The lungs are two huffing and puffing sponge-like organs that dominate the chest cavity, essential in their function as the principle structures of respiration. An understanding of their unique anatomy and physiology brings understanding of how diseases will affect the lungs, and how these changes will be manifest on x-ray and CT evaluation. In this first part of the series on the lung we emphasize principles, and outline how structure is integrated with function, disease and imaging.
The unique structural characteristics of the lungs include their:
- dominance in the chest cavity
- ability to accommodate the entire cardiac output with every heartbeat
- asymmetric nature
- irregular and dichotomous branch pattern of the bronchovascular bundle
- tubular transport system, with a single system functioning for both delivery and removal
- spongy air-filled character
- pyramidal or cone shape
- dual blood supply
The unique functional aspects include
- The ability to move air efficiently
- The ability to exchange gases efficiently
Lung Structure This collage reflects the anatomic range of the respiratory system from the macroscopic to the microscopic – a continuum of structure. Image 2 is a post-mortem specimen taken from the front and slightly above. It shows the trachea and bronchi supplying the two lungs above, with the aortic arch and cardiac structures in the middle and below. Note how pink the lungs are in this specimen from an unfortunate baby with congenital heart disease. Image 3, the chest x-ray, shows the lucent lungs within the thoracic cavity while image 4 is a diagram of the trilobed right lung and the bilobed left lung. Two respiratory units of the lung are shown in the next image each called a pulmonary lobule (5). The lobule consists of a central bronchiole (light blue) and pulmonary arteriole (dark blue), surrounded by the air filled acinus (teal) with its peripheral venules. (red) The acinus is magnified in the next image (6), showing first the tubular terminal bronchiole branching into the respiratory bronchioles, alveolar sacs, and finally the grape like alveoli. The organization of the connective tissues of the lung is shown in image 7. Finally we get down to the grapes or alveoli of the lung with surrounding vessels (8), and a single alveolus is seen in 9. It seems a long way for the air to travel but the system can deliver the air to and from the outside in a single breath, and exchange the gases at the capillary level even more rapidly. It is a remarkable system.
Courtesy Ashley Davidoff MD 42651c
As we progress through the module there is a recurring pattern of the dual function of the respiratory apparatus – an airway system that transports the air, and an exchange system that enables transfer of the gases across the alveolar membrane.
| There are two major functional components to the lungs; the transport system and the exchange system. The first image is a reformatted image in the coronal plane which reveals the transport system of the lungs. The second image is a surface rendering of the lungs which reveals the parenchymal component which functions as the gas exchange system. The tree like features of the lungs is exemplified by an infrastructure of dividing branches which subserve the leaves – in essence a tubular system serving the factories of gas exchange.
Courtesy Ashley Davidoff MD 32634d02 32634b02
The lungs are part of a larger system called the respiratory system. The role of the lungs is to deliver ambient air (ventilation) to the alveoli and to act as the agent for gas exchange by contributing one layer of epithelium to the bilayered membrane, a double layer which serves as the ultimate interface for gas exchange. The primary role of the pulmonary arterial circulation is to transport blood to the alveolar interface (perfusion) and to also play a part in the exchange of carbon dioxide and oxygen by providing the second layer of the bilayered filter. It is the homeostatic aim of the body to match the ventilation with the perfusion, in order to maintain uniform ventilation (V) to perfusion (Q) ratio (V/Q).
The lungs are exposed directly to the air in the atmosphere and to the blood within the circulatory system. Through the lungs the blood is therefore exposed to the atmosphere, which on the one hand contains life sustaining oxygen but on the other can present a hostile environment filled with microorganisms, industrial chemicals, and toxic fumes.
|In this diagram a single alveolus is outlined with its surrounding arteriole, venule, and capillary network. The process at the alveolar end is a simple exchange. Life sustaining oxygen is received by the hemoglobin and toxic carbon dioxide is excreted. Although this exchange occurs in the respiratory bronchioles alveolar ducts and alveolar sacs, the alveolus is the prime site of gaseous exchange.
Image courtesy Ashley Davidoff MD 42438b03
The lungs are made of expandable, sponge-like tissues. Their close proximity to the heart and circulatory system allows for rapid exchange of gases between the air and the circulatory system as noted above. This function is easily accomplished during rest, where resting respiratory rate in an adult is about 12 breaths per minute and heart rate 72 beats per minute. During exercise the respiratory rate can increase to 40-50 breaths per minute and the heart rate, increasing in concert, can reach 180-200 per minute. The interface of circulation with the respiratory system has to be sufficiently equipped to allow delivery, uptake, and exchange of gases at this accelerated pace. The key soldier in the exchange of oxygen is a complex protein called hemoglobin which lies in the red cell.
I imagine the hemoglobin molecule scurried by the forces of the right ventricle into the pulmonary circulation. As it enters the chambers of exchange, the open windows of the lungs herald the fresh air. Under basal conditions, the hemoglobin gnome can work at a leisurely pace filling his baskets with oxygen. Under exercise conditions he has to start working like crazy, grabbing molecules of oxygen and stacking these into his storage baskets, surrounded by an accelerated pace with gale-like air forces and flood-like blood conditions. Hemoglobin is a remarkable molecule and can adapt its function to these extremes in physiology.
Carbon dioxide from bodily metabolism is dissolved in the blood. Exchanges between the circulation and alveoli occur rapidly and efficiently across the alveolar membrane due to differences in the partial pressure of gas between the blood and the alveoli.
The presence of a structure in the body that is almost totally filled with air makes it a unique, challenging, and rewarding organ to examine clinically and radiologically. Examination of the lungs with a stethoscope enables the clinician to evaluate inspiratory and expiratory movement of air. Some pathologic conditions, including aspiration of a foreign body, collapsed lung, or inadvertent intubation of the bronchus, result in airway obstruction. In these conditions there is no air entry into the affected bronchus and subtended lung. The clinical finding of the lack of air entry, based simply on the lack of air sound, can be a life saving diagnostic maneuver.
Unusual but interesting words are used to describe the sounds of air character and movement on clinical examination. These include percussion, tactile fremitus, bronchophony, whispering pectoriloquy, and egophony and relate to the wayair moves through the airways, and how the transmission may change when there is fluid in the pleural space or in the lungs. When the air mixes with the fluid, characteristic sounds such as rales and crepitus will result.
|Chest of Fruit|
|This “Chest of Fruit” with the red pepper for the heart and clusters of grape-like oversized alveoli speaks for itself – The image provides a brief respite from the serious depths of our discussion about odd sounding terms for odd sounding breath sounds.