Cast of Trachea and Bronchial Tree

The cast below is from a large dog’s lungs and is approximately the same size as human lungs. The casts were created by forcing liquid latex into the respiratory passageways of the lungs and then letting the latex harden. The lungs were then placed in a weak acid until the organic tissue of the lungs was digested away. These views of the cast allow you to visualize the extensive nature of the bronchial tree as it branches out to the larger alveolar passageways within the lungs. The smaller alveolar spaces did not get incorported into the casts.

1 Trachea

2 Right main (primary) bronchus

3 Left main (primary) bronchus

4 Lobar (secondary) bronchus

5 Segmental (tertiary) bronchus

6 Branching bronchiole network

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Latex cast of respiratory passageways of trachea and lungs of a dog

Anterior view at left, posterior view at right

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18 Digestive System

The digestive system is the extensive environmental interface that makes it possible to transfer nutrients, water, and electrolytes from the food we eat into the body’s internal environment. This is made possible by a complex lining, which through a series of folds and a variety of small

to microscopic projections greatly increases the surface interface between the digested contents within the gastrointestinal organs and the numerous small capillaries beneath this lining. To better appreciate the degree of this surface increase, realize that the average total surface area of the skin of an adult human is about 20 square feet, while the surface area of the digestive system is approximately 2,500 square feet, or about the size of a tennis court. To make the transfer across this extensive surface area possible, the food we eat must be broken down into small molecules that can be absorbed from the digestive tract into the circulatory system, which then distributes the molecular metabolites to the cells. Therefore, the digestive organs also function in the mechanical and chemical breakdown of the food.

Developmentally the digestive system begins as a simple tube called the gut tube or gut. As this simple tube develops into the highly convoluted organs of the adult anatomy, it undergoes structural changes that account for its various functions. Though these structural changes lead to differences in the tube from one region to the next, there is a basic pattern of design throughout the

length of the tube. This structural pattern is responsible for the general function of the digestive system. Modifications of this pattern allow for the variation in structure and function along its length. This chapter will highlight the structural variation and underlying design of the digestive system.

Find more information about the digestive system in

R E A L A N AT O M Y

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The wall of the digestive tract has a basic pattern of design that is found throughout its length. This pattern consists of three tunics or layers of anatomy. The tunica

mucosa and its subdivisions, including the tela submucosa, form the inner layer of the wall and consist of an extensive epithelial lining with an underlying vascullar connective tissue. The middle layer, or tunica muscularis, consists of smooth muscle that provides for the varied types of movements that occur within the digestive organs. The majority of the organs have an outer layer, the tunica serosa, comprised of a lubricated meosthelial membrane that reduces friction as the organs move against one another. The image below, from the small intestine, illustrates the basic layers of the digestive tract wall.