The respiratory system is formed by a ducting portion (proximal) which is system of tubes and cavities that interconnect themselves and carry oxygen from the exterior to the lung; and a respiratory portion (distal) where gaseous exchange takes place.
CONDUCTION PORTION
Conduction portion functions are:
Conducting air
Heating, cooling and humidifying air
Catching and eliminating microorganisms and diverse suspended particles which entered the air.
The conductive portion embraces: nasal cavities, pharynges, larynges, trachea, bronchia, bronchiole and terminal bronchiole. We will analyze each structure separately.
Nasal Cavities
Its epistle it is characterized by possessing large rigid hairs. Its function is to retain dust particles which might be carried by the inspired air.
We find a series of characteristics which repeat themselves all along the respiratory epistle.
Sebaceous Glands contribute to retain particles through secretion. Mucus secreted by calceiform cells retains particles and also humidifies air. All these secretions are swollen.
Respiratory ducts as a whole are also covered by cilia epistles (200 cilia per cell which shake at a speed of 10 to 20 times per second). Their whip direction always faces the pharynges. This allows the mucus and sebaceous cover to flow towards the pharynges at a speed of 1 cm per minute. It will later be expelled by cough or swallowed.
Below the glands we find lot of vessels which will heat the air.
In the nose there are some bony formations called cornets. As the air flow becomes turbulent, they rise their temperature and favor the mucus task of catching a higher amount of particles.
This system impedes that particles with a size superior to 6 micrometers penetrate the respiratory system. Due to gravity, inferior ones (from one to five) usually deposit in the bronchioles. Those inferior to 0.5 usually make contact with alveolar air (trough diffusion) and remain in part suspended and others are partially expelled. Cigarette smoke particles, for instance, have a size of 0.3 um.
Pharynges
Digestive tube and respiratory ducts reach each other in that region. It can be divided into nasal-pharynges (connected with nasal ducts superior end), mouth- pharynges (same with mouth) and larynges-pharynges (which is linked to esophagus and trachea).
Laringe
It is located between the trachea and the pharynges. It possesses a cover of cartilage linked by the connective fiber elastic tissue: thyroid cartilage, cricoids, arytenoids and epiglottis.
We find here four muscular folds, two superior and two inferior. (True vocal strings). Any space between them is called Glottis.
Epiglottis
It is formed by elastic cartilages. It impedes food and liquid to enter the air duct and vice versa. During swallowing act larynges rises and this allows the epiglottis superior end to overlap the larynges superior tube, and this way the air duct which is located below remains sealed.
Trachea
It is a Tubular duct of a 10 cm length and a 2.5 cm, diameter. A cartilages shaped in C impedes its collapse. Its inferior end (carina) bifurcates into two primary bronchia which possess some 23 branches including the respiratory portion.
Bronchia
Main bronchia penetrate lung’s main hilia dividing themselves into lobar bronchia (3 to the right and two to the left) which in turn divide into segment and sub-segment ones. Lung arterial branches follow their path. As we get into the lung the cartilage rings disappear and they are finally replaced by straight muscle which finally constitutes bronchioles.
Bronchioles and terminal bronchioles
They constitute the last segment of the ducting portion. There are 65,000 of them.
RESPIRATORY PORTION
It is the portion where gaseous exchange takes place. It is formed by respiratory bronchioles, alveolar conducts, alveolar bags and alveoli.
Respiratory Bronchioles
Their walls are interrupted by alveoli. This is where gaseous exchange begins.
Conducts and Alveolar Bags
Respiratory bronchioles connect with alveolar conducts. After ramifying the alveolar conducts end in alveolar bags, and within this space 4 or 5 alveoli open up. There are 300 million alveoli within the lungs, and this is where gaseous exchange takes place.
Alveoli
Alveoli connect themselves through Kohn Pores. Each one possesses four or five. When the O2 molecule reaches the alveolus it goes trough some structures in order to reach blood. This is known as hemato alveolar barrier (or respiratory membrane) and its thickness reaches 0.5 micrometers.
Surfactant Layer
Alveolar Epithelia with its basal layer.
Interstice Space.
Capillary Endothelia with its basal layer.
The respiratory membrane total surface reaches 70 sq. meters. The blood amount we find in the lung capillary is of 60 to 140 ml. thus allowing gaseous exchange. In addition, the capillary diameter is of 5 um and erythrocytes have to fold in order to get in touch with the capillary wall.
IRRIGATION
Blood that reaches lungs comes from the lung artery and the bronchial arteries. Lung Artery blood is the one that will reach lung capillary in order to accomplish gaseous exchange with alveolar air. Path followed by this blood flow is the following: Lung artery-Lung-Gaseous exchange-Lung veins-Left auricle.
Bronchial arteries which have their origin in systemic circulation represent one or two per cent of the cardiac effort. Carried blood will irrigate the Pleura, lung walls and ganglia and end their way in the left auricle.
Bronchial arteries (direct branches of the aorta or intercostals arteries) irrigate pleura, walls and ganglia.
PRESSURES AND VOLUMES
Lung artery: 25 mmHg during diastole and 8 during systole. Average, then, is 15 mmHg.
Left auricle and Lung veins: Average 2 mmHg.
Lung blood volume: 450 ml or 9% of the body.
Lung capillary pressure is low: 7 mmHg and in the peripheral tissues raises to 17 mmHg. In order to produce an Edema this pressure must rise to 28 mmHg.
La presión capilar pulmonar es baja: 7 mmHg (en los tejidos periféricos es de unos 17 mmHg). Esta presión debe subir a 28 mmHg para producir EDEMA.
