The cardiovascular system is the heart and the network of blood vessels that circulate blood through the body. The heart acts as a double pump: its right side sends blood to the lungs, and its left side sends blood to the body. Chambers, valves and connecting vessels organise this flow.
How does the heart work as a double pump?
The heart lies between the lungs, behind the breastbone, with most of its bulk to the left of the midline. It has four chambers: two atria, which receive blood, and two ventricles, which pump blood out. A septum, or dividing wall, separates the right and left sides.
The right atrium receives blood returning from the body and passes it to the right ventricle. The right ventricle pumps this blood towards the lungs. The left atrium receives blood returning from the lungs and passes it to the left ventricle, which pumps it around the body.
Pulmonary circulation is the route between the heart and lungs. Systemic circulation is the route between the heart and the rest of the body. These circuits are connected in sequence rather than operating independently.
The left ventricle has a thicker muscular wall than the right ventricle because it pumps against greater resistance in the systemic circulation. Both ventricles eject blood during ventricular systole, the phase of ventricular contraction; they fill during diastole, the phase of relaxation.
What are the four heart valves?
Heart valves are folds of tissue that open and close as pressure changes on either side of them. They direct blood forwards and limit backward flow. The four valves occupy the entrances and exits of the ventricles:
- Tricuspid valve: between the right atrium and right ventricle.
- Pulmonary valve: between the right ventricle and the pulmonary trunk, the vessel leading towards the lungs.
- Mitral valve: between the left atrium and left ventricle.
- Aortic valve: between the left ventricle and the aorta, the main artery carrying blood into the systemic circulation.
The tricuspid and mitral valves are atrioventricular valves, meaning that they lie between an atrium and a ventricle. Their leaflets, or movable flaps, are attached by chordae tendineae, fibrous cords, to papillary muscles projecting from the ventricular walls. These attachments help prevent the leaflets from turning back into the atria during ventricular contraction.
The pulmonary and aortic valves are semilunar valves, named for their half-moon-shaped cusps. Unlike the atrioventricular valves, they have no chordae tendineae.
What is the path of blood through the heart?
An artery carries blood away from the heart; a vein carries blood towards it. These definitions concern direction, not oxygen content. Pulmonary arteries carry oxygen-poor blood towards the lungs, whereas pulmonary veins carry oxygen-rich blood back to the heart.
The main route can be followed in order:
- Blood from the upper body enters the right atrium through the superior vena cava; blood from the lower body enters through the inferior vena cava.
- Blood passes through the tricuspid valve into the right ventricle.
- It passes through the pulmonary valve into the pulmonary trunk and then into the right and left pulmonary arteries.
- In lung capillaries, the smallest blood vessels, blood exchanges gases with air in the alveoli across a thin barrier formed by the capillary and alveolar walls.
- Pulmonary veins return blood to the left atrium, from which it passes through the mitral valve into the left ventricle.
- Blood passes through the aortic valve into the aorta and its branches, reaches body capillaries, and returns through veins.
Most capillary beds connect the smallest arterial branches, called arterioles, with the smallest veins, called venules. Their thin walls allow exchange between blood and surrounding tissues.
What are the great vessels of the heart?
The great vessels are the large vessels entering or leaving the heart: the aorta, pulmonary trunk, pulmonary veins and venae cavae. Their positions reflect the separation of pulmonary and systemic blood flow.
The pulmonary trunk leaves the right ventricle and divides into right and left pulmonary arteries. Pulmonary veins enter the left atrium from the lungs. The superior vena cava enters the right atrium from above, while the inferior vena cava enters it from below.
The aorta begins as the ascending aorta, curves as the aortic arch, and continues downwards as the descending aorta. The descending aorta within the chest is the descending thoracic aorta; after passing through the diaphragm, the muscular partition between chest and abdomen, it continues as the abdominal aorta.
In the 3D model next to this article, rotate and zoom around the heart, then select and isolate a structure to examine its position relative to neighbouring vessels.
What are the main arteries and veins by region?
Systemic arteries branch from the aorta towards organs and limbs, while veins converge towards the heart. Deep veins lie beneath the deep fascia, the connective tissue investing muscles and other deep structures, and commonly accompany arteries; superficial veins lie in the tissue beneath the skin.
| Region | Main arterial route | Main venous return |
|---|---|---|
| Head and neck | Each common carotid artery divides into internal and external carotid arteries; vertebral arteries also supply the brain. | Internal jugular veins drain the brain and deep head and neck; external jugular veins drain many superficial regions. |
| Upper limb | The subclavian artery continues as the axillary and then brachial artery, which divides into radial and ulnar arteries. | Deep veins return through axillary and subclavian veins; cephalic and basilic veins provide superficial routes. |
| Chest | The thoracic aorta gives branches to the chest wall and chest organs. | The azygos venous system drains much of the chest wall into the superior vena cava. |
| Abdomen | The coeliac trunk and superior and inferior mesenteric arteries supply digestive organs; renal arteries supply the kidneys. | Renal veins enter the inferior vena cava; the hepatic portal vein carries blood from digestive organs and the spleen to the liver. |
| Pelvis | Common iliac arteries divide into internal iliac arteries for the pelvis and external iliac arteries towards the lower limbs. | Internal and external iliac veins join as common iliac veins, which unite to form the inferior vena cava. |
| Lower limb | The external iliac artery continues as the femoral artery, then as the popliteal artery behind the knee. The popliteal artery divides into anterior and posterior tibial arteries; the fibular artery usually arises from the posterior tibial artery. | Deep veins return through popliteal and femoral veins; great and small saphenous veins provide superficial routes. |
The aortic arch usually gives rise to the brachiocephalic trunk, left common carotid artery and left subclavian artery. The brachiocephalic trunk divides into the right common carotid and right subclavian arteries.
The hepatic portal route is distinctive because blood passes through capillaries in digestive organs and then through another vascular bed in the liver before hepatic veins return it to the inferior vena cava.
What is the coronary circulation?
A coronary artery supplies the heart muscle itself. The right and left coronary arteries arise from the aorta just above the aortic valve, rather than from the cavities of the heart chambers.
The left coronary artery usually divides into the anterior interventricular artery, which follows the groove between the ventricles on the front of the heart, and the circumflex artery, which curves around its left side. The right coronary artery follows the groove between the right atrium and right ventricle. Their branches supply the muscular walls and dividing septum.
Cardiac veins collect blood from the heart muscle. Most of this blood enters the coronary sinus, a large venous channel on the back of the heart that opens into the right atrium. Some cardiac veins drain directly into the heart chambers.
Where are arterial pulses felt?
A pulse is the rhythmic expansion of an artery as blood is ejected from the heart. Many common pulse sites are places where an artery lies relatively close to the surface and has a firm structure beneath it, although some palpable arteries lie deeper.
- Carotid: in the neck, beside the windpipe.
- Brachial: on the inner side of the elbow, medial to the biceps tendon, meaning towards the body's midline.
- Radial: at the wrist on the thumb side.
- Femoral: just below the inguinal ligament, the fibrous band spanning the front of the groin.
- Popliteal: deep in the hollow behind the knee.
- Posterior tibial: behind the medial malleolus, the bony prominence on the inner ankle.
- Dorsalis pedis: on the upper surface of the foot, usually just lateral to the tendon that extends the great toe; lateral means away from the midline.
These sites mark accessible points along continuous arterial routes. Some, particularly the popliteal artery, lie deeper than others.