Thoracic diaphragm

The thoracic diaphragm, or simply the diaphragm, is a big, flat muscle that sits at the bottom of the chest in humans and many other animals. It plays a very important role in breathing. When the diaphragm contracts, or tightens, it makes the space in the chest bigger. This helps pull air into the lungs so we can breathe.

This muscle separates the chest area, where the heart and lungs are, from the area below, called the belly. The diaphragm uses a lot of energy, which is why it has many tiny parts inside called mitochondria and lots of tiny blood vessels called capillaries—more than any other muscle in the body.

Structure of diaphragm shown using a 3D medical animation still shot

The word "diaphragm" was created a long time ago by a person named Gerard of Cremona. In the body, "diaphragm" can sometimes mean other flat structures, but when people talk about "the diaphragm," they usually mean the one in the chest. In humans, the right side of the diaphragm is a little higher than the left side because of the liver sitting underneath it.

Many animals, like other mammals, also have diaphragms to help them breathe. Even some other animals, like amphibians and reptiles, have something similar to a diaphragm, though it might look a little different.

Structure

The diaphragm is a muscle and tissue that separates the chest area from the belly. It looks like a dome and moves to help us breathe.

Definition of diaphragm in Blount's 1707 Glossographia Anglicana Nova

It connects to the ribs and spine, and all its muscles come together at a strong central point. There are openings in the diaphragm that let important structures pass between the chest and belly, like the aorta, esophagus, and inferior vena cava.

The diaphragm gets its nerve signals mainly from the phrenic nerve, which comes from the neck. It also receives blood from arteries above and below it.

Openings through the diaphragm and their content
! Description	Vertebral level	Contents
caval opening	T8	The caval opening passes through the central tendon of the diaphragm. It contains the inferior vena cava, and some branches of the right phrenic nerve. The outermost wall of inferior vena cava is fused with the central tendon.
esophageal hiatus	T10	The esophageal hiatus is situated in the posterior part of the diaphragm, located slightly left of the west central tendon through the muscular sling of the right crus of the diaphragm. It contains the esophagus, and anterior and posterior vagal trunks, left gastric artery and veins, and lymphatics.
aortic hiatus	T12	The aortic hiatus is in the posterior part of the diaphragm, between the left and right crus. It contains the aorta, the thoracic duct and Azygous vein.
Under the medial lumbocostal arch		sympathetic trunk, and least splanchic nerves
Under the lateral lumbocostal arch		Subcostal nerve and vessels
areolar tissue between the sternal and costal parts (see also foramina of Morgagni)		the superior epigastric branch of the internal thoracic artery and some lymphatics from the abdominal wall and convex surface of the liver
areolar tissue between the fibers springing from the medial and lateral lumbocostal arches		This interval is less constant; when this interval exists, the upper and back part of the kidney is separated from the pleura by areolar tissue only.

Function

The diaphragm is the main muscle that helps us breathe. When we breathe in, the diaphragm contracts and moves downward. This makes the space in our chest bigger and creates a partial vacuum. This vacuum pulls air into our lungs.

When the diaphragm relaxes and moves upward, we breathe out. The diaphragm also helps with other important body functions. It helps us throw up, pass waste, and even helps during childbirth by increasing pressure in our belly. It also helps stop stomach acid from coming back up into our esophagus.

Clinical significance

Paralysis

If the phrenic nerve, cervical spine, or brainstem gets damaged, the diaphragm can’t work properly. This often happens because of bronchial cancer, which usually affects just one side. Other reasons include Guillain–Barré syndrome and systemic lupus erythematosus.

Herniation

Main articles: Hiatus hernia and Congenital diaphragmatic hernia

X-ray of chest, showing top of diaphragm.

A hiatus hernia happens when part of the stomach moves up through the diaphragm into the chest area. This can cause discomfort because the normal pressure between the chest and belly isn’t there anymore. Not all of these hernias cause problems, but many people with certain stomach conditions have them.

Sometimes, babies are born with a congenital diaphragmatic hernia, where the diaphragm doesn’t close properly. This can let parts of the belly, like the intestines, move into the chest and affect lung development. This condition happens in a small number of births and often needs surgery right away.

Imaging

Because the diaphragm sits between the chest and belly, doctors can use an X-ray to look for problems. For example, they can see if there’s extra fluid around the lungs or air in the belly that shouldn’t be there. X-rays also help check for hernias.

Significance in strength training

Some people think that when we exercise, we breathe deeper to help our bodies take in more oxygen. But our bodies get oxygen from the lungs no matter how deep we breathe. When we exercise, we breathe faster and the diaphragm works harder. This helps more oxygen get into our blood. The diaphragm stays in the same place, but our need for oxygen goes up when we exercise hard. The diaphragm works mostly on its own during exercise, matching how much oxygen our body needs.

Other animals

The diaphragm is a structure found in many animals, including fish, amphibians, reptiles, and birds, though it works differently in each group. In some animals like fish and amphibians, the diaphragm helps move air or water, but it does not work the same way as in mammals. Birds, for example, do not have diaphragms at all. Instead, they breathe using a special system of air sacs and the movement of their chest bones to pull air in and out efficiently. This allows birds to get enough oxygen even though their lungs do not change size like mammalian lungs do.