The diaphragm sits directly beneath the heart, forming a dome-shaped muscular floor that separates the chest from the abdomen. Just below that partition lies a dense neighborhood of organs: the stomach and spleen on the left side, the liver’s large left lobe in the center and right, and deeper structures including the pancreas, kidneys, and adrenal glands. Several layers of abdominal muscle also wrap beneath and around these organs. The arrangement is tighter and more interactive than most people realize, with the heart’s performance linked in surprising ways to the pressure and movement of everything below it.
The Diaphragm Is the Dividing Line
Before you can map what lies “under” the heart, you need to know what separates the heart from those structures. The diaphragm is a broad, thin sheet of muscle and tendon that curves upward into a dome shape beneath the lungs and heart. Its central portion is a flat tendon (the central tendon), while its outer rim is skeletal muscle that anchors to the lower ribs, the breastbone, and the spine. In a study of over a hundred adult human diaphragms, the majority showed a tendon-to-muscle ratio between about 10 and 15 percent, though some specimens had a much larger tendinous area at the expense of muscle bulk.1SpringerLink (Surgical and Radiologic Anatomy). The clinical anatomy of the musculotendinous part of the diaphragm The heart actually rests on the central tendon, separated from the organs below by only this thin muscular sheet and the pericardial sac.
The diaphragm is not just a passive wall. It contracts with every breath, flattening downward to pull air into the lungs and then relaxing back into its dome shape during exhalation. That rhythmic pumping action also pushes on the abdominal organs below and influences blood flow back to the heart. It functions as a pump that facilitates venous and lymphatic return to the heart, modulates the pressure the left ventricle has to work against, and even helps regulate autonomic nervous system tone.2PubMed. Diaphragmatic Function in Cardiovascular Disease: JACC Review Topic of the Week In healthy people, this abdominal pumping action improves stroke volume and cardiac output both at rest and during exercise.3PubMed Central. Effects of diaphragmatic contraction on lower limb venous return and central hemodynamic parameters contrasting healthy subjects versus heart failure patients at rest and during exercise
The Stomach and Spleen on the Left
If you could look straight down through the heart, the left side of the abdomen is the first territory you’d encounter. The stomach sits in the left upper quadrant, tucked just below the diaphragm. Its uppermost portion, called the fundus, is the part closest to the heart. The diaphragm is all that separates the fundus of the stomach from the heart and the left lung.4Int J Cadaver Stud Ant Var. A New Look at the Gross Anatomy of the Stomach to Understand How It Functions This is why a very full meal can sometimes feel like pressure in the chest, and why gas trapped at the top of the stomach can mimic heart-related discomfort.
Tucked behind and to the left of the stomach is the spleen, a fist-sized organ involved in filtering blood and supporting immune function. The spleen normally stays entirely below the diaphragm, nestled against the left kidney and the tail of the pancreas. In certain congenital conditions, though, the spleen can herniate upward through a hole in the diaphragm and end up sitting behind the left atrium of the heart. This has been documented in fetuses with left-sided congenital diaphragmatic hernia, where the herniated spleen can reside posterior to the heart in the right side of the chest.5PubMed Central. Spleen Behind the Heart Complicates Lung-to-Head Ratio Measurement in Left-Sided Congenital Diaphragmatic Hernia That finding underlines just how close the spleen normally sits to the underside of the heart’s position.
The Liver’s Left Lobe Sits Almost Directly Below
Most people think of the liver as a right-sided organ, and the bulk of it does sit in the right upper abdomen. But the liver has a left lobe that extends across the midline, reaching beneath the heart and the lower part of the breastbone. This left lobe is the part of the liver most directly “under” the heart in a typical person.
The closeness of the left lobe to the heart and sternum has real clinical consequences. During CPR, chest compressions push the breastbone downward toward the spine, and the liver’s left lobe can get caught between. Liver injury during CPR occurs in up to about 11 percent of cases, and the left lobe is the most commonly damaged portion because of its proximity to the sternum.6PubMed Central. Rare Complication of Cardiopulmonary Resuscitation—Liver Injury The sternum itself has a pointed lower tip (the xiphoid process) that can act almost like a blade during deep compressions, and one review described the liver as being at risk of being pierced by the “sword-shaped” sternum during chest compressions.7CHEST. Liver Laceration Following Cardiopulmonary Resuscitation: A Case Report and Review of the Literature This risk is part of why proper hand placement during CPR matters so much.
Deeper Structures You Cannot Feel
Behind the stomach and liver, several organs sit against the back wall of the abdomen in what anatomists call the retroperitoneal space. These are not directly beneath the heart in the same intimate way as the stomach or liver, but they occupy the territory below and behind it:
- Pancreas: This long, flat organ stretches horizontally across the upper abdomen, sitting behind the stomach. Its head is cradled by the curve of the duodenum on the right, while its tail reaches left toward the spleen.
- Left kidney and adrenal gland: The left kidney sits behind the stomach and spleen, roughly at the level of the lower ribs. Perched on top of each kidney is a small adrenal gland that produces stress hormones and helps regulate blood pressure.
- Aorta and inferior vena cava: The body’s largest artery (the aorta) descends from the heart, passes through the diaphragm, and runs down the left side of the spine through the abdomen. The inferior vena cava, the largest vein returning blood to the heart, runs alongside it on the right. Both of these major vessels pass through openings in the diaphragm, meaning they physically connect the chest and abdominal compartments.
- Transverse colon: The middle segment of the large intestine crosses the abdomen horizontally, roughly at navel height but sometimes higher. In some people, a portion of the transverse colon sits just below the stomach.
The esophagus also passes through the diaphragm via its own opening (the esophageal hiatus) to connect the throat to the stomach. This opening is a weak point in the diaphragm’s partition and becomes relevant when things go wrong, as discussed below.
Muscles That Wrap Beneath and Around
Below the diaphragm, the abdominal wall is built from several layers of muscle. The rectus abdominis runs vertically down the front of the abdomen on either side of the midline. Flanking it are three layers of lateral muscles: the external oblique on the outside, the internal oblique beneath it, and the transversus abdominis as the deepest layer. These muscles don’t sit directly “under” the heart in the way the diaphragm does, but they form the muscular container that holds abdominal pressure around the organs below the heart.
The intercostal muscles, which run between the ribs, also deserve mention. The lower intercostals overlap with the territory directly around and below the heart. They assist breathing and help stabilize the rib cage, working in concert with the diaphragm. And deeper still, the psoas major and quadratus lumborum muscles line the back wall of the abdomen along the spine, providing structural support to the retroperitoneal space where the kidneys and aorta sit.
Why Abdominal Pressure Affects the Heart
Because only the thin diaphragm separates the heart from the abdominal cavity, pressure changes below the diaphragm transmit upward and directly affect how well the heart fills and pumps. This is not a minor side effect. In animal studies, raising abdominal pressure by 40 mmHg reduced cardiac output and stroke volume by about 36 percent, because the increased pressure caused the diaphragm to bulge upward into the chest and raised the pressure around the heart.8PubMed. Cardiovascular responses to elevation of intra-abdominal hydrostatic pressure
The relationship between abdominal pressure and cardiac output is not straightforward, though. At moderate levels of increased abdominal pressure, the squeeze on abdominal veins actually pushes more blood back toward the heart, temporarily boosting output. Only when pressure climbs higher does the heart start to struggle. Research in swine showed this pattern clearly: cardiac output initially rose slightly at modest abdominal pressure, then fell at higher pressures as venous return was choked off and the heart’s filling was impaired.9PubMed. Influence of increased abdominal pressure on steady-state cardiac performance This matters in clinical settings where abdominal pressure can spike, such as after major abdominal surgery, severe bloating, or abdominal compartment syndrome. Recent work has further clarified how elevated abdominal pressure reduces venous return, raises pressure inside the chest, and impairs both the heart’s filling and its ability to contract effectively.10PubMed Central. Cardiovascular effects of intra-abdominal hypertension: current perspectives
When the Stomach Pushes Into the Heart’s Space
The most dramatic example of abdominal organs interfering with the heart involves hiatal hernias. In a hiatal hernia, part of the stomach pushes upward through the esophageal opening in the diaphragm and enters the chest cavity. Small hiatal hernias are common and usually harmless, causing nothing worse than acid reflux. But large or “giant” hiatal hernias can allow a substantial portion of the stomach to sit directly behind the heart.
When that happens, the displaced stomach can physically compress the heart. The main mechanism behind serious cardiovascular effects of giant hiatal hernias is direct cardiac compression.11PubMed. The cardiovascular effects of large hiatal hernias: a narrative review of cases and studies In one documented case, imaging revealed a giant hiatal hernia with the stomach located just behind the heart, and echocardiography confirmed a mass compressing the heart, producing abnormal changes on the patient’s electrocardiogram.12Internal Medicine. Dynamic Electrocardiographic Changes due to Cardiac Compression by a Giant Hiatal Hernia Patients with these hernias can develop chest pain, heart rhythm disturbances, and even heart failure symptoms that resolve once the hernia is surgically repaired. It is a striking reminder that the stomach is normally just millimeters of diaphragm away from the heart.
Referred Pain and Shared Nerve Pathways
The closeness of abdominal organs to the heart also creates confusion in how the body reports pain. The vagus nerve, one of the longest nerves in the body, sends branches to both the heart and many abdominal organs, including the gallbladder. The heart and gallbladder share overlapping spinal nerve connections. The vagus nerve innervates both, and their respective spinal pathways enter the spinal cord at similar levels (around T4 for the heart and T6 for the gallbladder).13PubMed Central. Cholecystitis Masquerading as Cardiac Chest Pain: A Case Report In acute gallbladder inflammation, this overlap can cause chest pain that closely mimics a heart attack, because the inflammation increases vagal tone and may even trigger coronary artery spasm.
This is not a rare diagnostic headache. Emergency departments regularly see patients with gallbladder disease who present with what looks like cardiac chest pain. The stomach can cause similar confusion: severe acid reflux, gastric ulcers, or even just a large meal can produce discomfort that radiates into the chest. The diaphragm itself can refer pain to the shoulder tip (because the phrenic nerve that controls it shares spinal roots with nerves serving the shoulder), which is why conditions as diverse as a ruptured spleen, a liver abscess, or air trapped under the diaphragm after surgery can all produce shoulder pain that has nothing to do with the shoulder itself.
Nerve Connections Through the Diaphragm
The diaphragm is not just a passive partition. It contains nerve structures that physically link the chest and abdominal compartments in ways researchers are still mapping. The phrenic nerve, which controls diaphragm contraction, turns out to also carry autonomic nerve fibers. A cadaver study found that the right phrenic nerve serves as a conduit for autonomic nerves and connects to a structure called the caval body, located where the inferior vena cava passes through the diaphragm. Within the wall of the vena cava at this point, researchers found an extensive network of veins, large cell bodies, and nerve fibers, along with strands of heart muscle tissue that had migrated into the vein wall.14Nature (Scientific Reports). The human phrenic nerve serves as a morphological conduit for autonomic nerves and innervates the caval body of the diaphragm This finding suggests the diaphragm hosts a kind of sensory relay station between the abdomen and heart that may help the body monitor and adjust blood flow in real time.
Why the Diaphragm Evolved to Separate These Compartments
The tight packing of vital organs above and below the diaphragm is not an accident of cramped anatomy. From an evolutionary standpoint, the diaphragm as a muscular partition developed because it conferred specific biological advantages. In early reptilian ancestors, the body cavity was a single large space (a coelom), and separating it into distinct thoracic and abdominal compartments improved the efficiency of both breathing and circulation. The muscular diaphragm evolved from pleural and peritoneal membrane folds in reptiles, driven by the advantage of generating negative pressure in the chest for drawing air into the lungs, and positive pressure in the abdomen for pushing venous blood back toward the heart, as well as for expulsive behaviors like airway clearance, defecation, and childbirth.15PubMed Central. Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals
In other words, having a muscular wall between the heart and the abdominal organs is not just about keeping things organized. The diaphragm actively assists the heart by creating alternating pressure zones with every breath cycle. The organs beneath the heart are not merely below it. They are part of a coordinated pressure system that makes mammalian circulation work as efficiently as it does. That evolutionary context helps explain why the diaphragm is so richly supplied with nerves and blood vessels, and why problems with it, whether from hernias, paralysis, or trauma, can have such immediate consequences for the heart above.