Cardiovascular anatomy: the heart, arteries, veins and capillaries of the circulatory system

OpenStax describes eleven organ systems in the human body. One of them is the cardiovascular system, whose organs are the heart and the blood vessels. (OpenStax Anatomy and Physiology 2e, 1.2: Structural Organization of the Human Body.)

Circulatory system and cardiovascular system are two names for the same system. Its four main parts are the heart, arteries, veins and capillaries. Blood passes through the lungs, but the lungs themselves belong to the respiratory system. (Cleveland Clinic: Circulatory System.)

The fluid it moves is blood: about 5 to 6 litres in adult males and 4 to 5 litres in females, roughly 8 percent of body weight. Blood carries oxygen, nutrients, hormones and wastes, helps defend the body against infection, and helps control body temperature and pH. (OpenStax Anatomy and Physiology 2e, 18.1: An Overview of Blood.)

The lymphatic system is a separate but connected system of vessels, cells and organs, such as lymph nodes, the spleen and the thymus. It carries excess fluid from the tissues back into the bloodstream. (OpenStax Anatomy and Physiology 2e, 21.1: Anatomy of the Lymphatic and Immune Systems.)

Lymph vessels, lymph nodes and the thoracic duct are described in the guide to the Lymphatic system.

The pulmonary circuit takes blood to the lungs and back. Blood flows from the right atrium to the right ventricle, then through the pulmonary trunk, which splits into the right and left pulmonary arteries. These branch into arterioles and then the pulmonary capillaries around the alveoli, where gas exchange happens. Blood returns through pulmonary venules and four pulmonary veins, two on each side, to the left atrium. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

The systemic circuit takes blood to the rest of the body. Blood moves from the left atrium to the left ventricle and into the aorta, whose branches supply nearly every organ. It comes back through the systemic veins: the superior vena cava drains everything above the diaphragm and the inferior vena cava everything below it, and both empty into the right atrium. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

Whether a vessel is an artery or a vein depends on direction, not oxygen. An artery carries blood away from the heart. In the systemic circuit arteries carry oxygen-rich blood, but in the pulmonary circuit they carry blood low in oxygen to the lungs. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

The heart muscle has its own branch of the systemic circuit, the coronary circulation. The coronary arteries arise from the first part of the aorta and encircle the heart. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

Before birth the lungs are not used, so three shunts let most blood bypass them and the liver: the foramen ovale between the atria, the ductus arteriosus between the pulmonary trunk and aorta, and the ductus venosus past the liver. After birth they become the fossa ovalis, the ligamentum arteriosum and the ligamentum venosum. (OpenStax Anatomy and Physiology 2e, 20.6: Development of Blood Vessels and Fetal Circulation.)

Each step of the path through the chambers and valves is shown on an interactive 3D heart in Blood flow through the heart.

The heart lies in the mediastinum between the lungs. It has four chambers: the right atrium receives the superior and inferior venae cavae and the coronary sinus, the left atrium receives four pulmonary veins, and the two ventricles pump blood into the pulmonary trunk and the aorta. The left ventricle's wall is much thicker than the right's because it drives the whole systemic circuit. (OpenStax Anatomy and Physiology 2e, 19.1: Heart Anatomy.)

Each ventricle ejects about 70 mL of blood per beat at rest. At 75 beats a minute that is about 5.25 litres a minute, pumped through roughly 60,000 miles of blood vessels. (OpenStax Anatomy and Physiology 2e, 19.1: Heart Anatomy.)

Cleveland Clinic gives matching figures: more than 7,500 litres (about 2,000 gallons) of blood pumped a day through more than 60,000 miles of blood vessels. (Cleveland Clinic: Circulatory System.)

The chambers, valves, wall layers, great vessels and conduction system are covered in detail in Heart anatomy.

Arteries carry blood away from the heart and branch into smaller and smaller vessels. The smallest, the arterioles, have a lumen of about 30 micrometres or less and lead into capillaries, microscopic channels with a lumen of 5 to 10 micrometres. Capillaries drain into venules, about 8 to 100 micrometres across, which join to form veins, thin-walled vessels with large, irregular lumens that carry blood back towards the heart. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

Arteries come in two main kinds. Elastic (conducting) arteries, mostly more than 10 mm across, have plenty of elastic fibres so they stretch and recoil with each heartbeat. Muscular (distributing) arteries, about 0.1 to 10 mm across, have a thick layer of smooth muscle and do most of the narrowing (vasoconstriction). (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

The large elastic arteries are the aorta and the pulmonary arteries, closest to the heart; their middle layer holds 40 to 70 fenestrated elastic membranes. Muscular arteries have about 40 layers of smooth muscle, and arterioles, less than 0.1 mm across, usually about three. (TeachMeAnatomy: Blood Vessels.)

Blood pressure falls along the way from arteries to arterioles, capillaries, venules and veins. The biggest drop is in the arterioles, which offer the most resistance and so control blood pressure. Blood also slows sharply in the capillaries, which gives time for exchange, and speeds up again in the veins. (OpenStax Anatomy and Physiology 2e, 20.2: Blood Flow, Blood Pressure, and Resistance.)

There are three types of capillary. Continuous capillaries are the commonest; in the brain they have no gaps and form the blood-brain barrier. Fenestrated capillaries have pores and are found in the small intestine, kidneys, choroid plexus and many endocrine glands. Sinusoid capillaries, the least common, have large gaps and are found in the liver, spleen, bone marrow and lymph nodes. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

Each capillary bed of about 10 to 100 capillaries is supplied by a metarteriole. Precapillary sphincters at the entrance to the capillaries open when the tissue needs more oxygen or has waste to clear. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

Arteries and veins have the same three layers, called tunics. The tunica intima, the innermost, is lined by endothelium. The tunica media is smooth muscle and elastic fibres and is usually the thickest layer of an artery. The tunica externa (adventitia) is connective tissue and is usually the thickest layer of a vein. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

In cross-section an artery has a thick wall, a small lumen and a round shape, and its endothelium looks wavy. A vein has a thin wall, a large lumen and a flattened shape. Larger arteries have internal and external elastic membranes, which veins lack. Arterial pressure is high and venous pressure low, and only veins have valves. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

Compared with the artery it travels with, a vein is usually wider with a thinner wall, more connective tissue and fewer elastic and muscle fibres. (TeachMeAnatomy: Blood Vessels.)

The walls of large arteries and veins are too thick to be fed from the lumen alone, so they have their own small vessels, the vasa vasorum. In arteries these are limited to the outer layers, because the high pressure inside would squeeze them shut. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

Veins start as postcapillary venules, 10 to 30 micrometres across and leakier than capillaries. TeachMeAnatomy describes venules up to 1 mm wide with very thin walls and valves, and large veins more than 10 mm across, with a thicker intima and longitudinal smooth muscle in the adventitia. (TeachMeAnatomy: Blood Vessels.)

Venous valves are made of thickened endothelium reinforced with connective tissue. They let blood flow only towards the heart and stop it flowing back. They are commonest in the veins of the limbs. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

Veins are capacitance vessels: they stretch easily at low pressure and hold most of the blood. At rest about 64 percent of the blood is in the systemic veins, 13 percent in the systemic arteries, 7 percent in the systemic capillaries, 9 percent in the pulmonary circuit and 7 percent in the heart. About 21 percent of venous blood sits in the liver, bone marrow and skin as a reserve. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

Pressure in the veins is low, so two pumps help return blood. In the skeletal muscle pump, contracting muscles squeeze nearby veins; the valve above opens and the valve below closes, so blood can only move towards the heart. In the respiratory pump, breathing in lowers the pressure in the chest and draws blood up from the abdominal veins. (OpenStax Anatomy and Physiology 2e, 20.2: Blood Flow, Blood Pressure, and Resistance.)

Deep veins usually travel with an artery of the same name. Many deep arteries have a pair of veins, the venae comitantes, wrapped with them in one sheath, and the pulse of the artery helps push venous blood along. (TeachMeAnatomy: Blood Vessels.)

Superficial veins run in the fat under the skin and have no matching artery: examples are the cephalic and basilic veins of the arm and the great saphenous vein of the leg. Veins are also more variable from person to person than arteries. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

When valves fail, blood pools in the veins and they become stretched and twisted: these are varicose veins. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

The longest superficial vein, its valves and its role in varicose veins are covered on the page about the Great saphenous vein.

The aorta is the largest artery in the body. The ascending aorta rises about 5 cm from the left ventricle and gives off the coronary arteries. The aortic arch curves to the left and gives three branches: the brachiocephalic trunk (on the right only), the left common carotid artery and the left subclavian artery. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

The thoracic aorta gives small branches to the bronchi, pericardium, oesophagus and mediastinum, and intercostal and superior phrenic arteries to the chest wall and diaphragm. The abdominal aorta gives the coeliac (celiac) trunk, which divides into the left gastric, splenic and common hepatic arteries, the superior and inferior mesenteric arteries, and the inferior phrenic, adrenal, renal, gonadal and lumbar arteries. It ends at the level of L4 by dividing into the two common iliac arteries. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

Every part of the aorta, its branches and their vertebral levels are set out in the guide to The aorta.

Each common carotid artery divides into an external carotid artery, which supplies the face, lower jaw and neck, and an internal carotid artery, which with the vertebral artery is one of the two main suppliers of blood to the brain. The vertebral arteries arise from the subclavian arteries and, at the base of the brain, join the internal carotids in a ring of linked arteries called the arterial circle (circle of Willis). (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

The carotid and vertebral arteries in the neck are described in the guide to the Carotid arteries.

One artery runs the length of the arm but changes its name as it goes: the subclavian artery becomes the axillary artery in the armpit and the brachial artery in the arm. At the elbow the brachial artery divides into the radial and ulnar arteries, which join in the hand to form the palmar arches that supply the digital arteries of the fingers. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

Each artery and vein of the upper limb, with its branches, is traced in Arteries and veins of the arm.

In the lower limb the external iliac artery becomes the femoral artery in the thigh and the popliteal artery behind the knee. The popliteal artery divides into the anterior tibial artery, which continues on the top of the foot as the dorsalis pedis artery, and the posterior tibial artery, which gives off the fibular artery and ends as the medial and lateral plantar arteries in the sole. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

The iliac, femoral, popliteal, tibial and foot arteries and the leg pulses are covered in Arteries of the lower limb.

All systemic veins end in one of two great veins. The superior vena cava drains structures above the diaphragm; it is formed by the two brachiocephalic veins, which are fed by the subclavian and jugular veins, and it also receives the azygos vein. The inferior vena cava drains structures below the diaphragm. It forms at about L5 from the common iliac veins and receives the renal, hepatic, gonadal and lumbar veins; the renal veins are normally the largest veins entering it. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

The inferior vena cava runs up the right side of the vertebral column with the aorta on its left, passes through the diaphragm at the caval opening at the level of T8 and drains into the right atrium. (Radiopaedia: Inferior vena cava.)

The superior vena cava enters the upper right atrium at about the level of the third rib. (TeachMeAnatomy: Great Vessels of the Heart.)

The course, tributaries and variants of both venae cavae are described on the page about the Inferior and superior vena cava.

The internal jugular veins drain the brain. The external jugular veins drain the superficial parts of the head and empty into the subclavian veins. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

In the arm, digital veins drain into the palmar venous arches and then into the radial, ulnar and median antebrachial veins, which lead to the brachial vein. The superficial cephalic and basilic veins run up the outer and inner sides of the arm, linked at the elbow by the median cubital vein, the usual site for taking blood. The axillary vein becomes the subclavian vein. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

In the leg, the dorsal venous arch of the foot drains into the anterior and posterior tibial and fibular veins, which form the popliteal vein behind the knee. This becomes the femoral vein, which receives the great saphenous vein and continues as the external iliac and then the common iliac vein into the inferior vena cava. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

The main vein of the neck and its relations are described on the page about the Internal jugular vein.

Blood from the gut does not go straight back to the heart. The hepatic portal vein, formed at about the level of L2 by the superior mesenteric and splenic veins, carries blood from the stomach, intestines, spleen and pancreas to the liver. After passing through the liver sinusoids it leaves through the hepatic veins into the inferior vena cava. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

The tributaries, branches and clinical anatomy of the hepatic portal vein are covered on the page about the Portal vein.

The pulse is the expansion and recoil of an artery after each heartbeat. It is usually felt at the radial artery in the wrist or the common carotid artery in the neck. Other pulse points are the temporal and facial arteries of the head, the brachial artery in the upper arm, the femoral artery in the thigh, the popliteal artery behind the knee, the posterior tibial artery near the ankle and the dorsalis pedis artery on the foot. (OpenStax Anatomy and Physiology 2e, 20.2: Blood Flow, Blood Pressure, and Resistance.)

A normal adult blood pressure is about 120/80 mm Hg: 120 when the ventricles contract (systolic) and 80 when they relax (diastolic). The difference, about 40 mm Hg, is the pulse pressure, and the mean arterial pressure normally lies between 70 and 110 mm Hg. (OpenStax Anatomy and Physiology 2e, 20.2: Blood Flow, Blood Pressure, and Resistance.)

Where to feel the radial pulse, and the anatomy of the artery, are covered on the page about the Radial artery.

An aneurysm is a widening of an artery to more than 50 percent above its normal diameter. It can follow weakness of the wall, as in Marfan syndrome, or a dissection, in which a tear in the inner wall lets blood run inside the wall itself. The main danger is rupture. (TeachMeAnatomy: Great Vessels of the Heart.)

Fluid can also leak out of the vessels into the tissues, causing swelling (oedema, US edema). It has many causes, including high blood pressure and heart failure. (OpenStax Anatomy and Physiology 2e, 20.1: Structure and Function of Blood Vessels.)

Coronary artery disease, angina and heart attacks are explained with the anatomy of the Coronary arteries.

The 3D model comes from BodyParts3D, a whole-body model of an adult human male released as an anatomical reference database under a CC BY licence. (BodyParts3D, Database Center for Life Science.)

This page highlights the walls of the heart, the aorta and its main branches, the arteries of the neck, arms and legs, the pulmonary vessels, both venae cavae, the large veins of the neck and limbs, the renal veins and the hepatic portal vein. Switch off the skeleton to follow each vessel from the heart to the hands and feet, and click any vessel to open its own page. Capillaries and most small veins are too fine to be modelled, and the model has no external carotid artery or external jugular vein, so the diagrams show those.

Abdominal aorta

The abdominal aorta is the part of the aorta that runs through the abdomen and supplies the abdominal organs, pelvis and lower limbs.

Anterior tibial artery

The anterior tibial artery is the artery of the front compartment of the leg and the main source of the dorsalis pedis artery.

Arch of aorta

The aortic arch is the curved part of the aorta between the ascending and descending aorta, giving the great vessels to the head and arms.

Ascending aorta

The ascending aorta is the first part of the aorta, rising from the left ventricle.

Axillary artery

The axillary artery is the main artery of the armpit, continuing the subclavian artery.

Axillary vein

The axillary vein is the main vein of the armpit and carries blood from the upper limb.

Basilic vein

The basilic vein is a large superficial vein on the medial side of the arm.

Brachial artery

The brachial artery is the main artery of the arm, continuing the axillary artery.

Brachiocephalic artery

The brachiocephalic artery (trunk) is the first branch of the aortic arch and supplies the right head, neck and arm.

Brachiocephalic vein

The brachiocephalic veins are two large veins that unite to form the superior vena cava.

Celiac trunk

The celiac trunk is the first major unpaired branch of the abdominal aorta and supplies the foregut.

Cephalic vein

The cephalic vein is a large superficial vein on the lateral side of the upper limb.

Common carotid artery

The common carotid artery is the main artery of the neck and splits into the external and internal carotid arteries.

Common iliac artery

The common iliac arteries are the two terminal branches of the abdominal aorta.

Common iliac vein

The common iliac veins are formed by the external and internal iliac veins and unite to form the inferior vena cava.

Descending thoracic aorta

The descending thoracic aorta is the part of the aorta in the posterior mediastinum.

Dorsalis pedis artery

The dorsalis pedis artery is the continuation of the anterior tibial artery onto the top of the foot.

External iliac artery

The external iliac artery is the main artery of the lower limb in the pelvis and becomes the femoral artery.

External iliac vein

The external iliac vein is the continuation of the femoral vein above the inguinal ligament.

Femoral artery

The femoral artery is the main artery of the thigh, continuing the external iliac artery.

Femoral vein

The femoral vein is the main deep vein of the thigh and continues the popliteal vein.

Great saphenous vein

The great saphenous vein is the longest vein in the body and runs up the medial side of the leg and thigh.

Hepatic portal vein

The hepatic portal vein carries nutrient-rich blood from the gut and spleen to the liver.

Inferior pulmonary vein

The inferior pulmonary veins (one per lung) carry oxygenated blood from the lower lobes of the lungs to the left atrium.

Inferior vena cava

The inferior vena cava is the largest vein in the body and returns blood from the lower body to the right atrium.

Internal carotid artery

The internal carotid artery supplies the brain and the eye.

Internal iliac artery

The internal iliac artery supplies the pelvis.

Internal jugular vein

The internal jugular vein is the main vein draining the head and neck.

Median cubital vein

A superficial vein across the front of the elbow linking the cephalic and basilic veins.

Popliteal artery

The continuation of the femoral artery behind the knee.

Popliteal vein

A deep vein behind the knee formed by the joining of the anterior and posterior tibial veins.

Posterior tibial artery

The larger terminal branch of the popliteal artery in the posterior leg.

Pulmonary artery

An artery carrying deoxygenated blood from the heart to a lung.

Pulmonary trunk

The large artery arising from the right ventricle that carries blood to the lungs.

Radial artery

One of the two terminal branches of the brachial artery, running on the lateral forearm.

Renal artery

A large paired branch of the abdominal aorta supplying the kidney.

Renal vein

A large vein draining the kidney into the inferior vena cava.

Small saphenous vein

A superficial vein of the posterior leg.

Subclavian artery

The major artery to the upper limb, neck and part of the brain.

Subclavian vein

The large vein draining the upper limb into the brachiocephalic vein.

Superior mesenteric artery

An unpaired artery of the midgut.

Superior pulmonary vein

One of the pulmonary veins draining the upper lung lobes into the left atrium.

Superior vena cava

The large vein draining the upper body into the right atrium.

Ulnar artery

The larger terminal branch of the brachial artery.

Vertebral artery

A branch of the subclavian artery supplying the brain and spinal cord.

Wall of atrium

The muscular walls of the right and left atria, the thin-walled receiving chambers of the heart.

Wall of ventricle

The muscular wall of a ventricle of the heart.