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Respiratory System for A-Level Biology
A-Level Biology & Human Biology : Human respiratory system
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Gas Exchange in HumansExchange System
The gas exchange system in humans and other mammals consists essentially of a
conducting system for the conduction of inspired and expired gases, and an interface
for the exchange of gases between air and blood.
The conducting system consists of a pair of lungs connected to the atmosphere by a
series of air tubes. The lungs are situated inside a protective bony case, the thorax
or the rib cage, the walls of which are formed by the ribs and intercostal muscles,
and the floor by a muscular sheet, the diaphragm, which separates the thorax from
the abdomen. Each lung is surrounded by a very narrow pleural cavity lined by two
flexible, transparent and thin pleural membranes, which form an airtight barrier
around them. The inner visceral membrane is in contact with the lungs and the outer
parietal membrane lines the walls of the thorax and diaphragm. The pleural cavity
contains a thin layer of lubricating fluid – the pleural fluid – which reduces friction as
the membranes rub against each other during breathing movements.
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, The conducting system begins with the nasal passages and continues as the wind
pipe or trachea. Air enters the body through two external
nostrils, the internal surface of which possesses a border of
large hair that traps particles in the air and filter them out
of the system. The walls of the nasal passage are lined with
ciliated epithelium that also contains goblet cells secreting
mucus. The mucus traps any particles that have managed
to pass through the hair of the nostrils. It also moistens the
incoming air which also gets warmed by the superficial
blood vessels. In the roof of the posterior part of the nasal cavity is a mass of
olfactory epithelium consisting of neurosensory and supporting cells, richly supplied
with blood vessels. Here, odours in the air are detected. Air then traverses the
pharynx and past the epiglottis enters the larynx. Air from the larynx enters the
trachea that lies directly in front of the oesophagus. The trachea is wider than the
oesophagus and is strengthened and held open by horizontally arranged C-shaped
rings of cartilage. The open section of the C is applied against the oesophagus. The
cartilage prevents the trachea from collapsing when the intrapulmonary pressure
falls during inspiration. The trachea is lined internally with pseudostratified ciliated
columnar epithelium. Mucus secreted by goblet cells in the epithelium traps dust and
germs, and the rhythmic beating of the cilia wafts the unwanted material towards
the back of the buccal cavity, from where they can be spit out or swallowed.
The trachea divides to form the left and right primary or main bronchi. The right
primary bronchus divides into three secondary bronchi that enter the three lobes of
the right lung. The left primary bronchus divides into two secondary bronchi that
enter the two lobes of the left lung. The secondary bronchi further divide into tertiary
bronchi that supply segments of each lobe. The tertiary bronchi branch into
numerous bronchioles. The bronchial tree terminates in air passages called
respiratory bronchioles. These branch into many short tubes of equal diameter called
alveolar ducts, which end in tiny hollow bags called air sacs. In their walls, the air
sacs have many bubble-like pockets called alveoli. An alveolus is the functional unit
of the lung and forms the gas exchange surface of a human.
The rings of cartilage in the trachea are initially replaced in smaller tubes by
irregularly shaped plates of cartilage, but when the internal diameter of the
bronchioles is less than 1 mm, cartilaginous support ceases altogether.
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