Lungs and airways in asthma
In order to understand asthma, we first need to understand a little about the normal structure and function of the lungs. The most important function of the lungs is taking in of oxygen from the air into the blood stream and getting rid of the waste carbon dioxide. In order to do this, air has to be brought very close to the blood and this happens in tiny air sacs called alveoli. These alveoli are the end of an extensive branching structure which starts with trachea, the main airway into the lung.
Blood from the veins (from all over the body) drains into the right side of the heart and is then pumped through the lungs by way of the tiny vessels in the walls of the alveoli. There, it picks up oxygen (oxygenation of blood) and gets rid of carbon dioxide before returning to the left side of the heart. Oxygenated blood from the left side of the heart is pumped out to all the tissues in the body through the arteries.
The airways Various objects, including liquids and food particles, can enter the lungs via trachea. The large airways such as the trachea have a stiff wall, which contains cartilage, the same substance as that supports our nose and ears. These are present in the form of a ring. This is present only in the anterior and is absent in the posterior. This makes these large airways less likely to narrow in asthma. In children, of course, all the airways are smaller. All the airways are lined by an epithelium, which is like a thin skin, and on the top of this skin are tiny hairs called cilia which are constantly in motion shifting up the lung secretions from the outer portions of the lungs to the large airways.
Underneath the epithelium layer, there is a loose mass of tissue called connective tissue in which there are two important structures, the bronchial glands and the smooth muscle.
The bronchial glands
These have little tubes opening on to the inner surface of the airway. Through these, they pour secretions of mucus into the airway where again it is wafted up in the larger airways and then coughed up as abnormally sticky sputum. This adds to the narrowing of the airways and may play a crucial role in asthma patients. These thick and tenacious mucus plugs that block most of the airways, constitute the most remarkable feature in the lungs of patients who die of asthma.
The bronchi has smooth muscles wrapped around it like stripes on a candy stick. The state of tension of these muscles is an important factor in determining the diameter of the bronchus; contraction of bronchial muscles narrows the bronchus, while relaxation widens it.
Inspiration and expiration
Breathing in is also known as inspiration (inhalation) and breathing out is called expiration (exhalation). Under normal circumstances, little work is required for the breathing process. However, there are two things that make this work much harder and difficult:
- stiffness of the lungs
- narrowing of the airways
Increased bronchial reactivity in asthma is related to airway inflammation in which many cells participate. These include mast cells. Previously, it was believed that mast cells are crucial in the pathogenesis of bronchial asthma. However, it has been shown recently that other cells also participate. These are eosinophils, lymphocytes, macrophages, neutrophils and platelets, etc. These cells release various mediators which are responsible for symptoms of bronchial asthma. One such mediator released from mast cells is histamine, which produces immediate symptoms (acute broncho spasm or wheezing) of bronchial asthma.
Mediators released from other cells result in bronchial hyper-responsiveness, as a result of which, asthmatic airways exhibit an exaggerated response to agents such as pollen, often manifesting as increased non-specific reactivity for days or even weeks. Upper respiratory tract viral infection may lead to similar changes and may increase reactivity in non-asthmatic subjects. exhibit an exaggerated response to agents such as pollen, often manifesting as increased non specific reactivity for days or even weeks. Upper respiratory tract viral infection may lead to similar changes and may increase reactivity in non asthmatic subjects.
Lungs and airways in asthma
When the air passage is narrow, as in asthma, the lung it seems tries to keep the airway lumen as wide as possible by keeping more air in the lungs, this is called over inflammation or hyper-inflation. It means that there is less room for air with each breath and breathing in becomes difficult. Breathing out is also limited by the narrow airways. Asthmatics, during an acute attack, usually find it most comfortable to sit up so that their main muscle of respiration (the diaphragm) works best and they may even use extra muscles in the neck to help their breathing.Typical changes which involve the airways in asthmatics include: inflammation, broncho spasm and mucus production.
Researchers have paid much attention to contraction of the smooth muscle (broncho-constriction) in asthma. In fact, it was considered to be the crucial feature of bronchial asthma and all treatment modalities in the past were directed at reversing this broncho-constriction.
Currently, infiltrating of the airway wall by various inflammatory cells is considered the key abnormality in the pathogenesis of bronchial asthma. Broncho-constriction is certainly important; but so is the swelling of the airway wall by various inflammatory cells. It is composed of fluid and cells which release various substances that attract other cells. This causes more swelling and leads to contraction of the muscle. Research work has revealed that this inflammation persists to some degree in the walls of airways of asthmatics even when the disease has not given any trouble for 6 to 12 months. These cells lie quietly in the airway wall, waiting for the right stimulus to activate them again and spark off an attack of asthma. Corticosteroid medications are used to reduce this inflammation. Inhaled steroids may prevent it.
Many kinds of inflammatory cells in the wall of the airway are important in asthma. Those which have been best studied are the mast cells. These little packages contain performed mediators which are released if these cells are triggered, for instance, by a pollen grain. Other cells include eosinophils, lymphocytes, platelets, neutrophils and soon.
Another characteristic of asthma is increased sensitivity of the airways. This leads to broncho spasm, due to spasm of the smooth muscle around the airways. It causes further narrowing of the airways. Broncho-dilator drugs are very effective in reversing this muscle spasm.
In some asthmatics, the mucous glands in the airways produce excessive, thick mucus which further narrows the airways. Corticosteroids decrease swelling, thereby lessening mucus production. Drinking adequate fluids and deep coasting can also help to remove the mucus. Expectorants (medications which increase sputum production), and mycologist (medications which loosen the secretion) may also be beneficial.