Introduction:

The respiratory tract consists of two functional divisions with distinct structural elements that reflect their unique roles in the process of respiration: The conducting airways serve to clean, warm, moisten, and conduct air. This portion of the respiratory tract is composed of the nose, pharynx, larynx, trachea, bronchi, and terminal bronchioles. The respiratory airways have alveoli to facilitate gas exchange. They are located entirely within the lung and consist of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

In addition to the respiratory tract, numerous other components are necessary for respiration to occur. These include specialized blood vessels to conduct blood to and from the lungs and numerous muscles to control breathing.

Learning Objectives:

• Locate and identify the major gross structures of the respiratory tract.


• Understand the difference between the systemic and pulmonary circuits of the circulatory system.


• Distinguish between arteries of the body which carry oxygenated blood and those that carry deoxygenated blood.


• Locate and identify the majors muscles involved in inspiration and expiration.


• Locate and identify the phrenic and vagus nerves and their respective innervations within the respiratory system.

Build it to Learn it:

1. Add the nasal cavity. The nasal cavity is the first portion of the respiratory tract. It is the site of filtration, hydration, and warming of inspired air.


2. Add the pharynx (nasopharynx, oropharynx, and laryngopharynx) which is shared by the digestive and respiratory tracts. It conducts both swallowed food and air.


3. Add the larynx which conducts air from the pharynx to the trachea. It is composed of nine cartilages with numerous intrinsic and extrinsic muscles. The thyroid and cricoid cartilages help provide the "skeleton" of the larynx, while the epiglottis is a "flap" that closes off the airway entrance during swallowing. The larynx also contains the vocal cords which vibrate and make sound as air passes over them. The muscles of the larynx are innervated by branches of the vagus nerve (CN X).


4. Add the trachea which conducts air from the larynx to the two primary bronchi. It is supported by a framework of 16-20 C-shaped rings of cartilage.


5. Add the left and right primary bronchi. Each primary bronchus conducts air to one of the lungs. As it enter the lung, each primary bronchus branches into smaller secondary (lobar) bronchi and tertiary (segmental) bronchi.


6. Add the left and right lungs. The right lung is divided into inferior, middle, and superior lobes. The left lung is divided into inferior and superior lobes. Within each of the lobes, the bronchi branch into smaller and smaller bronchioles (terminal and respiratory) which continue branching into smaller airways (alveolar ducts and sacs) before finally reaching the alveoli, where gas exchange occurs. (Note: The bronchioles and smaller structures are all microscopic structures.)

1. The pulmonary circuit provides deoxygenated blood to the lungs in order for it to become oxygenated:


• Add the pulmonary trunk which conducts blood from the right ventricle of the heart. The pulmonary trunk splits into the left and right pulmonary arteries. The pulmonary arteries carry deoxygenated blood to the lungs. Branches of the arteries then travel with the bronchial system to reach the alveoli where gas exchange can occur. (Hint: Use the "Dissect" tool to remove the superior lobes of the lungs to better view the blood vessels.)


• Add the left and right pulmonary veins which carry oxygenated blood to the left atrium of the heart. (Hint: Rotate to a posterior view and use the "Dissect" tool to remove lobes of the lungs to better visualize the left atrium.)


2. The systemic circuit provides oxygenated blood to the lungs in order to supply oxygen to the tissues of the lungs:


• The bronchial arteries are regular, muscular arteries that conduct oxygenated blood to the lungs. They are usually branches of the descending thoracic aorta.


• The bronchial veins drain blood away from the lungs. Most of the blood is returned to the right atrium of the heart through the azygos system (azygos vein and the accessory hemiazygos vein).

1. The muscles of inspiration act to expand the thoracic cavity and thereby cause the lungs to inflate:


• Add the diaphgram which is the major muscle of inspiration. It separates the abdominal cavity from the thoracic cavity. As it contracts, it moves downward and expands the thoracic cavity, which causes the lungs to expand (inflate). The diaphgram is innervated by the phrenic nerves.


• Add the external intercostsal muscles which contract to raise the ribs and expand the thoracic cavity.


• Add the accessory muscles of inspiration which include the scalenes, sternocleidomastoid, serratus anterior, and pectoralis muscles. The accessory muscles are not normally involved in respiration, but they can become involved during periods of heavy breathing or in certain disease states.


2. Expiration is normally a passive process; however, several muscles of expiration can act to reduce the size of the thoracic cavity and thereby cause the lungs to deflate:


• Add the internal intercostsal muscles which contract to depress the ribs and shrink the thoracic cavity.


• Add the abdominal muscles which include the transversus abdominus, internal and external obliques, and the rectus abdominus. During active expiration, the abdominal muscle contract, increasing the pressure within the abdominal cavity to "push" the diaphgram up and shrink the thoracic cavity.

Check Your Understanding:

1. A 24-year-old bike messenger is hit by a car while riding her bike through traffic. Her left shoulder is seriously injured. She is immediately transported to the hospital. Before surgery, the anesthesiologist performs a nerve block of the brachial plexus to stop all sensation from her injured arm. The procedure is performed by injecting a nerve blocking agent into the tissue around the scalene muscles within the neck. The anesthesiologist is careful when injecting the block in order to minimize the chances of diaphragmatic paresis (weakness or partial paralysis). Explain how the nerve block administered in the neck may affect the diaphgram. (Give me a hint.)
   The brachial plexus emerges from between the anterior and middle scalene muscles. When an interscalene nerve block is admistered, it may diffuse in the tissues and affect adjacent structures. In this image, use the "Highlight" tool to identify the scalene muscles. What other nerve is located along the anterior scalene?
   
   Answer:
   The diaphgram muscle is innervated by the left and right phrenic nerves (C3-C5). The phrenic nerve descends along the anterior surface of the anterior scalene muscle before entering into the thoracic cavity to travel to the diaphragm. Since the brachial plexus emerges from the behind the anterior scalene muscle, injections in this area can affect the phrenic nerve, also.



Check Your Understanding:

1. A pediatric patient in the Emergency Department is found unresponsive and not breathing. A resident physician places a breathing tube down the child's throat through the trachea to facilitate mechanical ventilation. When a chest x-ray is performed after the procedure, the breathing tube is seen to be inserted too far, which has led to collapse of the lungs on the other side of the chest. In which primary bronchus is the tube most likely to be found? (Give me a hint.)
   This image shows the left and right primary bronchi branching from the trachea. Notice the difference in the angles at which the bronchi branch from the trachea. Considering the angles, an inserted tube would most likely pass into which bronchus?
   
   Answer:
   The right primary bronchus has a more straight path from the trachea; therefore, an incorrectly inserted intubation tube or accidental swallowing of food or other foreign objects will often enter into the right primary bronchus. This is especially true in children. The left primary bronchus branches from the trachea at a sharper angle due to the location of the heart.