Essentials of Anatomy and Physiology

the nervous system and the respiratory muscles. The
respiratory centers are located in the medulla and
pons. Their specific functions will be covered in a later
section, but it is the medulla that generates impulses
to the respiratory muscles.
These muscles are the diaphragm and the external
and internal intercostal muscles (Fig. 15–6). The
diaphragmis a dome-shaped muscle below the lungs;

when it contracts, the diaphragm flattens and moves

downward. The intercostal muscles are found between

the ribs. The external intercostal musclespull the

ribs upward and outward, and the internal inter-

costal musclespull the ribs downward and inward.

Ventilation is the result of the respiratory muscles pro-

ducing changes in the pressure within the alveoli and

bronchial tree.

348 The Respiratory System

Frontal sinuses

Sphenoidal sinuses

Nasal cavity

Nasopharynx

Soft palate

Epiglottis

Larynx and vocal folds

Trachea

Superior lobe

Right lung

Right primary

bronchus

Inferior lobe

Mediastinum

Cardiac notch

Pleural space

Pleural membranes

Inferior lobe

Bronchioles

Superior lobe

Left primary bronchus

Left lung

Venule

Alveolus

Alveolar duct

Arteriole

Pulmonary capillaries

B

A

Middle lobe

Diaphragm

Figure 15–4. Respiratory system. (A) Anterior view of the upper and lower respiratory

tracts. (B) Microscopic view of alveoli and pulmonary capillaries. (The colors represent the

vessels, not the oxygen content of the blood within the vessels.)

QUESTION:What are the first branches of the trachea, and how do they resemble the tra-

chea in structure?