The heart is a hollow muscular organ that pumps blood through the lungs and the rest of the body. It lies in the middle mediastinum, the subdivision of the inferior mediastinum containing the heart and its surrounding sac, between the lungs, behind the breastbone and above the diaphragm.
Where is the heart located?
The heart is enclosed by the pericardium, a sac with an outer fibrous layer and an inner membrane. A thin film of fluid between the membrane’s facing surfaces allows movement during each heartbeat. Most of the heart lies to the left of the body’s midline, although it extends to both sides.
The sternum, or breastbone, and adjoining rib cartilages lie in front. The diaphragm, the muscle separating the chest from the abdomen, supports the heart below. The lungs lie on either side, and the oesophagus, the tube carrying food towards the stomach, passes behind the heart.
What are the heart’s surfaces, base and apex?
The apex is the pointed end, formed by the left ventricle, a lower pumping chamber. It points forwards, downwards and to the left. The base is the broad posterior aspect, formed mainly by the left atrium, an upper receiving chamber, with a contribution from the right atrium.
The external surfaces are named for their neighbours:
- The sternocostal surface faces the sternum and ribs and is formed mainly by the right ventricle.
- The diaphragmatic surface rests on the diaphragm and is formed mainly by the left ventricle, with a contribution from the right ventricle.
- The right pulmonary surface faces the right lung and is formed mainly by the right atrium.
- The left pulmonary surface faces the left lung and is formed mainly by the left ventricle.
What are the heart’s chambers and blood-flow pathway?
The atria receive blood, and the ventricles propel it out of the heart. The interatrial septum is the wall between the atria; the interventricular septum separates the ventricles. The left atrial appendage is a small pouch projecting from the left atrium.
| Chamber | Receives blood from | Passes blood towards |
|---|---|---|
| Right atrium | Superior vena cava, inferior vena cava and coronary sinus | Right ventricle |
| Right ventricle | Right atrium | Pulmonary trunk and lungs |
| Left atrium | Pulmonary veins | Left ventricle |
| Left ventricle | Left atrium | Aorta and body |
The superior vena cava returns blood from the upper body, while the inferior vena cava returns blood from below the diaphragm. The coronary sinus is the main venous channel draining the heart wall into the right atrium.
The pulmonary trunk carries blood from the right ventricle and divides into arteries supplying the lungs. Each pulmonary vein returns blood from the lungs to the left atrium. The aorta carries blood from the left ventricle into the circulation supplying the body.
What makes up the heart wall and valves?
The endocardium is the smooth inner lining of the chambers. The myocardium is the muscular middle layer, and the epicardium is the outer layer, also forming the membrane closely covering the heart. The left ventricle has a thicker muscular wall than the right because it pumps against greater resistance.
The fibrous skeleton of heart is a framework of dense connective tissue supporting the valve openings. It also electrically separates atrial muscle from ventricular muscle, except where specialised conducting tissue passes through it.
A valve opens and closes in response to pressure differences. The tricuspid valve lies between the right atrium and right ventricle; the mitral valve lies between the left atrium and left ventricle. The pulmonary and aortic valves guard the ventricular outlets.
The inlet valves have flaps attached to chordae tendineae, strong fibrous cords. Each papillary muscle of heart is a muscular projection within a ventricle that tensions these cords, helping prevent the flaps from turning back into the atrium during contraction. The outlet valves have pocket-shaped cusps without these cords.
What supplies and drains the heart wall?
A coronary artery supplies the heart wall itself. The right and left coronary arteries arise from the root of the aorta, just beyond the aortic valve, and their branches spread over the heart before entering its wall.
Most venous blood from the heart wall reaches the coronary sinus. Other veins drain directly into the right atrium or open into the heart chambers. These routes are distinct from the large vessels carrying blood through the heart to the lungs and body.
What are the heart’s nerve supply and lymph drainage?
Autonomic nerves, which regulate involuntary activity, reach the heart through the cardiac plexus, a network near the great vessels. Sympathetic fibres from the upper thoracic spinal cord increase heart rate and force of contraction. Parasympathetic fibres carried by the vagus nerves slow the heart rate and conduction through the atrioventricular node.
Lymph, fluid collected from tissues, drains through vessels that accompany the coronary vessels. It passes towards tracheobronchial lymph nodes, small filtering structures near the division of the windpipe into the main airways.
How does the heart produce a heartbeat?
Specialised cardiac cells generate and conduct electrical impulses. The sinoatrial node in the right atrium normally initiates each heartbeat. The atrioventricular node delays transmission before the impulse travels through the atrioventricular bundle, its branches and Purkinje fibres to activate the ventricles.
During diastole, the relaxation phase, the ventricles fill. During systole, the contraction phase, they eject blood. The right side supplies the lungs, while the left side supplies the body.
How can you orientate the heart in the 3D model?
In the 3D model next to this article, rotate and zoom to explore the heart, then select and isolate it. You can also hide or fade selected structures, switch layers, cut a section along the sagittal, coronal or transverse plane, or pull selected parts apart.