- Superior vena-cava
- SVC collects blood from the upper half of the body and drains into the right atrium. It is formed by the union of twobrachiocephalic veinsat the level of lower border of 1st right costal cartilage. It passes vertically downwards behind the right border of sternum and piercing the pericardium at the level of the second costal cartilage, enters the upper border of the right atrium to end in the upper and posterior part of sinus venorum at the lower border of third right costal cartilage (opposite T5 vertebra).
- Behind the sternal angle (T4 vertebral level), SVC receives the azygos vein.
- SVC is divided into two parts -
- Upper half (extrapericardial):- It lies in the superior mediastinum and azygous vein open into it. It develops from caudal part of right anterior cardinal vein.
- Lower half (intrapericardial):- It lies in middle mediastinum and is covered by visceral pericardium. It develops from right common cardinal vein (Duct of Cuvier).
- Development anomalies
- Left superior vena cava: -This is due to persistence of left anterior cardinal vein and obliteration of anterior cardinal and common cardinal veins on the right side (which normally develop into SVC). The left SVC open into right atrium by way of coronary sinus.
- Double superior vena cava: - It is due to persistence of left anterior cardinal vein and failure of the left brachiocephalic vein to form. The persistent left anterior cardinal vein which is called the left SVC drains into the right atrium by way of coronary sinus.
Aorta receives oxygenated blood from left ventricle and distributes it to all parts ofthe body. Thoracic aorta is divided into three parts.
- Ascending aorta
- It is 5cm long, arises from the left ventricle and is entirely intrapericardial. It begins at the level of lower border of third costal cartilage, behind the left half of sternum. At sternal end of upper border of second costal cartilage it con tines as arch of aorta.
- There are three dilatations in the roots of ascending aorta, called as Aortic Sinuses of Valsalva;- One anterior and two posterior, Right coronary artery arises from anterior aortic sinus and left coronary artery arises from left posterior aortic sinus.
- Arch of aorta
It lies in the superior mediastinum, behind the lower half of manubrium. It begins as a continuation of ascending aorta at the level of upper border of right second sternocostal junction (sternal angle) and continues as descending aorta on the left side at lower border of T4 vertebra, Thus beginning and end of arch of aorta are at the same level, although it begins anteriorly and ends posteriorly. Arch of aorta has following features:-
- Left surface is crossed by 4 nerves from anterior to posterior: left phrenic, superior cervical cardiac branch of left sympathetic trunk, inferior cervical cardiac branch of left vagus, and left vagus nerve.
- Left recurrent laryngeal nerve winds round the arch.
- Three branches arise from convexity:-brachiocephalic trunk, left common carotid artery and left subclavianartery.
- Descending thoracic aorta
It begins as a continuation of arch of aorta to the left lower border of T4 vertebra. It descends through the posterior mediastinum and enters abdomen by passing through aortic hiatus of diaphragm at T12.
- Branches of descending aorta are:-
- Pericardial, mediastinal and esophageal branches.
- Left bronchial (right bronchial usually arises from 3rd posterior intercostal artery).
- Right and left lower nine posterior intercostal arteries,
- Right and left superior phrenic arteries,
- Right and left subcostal arteries.
Azygos venous system includes azygos vein, hemiazygous vein and accessory hemiazygous vein.
- Azygos vein
Azygos vein connects SVC and lVC. It also connects portal venous system and caval venous system, the azygosvein is formed by union of the lumbar azygos, right subcostal and right ascending lumbar veins. It enters the thorax by passing through the aortic opening ofthediaphragm. Then it ascends up to T4 vertebra where it arches forwards over the root of right lungand ends by joining the SVC, Its tributaries are:-
- Right superior intercostal vein (formed by union of2nd, 3rd and 4th posterior intercostal veins).
- 5th to 11th right posterior intercostal veins.
- Hemiazygous vein at lower border ofT5
- Accessory azygos vein at upper border ofT5
- Right bronchial vein, last tributary.
- Several oesophageal, pericardial and mediastinal veins.
- Hemiazygous vein
It is also called inferior hemiazygous vein. It is formed by union ofleft lumbar azygos, left ascending lumbar and left subcostal veins. It pierces the left crus of diaphragm and ascends up to T8 level where it turns to right and joins azygosvein. It drains 9th to 11th left posterior intercostal veins and oesophageal veins.
- Accessory hemiazygous vein
It is also called superior hemiazygous vein. It descends from 4th or 5th intercostal space down to upper border of T8 where it turns to right and joins azygos vein. It receives 5thto 8th left posterior intercostal veins and sometimes left bronchial vein.