Structure of the respiratory system
air is usually drawn into your body via the nose, but sometimes via the mouth, and passes through a series of airways to reach the lungs. This series if airways us referred to as the respiratory tract, and can be divided into two main parts. The upper respiratory tract includes the nose, nasal cavity, mouth, pharynx and larynx; and the lower respiratory tract consists of the trachea, bronchi and lungs.
Nasal cavity
your nose is divided into the external nose and the internal nasal cavity. when you breathe in, air enters the cavity by passing through the nostrils. hairs within the cavity filter out dust, pollen and other foreign particles before the air passes into the two passages of the nasal cavity. here the air is warmed and moistened before it passes into the nasopharynx. A sticky mucous layer traps smaller foreign particles, which tiny hairs called cilia transport to the pharynx to be swallowed.
Epiglottis
a small flap of cartilage at the back of the tongue, the epiglottis closes the top of the trachea when you swallow to ensure food and drink pass into your stomach and not your lungs.
Pharynx
the funnel-shaped pharynx connects the nasal cavity and mouth to the larynx (air) and oesophagus (food). commonly called the throat, the pharynx is a small tube that measures approximately 10-13 cm from the base of the skull to the level of the sixth cervical vertebra. the muscular pharynx wall is composed of a skeletal muscle throughtout its length. it is a passageway for food as well as air, so special adaptations are required to prevent choking when food or liquid is swallowed.
Larynx
The larynx or voice box has rigid walls of muscle and cartiliage, contains the vocal cords and connects the pharynx to the trachea. it extends for about 5 cm from the level of the third to sixth vertrebra.
Trachea
the trachea or windpipe denotes the start of the lower respiratory tract. it is about 12cm long by 2cm in diameter. it contains rings of cartilage to prevent it from collapsing and is very flexible. it travels down the neck in front of the oesophagus and branches into the right and left bronchi.
Bronchus
the right and left bronchi are formed by the division of the trachea, they carry air to the lungs. the right bronchus is shorter and wider than the left is a more common site for foreign objects to become lodged. By the time inhaled air reaches the bronchi, it is warm, clear of most impurities and saturated with water vapour. once inside the lungs, each bronchus subdivides into lobar bronchi: three on the right and two on the left. the lobar bronchi: three on the right and two on the left. the lobar bronchi branch into segmental bronchi, which divide again into smaller and smaller bronchi. overall, there are approximately 23 orders of branching bronchial airways in the lungs. because of this branching pattern, the bronchial network within the lungs is often called the bronchial tree.
Bronchioles
Bronchioles are small airways that extend from the bronchi. they are about 1 mm in diameter and are the first airway branches of the respiratory system that do not contain cartilage. bronchioles end in clusters of thin-walled air sacs, known as alveoli.
Lungs
the paired right and left lungs occupy most of the thoracic cavity and extend down to the diaphragm. they hang suspended in the right and left pleural cavities straddling the heart. the left lung is smaller than the right.
Lobes
each lung is divided into lobes; the right lung has three lobes and the left has two.
Pleural membrane and cavity
the lungs are surrounded by membranes known as pleura. these contain a cavity with fluid that lubricates the pleural surfaces as the lungs expand and contract, preventing friction and keeping them airtight.
Thoracic activity
this is the chamber of the chest that is protected by the thoracic wall. it is separated from the abdominal cavity by the diaphragm.
Visceral pleura
the visceral pleura is the innermost of the two pleural membranes. it covers the surface of the lung and dips into the spaces between the lobes.
Pleural fluid
the pleural membranes produce pleural fluid which fills the space between them. this lubricating fluid allows the lungs to glide easily over the thoracic wall during respiration. although the membranes slide easily over each other, their separation is resisted by the surface tension of the pleural fluid that keeps the lung surface in contact with the chest wall.
Alveoli
the bronchioles end in air sacs called alveoli. the 300 million gas-filled alveoli in each lung amount for most of the lung volume and provide an enormous area for gaseous exchange - roughly the size of a tennis court. a dense network of capillaries surrounds the alveoli to facilitate this process. together, the alveolar and capillary walls form the respiratory membrane that has gas on one side and blood flowing past on the other. gasesous exchange occurs readily by simple diffusion across the respiratory membrane. oxygen passes from the alveoli into the blood and carbon dioxide leaves the blood to enter the alveoli.
Diaphragm
the diaphragm separates the chest from the abdomen, it is the most important muscle involved in breathing. contraction of the diaphragm increases the volume of the chest cavity, drawing air into the lungs during respiration, while relaxation involves recoil of the diaphragm and decreases the volume of the chest cavity, pumping out air.
Internal and external intercostal muscles
- the intercostal muscles lie between the ribs. to help with inhalation and exhalation, they extend and contract.
- the internal intercostal musles lie inside the ribcage, they draw the ribs downwards and inwards, decreasing the volume of the chest cavity and forcing air out of the lungs during expiration.
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