Muscles of Respiration

We breathe primarily by means of muscles that enclose the thoracic cavity—the diaphragm, which forms its floor; 11 pairs of external intercostal muscles, which lie superficially between the ribs; and 11 pairs of internal intercostal muscles between the ribs deep to the external intercostals (fig. 10.13; table 10.5). The lungs themselves contain no skeletal muscle; they do not play an active part in their own ventilation.

The diaphragm36 is a muscular dome between the abdominal and thoracic cavities. It has openings that allow passage of the esophagus and major blood vessels. Its fascicles converge from the margins toward a fibrous central tendon. When the diaphragm contracts, it flattens slightly, increasing the volume of the thoracic cage and

5spleni = bandage

6dia = across + phragm = partition

5spleni = bandage

6dia = across + phragm = partition

Diaphragm Viewed From Below
Figure 10.13 Muscles of Respiration. (a) The intercostal muscles, viewed from the left. (b) The diaphragm, viewed from below.

Saladin: Anatomy & I 10. The Muscular System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

346 Part Two Support and Movement

Table 10.5 Muscles of Respiration (see fig. 10.13)

O = origin, I = insertion, N = innervation (n. = nerve, nn. = nerves)

Diaphragm (DY-uh-fram)

Prime mover of inspiration; compresses abdominal viscera to aid in such processes as defecation, urination, and childbirth O: xiphoid process, ribs 10-12, costal cartilages 5-9, lumbar vertebrae I: central tendon

N: phrenic n.

External Intercostals (IN-tur-COSS-tulz)

When scalenes fix rib 1, external intercostals draw ribs 2-12 upward and outward to expand thoracic cavity and inflate lungs O: inferior margins of ribs 1-11 I: superior margins of ribs 2-12

N: intercostal nn.

Internal Intercostals

When quadratus lumborum and other muscles fix rib 12, internal intercostals draw ribs downward and inward to compress thoracic cavity and force air from lungs; not needed for relaxed expiration O: inferior margins of ribs 1-11 I: superior margins of ribs 2-12 N: intercostal nn.

creating a partial vacuum that draws air into the lungs. Its contraction also raises pressure in the abdominal cavity below, thus helping to expel the contents of the bladder and rectum and facilitating childbirth—which is why people tend to take a deep breath and hold it during these functions.

The external intercostals37 extend obliquely downward and anteriorly from each rib to the rib below it. When the scalenes fix the first rib, the external intercostals lift the others, pulling them up somewhat like bucket handles. This action pulls the ribs closer together and draws the entire rib cage upward and outward, expanding the thoracic cage and promoting inhalation.

When the diaphragm and external intercostals relax, the thoracic cage springs back to its prior size and expels the air. The only muscular effort normally expended in exhaling is for the inspiratory muscles to maintain partial tension (tonus) and exert a braking action, so exhalation is smooth and not explosive. However, forced expiration— exhaling more than the usual amount of air or exhaling quickly as in blowing out a candle—is achieved mainly by the internal intercostals. These also extend from one rib to the next, but they lie deep to the external intercostals and have fascicles at right angles to them. The abdominal muscles also aid in forced expiration by pushing the viscera up against the diaphragm.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


  • demsas
    How we breathe and involvement of the brain, intercostal muscles and diaphragm?
    5 years ago
  • onni
    What muscles May aid enforced expiration by pushing viscera against the diaphragm?
    4 years ago
  • carmen
    What muscle may aid in forced expiration by pushing viscera against the diaphragm?
    1 month ago

Post a comment