A Brief Look at Respiration - Pharmacology

Before learning about respiratory diseases and the medications used to treat them, let’s take a few moments to briefly review the anatomy and physiology of the respiratory tract. This review will help you better understand the disease and treatment.

The respiratory tract is divided into the upper and the lower tracts. The upper respiratory tract contains the nares, nasal cavity, pharynx, and larynx and the lower tract consists of the trachea, bronchi, bronchioles, alveoli, and alveolar-capillary membrane.

During respiration, air is inhaled and makes its way through the upper respiratory tract and travels to the alveoli capillary membrane in the lower respiratory tract, which is the site of gas exchange. Oxygen from the air attaches to the hemoglobin of the blood while carbon dioxide leaves the blood and is expelled through the lower and upper respiratory tracts during expiration.

RESPIRATION

There are three phases of respiration:

Ventilation

Ventilation is the process by which oxygenated air passes through the respiratory tract during inspiration.

Perfusion

Perfusion is when blood from the pulmonary circulation is sufficient at the alveolar-capillary bed to conduct diffusion. In order for perfusion to occur, the alveolar pressure must be matched by adequate ventilation. The presence of mucosal edema, secretions and bronchospasm increase resistance to the airflow, which results in decreased ventilation. Decreased ventilation causes a decrease in diffusion.

Diffusion

Diffusion is the process where oxygen moves into the capillary bed and carbon dioxide leaves the capillary bed.

COMPLIANCE AND THE LUNGS

There are two lungs inside the chest cavity. Each is surrounded by a membrane called the pleura. Each lung is divided into parts called lobes. The right lung has three lobes and the left lung has two lobes.

You’ll frequently hear the term “lung compliance” used when measuring the functionality of the lungs. Compliance is the ability of the lungs to be distended and is expressed as a change in volume per unit change in pressure. That is, a measurement of how well the lungs can stretch when filling with air.

There are two factors that affect compliance. These are the connective tissue that consists of collagen and elastin and surface tension in the alveoli, which is controlled by surfactant. Surfactant is a substance that lowers surface tension in the alveoli, thereby preventing interstitial fluid from entering the alveoli.

Compliance is increased in patients who have chronic obstructive pulmonary disease (COPD). Compliance is decreased with patients who have restrictive pulmonary disease. A decrease in compliance results in a decreased lung volume. That is, the lungs become stiff requiring more-than-normal pressure to expand the lungs. This is typically caused by an increase in connective tissue or an increase in surface tension in the alveoli.

CONTROLLING RESPIRATION

Respiration is controlled by three factors that sense the need for the body’s increased or decreased requirement for oxygen. These are the concentration of oxygen (O2), carbon dioxide (CO2), and hydrogen (H+) ion concentration in the blood.

Throughout the body chemoreceptors sense the concentration of oxygen, carbon, and carbon dioxide and then send a message to the central chemoreceptors located in the medulla near the respiratory center of the brain and through cerebrospinal fluid to respond to changes.

When an increase in carbon dioxide is detected and there is an increase in hydrogen ions, the message goes out to increase ventilation. Hydrogen ions are measured using the pH scale. The pH of normal blood is between 7.35 and 7.45. A pH lower than 7.35 means the blood is acidic and a pH higher than 7.45 means pH is alkaline. The chemoreceptors respond to an increase in CO2 and a decrease in pH by increasing ventilation. If the CO2 level remains elevated, the stimulus to increase ventilation is lost.

There are chemoreceptors located in carotid arteries and aortic arteries that monitor changes in oxygen pressure (PO2) levels in the arteries. These are called peripheral chemoreceptors. Once the oxygen pressure falls below 60 mmHg, the peripheral chemoreceptors send a message to the respiratory center in the medulla to increase ventilation.

TRACHEOBRONCHIAL TUBE

The tracheobronchial tube connects the pharynx to the bronchial tree that extends into the terminal bronchioles in the lungs providing an unobstructed pathway for air to enter the body and carbon dioxide to leave the body.

The tracheobronchial tube is a fibrous spiral of smooth muscles that become more closely spaced as they near the terminal bronchioles. The size of the air way can be increased or decreased by relaxing or contracting the bronchial smooth muscle. This is controlled by the parasympathetic nervous system particularly the vagus nerve.

The vagus nerve releases acetylcholine when it is stimulated, which causes the tracheobronchial tube to contract. This is referred to as bronchoconstriction. The opposite effect is created when the sympathetic nervous system releases epinephrine that stimulates the beta2 receptor in the bronchial smooth muscle. This causes the tracheobronchial tube to dilate. This is called bronchodilation. In a healthy patient the sympathetic and parasympathetic nervous systems counter balance each other to maintain homeostasis.


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