Respiratory Health

All animals need oxygen to metabolize nutrients to generate cellular energy.

The above equation illustrates how nutrients (glucose) are converted to cellular energy in the form of adenosine triphosphate (ATP). Note carbon dioxide is produced as a by-product of metabolism.

The lungs are the interface between our blood stream and the external environment and the site where O2 is taken up and CO2 leaves the bloodstream to be expelled into the environment. The intimacy of this interface between our internal and external environment with the need to exchange air has a major impact on health for several reasons:

• Continual exposure of the respiratory tract to air from the environment provides an important portal for entry of viruses and bacteria that can cause infection.
• Disease processes that impede the mechanisms of ventilation (e.g. asthma and emphysema) can cause severe illness or death; these processes can also be triggered or exacerbated by substances in the air we breath.
• Air is a variable and complex mixture of many substances including particulate matter and chemicals that can have a detrimental effect on lung function; moreover many of these chemicals can be absorbed into the blood and cause a wide range of health problems.

Learning Outcomes

1. Describe the function of alveoli with respect to exchange of oxygen and carbon dioxide
2. Describe the role of hemoglobin in oxygen transport and the adverse effects of CO
3. Explain how the occurrence and severity of chronic obstructive pulmonary disease (COPD) and asthma relates to environmental factors and be able to give examples of factors known to trigger asthma attacks
4. List the six criteria air pollutants and discuss the impact of air pollution on health
5. Discuss the impact of environmental tobacco smoke on health

The Respiratory Tract

Inspired air enters through the nose or mouth, passes through the pharynx (throat) and larynx (voice box) and then enters the tracheo-bronchial tree. The trachea bifurcates into the rate and main stem bronchi which branch again and again into increasingly smaller conduits called bronchioles.

Respiration continually brings air from the environment in contact with the delicate cells in our lungs to provide oxygen and to expel carbon dioxide. The air must be warmed, moistened and filtered for the lungs. When harmful contaminants are carried in the air there are several mechanisms to mitigate the effects of these contaminants.

Nasal hairs and mucus can trap dust and other particulate matter, and goblet and ciliated cells work in tandem to remove particulate matter. Goblet cells create a mucus layer on the lining of the surface that protects the cells and traps dust and other foreign material  while ciliated cells have hair-like projections that move rhythmically and sweep the mucus and particulate matter upwards where it can be swallowed or expectorated.

Supplying Oxygen and Expelling CO2

The above figure shows a bronchiole terminating in several clusters of bronchiole. The bronchiole has a vacular supply of oxygenated blood (in blue) and nervous enervation (nerves in yellow) and the bronchiole is wrapped by smooth muscle cells that can contract or relax in response to physiologic conditions.

Contraction of smooth muscle narrows the airways, and relaxation increases their diameter. Abnormally severe contraction of this smooth muscle is of central importance to asthmatic attacks.

Eventually, the terminal bronchioles open into thousands of alvelo - delicate grape-like clusters of air sacs where gas exhange occurs. There are several types of alveolar cells: type 1, type 2, and macrophages.

The above image focuses on the details in a single alveolus. Note the capillaries surrounding the alveolar cells are in very inimate contact, which facilitates the exchange of oxygen and carbon dioxide between the air in the alveolus and blood in the capillary.

The alveolus is lined with alveolar cells; type 1 are for exchange of O and CO2, while type 2 can divide to give rise to new type 1 cells and also synthesize and secrete a substance called surfactant which reduces surface tension in the alveolus and prevents collapse during expiration.

Within the alveolus there are also macrophages (also called "dust cells") - phagocytic cells that engulf particulate matter then migrate up the bronchioles where the ciliated cells sweep them up into the pharynx where they are swallowed or expelled.

The graphic above illustrates that blood returning to the heart is de-oxygenated (in blue) and returns to the right atrium of the heart and then the right ventricle. When the heart contracts blood in the right ventricle is pumped to the lungs where it enters capillary vessels that circulate around the alveoli where gas exchange takes place. As blood circulates around the alveoli it becomes re-oxygenated and CO2 moved from the pulmonary arterial blood into the alveolus to be exhaled. Blood returning to the left atrium of the heart then to the left ventricle is fully oxygenated and when the heart contracts it is pumped into the aorta to be distributed throughout the body via the arterial system.

Blood returning from the peripheral tissues has a relatively low concentration of oxygen but is rich in CO2 as a result of cellular metabolism. As a result, the oxygen readily diffuses from the alveolus across the alveolar cells and into the capillary where it binds to the hemoglobin in red blood cells. Conversely, CO2 diffuses from the capillary blood into the air in the alveolus which is exhaled.

Cellular Gas Exchange

Cells use oxygen to metabolize, or create cellular energy from glucose or fatty acids. Arteries branch into arterioles which eventually branch into capillaries.

The image above shows an arteriole branching into smaller capillaries which supply nutrients and red blood cells to metabolizing cells. Capillaries are often so thin that often red blood cells can only pass one at a time.

Smoking

Tobacco smoke is a complex mix of toxic and carcinogenic chemicals and particulate matter. Emission levels of 98 components in mainstream smoke constitute a human inhalation risk with potential to contribute to many adverse effects.