. These three principles — surface area,
diffusion distance and concentration gradient — recur throughout this topic, whether the exchange surface in
question is an alveolus, a gill lamella or a tracheole.
2. The Mammalian Gas Exchange System
2.1 Structure of the airways
Air passes from the trachea into the two bronchi and then into progressively smaller bronchioles before reaching
the alveoli, where gas exchange actually takes place. The trachea and bronchi share several structural features.
Both are lined with a ciliated epithelium interspersed with goblet cells; the goblet cells produce and secrete
mucus, which traps pathogens and dust particles, while the cilia beat to move that mucus up and out of the airway
so that it can be swallowed rather than reaching the lungs. Both are also supported by rings of cartilage, which
are C-shaped rather than complete circles — this keeps the airway held open while still allowing enough flexibility
for the oesophagus, which sits directly behind the trachea, to expand as food passes down it. Smooth muscle in
the walls of the airways can contract involuntarily in response to harmful substances; when it does so, the lumen
, of the bronchioles narrows and this restricts airflow to the alveoli beyond. Cartilage becomes progressively sparser
moving down the airways and is generally absent from the smallest bronchioles, which instead rely on elastic
fibres to stay open; the smallest bronchioles can be identified under the microscope by the absence of goblet cells
and ciliated epithelium, which are only found in the trachea and bronchi.
A bronchiole in cross-section — note the absence of goblet cells and cartilage seen further up the airway.
Airway histology: ciliated epithelium with goblet cells (top) and smooth muscle (bottom).
2.2 The alveoli
The alveoli are the tiny air sacs where gas exchange occurs, and they are extremely well adapted to the three
requirements of an efficient exchange surface. Collectively they provide an enormous surface area. The diffusion
distance between the air in an alveolus and the blood in the surrounding capillary is less than a micrometre,
because both the alveolar wall and the capillary wall are only one cell thick and are made of flattened (squamous)
