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. 
 
INERVATION
Lung innervations are automatic, both Para sympathetic and sympathetic and produce bronco constriction and bronco dilation due to the straight muscle presence. Para sympathetic innervations mainly reach through the pneumo-gastric nerve.  However, the sympathetic fiber amount is low.  Fortunately, the bronchial arbola is very much exposed to noradrenalin and adrenalin which come from blood and produce bronchial dilation (simile sympathetic effect).  

PLEURA
It is serous membrane which covers the lungs. There are two leaves: parietal and visceral and both reflect each other in the lung hilio. A pleural space lies between them. It is considered a virtual space since it possesses very few liquid and some free cells.  It could transform into real during pathological conditions such as pneumo thorax, hemo thorax and hydro thorax. 
 
 
LUNG PHYSIOLOGY
The main function of the lungs is to provide tissues with O2 and eliminate CO2. Its physiology could be divided into four main functional facts.
1.      Lung ventilation: air flow, entrance and exit between atmosphere and alveoli.
2.      Gas diffusion between alveoli and blood.
3.      Gas transportation from and to the cells.
4.      Regulation.
 
 
 
VENTILATION
 
 
 Lungs might expand and contract and that is how they are filled and emptied of air. The organs which cause inspiration and expiration are:
The diaphragm muscle pulls the lung inferior surfaces in order to inspire. During expiration it simply relaxes, lungs elastically draw back. During energetic respiration the abdominal muscles also participate.
Ribs: its aperture expands the thoracic cavity. Anterior-posterior diameter rises a 20%.  Muscles which produce this expansion are intercostals, esternocleidomastoid, serratum, escalenum and abdominal rectum.
Lung is an elastic structure; if nothing keeps it inflated it expels the air.
 
  
Alveolar Pressure

Air pressure in the interior of the alveoli: In order to originate an inside air flow (inspiration), the alveolar pressure must be inferior as compared with atmospheric pressure., this is why it reaches to minus 1 cm of water. This is enough to displace 500 ml of air to the interior of the lungs during the two seconds that a normal inspiration lasts.  During expiration ( 2 to 3 seconds) pressure amounts to plus 1 cm of water.
Lung Volumes
Current volume: inspired and expired air during every normal respiration (500 ml).
Inspiratory reserve volume: maximum additional volume which might be inspired over the current volume (3,000 ml)
Reserve Expiratory volume: maximum additional air volume which might be expired through forced expiration after a normal current expiration (1,100 ml).
Residual Volume:  Air volume which remains in the lungs after a forced expiration (1,200 ml)
 
Lung Capacities

Inspiratory capacity: CV plus IRV. It is the total amount of air that a person might breathe starting at a normal expiratory level and expanding his lung to reach a maximum capacity (3,500 ml).
Functional residual Capacity: ERV plus RV. It is the air remaining inside the lungs after normal expiration, 2,300 ml.
Vital Capacity: IRV plus CV plus ERV. It is the maximum amount of air that can be expelled by a person after he accomplishes his maximum inspiration effort and after he makes a maximum expiration effort, 4,600 ml.
Total Lung capacity: Vital capacity plus RV which is the maximum volume that lungs can reach, 5,800 ml.
In women, these volumes are 20-25% inferior and athletic or tall persons might increase it.
 
Respiratory Volume Minute

It is the amount of new air that penetrates the respiratory conducts each minute. It is current volume x respiratory frequency. Normal respiratory frequency reaches 12 per minute.
RMV equals 6,000 ml per minute.
 
 
 
Alveolar Ventilation

During current respiration, the air volume only gathers to fill the respiratory conducts up to the terminal bronchioles and only a small portion reaches the alveoli. Thus, alveolar ventilation is mainly produced through diffusion.
The space in the respiratory conducts where there is no gaseous exchange is usually called dead space. A young man shows some 150 ml, but this figure rises with age.
Alveolar ventilation rate (usually called alveolar ventilation) is the total volume of new air that penetrates the alveoli and close areas in which gaseous exchange takes place.
That is, AV equals Respiratory frequency x (CV minus Dead Space V) which yields 4,200 ml per minute.
 
GAS DIFFUSION BETWEEN ALVEOLI AND BLOOD

The diffusion process is a molecule random movement which crisscrosses their paths in both directions through the respiratory membrane and adjacent liquids.  For diffusion to take place a source of energy must exist and this is furnished by the molecules’ kinetic movement.  However, a gas net diffusion towards a direction has its origin in what we know as concentration gradient. In other words: one side of the membrane possesses more molecules than the other side.
 
 

Net gas diffusion is equal to the molecules amount going towards anterogradic direction minus the ones going in the opposite direction.

Pressures

Pressure begins through the constant impact of molecules in movement against a surface. It is directly proportional to molecules’ concentration within a gas.  In respiratory physiology we work with gas mixture (O2, N2, and CO2). Diffusion rate of each one is proportional to that determinate gas originated pressure which is denominated as Gas Partial Pressure.
Air is made as follows: Nitrogen (79%) and Oxygen (21%). At sea level, this mixture’s total pressure is of 780 mmHg. Thus N2 partial pressure is of 600 and oxygen’s 160 mmHg.
Gases dissolved in water or corporal tissues also exercise pressure.
In solution, a gas pressure is not only determined by its concentration but also by the gas solubility coefficient.
Pressure equals Concentration over Solubility Coefficient.
 
 
 Solubility coefficients

   
Oxygen  
0.024
 
Carbon Dioxide  
0.57
 
Carbon Monoxide
0.018

Nitrogen
0.012
 
Helium
0.008

   
Net Diffusion is given by the difference of partial pressures.
Once air enters respiratory ducts, water evaporates and they are humidified. These water molecules exercise a Water steam pressure which at 37 º C is of 47 mmHg and this will represent water’s partial pressure within this gaseous mixture.
 

 
Alveolar and Atmospheric Air
Alveolar air is only partially substituted by atmospheric air with each breathe. As a matter of fact, the substituted volume represents only one seventh of the total.  Therefore, during normal respiration, gas eliminates itself every 34 seconds. This alveolar air slow renovation is the key to avoid sudden variations of blood gas concentrations.
During normal alveolar ventilation, alveolar PCO 2 is of 40 mmHg.
Then, the factors which determine a gas pass speed through a membrane are: Membrane thickness, Membrane area, Gas diffusion coefficient in membrane’s substance and both sides’ pressure difference.
Diffusion Capacity
Gas volume which diffuses through a membrane per minute for a pressure difference of 1 mmHg.  In a medium aged man, diffusion capacity of resting O2 is of 21 ml/min/mmHg. Medium difference pressure through the respiratory membrane is of 11 mmHg.  Thus, every minute, some 230 ml of O2 diffuse through the respiratory membrane. This is our body’s oxygen consumption. During some vigorous exercise, this might reach 65 ml/min/mmHg. This is due to:
a) Opening of the previously inactive capillaries and additional dilation of those which were already open.
b) Better adjustment of the ventilation/perfusion relationship.
 In the case of smoking persons, this ventilation-perfusion relationship will be very much deteriorated.
 
GAS TRANSPORTATION FROM AND TO THE CELLS

Pressures
Both O2 and CO2 diffuse themselves from alveoli to blood and from blood to tissues due to Partial Pressure Gradients.
O2 P in alveolus: 104 mmHg
O2 P in venous blood: 40 mmHg
O2 P in cells: 23 mmHg
During exercise any organism might require twenty times the usual oxygen. Besides, due to cardiac requests, blood permanency in capillary could reduce by half.  However, there is a security factor: O2 diffusion capacity raises three times during exercise since the area (capillary surface) also rises and the ventilation-perfusion relationship is closest to the ideal.
CO2 might diffuse twenty times faster than O2. This is why pressure differences needed in order to diffuse gas are lower than oxygen’s.
Intracellular PCO2: 46 mmHg
Insterticial PCO2: 45 mmHg
Arterial PCO2 arterial which penetrates tissues: 40 mmHg
Venous PCO2 which abandons tissues: 45mmHg
Lung capillary PCO2: 45 mmHg
Alveolar PCO2: 40 mmHg
Transportation
97% of oxygen is carried by erythrocytes combined with hemoglobin. The rest (3%) travels dissolved in plasma and cell water. O2 joins hemoglobin in a reversible and lax manner.
O2 Saturation of systemic arterial blood: 97%.
O2 saturation of venous blood: 75%.
Every 100 ml of blood some 5 ml of O2 are carried to the tissues. During intense exercise this figure might raise up to 15 ml. If we add the rise of the cardiac effort, we obtain a 20 times rise of the O2 transportation towards the tissues.
Carbon Monoxide combines with hemoglobin at the very same point O2 does and its joining strength is 250 times superior. A 0.1% CO concentration in air, thus, evenly competes with oxygen.
During normal conditions 4 ml of CO2 every 100 blood ml are transported from tissues to the lungs. A seven per cent travels dissolved. A 70% combines with H2O and dissociates with H+ (hydrogen ions) and HCO 3 (bicarbonate ion) and so it travels. Twenty per cent travels as carboxyl- hemoglobin.
 
 
REGULATION
The nervous system adjusts the alveolar ventilation system rate almost exactly to the organism demands. The respiratory system is composed by several groups of neurons bilaterally located within the rachides bulb and within the protuberance.
The vague and the gloso-pharyngeal nerve transmit to the respiratory center sensitive signals of: Peripheral chemic-receptors (in carotid and aortic bodies), baro receptors, and distension and contraction receptors.
When O2 concentration in the alveoli lowers below normal (especially below 70%), adjacent blood vessels slowly constrict raising by five the vessel resistance.  The opposed effect occurs during systemic circulation where vessels dilate if oxygen is low. This means that the blood flow redistributes itself to places where its efficiency is maximized.
During exercise lung’s blood flow rises 4 to seven times mainly due to three reasons.

A)    The amount of open capillary rises.
B)     Capillary distend and flow rate through them also rises.
C)    Lung arterial pressure rises.