The process of gaseous exchangement in the body called respiration. In other words, respiration is the process of obtaining energy ATP with the intake of oxygen and release of carbon dioxide which is produced from the oxidation of organic substances.
Respiration phase: -
Respiration occur in two phases namely inhalation / inspiration and exhalation / expiration.
During inhalation, thoracic cage and lungs expand so that air enters the lungs easily from the atmosphere.
During exhalation, the thoracic cage and lungs decrease in size and attain the prior position so that air is released outside easily
During normal active breathing, inhalation is an active process and exhalation is a passive process.
Types of respiration:-four major types of respirations are as follow: 
1)     pulmonary ventilation
2)     external respiration 
3)     internal respiration
4)     cellular respiration

Pulmonary ventilation: - The inflow and outflow of air between the atmosphere and the alveoli of lungs
External respiration: - Exchange of the gases between the alveoli of the lungs and blood in pulmonary capillaries across the respiratory membrane.
Internal respiration: - Exchange of the gases between the blood and tissue cells by the step in which blood loss Oxygen and give carbon dioxide.
Cellular respiration: - This process involves metabolic action that consumes Oxygen and gives carbon dioxide during the production of ATP.
Anatomy of the respiratory system: -
It includes several organs that facilitate the inhalation and exhalation of gases in Living Organism. The system is composed of following biological structures:- nose and nasal cavity, mouth, pharynx, larynx (voice box), trachea (windpipe), bronchi and bronchioles lungs and the muscles of respiration

1)     External Nose
This is the entryway of the respiratory tract. It opens into the nasal cavity and provides support and protection to the nasal cavity.
External nose consists of supporting framework of bone and Hyaline cartilage.
In External Nose :-
Bone - parietal bone, nasal bones and maxillae
Cartilage-septal cartilage, lateral nasal cartilage, alar cartilage.
External nose posses two opening called nostrils. The internal nose is a large cavity called nasal - cavity. 
Nasal cavity and mouth:- 
Mouth supplements inhaling along with the nose. The mouth does not possess hairs and mucous membrane but it has wider diameter space for passing air that enter the body at the same speed.
pharynx:- It is an intermediate region between the nasal cavity, larynx and oesophagus. It is defined into three separate sections, i.e. nasopharynx, oropharynx and laryngopharynx.  laryngopharynx is also known as epiglottis. epiglottis play a role to divert the food to the oesophagus.
Larynx:- It is the intermediate organ between laryngopharynx and trachea. It is termed as voice box and locates in the anterior part of the neck just below the hyoid bone. The epiglottis is part of the larynx. Larynx includes the thyroid cartilage and vocal cords. Cartilage supports and protects other components of the larynx. The thyroid cartilage is commonly known as Adam's  Apple.
Trachea: - It is a large part of the respiratory tract. Its shape is like a long tube including several c-shaped hyaline cartilages links which are lined with pseudostratified ciliated epithelium. C shaped rings keep the trachea open during breathing time. Trachea has an important role in filtering the air.
Bronchi: - At the end of the trachea, two branches like structure arises which are termed as bronchi. Bronchi connect at the edge of the lungs then further divided into tertiary bronchi. The wall of bronchi is lined with muscle tissue. Muscles and Cilia filter environmental contaminants.
Alveoli: - Bronchioles open into the ducts which leads into an expanded passage, i.e. the atria which open into alveolar sacs or air sacs. The membrane of alveoli is very thin, irregular and richly supplied with blood vessels so that,  exchange of gases takes place easily.
Lungs: - These are the main part of the respiratory system. These are paired organs named left lung and right lung. The left lung has two lobes and the right lung has three lobes. Each lobe has small vacuoles, uncountable sacs, which terms as alveoli. alveoli are surrounded by an epithelium layer. This layer allows the exchange of gases from the blood through the capillaries.
Muscles of respiration:- 
There are many muscles which surround the lungs and help to exhale and inhale the air. The main muscles of the lungs are known as the diaphragm. It pulls in the air into the lungs by contraction with each breath. Many intercostal muscles are located between the ribs and help in the contraction and relaxation of lungs. Hence, there are two zones in the respiratory system. One is a contracting zone which includes air passing area like a nostril, Nosal chamber, pharynx, trachea and bronchial tree 2nd zone is called exchange zone, where the gaseous exchangement occurs. It includes bronchioles, alveolar duct, alveoli.  

Chart:- Respiratory tract

Chart:- Bronchial tree

Mechanism of pulmonary ventilation or breathing:-
In pulmonary ventilation on breathing air flows between the atmosphere and alveoli of the lungs because of alternating pressure difference created by contraction-relaxation of respiratory muscles.
It is including 2 stages 
1     Inhalation / Inspiration
2     Exhalation / Expiration
1)     Inhalation /inspiration
Inhalation follows Boyle's law According to this law, the volume is inverse by proportional to pressure. 

The difference in pressure caused by the change in lungs volume forces the air into the lungs during inhalation and during out, of the lungs exhalation.
For inhalation, the lungs must expand to increase its volume and thus decrease the pressure in the lungs below the atmospheric pressure.
The first step in Expanding
The lungs during normal inhalation involve contraction of the main muscles of inhalation. i.e. The diaphragm and intercoastal.
Role of the diaphragm in inhalation
The diaphragm is a dome-shaped skeletal muscle that forms a floor thoracic cavity. The contraftion of muscle fibre of diaphragm causes it to become flat and lower down, thereby increasing the volume of the thoracic cavity in the anterior-posterior axis.

During normal quiet inhalation, the diaphragm depends about 1 cm producing a pressure difference of 1-3 mm Hg and inhalation of about 500 ml of air.
Role of intercostal muscles 
These muscles are present between the ribs. The contraction of these muscles lifts ribs sternum up and outward causing an increase in the volume of the thoracic cavity in the dorsoventral axis. 

Contraction of the diaphragm is responsible for about 75% and the concentration of intercostal muscles about 25% of the air that enter the lungs during normal quiet breathing. During quiet inhalation, the pressure betulus two pleural layers in the pleural cavity is called inter plural pressure. i.e. always lower than atmospheric pressure. Just before inhalation, it is about 4 mm Hg less than the atmospheric pressure. i.e. about 756 mmHg at an atmospheric pressure of 760 mmHg. As the diaphragm and external intercostal contract and the overall size of the thoracic cavity increase the volume of the pleural cavity also increases which cause intra pleural pressure to decrease to about 754 mmHg.

The accessory muscles of inhalation include the sternocleidomastoid muscle. Which elevate the sternum, the scalene muscle. which elevate the first two ribs, and the pectoralis minor muscle which elevates the 3rd, 4th and 5th ribs. 
Exhalation/expiration: It is the expelling of air out of lungs if the pressure of lungs is more than the atmospheric pressure.
Relaxation of the diaphragm and external intercostal muscle cause returning of diaphragm and sternum to the relaxing phase possessing normal thoracic volume and thereby pulmonary volume.
Diaphragm the relaxation of muscle fibre of diaphragm cause it to come in the normal position which reduces the volume of the thoracic cavity

Forceful expiration: We have the ability to increase the strength of expiration. It is due to the contraction of internal intercostal and abdominal muscles.
1.    Internal Intercostal muscles 
Contraction of these muscles leads to the pulling of ribs downward and inward decreasing in the volume of the thoracic cavity.

Abdomen muscles
Abdominal muscles contract thereby compress of the abdomen and pushes its contents towards the diaphragm.

Factors affecting pulmonary ventilation.
Three-factor effecting the rate of airflow and the ease of Pulmonary ventilation.
1)     surface tension of alveolar fluid
2)     competence of the layer
3)     airway resistance
A thin layer of alveolar fluid coat the luminal surface of alveoli and extras force known as surface tension. In the lungs, surface tension causes the alveoli to assume the smallest possible diameter. During breathing, surface tension must be overcome to expand the lungs on every inhalation. Surface tension also accounts for two-thirds of lung elastic recoil which decreases the size of alveoli during exhalation in which surface tension of alveolar fluid greatly increased so that many alveoli collapse at the end of each exhalation.
Compliance: It referred to how much effort is required to stretch the lungs and chest wall. High compliances mean that lungs and chest wall expand easily whereas low complainer means that they resist expansion. In the lungs, compliance is related to 2 principal factors. i.e. elasticity and surface tension
The lungs normally have high compliance and expand easily because, in the lungs, the tissue is easily stretchable and in surfactant alveolar fluid reduce surface tension.
Decreased compliance is a common feature in pulmonary conditions like tuberculosis pulmonary oedema, produce a deficiency in surfactant and impede lungs expansion in any way. 
Airway resistance is the resistance in the flow of air through respiratory trait. The rate of airflow through the airways depends on both pressure difference and resistance.
Airflow equals the pressure difference between the alveoli and Atmosphere divided by resistance.
The wall of airways especially branchiolse, offer some resistance to the normal flow of air into and out of the lungs. As the lungs expand during inhalation, the bronchioles enlarge because their walls are pulled outward in all directions. Large diameter airways decreased resistance.
Airway resistance increase during exhalation as the diameter of bronchioles decreases. Airway diameter is also regular by walls of the airway. Signals from sympathetic division of autonomic nervous system cause. Relaxation of these smooth muscles which results in bronchodilation and decreased resistance.
Resistance volume and capacities
Spirometer measure the volume of air exchanged during breathing and the respiratory rate.
Anatomic dead space 
Collectively, the conducting airway that does not undergo respiratory exchange is known as the anatomic dead Space. In a typical adult, about 70% of tidal volume (nearby 350 ml) actually reaches the respiratory zone of the respiratory system. ie.e respiratory bronchioles, alveolar duct, alveolar cars sacs and alveoli participate in external respiration. The Other 30% (nearby 150 ml) remain in the conducting Airway of nose, pharynx, larynx, trachea.
Tidal volume (TV) the volume of one breath is called the tidal volume. The TV is about 500 ml. A healthy man can inspire or expire approximately 6000 to 8000 ml of air per minute. 
Inspiratory reserve volume (IRV): The volume of air that we can inhale more than 500 ml Forcefully during inspiration. It is about 3100 ml in an average adult male.
Expiratory reserve volume (ERV): Additional volume of air, a person can be expired by forceful expiration. It is 1200 ml in male and 700 ml in the female.
Residual volume (RV): The volume of air remaining in lungs even after a forceful expiration residual volume is average 1200 ml in male and 1100 ml in the female.
Capacities :
*    Inspiratory capacity (IC) ⇒ (TV + IRV), 500 ml + 3100 = 3600 ml
*    Expiratory capacity (EC) ⇒ (TV + ERV), 500 + 1200 = 1700 ml
*    Functional residual capacity (FRC) ⇒ Volume of air that will remain in the lungs after a normal expiration.
(FRC = IRV + RV), (1200 ml + 1200 ml = 2400 ml) in male (1100 ml + 700 ml = 18 ml) in Female
*    Vital Capacity (VC) = TV + IRV + ERV 
        (4800 ml in male and 3100 ml in female)
*    Total lungs capacity (TLC) = (VC + FRC) (4800 ml + 1200 ml = 6000 ml) in male and (3100 ml + 1100 ml = 4200 ml in Female)

Exchange of Oxygen and Carbon dioxide
The exchange of Oxygen and Carbon dioxide between alveolar air and pulmonary blood via passive diffusion. Gases are exchanged by simple diffusion. It is a passive activity. This diffusion pressure is also called partial pressure. Gases move from high partial pressure to low partial pressure. Two laws explain the exchange of gases. i.e. dalton's law of partial pressure and Henry's law.
1.    Dalton's law of partial pressure
The pressure of a specific gas in a mixture of gases is called its. 
Partial pressure

Atmospheric air is a mixture of gases – N2, O2, H2O (water vapour) CO2 and other gases included in air gases.
Atm. pressure (760 mmHg) = PN2 + PO2 + PH20 + PCO2 + Pothergases
P = Partial pressure.
The composition of alveolar air  & its relation to atmospheric air ⇒
Alveolar air does not have the same concentration of gases as atmospheric air. There are several reasons for the differences:-
1.    The alveolar air is only partially replaced by atmospheric air with each breath.
2.    O2 is constantly being absorbed into the pulmonary blood from the alveolar air.
3.    CO2 is constantly diffusing from the pulmonary blood into the alveoli.
4.    Dry atmospheric air that enters the respiratory passage is humidified even before it reaches the alveoli.
Partial pressures of respiratory gases as they enter and leave the lungs.

Henry's law 
This law explains how the solubility of a gas is related to its diffusion. Henry's law states that the quantity of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas and its solubility.
In comparison to O2, much more CO2 is dissolved in blood plasma because the solubility of CO2 is 24 times greater than that of O2.

Gas dissolution a Partial pressure
Gas dissolution a Solubility
So: Partial pressure a Solubility

External and internal respiration :
1.    External respiration :
External respiration is also called pulmonary gas exchange. In this, the diffusion of oxygen from the air to alveoli of lungs to blood in capillaries and the diffusion of carbon dioxide to the opposite direction. External Respiration in the lungs converts deoxygenated blood coming from the right side of the heart into Oxygenated blood (saturated with oxygen) that returns to the left side of the heart.

The partial pressure of oxygen in pulmonary capillaries is 40 mm Hg and in alveoli, it is 100 mmHg. The pressure gradient is 64 mmHg it facilitates the diffusion of oxygen from alveoli into the blood.
In atmospheric air potential pressure of carbon dioxide is insufficient and is only about 46 mmHg where is in alveoli it is 40 mmHg so carbon dioxide enters suggest to blood into alveoli easily.
Internal respiration
Internal respiration or systematic gas exchange. The exchange of carbon dioxide and oxygen between systemic capillaries and tissue cells is called internal respiration or systemic gas exchange. As this level the, blood reverse oxygenated types of blood converted into deoxygenated blood

The partial pressure of oxygen of blood is higher (100 mmHg) in systematic capillaries than the partial pressure of oxygen in tissue cell (about 40 mmHg) because the cells constant use oxygen to produce ATP Due to this pressure difference, oxygen diffuses out of the capillaries in cells and blood. PO2 drop to 40mmHg when blood exits systemic capillaries.
While oxygen diffuse from systemic capillaries into tissue carbon dioxide diffuses in the opposite direction because tissues cell are constantly producing carbon dioxide. The partial pressure of carbon dioxide of the cell (46 mmHg) is higher than the systemic capillary blood (40 mmHg). As a result, carbon dioxide diffuses from tissue cells through interstitial fluid into systemic capillaries until the pressure of carbon dioxide in blood increase to 46 mmHg.
The deoxygenated blood then returns to the heart and pump to the lungs for another cycle of external respiration.

Factor affecting  of external or internal respiration (Factor affecting the diffusing capacity of CO2 and O2)
Pressure gradient :
Diffusing capacity is directly proportional to the pressure gradient. The pressure gradient is the difference between the partial pressure of the gas in alveoli and pulmonary capillary blood.
The solubility of a gas in a fluid medium :
Diffusing capacity is directly proportional to the solubility of the gas. If the solubility of gas is more in fluid medium then a large number of the molecule would be dissolved in it and diffuse easily.
The total surface area of respiratory membrane :
Diffusing capacity is directly proportional to the surface area of the respiratory membrane. A surface area of the respiratory membrane in each lung is about 70 square meter. Diffusion capacity is decreased in emphysema in which many of the alveoli collapses are because of heavy smoking or oxidant gases
The molecular weight of gases :
The molecular weight was inversely proportional to. Diffusing capacity.
The thickness of respiratory membrane :
Diffusion is inversely proportional to the thickness of the respiratory membrane. More the thickness of respiration membrane, less is the diffusion. It is because the distance through which the diffusion takes place is long.
In a condition like Fibrosis and oedema, the diffusion rate is reduced because the thickness of the respiratory membrane is increased.

Transport of Oxygen and Carbon dioxide in blood and tissue fluids.
Blood serves to transport the respiratory gases. Oxygen which is essential for the cells is transported from alveoli of lungs to the cells. Carbon dioxide which is the waste product is transported from cells to lungs.
Transport of Oxygen and Carbon dioxide is completed into 2 step 
1)     transport of oxygen from the lungs to the body tissue
2)     transport of carbon dioxide from body tissue to lungs
Both processes are completed in the following step

Transport of oxygen: -
Oxygen does not dissolve easily in water. So only about 1.5 % of inhaled oxygen is dissolved in blood plasma, which is mostly water (98.5% of blood). Oxygen is bounded by haemoglobin in red blood cells.
Oxygen is transported from alveoli to tissue by blood in two forms 
1)     As a simple physical solution
2)     In combination with haemoglobin
1)     Simple physical solution: It forms only about 1.5 % of total oxygen transport via blood plasma. Transport of oxygen in this form becomes important during the condition like muscular exercise to meet excess demand for oxygen by the tissue
2)     In combination with haemoglobin: Oxygen combined with haemoglobin in blood and is transported as oxyhaemoglobin (HbO2

95% of the oxygen that is bonded to haemoglobin is trapped inside red blood cells. So only dissolved oxygen i.e. 1.5%, can diffuse out of tissue capillaries into tissue cells.
The most important factor that determines how much oxygen binds to haemoglobin is the partial pressure of oxygen. Higher the partial pressure of oxygen, more oxygen combines with haemoglobin. When reduced haemoglobin is completely converted in oxyhemoglobin, it is said to be fully saturated.
When haemoglobin consists of a mixture of haemoglobin and oxyhemoglobin, it is partially saturated.
Each haemoglobin molecule bind with is molecules of oxygen. If each haemoglobin molecule has bound to two oxygen molecule then the haemoglobin is 50% saturated
Oxygen-haemoglobin dissociation curve : - 
The relation between the percent saturation of haemoglobin and partial pressure of oxygen illustrated in the oxygen-haemoglobin dissociation curve. It explains haemoglobin's affinity for oxygen. Relation carrying capacity between partial pressure and haemoglobin.
Haemoglobin saturation (potential pressure directly proportional to haemoglobin solution)

Partial Pressure \alpha Hb saturation    
PO2 is high  →  Hb 100% saturated
PO2 is low →  Hb is partially saturated

The relation between the percent saturation of haemoglobin is illustrated in the oxygen-haemoglobin dissociation curve. A curve peloton percent oxygen saturation of haemoglobin versus the partial pressure of oxygen for haemoglobin is as shaped or sigmoidal curve.

Curve – I    

Curve – II

Curve – III

Factor affecting the oxygen-haemoglobin dissociation curve: -
Oxygen-haemoglobin dissociation curve is shifted to left or right by various factors.
1)     Shift to left indicates a competence means Association of oxygen by hemoglobin.
2)     Shift to right indicates dissociation of oxygen from haemoglobin.
1)     Shift to left : - oxygen hemoglobin dissociation curve is shifted to left in the following conditions :-
A)     In fetal blood, because fetal hemoglobin has got more affinity for oxygen than the adult hemoglobin.
B)     Decrease in hydrogen ion concentration and an increase in pH (alkalinity).
2)     Shift to right: - Oxygen hemoglobin dissociation curve is shifted to the right side in the following conditions :-
A)     Acidity: - Increase in Hydrogen ion concentration and a decrease in pH. In other words, as acidity increases (pH decreases) affinity of hemoglobin for oxygen decrease and oxygen dissociates more readily from hemoglobin. The effect of pH on Oxygen affinity of hemoglobin is known as the “bohr effect”. 
B)     Partial pressure of carbon dioxide: Carbon dioxide also can bind to hemoglobin and effect is similar to that of hydrogen ion. As the partial pressure of carbon dioxide rises hemoglobin to release oxygen more easily. Partial Pressure of carbon dioxide and pH are related factors because low blood PH (acidity) results from the high partial pressure of carbon dioxide. As carbon dioxide enters the blood, much of it is temporarily converted to carbonic acid. 

3)     Increase the body temperature because heat is a product of the metabolic reaction of all cells and the heat released by contracting muscle fibrestends to raise body temperature and shift the curve to the right side.
4)     Excess of 2,3 diphosphoglycerate (DPG) in RBC. it is also called 2,3- biphosphoglycerate (BPG). DPG is a by product in embde.- meyer- hof pathway of carbohydrates metabolism. It combines with beta- chain of hemoglobin. In a condition like muscles exercise and high attitude, the DPG increase in RBC. So, the oxygen hemoglobin dissociation curve shift right to a great extent.

Transport of carbon dioxide
Carbon dioxide is transported by the blood from cells to the alveoli.
Carbon dioxide is transported in the blood in three ways
1)     as dissolved form (7%)   
2)     as Bicarbonate (63%) 
3)     as carbamino compounds (30%)
1)     as dissolved form: - carbon dioxide diffused into the blood and in the fluid of Plasma forming a simple solution. About 7% is dissolved in blood plasma
2)     as carbamino compounds :- About 30% of carbon dioxide is transported as amino compounds. Carbon dioxide is transported in the blood in combination with haemoglobin and plasma proteins. Carbon dioxide combined with haemoglobin to form carbaminohemoglobin or carbhaemoglobin.

The main carbon dioxide binding site are the terminal amino acid in the 2 Alpha 2 beta globin chains.
Bicarbonate ions: -
About 63% of carbon dioxide is transported in blood plasma as bicarbonate Ion. Carbon dioxide diffuse into systemic capillaries and enters RBC. It react with water in the presence of enzyme carbonic anhydrase (CA) to form carbonic acid which dissociate into H+ and HCO3.
Chloride shift:-
Chloride shift hamburger phenomenon is the exchange of a chloride ion and for a bicarbonate ion across the RBC membrane.
Chloride shift occur when carbon dioxide enters the blood from the tissue. In plasma, plenty of sodium chloride is present. It dessociate into Sodium and chloride ions when the negative charged bicarbonate ions moves out of RBC into the plasma, the negative charged chloride Ion moves into the RBC in order to maintain the electrlyte equilibrium( ionic balance) .
Bicarbonate ions combine with sodium ions in the plasma and form Sodium Bicarbonate.
In this form, it is transported in the blood. Hydrogen ion dissociated from carbonic acid are buffered by hemoglobin inside the cell. 
Reverse chloride shift :
This is the process by which chloride Ion moves back into the plasma from RBC shift. It is occur in the lungs. It help in the elimination of carbon dioxide from the blood. Bicarbonate is converted back into carbon dioxide which is be expelled out. It take place by the following mechanism :-
When blood reach the alveoli sodium bicarbonate in plasma dissociate the sodium ion and HCO3 . It moves into RBC and make chloride ions to move out of the RBC into plasma where it combine with sodium ion and form sodium chloride. HCO3  ion inside the RBC combines with a hydrogen ion and form carbonic Acid which dissociate into water and carbon dioxide. Carbon dioxide is then expelled out.
Carbon dioxide dissociation curve: - 
Carbon dioxide dissociation curve show the relationship between the partial pressure of carbon dioxide and the quantity of carbon dioxide that combine with blood
Control of respiration: -
Respiration is the reflex process. But it can be controlled  voluntarily for a short period of 40 seconds. Breathing occurs in the cycle pattern to sustain life processes. Normally, quiet regular breathing occurs because of two regularrory mechanism
1)     Neural mechanism
2)     Chemical mechanism
1)     Neural mechanism
Neural mechanism that regulate the respiration includes
A)     respiratory centres
B)     afferent nerve
C)     efferent nerve
A)     respiratory centre
Respiratory centre are a group of neurons which control the rate, Rhythm and force of respiration. These centres are by bilaterally situated in the reticular formation of the brainstem. 
The respiratory centres are classified into two groups.
i)     Molecular centre consist of 
a)     Dorsal respiratory group of neurons
b)     Ventral respiratory group of neurons
c)     apneustic centre
d)     pneumotaxic centre
Dorsal respiratory group of neurons :  It consist of mostly respiratory neurons. These neurons are diffusely situated in the nucleus of tractus solitarius which is present in the upper part of the medulla oblongata.

Ventral respiratory group : It is composed of both inspiratory neurons and expiratory neurons. Both of these neurons remain inactive during quiet breathing and become active during forced breathing. Neurons stimulated both inspiratory muscles and expiratory muscles. Ventral respiratory group is present in the nucleus ambiguous and nucleus retroambiguous. It these two nuclei are situated in the medulla oblongata. 
Apneustic centre : It is situated in the reticular formation of lower pons. Stimulation of apneustic centre cause apneusis . apneusis is an abnormal pattern of respiration characterized poolonged inspiration followed by short efficient expiration.
Pneumotaxic centres : It is situated in the dosso-lateral part of the reticular formation in upper pons. It is formed by neurons of medial parabranchial and subparabranchial nuclei .
When the pneumotaxic is more active, the breathing rate is more rapid .
Chemoreceptor:- The respiratory centre knows how to control the rate and depth of breathing. But its activity is influenced by input from chemoreceptors which is sensitive to change in pH, partial pressure of carbon dioxide and pressure of oxygen.
Chemoreceptors are classified into groups.
1)     central chemoreceptors
2)     peripheral chemoreceptors
1)     central chemoreceptors
Central chemoreceptors are situated in the deeper part of medulla abrogation, close to the dorsal respiratory group of neurons. This area is known as chemosensitive area neurons are called chemoreceptors. 
Central chemoreceptors are connected with respiratory centres, particularly the dorsal respiratory group of neurons through synapses. Main stimulant for Central chemoreceptors is the increased hydrogen ion concentration. However, if hydrogen Ion concentration increase in the blood, it cannot stimulate the central chemoreceptors because the hydrogen ions from blood can not cross the blood brain barrier and blood cerebrospinal fluid barrier.
On the other hand, if carbon dioxide increase in the blood, It can easily cross the blood brain barrier and blood cerebrospinal fluid barrier and enter in interstitial fluid of the brain or the cerebrospinal fluid.

CO2  +  H2O   →   H2CO3  →  H+  +  HCO3

Carbon dioxide and water combine with water to form carbonic acid and immediately dissociate into hydrogen ion and bicarbonate ion. Hydrogen ion stimulate central cameo Transporters from receptors the excitatory impulse is the dorsal respiratory group of neurones, resulting in increased ventilation (increase rate and a force of breathing), because of this excess carbon dioxide is coushed out and respiration is brought back in normal. Lack of oxygen does not have signification effect on Central chemoreceptors expect it generally decrease the overall function of the brain.
Peripheral chemoreceptors:-
This is present in carotid and erotic region hypoxia is the most potent stimulant for peripheral km receptor presence of oxygen sensitive potassium ion channels in the glomus cell of peripheral chemoreceptors.
Hypoxia causes closure of oxygen sensitive potassium ion channels and prevent potassium efflux. This lead to depolarization of glomus cells and generation of action potential in a nerve ending. These impulses pass through aortic and hering's nerve and excite a dorsal group of neurons. Dorsal group of neurons in tush send the excitatory impulse to respiratory muscles resulting in increase ventilation. This provides enough Oxygen and rectifies the lack of oxygen.
In addition to hypoxia, peripheral chemoreceptors are also stimulated by hypercapnea and increased hydrogen ion concentration. However, the sensitivity of peripheral chemoreceptors to hypercapnea and increased hydrogen ion concentration is mild.
Disorders of the respiratory system
1)     Hyperventilation: - Hyperventilation means increased pulmonary ventilation due to forced breathing. It is also called over ventilation.
In hyperventilation, both the rate and force of breathing are increased and a large amount of air move in and out of lungs. During hyperventilation, the partial pressure of carbon dioxide is the increase. So excessive carbon oxide is washed out in blood.
2)     hypoventilation: - Hypoventilation is the decrease in pulmonary ventilation caused by the decrease in the rate of the force of breathing. Thus, the air moving in and out of the lungs is reduced.
Hypoventilation result in the development of hypoxia. It increase the rate of respiration leading to dyspnea, severe condition results in lethargy, coma and death.
3)     hypercapnea : - Hypercapnea  is the increased carbon dioxide content of blood. Hypercapnea occurs in conditions which leads to blockage of the respiratory pathway. During hypercapnia, the pH of blood get reduced and blood become acidic and blood pressure increases. During hypercapnea, the nervous system is also affected, resulting in headache depression and laziness.
4)     hypocapnea:- hypocapnea is the decrease in carbon dioxide content in blood. During hypocapnea, the pH of blood increases, calcium concentration decrease. In hypocapnea diseases, mental confusion, muscular twitching and loss of consciousness are the common feature.
A)     asthma:- asthma is a chronic lung disease characterized by inflammation in Airway. Inflammation in Airway makes the airways swollen and very sensitive to a variety of stimuli. Airway obstruction may be due to smooth muscle spasms in the walls of smaller bronchi and bronchioles, edema of the mucosa of the Airway, increased mucus secretion and damage to the epithelium of airway.
B)     emphysema:- This is the disorder characterized by destruction walls of alveoli, producing as normally large air space that remain filled with air during exhalation. With less surface area of the gas exchange, oxygen diffusion across the damage respiratory membrane is reduced. It also cause an increase amount of air strap in the lungs at end of exhalation. Emphysema is generally caused by long term irritation, cigarette smoking, air pollution and occupation exposure to industrial dust are the most common irritants.
Chronic obstructive pulmonary disease (COPD):-
This is the type of respiratory disorder characterized by chronic and virulent obstruction of airflow. The principal type of COPD are emphysema and chronic bronchitis. In most case, COPD is preventable because it is most common by caused is cigarette smoking or breathing secondhand and water airpollution, pulmonary infection etc. 
Bronchitis:- bronchitis is an inflammation or swelling of the bronchial tube( bronchi), the air passage between the mouth and nose and lungs. More specifically, bronchitis describe a condition where the lining of the bronchial tube becomes inflammated. 
Bronchitis can be caused by various bacteria, and other particles that irritates the bronchial tube.
Acute bronchitis is a short term illness that often follows a cold or viral infection.
Chronic bronchitis is a long term illness and can be the result of environmental factors or extrnded illness. Major cause are cigarette smoking, chest X Ray, lungs function testing and blood testing used to diagnose bronchitis.

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