Ducts are small tubes throughout your body that transport fluids from one place to another. They carry everything from bile and breast milk to tears and sweat, connecting the organs that produce these fluids to the surfaces where they’re needed. Almost every body system relies on some form of ductal network, and understanding how they work helps explain a surprising range of common health conditions.
How Ducts Work
A gland is a group of cells that produces and releases a substance. The two main types, exocrine and endocrine, are defined by how they deliver their products. Endocrine glands (like the thyroid or adrenal glands) release hormones directly into the bloodstream with no duct involved. Exocrine glands use ducts to channel their secretions to a specific surface, whether that’s the inside of your intestine, the surface of your skin, or the inside of your mouth.
Each duct system has two basic parts: the acinus, which is the cluster of cells that actually makes the fluid, and the duct itself, which acts as the delivery tube. Some ducts are microscopic. Others, like the main bile duct, are large enough to measure on an ultrasound, with a normal diameter of about 6 millimeters.
Ducts in the Digestive System
The digestive system has some of the body’s most critical ducts, organized into a network called the biliary tree. Your liver continuously produces bile, a fluid that breaks down fats. Tiny bile ducts inside the liver collect this bile and merge into progressively larger tubes until they form the common bile duct, essentially the trunk of the tree.
Meanwhile, your pancreas produces digestive enzymes and sends them through the pancreatic duct. This duct connects with the common bile duct, and together they empty into the first section of your small intestine (the duodenum) through a shared opening called the ampulla of Vater. This is where bile acids and pancreatic enzymes mix with food and begin breaking it down for absorption.
This ductal intersection is also where problems frequently arise. If a gallstone lodges in the common bile duct, it can block the flow of bile entirely, causing sudden intense pain in the upper right abdomen or just below the breastbone, pain between the shoulder blades, nausea, and vomiting. A gallstone blockage that persists can lead to jaundice (yellowing of the skin and eyes), high fever, and chills, all of which require emergency care.
Salivary Ducts
You have three pairs of major salivary glands, each with its own duct emptying into your mouth. The parotid gland, the largest, sits just in front of each ear and delivers saliva through a duct about 50 millimeters long that opens inside your cheek near the upper molars. The submandibular gland, tucked beneath the jaw, uses a slightly longer duct (about 58 millimeters on average) that opens under the tongue.
These ducts can develop calcium deposits called salivary stones, similar in concept to kidney stones. When a stone blocks a salivary duct, the gland swells painfully, especially during meals when saliva production ramps up. Most salivary stones are small enough to pass on their own or with gentle massage, though larger ones sometimes need to be removed.
Breast and Milk Ducts
The breast contains a network of milk ducts that carry milk from the glandular tissue to the nipple. Older anatomy textbooks describe 15 to 20 ducts per breast, but ultrasound imaging in lactating women tells a different story. The actual number is closer to nine on average, with a range of about 4 to 18 per breast. During breastfeeding, as few as one to five ducts may be actively conveying milk at any given time.
These ducts are also where the most common type of breast cancer begins. Ductal carcinoma in situ (DCIS) starts when cells lining the inside of a milk duct develop abnormal DNA changes. In DCIS, the abnormal cells remain confined inside the duct and haven’t spread into surrounding breast tissue, which is why it’s classified as stage 0 or noninvasive breast cancer. The term “ductal” in a breast cancer diagnosis simply means the cancer originated in the duct lining rather than in the milk-producing lobules themselves.
Tear Ducts
Your tear ducts don’t produce tears. They drain them. The lacrimal glands above each eye produce the tear film that keeps your eyes moist. After tears wash across the eye’s surface, they exit through two tiny openings (punctae) at the inner corners of your eyelids, flow through small channels within the eyelid, collect in the lacrimal sac near the bridge of your nose, and then travel down through a bony canal that empties into your nasal cavity. This is why your nose runs when you cry.
Blocked tear ducts are common in newborns because the lower end of the duct sometimes hasn’t fully opened at birth. In adults, blockages can result from infection, inflammation, or age-related narrowing, leading to persistent watery eyes, crusting, and recurrent eye infections.
Sweat Gland Ducts
Your skin contains two types of sweat glands, each with a different ductal arrangement. Eccrine glands cover most of the body and open directly onto the skin’s surface through their own tiny pores. These are your primary cooling system, producing the watery sweat that evaporates to regulate body temperature.
Apocrine glands are concentrated in the armpits and groin. Instead of opening directly onto the skin, their ducts empty into hair follicles, which then lead to the surface. Apocrine sweat is thicker and contains proteins and lipids that skin bacteria feed on, which is why these areas are more prone to body odor. When sweat ducts become blocked, the result is heat rash: small, itchy bumps that appear when trapped sweat irritates the surrounding skin.
Reproductive Tract Ducts
In the male reproductive system, sperm travel through a series of four ducts to reach the outside of the body. After being produced in the testes, sperm first enter the epididymis, a tightly coiled tube where they mature and gain the ability to swim. From there, they pass into the vas deferens (also called the ductus deferens), a muscular tube that stores sperm and uses wave-like contractions to propel them forward during ejaculation. The vas deferens connects to the ejaculatory duct, which merges with the urethra for the final exit. A vasectomy works by cutting or blocking the vas deferens, interrupting this ductal chain.
In the female reproductive system, the fallopian tubes serve as the primary ducts, transporting eggs from the ovaries toward the uterus. Fertilization typically happens inside the fallopian tube itself. Blockages in these tubes are a significant cause of infertility.
The Lymphatic System’s Main Duct
The thoracic duct is the largest lymphatic vessel in the body, running from the abdomen up through the chest. It collects lymph fluid, a clear fluid that carries immune cells and filters waste, from roughly three-quarters of the body. The thoracic duct empties this fluid back into the bloodstream near the left shoulder, completing a circuit that helps maintain fluid balance and supports immune function. A smaller duct on the right side handles lymph from the right arm, the right side of the chest, and the right side of the head.