Biomechanics of respiration

  1. The bottom line
  2. Intrathoracic volume and pressure
  3. Thoracic anatomy
  4. Thoracic movement during inspiration and expiration
  5. Muscles of respiration
  6. The diaphragm

1. The bottom line

2. Intrathoracic volume and pressure

3. THORACIC ANATOMY

The T10 spinous process is a posterior surface landmark for the inferior boundary of lung substance (Clemente, 1981, Fig.129).

  • base of scapular spine is at T4
  • inferior angle of scapula is at T8

The xiphoid process is an anterior surface landmark for the inferior boundary of lung substance (Clemente, 1981, Fig.128).

4. Thoracic movement during inspiration and expiration

INSPIRATION

EXPIRATION

DIAPHRAGM DESCENDS

RIBCAGE ELEVATES AND/OR EXPANDS

INCREASED INTRATHORACIC VOLUME

DECREASED INTRATHORACIC PRESSURE

'HIGH PRESSURE' EXTERIOR AIR FLOWS INTO 'LOW PRESSURE' LUNG.

DIAPHRAGM ASCENDS

RIBCAGE DESCENDS AND/OR CONTRACTS

DECREASED INTRATHORACIC VOLUME

INCREASED INTRATHORACIC PRESSURE

'HIGH PRESSURE' AIR IN LUNG FLOWS OUT TOWARD 'LOW PRESSURE' EXTERIOR.

5. Muscles of respiration

Inspiration

Expiration

Quiet
(primary muscles)

diaphragm
external intercostals

elastic recoil of lung tissue
surface tension
gravity on ribs
internal intercostals

Forced
(secondary or accessory muscles)

sternocleidomastoideus
scalenes

pectoralis major
pectoralis minor

serratus anterior
serratus posterior superior

upper iliocostalis

abdominals external oblique
internal oblique
rectus abdominus

lower iliocostalis
lower longissimus
serratus posterior inferior

Some authors classify the quadratus lumborum and latissimus dorsi as secondary muscles of expiration, but the reasoning is not as clear as with the muscles listed in the table.

Diagrams of secondary (accessory) muscles of respiration

6. THE DIAPHRAGM

Shortening of diaphragmatic fibers pulls
  1. inferiorly on central tendon
  2. superiorly on lower ribs

Anterior view of diaphragm (Rasch & Burke, 1974, Fig.14-3) showing its lines of application where it attaches to the ribs and to the central tendon.

"Top down" view of diaphragm (Clemente, 1981, Fig.193) showing location of central tendon.

During inspiration, the diaphragm's central tendon descends until it is fixed or stabilized by forces that develop in:

  1. elongated mediastinal structures, which pull upward on the diaphragm

  2. compressed abdominal contents, which push upward on the descending diaphragm

When the central tendon becomes stable, it is still superior to the diaphragm's mobile attachments on the lower ribs. Therefore, the diaphragm's muscular lines of application elevate the lower ribs. Because of the orientation of the lower ribs' attachments to the vertebrae, rib elevation expands the thorax' lateral dimensions.

References: Blaber, M. (1996). The ideal gas equation. Retrieved from October 31, 2001 from Florida State University, General Chemistry 1, A Virtual Textbook Web site: http://wine1.sb.fsu.edu/chm1045/notes/Gases/IdealGas/Gases04.htm

Clemente, C.D. (1981). Anatomy. (2nd ed.). Baltimore: Urban and Schwarzenberg.

Kapandji, I.A. (1974). Functional components of the vertebral column. In I.A. Kapandji, The physiology of the joints: Vol. 3. The trunk and the vertebral column. New York: Churchill Livingstone.

Poole, D.C., Sexton, W.L., Farkas, G.A., Powers, S.K., Reid, M.B. (1997). Diaphragm structure and function in health and disease. Medicine and Science in Sports and Exercise, 29, 738-54. (full text version is available on Medline; Unique Identifier: 97362737).

Rasch, P.J., & Burke, R.K. (1978). Kinesiology and applied anatomy (6th ed.). Philadelphia: Lea and Febiger.

Last updated 10-31-01 Dave Thompson PT
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