The following are some major concepts relating to the anatomy of the thoracic cavity.
1) The 'thoracic cavity" is, in fact, three separate cavities - the right and left pleural cavities, and the mediastinum. While the pleural cavities encase each lung, the mediastinum contains the heart & thymus (or remnants of the thymus - depending on the age of the individual). In a normal healthy individual, these three cavities are functionally separate from each other. Thus, a pneumothorax (entry of air into the pleural space) of the right side does not have to result in a similar condition on the left. Clinical note: Air can enter either pleural cavity without the presence of an externally visible chest wound. The most common way for this to happen is to have the surface of the lung compromised, thereby allowing air to enter the pleural cavity from the lung. Such lung damage can occur spontaneously (rupture of surface blebs), or as the result of a fractured rib puncturing the lungs surface. Another variant of a pneumothorax is what is known as a tension pneumothorax. When this condition occurs, a portion of tissue at the trauma site acts like a one-way valve. Thus, air is allowed into the pleural cavity during the negative pressure phase of inspiration, but it cannot exit during expiration. The result of a tension pneumo is the accumulation of air in the pleural space and a resulting positive pressure in the cavity that is normally sub-atmospheric. This positive pressure can result in the thoracic contents being displaced to the opposite side of the thoracic cavity, which is easily detected with any type of imaging modality. Finally, there is the condition known as a hemothorax, or blood accumulation in the thoracic cavity. Once again, accumulation of the blood causes a displacement of the lung, as well as compression. Thus, the patient has a very difficult time breathing. Insertion of a chest tube to allow drainage of the blood will normally improve the condition, and the patient will have a much easier time breathing.
2) If you are unfamiliar with the pressure/volume changes associated with ventilation/respiration - please refer back to your basic anatomy and physiology text for a review.
There is an inverse relationship between pressure and volume. As intrapulmonic pressure (Ppulm) is decreased secondary to an increase in thoracic volume, the Ppulm reaches a value that is less than Patm and inhalation occurs. Relaxation of the inspiratory muscles results in elastic recoil of the thorax, a decrease in thoracic volume, and a resulting exhalation secondary to the decreased volume and increased Ppulm. Control of this system is via the respiratory control center in the brainstem. Innervation of the diaphragm and intercostal muscles is accomplished by the phrenic nerves and intercostal nerves, respectively. Recall that the phrenic nerves originate from the spinal cord at C3, C4, and C5. Thus, a transected spinal cord at the level of C7 will still allow control of the diaphragm via the phrenic nerves, whereas a similar transection at C2 will not.
The phrenic nerve is also the pathway by which pain signals from the diaphragm reach the CNS. Thus, pain originating in the diaphragm will not reach the CNS until the level of C3 - C5. This explains why diaphragmatic pain is often perceived in the neck or shoulder region - a version of referred pain. Clinical note: Many laparoscopic procedures of the abdominal cavity require gas insufflation, i.e., the injection of an inert gas into the abdominal cavity so as to enlarge the cavity and force the anterior abdominal wall away from the viscera. This is often associated with neck and shoulder pain during and after the procedure. This pain is due to the fact that the gas insufflation also causes distension of the diaphragm, and the resulting pain signals being sent to through the phrenics to the cervical region of the spinal cord.
3) If we ignore the spinal cord passing through the vertebral column, there are three major systems in close proximity within the thoracic cavity - the cardiovascular, respiratory, and digestive systems. While most people consider the thorax to be a cardio-respiratory region, the transit of the esophagus cannot be overlooked. While not common, conditions such as esophageal erosion and/or penetration can result in significant clinical complications within the chest. When studying the scans of this region, note the close juxtaposition of the esophagus to the descending aorta. While relatively rare, it is not unheard of to have some swallowed foreign body (e.g., a chicken or fish bone) penetrate the espohagus and cause a concomitant puncture of the aorta.
4) Selected clinical topics related to thoracic anatomy:
• The superior aspect of the pleural cavity is in close approximation to the subclavian artery and vein. Thus, insertion of a subclavian line can, on occasion, result in a pneumothorax secondary to the needle puncturing the pleural membrane.
• There are three openings through the diaphragm. One is for the descending aorta, one for the inferior vena cava, and one for the esophagus. (The vertebral column and spinal cord technically pass behind the diaphragm - not through it.) In a significant portion of the population, the hiatus for the esophagus is larger than necessary, resulting in herniation of the abdominal viscera (usually the stomach) into the thoracic cavity. This is commonly known as a hiatal or diaphragmatic hernia. Additionally, a significant portion of these patients have a lack of tone in the LES (lower esophageal sphincter), resulting in a reflux of gastric juices and contents into the lower esophagus. This condition is known as GERD (gastro-esophageal reflux disease).
• While the distal colon is best known for the development of outpockets or diverticula, it must be noted that the esophagus can also present with the same category of pathology. These diverticula accumulate food during the swallowing process, which then undergoes putrefaction over time. The patient often presents with "terrible halitosis", although a dental exam is usually unremarkable. Thoracic imaging will easily show the diverticula and their contents. Surgical intervention is the therapy of choice.
• Pain perception: While the visceral component of the pleural membrane, as well as the lung itself, is relatively devoid of pain receptors, the parietal viscera is very well innervated. Thus, pleurisy is often associated with fairly severe pain.
• Diaphragmatic pain is carried to the CNS by the phrenic nerve, which enters the spinal cord at C3, C4, and C5. Thus, pain originating in the diaphragm is often perceived as occurring in the neck, i.e., the location where the diaphragmatic pain signals are entering the spinal cord.
5) Relationships with the neck:
• The left laryngeal recurrent nerve (which innervates the larynx for speech) descends into the thoracic cavity, runs beneath the arch of the aorta, and then ascends upwards to eventually innervate the larynx. Thus - a growing aneurysm of the arch of the aorta will sometimes apply traction to this nerve, leading to a dysfunction of the speech function of the larynx. So - 99% of the time, a little "hoarseness" of the larynx simply means the beginning of laryngitis. Occasionally, however, it might indicate the beginning of something far more serious - an aortic aneurysm.
What's the next step??????
You should now begin studying the cross-sectional images from your text. Remember - it is best if you:
• Look at the diagrams in your text first, and highlight those structures (from your Structure List) that are required for Exam I.
• Once you feel that you are familiar with the structures in your text, go to the Thorax Anatomy section of this webpage and begin looking at the unlabelled images. Name as many structures as you can on each image, and then check your text to confirm (hopefully:) that you named them correctly.
