The fundus isn’t a single organ. It’s an anatomical term for the part of a hollow organ that sits farthest from its opening. Your stomach, uterus, eye, gallbladder, and bladder all have a fundus, and each one serves a different purpose. The stomach fundus is the most commonly referenced, but the eye and uterine fundus play important roles in diagnostics and pregnancy monitoring.
The Stomach Fundus: Storage and Hunger Signals
The fundus of your stomach is the dome-shaped upper portion that sits above where your esophagus connects. Its primary job is acting as a reservoir. When you swallow food, your vagus nerve triggers what’s called a “receptive relaxation reflex,” which causes the fundus to expand and make room. This is remarkably efficient: your stomach can increase its volume by 1.5 liters with only a small rise in internal pressure.
Once food arrives, it sits largely unmixed in the fundus for up to an hour. During that time, the contents naturally separate by density. Fats float to the top, while liquids flow around solid food and settle at the bottom. This layering helps your stomach process different types of food at different rates before pushing them into the lower stomach for more aggressive churning and digestion.
The fundus also plays a role in appetite. Your stomach is the primary source of ghrelin, the hormone that tells your brain you’re hungry. Ghrelin levels spike when your stomach is empty, peaking right before mealtimes, and drop after you eat. This signaling loop between your stomach and brain is one reason you feel hungry on a predictable schedule.
The Stomach Fundus in GERD Surgery
The fundus has a second life in surgical treatment for chronic acid reflux. In a procedure called Nissen fundoplication, a surgeon wraps the fundus around the bottom of the esophagus to reinforce the valve (lower esophageal sphincter) that normally keeps stomach acid from flowing upward. The wrap acts like a tightened collar, compensating for a sphincter that has become too weak or relaxes too often. This is one of the most common surgical treatments for gastroesophageal reflux disease when medications aren’t enough.
The Eye Fundus: A Window Into Your Health
In the eye, the fundus refers to the entire inner back surface. It includes the retina, the macula (responsible for sharp central vision), the fovea (the tiny pit at the center of the macula), the optic disc (where the optic nerve enters), and a network of blood vessels. Unlike any other fundus in the body, this one can be directly observed without surgery, using a standard ophthalmoscope or a fundus camera.
That visibility makes the eye fundus one of the most valuable diagnostic surfaces in medicine. The blood vessels on the retina reflect what’s happening in your circulatory system overall. Doctors can spot signs of diabetic damage, high blood pressure, and blood vessel blockages just by looking at the back of your eye. More recently, researchers have used images of the eye fundus to predict five-year cardiovascular disease risk and even detect early signs of neurological conditions like Alzheimer’s and Parkinson’s disease.
Fundus photography captures 30- to 50-degree views of the retina, and newer ultra-widefield systems can image up to 200 degrees in a single shot. For measuring retinal thickness and detecting fluid buildup, optical coherence tomography (OCT) has become the standard. These tools together give clinicians a detailed, non-invasive look at structures that would otherwise require surgery to examine.
The Uterine Fundus: Tracking Pregnancy Growth
The fundus of the uterus is its topmost part, opposite the cervix. Outside of pregnancy, it’s not something most people think about. During pregnancy, it becomes a key measurement tool.
As the fetus grows, the uterine fundus rises predictably through the abdomen. At about 12 weeks, the fundus sits near the pubic bone. By 20 weeks, it reaches the belly button. At 36 weeks, it climbs all the way to the breastbone, the highest point it will reach. After 36 weeks, the fundus drops as the baby descends into the pelvis in preparation for labor.
Healthcare providers measure “fundal height,” the distance in centimeters from the pubic bone to the top of the fundus, as a simple way to track fetal growth. After 20 weeks, the number in centimeters should roughly match the number of weeks pregnant you are, plus or minus two. If you’re 32 weeks along, a fundal height between 30 and 34 centimeters is considered normal. A measurement outside that range can be the first sign of growth problems or an unusual fetal position, prompting further evaluation with ultrasound.
The Gallbladder and Bladder Fundus
The gallbladder fundus is the rounded bottom portion that projects slightly beyond the edge of the liver. The gallbladder itself stores and concentrates bile, which your liver produces to help digest fats. The fundus doesn’t have a specialized role separate from the rest of the gallbladder; it’s simply the anatomical landmark for the organ’s lowest, most visible tip, often the part a surgeon identifies first during imaging or an operation.
The bladder fundus sits at the back, bottom part of the organ. Like the gallbladder fundus, it’s more of a positional label than a functionally distinct region. It’s worth knowing primarily because it appears in imaging reports and surgical notes, where its location helps describe where a finding or abnormality sits within the organ.