Lumina are the spaces inside of tubular-shaped structures within the body. For example, the "open cavity" through which food travels down the esophagus to the stomach is a lumen. A lumen in anatomy can also refer to an aperture or an opening within a fixed structure, such as the circular hole in a vertebral bone through which the spinal cord courses.
One of the largest lumina in anatomy is the open space within the aorta, which is the largest vessel of the body. Blood flows from the left side of the heart through the aorta to the rest of the body. Tears in the aorta can cause serious and even life-threatening conditions. For instance, a small tear into the aorta's tunica intima, the innermost tissue layer of the blood vessel, can result in a collection of blood between the wall tissue layers called an aneurysm. An aneurysm can increase in size until either it blocks off the entire lumen of the vessel or can rupture open. The rupturing of the aorta, known as aortic dissection, can result in death.
Another example of a massive lumen is the foramen magnum, the largest aperture at the base of the skull. This bony hole is the anatomical demarcation of where the brain stem becomes the spinal cord. This is an important structure through which the transmission of nerve impulses to the body occurs. Any major swelling or increases in pressure within the skull cavity can displace the brain downward through the foramen magnum, resulting in death. This condition is known as the Arnold-Chiari malformation.
Medium-sized lumina can be represented by open cavities within the esophagus, the small intestine, the large intestine and the colon. The stomach would not normally be considered a lumen due to the bulbous shape of the organ, but technically, due to the passageway or channel-like nature of it, one could call the open space within it a lumen. The transmission of nutrients passes through the walls of the gastrointestinal tract. Perforations within this lumen can result in the need for emergency surgery.
Smaller examples of lumina include ducts and channels traveling between organs. A prime specimen of this group is the Cystic duct, which courses from the gallbladder, an organ that collects bile, to the bile duct, which empties into the duodenum at small opening in the duodenum's wall called the major duodenal papilla. This lumen allows for the passage of bile from the gallbladder to the intestines in order to help with the digestion of food.
Tiny lumina make up the majority of the lumen structures within the body. This group of lumina include the open spaces within anatomical tubes, such as arteries and veins, where blood can pass from one area of the body to the next. The tiniest of these lumina can be found in the kidney. Lumina of this size make up the glomeruli apparatuses, the extraordinarily tiny blood vessels that allow for the filtration of sodium, water and ammonia from the blood to form urine.