The heart sits in the thoracic cavity, which is the chest cavity. That answer is only the outermost layer, though. The heart is also inside the central compartment between the lungs, called the mediastinum, and more precisely inside its own small space, the pericardial cavity, enclosed by a protective sac called the pericardium. Which name counts as correct depends on how precise the question wants you to be, and anatomy courses usually expect the most specific one.1Anatomy and Physiology. Location of the Heart
The Three Nested Spaces That Hold the Heart
Each space sits inside the next, like boxes within boxes. From largest to smallest:
- Thoracic cavity: the whole chest space inside the ribs and above the diaphragm, holding the lungs and heart.2Anatomy and Physiology. Location of the Heart
- Mediastinum: the central compartment between the two lungs, where the heart sits in the middle.3Anatomy and Physiology. Location of the Heart
- Pericardium: a tough membrane sac (not itself a space) that separates the heart from the other structures in the mediastinum.4Anatomy and Physiology. Location of the Heart
- Pericardial cavity: the heart’s own space inside that sac.5Anatomy and Physiology. Location of the Heart
The mediastinum is split into parts. A superior part lies above an imaginary line, and an inferior part lies below it. The inferior part is divided again into front, middle, and back sections. The front section is a narrow zone behind the breastbone, and the back section lies in front of the spine. The heart and its sac make up the middle one.6TeachMeAnatomy. The Middle Mediastinum – Borders – Contents – TeachMeAnatomy
The middle mediastinum is bounded in front and behind by the sac itself, on each side by the lining of the lungs, and below by the top of the diaphragm.7TeachMeAnatomy. The Middle Mediastinum – Borders – Contents – TeachMeAnatomy Its upper edge is the imaginary line running from a bony ridge on the breastbone back to the fourth chest vertebra.8TeachMeAnatomy. The Middle Mediastinum – Borders – Contents – TeachMeAnatomy Roughly speaking, it spans the level of the fourth to the ninth chest vertebrae.9Kenhub. Mediastinum Alongside the heart it holds the start of the great vessels, such as the aorta and the main vein returning blood from the upper body.10Kenhub. Mediastinum
A common mix-up is the pericardium versus the pericardial cavity. The pericardium is the sac, a membrane rather than a space. The pericardial cavity is the narrow gap inside it, not a big hollow. Saying the heart is “in the pericardium” is common shorthand, but it is less exact.
For a quiz or homework answer, match the wording of the question. If it asks for a body cavity in a general sense, thoracic cavity is correct. If it asks for the compartment, answer mediastinum, and specifically the middle mediastinum. If it asks for the heart’s own space, the precise answer is pericardial cavity.
Where the Heart Sits in Your Chest
The heart lies between the lungs in the middle of the chest, behind and slightly to the left of the breastbone.11The Texas Heart Institute®. Heart Anatomy Its front surface sits just beneath the breastbone and the cartilage that joins the ribs to it, and its back surface rests near the bones of the spine.12Anatomy and Physiology. Cardiac Anatomy Its tip, called the apex, tilts toward the left side, and the human heart leans further left than the hearts of most four-legged mammals.13vhlab.umn.edu. Comparative Heart Anatomy Reference People think the heart is on the left because the beat is felt strongest there, where the tip points.
A better picture than the cartoon shape is a cone lying on its side, with the wide base toward the back and right and the pointed tip toward the front and left. Roughly two-thirds of the heart’s mass lies left of the midline, so “left-leaning” is accurate while “on the left” overstates it.
You can find the tip of your own heart. In adults, the heartbeat there is felt in the fifth space between the ribs, on a line straight down from the middle of the left collarbone.14Vital Sign Measurement Across the Lifespan. Apical Pulse The normal spot is usually just inside that line.15Stanford Medicine 25. Precordial Movements in the Cardiac Exam In infants and young children it sits one space higher, in the fourth gap between ribs.16Vital Sign Measurement Across the Lifespan. Apical Pulse To count ribs, find the bump where the top piece of the breastbone meets the body of the bone. It lines up with the second rib.
To feel the beat, sit or lean slightly forward, which brings the heart closer to the chest wall. Place your fingertips below the left nipple area and move them around until you feel a tap. In people with more chest tissue it can be hard to feel, which is normal.
The Pericardium and the Fluid Around the Heart
The sac has a tough outer layer, the fibrous pericardium, and a thinner inner one, the serous pericardium.17Folia Histochemica et Cytobiologica. Pericardium: structure and function in health and disease The inner layer folds into two sheets, one stuck to the heart’s surface and one lining the outer sac. The pericardial cavity is the thin gap between those two sheets, the visceral layer on the heart and the parietal layer lining the sac.18Folia Histochemica et Cytobiologica. Pericardium: structure and function in health and disease Normally that gap holds only about 20 to 60 mL of fluid, roughly one to four tablespoons.19Folia Histochemica et Cytobiologica. Pericardium: structure and function in health and disease The fluid acts as a lubricant so the heart’s surface glides as it beats. The inner layer is a single sheet of flat cells that also help with healing, inflammation, and clotting.20Folia Histochemica et Cytobiologica. Pericardium: structure and function in health and disease
The visceral layer is also known as the epicardium. Exam questions often ask for these two names, and the fibrous layer is a separate, outermost third piece.
The sac also holds the heart in place. The outer layer wraps around the roots of the heart’s major blood vessels, and ligaments tie it to the spine, the diaphragm, and other structures.21The Texas Heart Institute®. Heart Anatomy One of these connects the sac directly to the diaphragm.22PubMed Central. Anatomic connections of the diaphragm: influence of respiration on the body system Because of these ties, the heart’s position shifts a little when the diaphragm moves. In water immersion, for example, the diaphragm rises and the heart moves slightly left.23Journal of UOEH. The Effects of Breathing on Changes in Heart Position in Humans
How Breathing Affects the Heart’s Space
Breathing in happens as the diaphragm moves down and the rib cage expands, which makes the chest cavity larger. The larger space has lower pressure, which is why air flows in, and that pressure drop reaches the sac and heart chambers too.24PubMed Central. Using a human cardiovascular-respiratory model to characterize cardiac tamponade and pulsus paradoxus Veins near the heart are thin-walled and soft, so the lower pressure acts like a gentle suction. More blood flows from veins outside the chest into the right side of the heart.25PubMed Central. Using a human cardiovascular-respiratory model to characterize cardiac tamponade and pulsus paradoxus That extra blood reaches the left side two to three beats later, after passing through the lungs.26PubMed Central. Using a human cardiovascular-respiratory model to characterize cardiac tamponade and pulsus paradoxus Meanwhile, pressure in the sac rises and falls with each beat, which helps blunt breathing-driven swings in how much the heart fills.27PubMed. Influence of the pericardium on ventricular loading during respiration Breathing out reverses the pattern, and blood return to the right side eases slightly.
You may notice that pulse strength varies slightly with breathing, which is normal. A large drop in pulse strength when breathing in is a warning sign doctors check for.
When the Heart Is in a Different Place
In dextrocardia with situs inversus, the heart’s tip points to the right and the other organs are mirrored.28National Organization for Rare Disorders. Dextrocardia with Situs Inversus – Symptoms, Causes, Treatment The liver and spleen switch sides too.29National Organization for Rare Disorders. Dextrocardia with Situs Inversus – Symptoms, Causes, Treatment It is a rare condition present from birth.30PubMed Central. Incidental Finding of Dextrocardia with Situs Inversus in a 59-Year-Old Man Most people with it live normal lives without symptoms.31National Organization for Rare Disorders. Dextrocardia with Situs Inversus – Symptoms, Causes, Treatment About 3 to 5 in 100 have an additional heart defect, most often swapped main arteries.32PubMed Central. A Case Report of Dextrocardia with Situs Inversus: A Rare Condition and Its Clinical Importance Because it causes so few problems, it is sometimes found by chance on an X-ray in adulthood.33PubMed Central. Incidental Finding of Dextrocardia with Situs Inversus in a 59-Year-Old Man Dextrocardia can also occur with the other organs in their usual places, and then the chance of additional heart problems is higher.
It is often noticed when a doctor listens to the chest and hears the heartbeat loudest on the right. A chest X-ray then shows the heart shadow on the right, and further imaging confirms whether other organs are mirrored.
Day to day, people with a mirrored heart should tell doctors, since chest pain and appendix pain may show up on unexpected sides. Heart tracing tests read differently and need adjusting. Usually no treatment is needed if there are no other defects.
A far more serious situation is ectopia cordis, a birth defect where the heart sits partly or fully outside the chest.34PubMed Central. Thoracic ectopia cordis It happens when the front of the chest wall fails to close during development.35PubMed Central. Thoracic ectopia cordis It affects roughly 5.5 to 7.9 babies per million born.36PubMed Central. Thoracic ectopia cordis The heart may lie in the neck, chest, or belly area, and usually pushes out through a split breastbone.37PubMed. Ectopia cordis: a rare congenital anomaly The outlook is generally very poor, especially when the heart is in the neck or has other defects.38PubMed Central. A Rare Case Report of Thoracic Ectopia Cordis: An Obstetrician’s Point of View in Multidisciplinary Approach Some forms can be repaired surgically in infancy.39PubMed Central. A Rare Case Report of Thoracic Ectopia Cordis: An Obstetrician’s Point of View in Multidisciplinary Approach
When Fluid Fills the Heart’s Space
Cardiac tamponade is an emergency where fluid, blood, pus, or air in the sac squeezes the heart.40PubMed. Cardiac tamponade It can cause shock, cardiac arrest, and death.41PubMed. Cardiac tamponade Speed matters more than amount. A fast buildup of as little as 150 mL can sharply cut how much blood the heart pumps.42Escardio. Cardiac tamponade: a clinical challenge Up to 1,000 mL that builds slowly may cause little effect because the sac stretches over time.43Escardio. Cardiac tamponade: a clinical challenge The tough outer sac does not stretch quickly, so sudden fluid squeezes the heart and stops the chambers from filling properly.
Several things can cause fluid or blood to collect. Injury to the chest, such as a stab wound or a crash, can cause bleeding into the sac. Heart surgery and some heart procedures can lead to bleeding into the space afterward. Inflammation of the sac from infection or other illness can produce fluid, and some cancers and kidney failure can too.
Rapidly worsening shortness of breath, lightheadedness or fainting, and chest pressure, especially after injury or heart surgery, need emergency care. Call emergency services rather than waiting.
Doctors usually reach the space through the front of the chest. The most common surgical approach is a small cut just below the tip of the breastbone.44PubMed Central. Surgical Management Strategies for Pericardial Effusion—A Systematic Review Through a 3 to 5 cm cut, surgeons drain the fluid and remove a small piece of sac to make a drainage window.45PubMed Central. Contemporary outcomes after pericardial window surgery: impact of operative technique Going in below the breastbone avoids opening the lung and belly linings.46PubMed Central. LIVING ANATOMY OF THE PERICARDIAL SPACE: A GUIDE FOR IMAGING AND INTERVENTIONS Less often, surgeons go in between ribs below the left nipple.47PubMed Central. Contemporary outcomes after pericardial window surgery: impact of operative technique
How Doctors See the Heart’s Size and Position
A chest X-ray is a quick, painless first test showing the heart’s size and position next to the lungs.48Innovative MRI Partners, LLC. Cardiac Assessment through Chest X-rays On a standard X-ray the heart is normally less than half the chest’s width.49PubMed Central. Comparison of Radiological Findings of Chest X-Ray With Echocardiography in Determination of the Heart Size A heart wider than half the chest may prompt further tests, though this measure alone is not a dependable sign of disease. A large shadow can come from an enlarged heart or extra fluid in the sac, and a poor breath-in during the X-ray can make the heart look bigger.
An X-ray shows the heart’s outline as a single shadow, so it cannot show the individual chambers or valves. It also cannot tell whether a wide shadow is caused by muscle, chamber size, or fluid. That limits it to a first look, and an ultrasound is usually the next step when the outline looks abnormal.
A heart ultrasound measures each chamber directly.50PubMed Central. Comparison of Radiological Findings of Chest X-Ray With Echocardiography in Determination of the Heart Size A technician places a probe with gel on the chest, and sound waves create a moving picture of the heart. It is painless and uses no radiation, so it can be repeated safely. It can show how the chambers contract, how the valves open, and whether fluid has collected in the sac.