Key Features of the Respiratory System
Parts: Respiratory passages, Lungs, Generations of the Bronchial Tree, Respiratory Membrane, Respiratory Muscles, Conducting zone, (Transitional and) Respiratory zone
Major Tissue Types: Connective tissue, Smooth muscle, Epithelial tissue
Characteristic Cell Types: Epithelial cells including Septal cells
Functional Unit: Respiratory Membrane
However, some Anatomists prefer to identify the acinus as the functional unit. The acinus includes all of the structures of the respiratory portion of the lung: namely, the respiratory bronchiole and the alveolar ducts and alveolar sacs.
The primary function of the respiratory system is gaseous exchange, that is, to bring oxygen into the body during inspiration and to expel carbon dioxide during expiration. An extensive, respiratory pathway begins at the nose and ends in the lungs. As you read the chapter in your textbook, trace the movement of air through the nasal cavity and all three parts of the pharynx and into the lungs. Notice the points of overlap or possible confusion with the digestive system. Let's review.
Now think about the position of the lungs lying in the thoracic cavity and bordering the mediastinum. Like many organs, the lungs contain a region called a hilus. Through this region the major structures, vessels, nerves and lymphatics enter and leave the lung.
Skeletal muscles are an important part of the respiratory process. The principal muscles of inspiration, the intercostals and the diaphragm contract to increase the size of the thoracic cage during normal breathing.
Two mechanisms for expanding the size of the thoracic cavity have been described: the pump handle and the bucket handle. The thoracic cage expands in the anteroposterior dimension when the sternum moves forward. This movement reminded the old anatomists of the movements of the pump handle used to bring water to the surface on the farm. (I have no personal experience with this, do you?)
On inspiration, the ribs also separate from each other and move apart. Remember the curved shaped of the ribs and that they are anchored anteriorly by the costal cartilage and posterior by the costovertebral joints. If you look at the movements of the ribs laterally, their movements during inspiration and expiration are reminscent of the movements of a bucket handle that is moved from one side of the bucket to the other. (Play with the handle of a bucket when you wash the floor this weekend!) :)
The diaphragm is dome-shaped at rest. When the diaphragm contracts during inspiration, it flattens as it descends into the abdominal cavity, greatly increasing the superoinferior dimension of the thoracic cavity.
Normal expiration is passive, and relies on characteristics of both the diaphragm and the lungs. The diaphragm relaxes and ascends, decreasing the size of the thoracic cage. The lungs contain large amounts of elastic connective tissue; they recoil automatically and return to their original size as exhalation occurs. Remember the elastic fibers you saw in lab.
Any muscle that attaches to the rib cage can become an accessory muscle of inspiration if the proper training is given. Generally, we assign sternocleidomastoid, the scalenes, and pectoralis minor to this category. These muscles are called accessory because they contract during breathing only when you take that large breath and expand your thoracic cage more than you do during normal breathing. If you take a larger breath in, you must blow a larger breath out. The accessory muscles of expiration are the abdominals. Their contraction forces the abdominal cavity contents to push against the diaphragm forcing it to rise.
When air moves into the respiratory passages, it enters and fills the nasal cavity, nasopharynx, and larynx first. Then air moves into the trachea and into the two primary bronchi. Remember that, in Anatomy, we often refer to structures that branch and rebranch as a "tree". Thus, respiratory therapists refer to the branching pattern inside the lungs as the generations of the bronchial tree.
The conductive zone is described as the first 16 branchings. The only function of this zone is to drive air further and further into the lungs. It ends at the terminal bronchioles. The transitional and respiratory zones begin with the respiratory bronchioles which correspond to generations 17 through 19 in this system. Generations 20-22 consist of the alveolar ducts; the alveolar sac is generation 23.
The alveolar sac participates in the formation of the functional unit of the lung, the respiratory membrane. The respiratory membrane is also called the alveolar-capillary membrane, but only in Anatomy books!