120 Circulatory Routes

The Aorta and Its Branches

The aorta is the largest artery in the body and is divided into 3 parts: the ascending aorta, arch of the aorta, and descending aorta.

The aorta is the largest artery in the systemic circulatory system. Blood is pumped from the left ventricle into the aorta and from there branches to all parts of the body. The aorta is an elastic artery, meaning it is able to distend. When the left ventricle contracts to force blood into the aorta, the aorta expands. This stretching generates the potential energy that helps maintain blood pressure during diastole, since during this time the aorta contracts passively.

The aorta is divided into three parts: the ascending aorta, where the aorta initially leaves the heart and points superiorly toward the head; the arch of the aorta where the aorta changes direction; and the descending aorta where the aorta points inferiorly toward the feet.

The ascending aorta has two small branches, the left and right coronary arteries. These arteries provide blood to the heart muscle, and their blockage is the cause myocardial infarctions or heart attacks.

The arch of the aorta has three branches: the brachiocephalic artery, which itself divides into right common carotid artery and the right subclavian artery, the left common carotid artery, and the left subclavian artery. These arteries provide blood to both arms and the head.

The descending aorta is the largest artery in the body; it runs from the heart down the length of the chest and abdomen. It is divided into two portions, the thoracic and abdominal, in correspondence with the two great cavities of the trunk in which it sits. Within the abdomen, the descending aorta branches into the two common iliac arteries that provide blood to the pelvis and, eventually, the legs.

Ascending Aorta

The ascending aorta is the first portion of the aorta; it includes the aortic sinuses, the bulb of the aorta, and the sinotubular junction.

The ascending aorta is a portion of the aorta beginning at the upper part of the base of the left ventricle, on a level with the lower border of the third costal cartilage behind the left half of the sternum; it passes diagonally upward, forward, and to the right, in the direction of the heart’s axis, as high as the upper border of the second right costal cartilage. Its total length is about five centimeters.

The aortic root is the portion of the ascending aorta beginning at the aortic annulus, the fibrous attachment between the heart and the aorta, and extending to the sinotubular junction. Just above the aortic valve are three small dilations called the aortic sinuses. The two anterior sinuses give rise to the coronary arteries, while the third posterior sinus does not usually give rise to any vessels and so is known as the non-coronary sinus.

The sinotubular junction is the point in the ascending aorta where the aortic sinuses end and the aorta becomes a tubular structure.

At the junction of the ascending aorta with the aortic arch, the caliber of the vessel increases with a bulging of its right wall. This dilatation is termed the “bulb of the aorta.” The ascending aorta is contained within the pericardium. It is enclosed in a tube of the serous pericardium, which also encloses the pulmonary artery.

The ascending aorta is covered at its beginning by the trunk of the pulmonary artery and, higher up, is separated from the sternum by the pericardium, the right pleura, the anterior margin of the right lung, some loose areolar tissue, and the remains of the thymus. Posteriorly, it rests upon the left atrium and right pulmonary artery.

Arch of the Aorta

The arch of the aorta follows the ascending aorta and begins at the level of the second sternocostal articulation of the right side.

The arch of the aorta, or the transverse aortic arch, is continuous with the upper border of the ascending aorta and begins at the level of the upper border of the second sternocostal articulation of the right side. The arch of the aorta runs at first upward, backward, and to the left in front of the trachea; it is then directed backward on the left side of the trachea and finally passes downward on the left side of the body of the fourth thoracic vertebra. At the lower border, this vertebra becomes continuous with the descending aorta.

Three vessels come out of the aortic arch: the brachiocephalic artery, the left common carotid artery, and the left subclavian artery. These vessels supply blood to the head, neck, thorax and upper limbs. In approximately 20% of individuals, the left common carotid artery arises from the brachiocephalic artery rather than the aortic arch, and in approximately 7% of individuals the left subclavian artery also arises here.

Thoracic Aorta

The thoracic aorta is the section of the aorta that travels through the thoracic cavity to carry blood to the head, neck, thorax and arms.

The thoracic aorta forms part of the descending aorta and is continuous with the aortic arch at its origin before becoming the abdominal aorta. Contained within the posterior mediastinal cavity, it begins at the lower border of the fourth thoracic vertebra where it is continuous with the aortic arch, and ends in front of the lower border of the twelfth thoracic vertebra at the aortic hiatus in the diaphragm. At its commencement, the thoracic aorta is situated on the left of the vertebral column; it approaches the median line as it descends, and at its termination lies directly in front of the column.

The thoracic aorta’s relation, from above downward, is as follows: anteriorly with the root of the left lung, the pericardium, the esophagus and the diaphragm; posteriorly with the vertebral column; on the right side with the hemiazygos veins and thoracic duct; and on the left side with the left pleura and lung. The esophagus lies on the right side of the aorta for most of its length, but at the lower part of the thorax is placed in front of the aorta and close to the diaphragm, situated on its left side.

As it descends in the thorax, the aorta gives off several paired branches. In descending order these are the bronchial arteries, the mediastinal arteries, the esophageal arteries, the pericardial arteries, and the superior phrenic artery. The posterior intercostal arteries are branches that originate throughout the length of the posterior aspect of the thoracic aorta.

Abdominal Aorta

The abdominal aorta is the largest artery in the abdominal cavity and supplies blood to most of the abdominal organs.

The abdominal aorta is the largest artery in the abdominal cavity. As part of the descending aorta, it is a direct continuation of the thoracic aorta.

It begins at the level of the diaphragm, crossing it via the aortic hiatus. This hole in the diaphragm that allows the passage of the great vessels at the vertebral level of T12. The abdominal aorta travels down the posterior wall of the abdomen, anterior to the vertebral column, following the curvature of the lumbar vertebrae. The abdominal aorta runs parallel to the inferior vena cava, located just to the right of the abdominal aorta.

The abdominal aorta lies slightly to the left of the midline of the body. It is covered anteriorly by the lesser omentum and stomach. Posteriorly, it is separated from the lumbar vertebrae by the anterior longitudinal ligament and left lumbar veins.

The abdominal aorta supplies blood to much of the abdominal cavity through numerous branches that become smaller in diameter as it descends. Terminally it branches into the paired common iliac arteries, which supply the pelvis and lower limbs.

Arteries of the Pelvis and Lower Limbs

The abdominal aorta divides into the major arteries of the leg: the femoral, popliteal, tibial, dorsal foot, plantar, and fibular arteries.

The pelvic cavity is largely supplied by the paired internal iliac arteries, formed when the common iliac artery divides the internal iliac artery at the vertebral level L5 descends inferiorly into the lesser pelvis. The external iliac artery passes into the thigh, becoming the femoral artery.

At the most superior border of the greater sciatic foramen, the large opening to the rear of the pelvis, the internal iliac artery divides into anterior and posterior trunks.

The anterior trunk gives rise to numerous arteries that supply the organs of the pelvis and the gluteal and adductor muscles of the leg. Key branches include the obturator artery, the inferior vesical artery in men and the equivalent vaginal artery in females, and the rectal and gluteal arteries.

The posterior trunk gives rise to arteries that supply the posterior pelvic wall and the gluteal region, including the iliolumbar artery that supplies the psoas major muscle, the lateral sacral arteries, and the superior gluteal artery.

Principal Veins

Veins are blood vessels that carry blood towards the heart, have thin, inelastic walls, and contain numerous valves.

In the circulatory system, veins are blood vessels that carry blood towards the heart. Veins have thin, inelastic walls, and contain numerous valves in order to prevent backflow of blood. Most veins carry deoxygenated blood from the tissues back to the heart with the exceptions of the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart.

Veins can be broadly classified based on their depth within the body. Superficial veins are located close to the surface of the body and have no corresponding arteries, such as the great saphenous vein which runs the length of the leg. The deep veins lie deeper in the body and often run adjacent to corresponding arteries, such as the femoral vein which sits adjacent to the femoral artery in the thigh. Deep veins are often of larger caliber than superficial veins and carry the majority of the blood within the circulatory system. Communicating veins, or perforator veins if they pass through a large muscle mass, directly connect superficial and direct veins. The above veins form part of the systemic circulatory system. The pulmonary veins and venules that run from the lungs to the heart form part of the pulmonary circulatory system and are distinct from other veins in that they carry oxygenated blood.

Venae Cavae

The venae cavae are the veins with the largest diameter. Both enter the right atrium of the heart with the superior vena cava carrying blood from the arms, head, and thoracic cavity and the inferior vena cava carrying blood from the legs and abdomen. The inferior vena cava runs parallel to the abdominal aorta.

The superior vena cava is formed from the brachiocephalic veins which are in turn formed from the subclavian and internal jugular veins that serve the arm and head respectively. The inferior vena cava is formed from the common iliac veins that serve the legs and abdomen. The renal and hepatic veins from the kidneys and liver respectively also feed into the inferior vena cava.

Other Important Veins

Other important venous systems include the cardiac veins, which return blood from the heart tissue back to the general circulation. The cardiac veins merge into the coronary sinus, which empties directly into the right atrium.

The pulmonary veins are large blood vessels which receive oxygenated blood from the lungs and return it to the left atrium of the heart. There are four pulmonary veins, two from each lung, each of which forms from three to four bronchial veins. In approximately 25% of individuals, the left pulmonary veins may merge into a single vein; the same effect on the right side is only seen in approximately 3% of individuals.

The hepatic portal vein carries blood from the gastrointestinal tract to the liver. The portal vein is often described as a false vein because it conducts blood between capillary networks rather than between a capillary network and the heart. It functions to supply the liver with blood and required metabolites, but also ensures that ingested substances are first processed in the liver before reaching the wider systemic circulation.

Veins of the Head and Neck

In the head and neck, blood circulates from the upper systemic loop, which originates at the aortic arch.

Two main jugular veins are responsible for the venous draining of the head and neck.

The superficial external jugular vein is formed from the retromandibular vein and the posterior auricular vein at a point adjacent to the mandible. The external jugular vein passes down the neck and underneath the clavicle before draining into the subclavian vein.

The deep-lying internal jugular vein receives blood from the dural venous sinuses in the brain as well as the cerebral and cerebellar veins. Dural sinuses are composed of dural mater lined with endothelium, making them distinct from arteries, veins, and capillaries. The dural sinuses receive blood from the veins that drain the brain and skull.

Formed at the base of the brain from the inferior petrosal sinus and the sigmoid sinus, the internal jugular vein runs down the side of the neck adjacent to the internal carotid artery. As well as removing blood from the brain, the anterior retromandibular, facial, and lingual veins also drain into the internal jugular. Upon exiting the neck, the internal jugular vein merges with the subclavian vein to form the brachiocephalic vein.

Additional veins such as the occipital, deep cervical, and thyroid veins drain directly into the brachiocephalic vein.

Veins of the Upper Limbs

The veins of the upper extremity are divided into superficial and deep veins, indicating their relative depths from the skin.

Veins of the arm are either deep or superficial and are responsible for draining the hand and arm.

The major deep veins of the arm are the radial and ulnar veins, which run along the length of their respective bones and merge at the elbow to form the paired brachial vein. The brachial vein runs from the elbow up to the shoulder parallel to the brachial artery.

The major superficial veins of the upper limb are the cephalic, median cubital and basilic veins. The cephalic vein arises from the dorsal venous network of the hand and passes the elbow anteriorly, continuing up the upper arm to the shoulder. The basilic vein follows a similar path but is located medially to the cephalic vein. At the elbow, the basilic and cephalic veins are linked by the median cubital vein, from which blood is often drawn.

At the shoulder, the basilic vein passes deep into the arm and merges with the brachial veins to form the axillary vein, to which the cephaliac vein merges, forming the subclavian vein.

Veins of the Thorax

The veins of the thorax drain deoxygenated blood from the thorax region for return to the heart.

Two venae cavae return deoxygenated blood from the systemic circulation to the right atrium of the heart.

The superior vena cava, formed from the left and right brachiocephalic veins, returns deoxygenated blood from the upper half of the body and carries blood from the upper limbs, head, and neck via the thyroid and jugular veins. It is joined just before entering the heart by the azygos vein, which runs up the right side of the thoracic vertebral column and transports blood from the external thoracic cavity.

The internal thoracic vein is a vessel that drains the chest wall and breasts. Bilaterally, it arises from the superior epigastric vein, accompanies the internal thoracic artery along its course, and terminates in the brachiocephalic vein.

The supreme intercostal vein is a paired vein that drains the first intercostal space on its corresponding side. It usually drains into the brachiocephalic vein.

The inferior vena cava returns blood from the abdomen and lower limbs to the right atrium of the heart. The renal veins from the kidney and hepatic veins of the liver drain directly into the inferior vena cava. Additionally, the superior and inferior phrenic veins drain the diaphragm and usually open into the internal mammary vein and inferior vena cava, respectively.

Veins of the Abdomen and Pelvis

The major veins of the abdomen and pelvis return deoxygenated blood from the abdomen and pelvis to the heart.

A number of veins remove deoxygenated blood from the abdomen and pelvis. The external iliac vein, the upward continuation of the femoral vein, passes upward along the pelvis and ends to form the common iliac vein.The tributaries of the external iliac vein are the inferior epigastric, deep iliac circumflex, and pubic veins.

The internal iliac vein begins near the upper part of the greater sciatic foramen, the large opening at the rear of the pelvis, passes upward behind and slightly medial to the internal iliac artery and, at the brim of the pelvis, joins with the external iliac vein to form the common iliac vein.

The left and right common iliac veins come together in the abdomen at the level of the fifth lumbar vertebra, forming the abdominal vena cava. They drain blood from the pelvis and lower limbs.

The superior epigastric vein refers to a blood vessel that carries deoxygenated blood and drains into the internal thoracic vein. It anastomoses with the inferior epigastric vein at the level of the umbilicus and drains the anterior part of the abdominal wall and some of the diaphragm.

The inferior epigastric vein refers to the vein that drains into the external iliac vein and arises from the superior epigastric vein.

The deep circumflex iliac vein is formed by the union of the venae comitantes of the deep iliac circumflex artery, and joins the external iliac vein about 2 cm above the inguinal ligament.

Veins of the Lower Limbs

The deep veins of the lower extremity have valves for unidirectional flow and accompany the arteries and their branches.

Running the full length of the leg, making it the longest vein in the body, the great saphenous vein is a superficial vein that returns blood from the foot and superficial muscles of the leg before merging with the femoral vein to form the external iliac vein.