Bronchial asthma:

Bronchial asthma is characterised by bouts of  dyspnoea as a result of  temporary narrowing of bronchi due to bronchial spasm, mucosal edema and thick secretions. It is caused due to hyper-reactivity by bronchial tree to a variety of  stimuli. Asthma may be extrinsic or allergic is. when symptoms are induced by hyperimmune response to the inhalation of a specific allergen. It may be intrinsic asthma or non-allergic type in which symptoms are produced in response to non-specific factors  in  the environment. 

Etiology:

The exact etiopathogenesis  is not known but there are some excitatory factors and predisposing factors which are as follows: 

Excitatory Factors 

i)  Allergic factors/Allergy to certain foreign substances: 

a)  Inhalants like pollen, smoke, dust and powder 

b) Foods like egg, meat, wheat and chocolate 

c)  Drugs like aspirin and morphine

ii)  Respiratory infections

iii) Emotional disturbances like "row" with the siblings or the parents and fear of punishment 

iv)  Exhaustion 

v)  Change of  climate  

Predesposing Factors 

i)  Heredity: A  family histroy of asthma or some other allergic disorder is usually present 

ii)  Childhood infections like measles and pertussis (i)  Constitution: Asthmatics  are high stung, emotional and intelligent Pathology The major pathologic mechanism responsible for airways obstruction, associated with asthma  are caused by (i) Spasm of smooth muscles of  the bronchi,  (ii) Edema of the bronchial mucosa, (iii)  Increased secretion and accumulation of the tenacious mucus within the  lumen of the bronchi and bronchioles. As a result of this the resistance to outflow increase, manifesting as reduction  in forced expiratory volume and flow rate. The  airway closes prematurely during expiration which results  in hyperinflation  of lungs and increase in functional residual capacity, the patient has  to breath in while  the lungs are already hyperinflated. Therefore there is a marked distress in breathing and compliance of  lung  is decreased. Initially the patient  tries  to  hyperventilate and therefore arterial PC02 rises. As the compensation by buffering mechanism fails, PH of blood falls causing respiratory acidosis, All the obstructive processes interfere with ventilation and result in the characteristic symptoms  of coughing, shortness of breath, and wheezing.