Thoracic diaphragm

The thoracic diaphragm, or simply the diaphragm, is a big, flat muscle at the bottom of the chest in humans and many animals. It helps us breathe. When the diaphragm tightens, it makes more space in the chest. This pulls air into the lungs.

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

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

The word "diaphragm" was created long ago by a person named Gerard of Cremona. In the body, “diaphragm” can sometimes mean other flat structures, but usually people 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 below it.

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

Structure

The diaphragm is a muscle that separates the chest from the belly. It looks like a dome and helps us breathe.

It connects to the ribs and spine, and all its muscles meet 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 down. This makes the space in our chest bigger and pulls air into our lungs.

When the diaphragm relaxes and moves up, we breathe out. The diaphragm also helps with other body functions. It helps us throw up, pass waste, and helps during childbirth by pushing on our belly. It also helps stop stomach acid from moving back up into our esophagus.

Clinical significance

Paralysis

If the phrenic nerve, cervical spine, or brainstem gets damaged, the diaphragm can’t work well. This can happen 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. This can cause discomfort because the normal pressure between the chest and belly changes. Not all of these hernias cause problems.

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. This condition happens in a small number of births and often needs surgery.

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