The chest cavity, or thoracic cavity, contains and your heart, lungs and your body's major blood vessels and airways. Your rib cage surrounds your chest cavity and shields your heart, lungs and other organs. Muscles around and at the bottom of the chest cavity aid in respiration and increasing intra-abdominal pressure.
Diaphragm
The diaphragm muscle makes up the entire bottom of your chest cavity. The diaphragm is a large, broad muscle that forms a muscular partition between your chest and abdominal cavities. In a relaxed state, your diaphragm is dome shaped, but when it contracts, it moves downward and flattens, increasing the volume of your chest cavity. Your diaphragm contracts automatically in a rhythmic fashion during respiration, but you can also contract your diaphragm voluntarily to increase pressure in your abdominal cavity. This ability is useful when evacuating feces, urine or a baby and also during weight lifting.
External Intercostal Muscles
The external intercostal muscles lie between your ribs and surround your chest cavity, including the bottom of your chest cavity that houses your diaphragm. These muscles are considered inspiratory muscles and work to lift your rib cage, pulling your ribs toward one another. This action increases the dimensions of your chest cavity, allowing you to inhale.
Internal Intercostal Muscles
The internal intercostal muscles surround your entire chest cavity, including the bottom. These muscles also lie between your ribs, lying underneath your external intercostals. Your internal intercostals act to draw your ribs together and depress your rib cage while also aiding in active, or forced, expiration.
Rectus Abdominis
The rectus abdominis muscles, although part of your abdomen, connect to your xiphoid process and fifth, sixth and seventh ribs, which are part of the bottom of your chest cavity. Your rectus abdominis muscles act to fix and depress your ribs as well as increase intra-abdominal pressure.
External Obliques
The external obliques, like your rectus abdominis, are also part of your abdominal cavity. However, the muscles' origin is at the bottom of your chest cavity. Your external obliques aid your rectus abdominis in flexing your vertebral column and in compressing your abdominal wall and increasing intra-abdominal pressure.
References
- "Anatomy & Physiology, Second Edition"; Elaine N. Marieb; 2005
- "Human Body: An Illustrated Guide to Every Part of the Human Body and How It Works"; Martyn Page; 2001
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