The Lungs

The lungs are 2 sponge-like organs in your chest cavity. Your right lung has 3 sections, called lobes. The left lung has 2 lobes. It is smaller because the heart takes up more room on that side of the body. The lungs bring air in and out, taking in oxygen and getting rid of carbon dioxide gas, a waste product of the body.

The lining, which surrounds the lungs and helps to protect them and ease the sliding motion during breathing, is called the pleura. The chest cavity is called the pleural cavity. The trachea (windpipe) brings air down into the lungs. It divides into tubes called the bronchi, which divide into smaller branches called the bronchioles. At the end of the bronchioles are tiny air sacs known as alveoli.

The Diffuse Neuroendocrine System

The diffuse (spread out or widely scattered) neuroendocrine system is made up of cells that are in certain ways like nerve cells and in other ways like cells of endocrine (hormone-producing) glands. These cells do not form an actual organ like the pancreas, adrenal, or thyroid. Instead, they are scattered throughout your body in organs like the lungs, stomach, and intestines.

Neuroendocrine cells produce hormones like adrenaline and adrenaline-like substances. This may help control air flow and blood flow in the lungs and may help control growth of other types of lung cells. These neuroendocrine cells may detect decreased oxygen or increased carbon dioxide in the air we breathe and then release chemical messages to help the lungs adjust to changes in air composition. People who live at higher altitudes have more lung neuroendocrine cells, apparently because there is less oxygen in the air they breathe.

Neuroendocrine Cancers

Like most cells in your body, lung neuroendocrine cells sometimes undergo certain changes that cause them to grow too much and form tumors. The tumors that develop from neuroendocrine cells are known as neuroendocrine tumors or neuroendocrine cancers. These tumors can develop anywhere in the body.

There are 4 types of neuroendocrine lung tumors. The most serious type, small cell lung cancer (SCLC), is one of the most rapidly growing and spreading of all cancers. It is discussed in a separate American Cancer Society lung cancer document. Large cell neuroendocrine carcinoma is a rare cancer that, except for the size of the cells forming the cancer, is very similar to SCLC in its prognosis (outlook for survival) and in how patients are treated. Carcinoid tumors, also known as carcinoids, comprise the other 2 types of lung neuroendocrine cancer: typical carcinoid and atypical carcinoid. This document will only cover these 2 types of tumors. Another ACS document discusses carcinoid tumors that begin in the gastrointestinal tract, another common site for these tumors.

Carcinoid Tumors

Typical and atypical carcinoid tumors are distinguished from each other by their appearance under the microscope.

• Typical carcinoids grow slowly and only rarely spread beyond the lungs. They are 9 times as common as atypical carcinoids.

• Atypical carcinoids grow a little faster and are somewhat more likely to spread to other organs. They have more cells in the process of dividing and look more like a fast-growing tumor. They are much less common than typical carcinoids.

In addition to being classified as typical or atypical based on how they look under a microscope, carcinoids are sometimes also classified according to where they form within the lung.

• Central carcinoids form in the walls of large airways near the center of the lungs.

• Peripheral carcinoids develop in the narrower airways toward the edges of the lungs.

This distinction is important because the tumor’s location determines which symptoms a patient will have. (See the section "How Are Lung Carcinoid Tumors Diagnosed?.") Nearly all central carcinoid tumors are also typical carcinoids. Most peripheral carcinoids are also typical carcinoids.

Revised: 08/07/2006