Summary notes for A-level Biology OCR B (Advancing Biology). Chapter 7 - Gas exchange in mammals and plants (2.2.3 on specification). In-depth detailed notes covering all required content.
Mammalian Gas exchange system
gas exchange A tissue is a group of similar cells which are specialised to carry out
a function. Tissues are grouped into organs.
system
The functions of the respiratory system are to provide a large
surface area for gas exchange and to move air to and from gas
exchange surfaces. It has a close relationship with the circulatory
system, which carries the exchanged gases to and from the lungs.
Pharynx is another name for the throat. It is a shared passage for
food and gases.
Trachea
The trachea is kept permanently open with C shaped rings of
cartilage around its walls. These rings also help with pressure
changes. Under the cartilage is a layer of smooth muscle, elastic
fibres, connective tissue, and blood vessels.
The trachea is lined with a layer of ciliated columnar epithelium
cells and goblet cells, which are in between the epithelium and
produce mucus. The ciliated cells move the mucus and any
trapped bacteria back up the trachea into the throat, where it is
swallowed and destroyed by the stomach acid.
-The role of the columnar epithelium is to protect the lungs from
pathogens.
The lungs
The right lung has 3 lobes and the left lung has 2, so the right lung is
bigger. Elastin surrounds everything in the lungs.
At the bottom the trachea branches into two bronchi. These bronchi
branch off into smaller bronchioles.
Bronchioles
-Smooth muscle lines the airways of the bronchioles. This copes
with pressure changes, and can contract, narrowing the lumen of
the airway. This can cause problems by closing the airway, which
occurs in an allergic reaction.
-Elastic fibres also line the walls, which allow recoil back to normal
size when the muscle relaxes after the smooth muscle contracts.
, -Bronchitis is inflammation and infection of the bronchi.
-The terminal bronchioles are the final ones, and at the end of
these sacs of alveoli are formed.
Alveoli
The alveoli form the gas exchange surface.
-Squamous epithelium makes up the alveolar wall. This is thin
which allows for efficient gas exchange. These cells are type I
pneumocytes.
-Elastic fibres are also found for expansion and recoil, as well as
some collagen and stretch receptors to assist in controlling the
breathing cycle.
-Type II pneumocytes are also found in the alveolar wall. These cells
secrete the fluid surfactant, which reduces the surface tension and
prevents the collapse of the alveolus. It also makes it easier to
inflate the lungs as well as acting as an antibacterial substance.
-The liquid allows for efficient gas exchange across the surface by
allowing oxygen to dissolve and then diffuse into the blood.
Adaptations of the mammalian gas exchange system
Large surface area
-There are many alveoli, and the elastic fibres allow them to
expand during inhalation. This further increases the surface area.
-A large surface area enables many molecules to cross at the
same time.
Short diffusion distance
-Squamous epithelial layer is extremely thin.
-The capillary walls consist of a single layer of cells so the total
distance between the blood in capillary and air in alveoli is extremely
small. This means molecules can cross in less time.
Steep diffusion gradient
-Pulmonary circulation rapidly moves oxygenated blood away and
brings deoxygenated blood.
-Pulmonary ventilation replaces the carbon dioxide rich air in the
alveoli with oxygen rich air maintaining the diffusion gradient.
Pulmonary Breathing
ventilation Lung ventilation is vital to maintain the diffusion gradient. Muscle
contraction causes the thorax to move and so it changes its volume
and the volume of the lungs within it.
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