Asthma Medicines

To understand asthma you must first consider the normal structure of the lung. and we will define asthma.

The Normal Lung

The Bronchial Tubes

In a normal lung, two systems coexist to permit its function of enriching the blood with oxygen and excreting carbon dioxide. Oxygen is the fuel for metabolism while carbon dioxide represents the waste product.

The first system is a series of hollow connecting airways that form the bronchial tubes. Imagine that this system resembles a large branching tree with the trunk being the windpipe (trachea) that begins below the voice box (larynx). Branching of this bronchial “tree” occurs frequently to the smallest imaginable airway, a bronchiole. The inner lining of the bronchial tubes is a delicate membrane called the bronchial mucosa. Although normally pale and innocent in appearance, this membrane is capable of becoming inflamed and swollen in an asthmatic attack. Mucus glands within the layers of the bronchial lining can produce large quantities of extremely thick secretion when stimulated by infection or asthma triggers. In the wall of the bronchial tubes is a smooth muscle layer capable of contracting, producing narrowing or bronchoconstriction, and relaxing, producing widening or bronchodilatation.

The Alveoli

The second system within the lung consists of millions of tiny air sacs or alveoli. Think of these sacs surrounding the small bronchial tubes like grapes surrounding a stem. A bronchiole is surrounded by alveoli. Each of the smallest bronchioles allows air to enter and leave multiple alveoli. Within these small air sacs the vital gas exchange takes place. Within the wall of these sacs blood courses within small vessels called capillaries. This blood has been returned from all parts of the body where it has been used for metabolism. As it enters the capillaries it is low in oxygen and high in carbon dioxide. The extremely thin walls of these blood vessels allow the exchange of oxygen and carbon dioxide. The blood leaves the alveoli oxygen rich with lower carbon dioxide levels.

The Nervous System And The Lung

Besides reviewing the normal structure of the lung, it is important to understand the role of the nervous system and how it relates to bronchial asthma. The nervous system is generally divided into the centre tures of the brain and spinal cord and peripheral nerve structures distributed throughout the body.

The Autonomic Nervous System

One major subdivision of the nervous system is called the autonomic nervous system, which is responsible for the unconscious control of major body functions and is divided into a parasympathetic and a sympathetic branch. These systems extend throughout the body but are extremely important in lung function. Just think of these two systems as balancing each other. For example, stimulating the parasympathetic system causes the bronchial tubes to constrict while stimulating the sympathetic nervous system produces the opposite reaction (dilatation). In a normal lung a balance of these two systems maintains open airways. In an asthmatic lung, an imbalance occurs, favoring the parasympathetic system that produces narrowing or constriction of the air tubes.

Adrenergic Versus Cholinergic Effects

The effects produced through the nerve pathways are mediated by chemicals called neurotransmitters. These chemicals act at nerve endings or receptor sites throughout the body. In the parasympathetic nervous system the neurotransmitter is a chemical substance known as acetylcholine. Agents or medications that mimic the effects of this substance are called cholinergic agents. In the sympathetic nervous system the neurotransmitter is a substance known as epinephrine or adrenaline. Agents or medications that mimic the effects of adrenaline are called adrenergic agents.

Receptors: Alpha and Beta

It is important to familiarize yourself with the different types of receptors that exist in the nerve endings. These receptors are divided into alphas and betas based on how they respond to medication. In general, alpha receptors excite and beta receptors usually inhibit or relax. Alpha receptors seem to be less important in regulating bronchial tubes than beta receptors. Beta receptors are classified as B1, found in the heart muscle, and B2, found in the bronchial tubes and other parts of the body.

Definition of Asthma

That asthma has been defined in so many ways reflects the complexity of this illness. Several features of asthma are covered by the currently accepted definition.

Airway Obstruction: Reversible

First, the airways of the lung or bronchial tubes are narrowed. This is called “airway obstruction” since air can no longer flow smoothly through elaborate system of branching tubes. Since these tubes can dilate a asthma this obstruction is called reversible, an important aspect of definition since it may distinguish asthma from other bronchial illnesses fixed or irreversible obstruction such as bronchitis and emphysema

This narrowing within the bronchial tubes has occurred due to a tightening or constriction of the muscle that exists in the bronchial, reaction may be thought of as a muscle “spasm” that results in narrowing of the bronchial tubes, similar to any muscle cramp.

It is these narrowed or “obstructed” airways that produce one of the common features of asthma, the wheeze. As air is exhaled through these tubes, its movement is turbulent and produces this sound.

Inflammation

The second element included in defining asthma is the presence of inflammation, the red, swollen appearance of the inside of the tubes. This characteristic of asthma has received a great deal of attention recently and has become the focus of much of asthma therapy. The inflammation is present in the lining (the mucosa) of the bronch which can be examined by inserting into them a lighted scope bronchoscope. With this instrument a physician can also obtain of the bronchial lining and its secretions. Under a microscope these samples may show large numbers of cells that carry substances called mediators capable of causing inflammation. Using these techniques, the mediators released by inflammatory cells when an attack occurs can be measured and identified.

Hyperirritability

The third defining feature is increased responsiveness or hyperirritability the bronchial tubes and their tendency to”overreact” and narrow. ‘”twitchy” has also been used in this regard. This irritability is often demonstrated by the sudden, severe attacks patients can experience when exposed to substances such as pollen, animal dander, dust, and fumes. This hyperreactivity forms the basis for bronchial provocation or challenge testing that is used by physicians to diagnose asthma in patients whose illnesses do not fit easily into the other definitions.

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