Aorta

Largest arterial trunk receives oxygenated blood from the left ventricle and distributes it to all parts of the
body. It is divided into 3 parts for descriptive purposes.
1. Ascending aorta
2. Arch of aorta
3. Descending aorta – which in turn has a thoracic part and an abdominal part

Ascending aorta

Origin and course

It arises from the left ventricle and lies within the pericardium. It is 5 cm long beginning behind the left
half of the sternum at the level of lower border of the left 3rd costal cartilage. Passes upwards and to the
right continues as arch of aorta at the level of second right costal cartilage close to the sternum.
At the root of the ascending aorta above each cusp of the aortic valve, there are slight dilatations of the
vessel wall - aortic sinuses [three aortic sinuses – anterior, left posterior & right posterior].

Branches
1. Right coronary artery from the anterior aortic sinus
2. Left coronary artery from the left posterior aortic sinus
Note: Right posterior aortic sinus is non-coronary

Arch of aorta
It is the continuation of the ascending aorta, situated in the superior mediastinum, behind the lower half of
manubrium sterni.

Course
Begins behind the upper border of the 2nd right sternochondral joint. It runs initially upwards, backwards
and to the left, then downwards by arching over the left lung root.
It ends at the lower border of the 4th thoracic vertebra, where it continues as the descending thoracic aorta.
[Beginning and termination are at the same level, although beginning is anterior and ending posterior].

Relations
A. Anterior and to the left
1. Left pleura and Lung
2. Four nerves – Left phrenic, inferior cervical cardiac branch of left vagus, superior cervical cardiac
branch of left sympathetic trunk and left vagus
3. Left superior inter-costal vein

B. Posterior & to the right

1. Deep cardiac plexus
2. Trachea
3. Oesophagus
4. Left recurrent laryngeal nerve
5. Thoracic duct

C. Inferior
1. Bifurcation of the pulmonary trunk
2. Left principal bronchus
3. Ligamentum arteriosum with superficial cardiac plexus.
4. Left recurrent laryngeal nerve
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D. Superior
1. Three branches – Brachio-cephalic trunk, left common carotid and left subclavian arteries
2. Left brachiocephalic vein

Branches
1. Brachiocephalic artery
2. Left common carotid artery
3. Left subclavian artery

Applied Anatomy
1. Coarctation of the aorta – is localized narrowing of aorta opposite or just beyond the ductus arteriosus.
It may be due to hyper-involution of the ductus arteriosus. Collateral circulation is established
between the branches of the subclavian and those of the descending thoracic aorta.
Clinical features include – (A) Hypertension in the upper limbs, but low BP in the lower limbs. (B)
Grooving or notching of the ribs by the dilated posterior inter-costal arteries. (C) Pulsating scapulae,
visible arterial pulsations and systolic bruit on the back.
2. Aortic knuckle – In PA view X- ray of the chest, arch of aorta is seen as a projection beyond the left
margin of the mediastinal shadow. This projection is called the aortic knuckle.
3. In the left anterior oblique [LAO] direction, a radiolucent area known as the ‘aortic window’ is seen
between the aortic arch above and the bifurcation of the pulmonary trunk below.
4. Aortic aneurysm – localized dilatation, which may press upon mediastinal structures or may rupture.
5. Patent ductus arteriosus [PDA] – During embryonic life, left pulmonary artery is connected to the arch
of aorta immediately distal to the origin of left subclavian artery by ductus arteriosus*, which
conducts blood from pulmonary artery to the aorta. After birth it gets closed and remains as a fibrous
band called the ligamentum arteriosum. If it remains patent then called PDA [*developmentally
derived from the left 6th aortic arch artery]. In this condition blood flows in a reverse direction from
the aorta into the left pulmonary artery [Clinically - Retarded body growth, left ventricular
hypertrophy; Radiologically - increased vascularity and hilar shadows of lungs].

Descending thoracic aorta

It is the continuation of the arch of aorta, present in the posterior mediastinum. Begins at the lower border
of the 4th thoracic vertebra, descends down in the posterior mediastinum, and terminates in front of the T12
by passing through the diaphragm.

Branches
1. Nine pairs of posterior inter costalarteries for 3 -11 spaces.
2. Subcostal artery on each side.
3. Two left bronchial arteries [Right bronchial artery usually is a branch of 3rd posterior intercostal
artery]
4. Oesophageal branches for the middle third of the oesophagus.
5. Pericardial branches.
6. Mediastinal branches for the lymph nodes and areolar tissue of the posterior mediastinum.
7. Superior phrenic arteries.

Common Carotid Artery [CCA]

The origin of CCA differs on the two sides. Right arises from the brachiocephalic trunk, behind the right
sternoclavicular joint. Left artery arises directly from the arch of aorta, behind the manubrium sterni, so
has an additional short course in the thorax.

asdsouza 2
In the neck each CCA extends upwards and slightly laterally within the carotid sheath from the level of
sternoclavicular joint to the upper border of the lamina of the thyroid cartilage, where it ends by dividing
into external and internal carotid arteries [at the level of C4].

Note: CCA does not give any other branches.

Development: 3rd aortic arch

Carotid sinus
It is a fusiform dilatation involving the terminal part of the CCA and /or beginning of ICA. The tunica
media is thinner at this part and receives sinus branches from the glossopharyngeal, vagus and
sympathetic trunk. Carotid sinus acts as a baro-receptor and regulates blood pressure. Stimulation of the
sinus produces fall of BP and slowing of heart rate.

Carotid sinus syndrome: Sudden attacks of syncope and slowing of heartbeat during rotation of the head
are occasionally observed in patients with a hypersensitive sinus.

Carotid Body
It is a small, oval neuro-vascular structure situated close to the carotid sinus on the posterior part of the
bifurcation of the CCA. It receives nerve supply from the same nerves as that of the sinus. Carotid body
acts as a chemoreceptor monitoring oxygen tension within the artery. Stimulation of the carotid body [by
hypoxia] produces a rise in BP, heart rate, and changes in the depth and rate of respiration.

Development: Mesoderm of the 3rd pharyngeal arch.

External carotid artery [ECA]

It is one of the terminal branches of CCA

Extent: From the level of the upper border of the lamina of the thyroid cartilage to a point behind the neck
of the mandible, where it divides within the substance of the parotid gland into the terminal branches -
Maxillary and Superficial temporal arteries.

Course: It begins in the carotid triangle, runs upwards and slightly laterally. It passes anteromedial to the
ICA in the lower part and anterolateral to it in the upper part. Above the carotid triangle, ECA lies deep
within the substance of the parotid gland.

Branches
A. Anterior
1. Superior thyroid
2. Lingual
3. Facial
B. Posterior
1. Occipital
2. Posterior auricular
C. Medial
1. Ascending pharyngeal
D. Terminal
1. Maxillary
2. Superficial temporal.

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Superior Thyroid artery
Arises from the ECA just below the level of the greater cornu of the hyoid bone. It runs downwards and
forwards [along with the external laryngeal nerve]. It reaches the upper pole of the lateral lobe of the
thyroid gland and divides into terminal branches.

Note: Artery and the nerve are close to each other higher up, but diverge near the gland. During surgery of
the thyroid gland, artery to be ligated as near to the gland as possible, to avoid injury to the nerve.

Branches
1. Infrahyoid
2. Cricothyroid
3. Sternomastoid
4. Superior laryngeal
5. Terminal – glandular

Superior laryngeal - accompanied by the internal laryngeal nerve, pierces the thyrohyoid membrane,
supplies the larynx and anastomoses with the inferior laryngeal branch of the inferior thyroid artery.
Glandular branches are two - anterior and posterior. Anterior anastomoses with the similar branch of the
opposite side. Posterior branch anastomoses with the ascending branch of the inferior thyroid artery.

Facial artery
It is a branch of ECA given off just above the tip of the greater cornu of the hyoid bone. It has a tortuous
course both in the carotid triangle [cervical part] and in the face [facial part]. (tortuous course is to
accommodate movements of hyoid bone and mandible during deglutition and mastication respectively)

Artery runs upwards and medially to the posterior border of the submandibular gland. Then it winds
round the gland to appear at the lower border of the mandible to enter the face, at the antero-inferior angle
of the masseter.
In the face - artery passes about 1.25 cm lateral to the angle of mouth, continues to ascend to reach the
side of the nose and then to the medial angle of the eye, where it terminates as the angular artery.

Branches
Cervical part
1. Ascending palatine artery – Supplies the soft palate, tonsil, wall of the pharynx and auditory tube.
2. Tonsillar artery – Major artery to the palatine tonsil [after piercing the superior constrictor muscle].
3. Glandular branches – 3 to 4 in number supply the sub-mandibular gland.
4. Sub mental artery – runs in the sub mental triangle.
Facial part
5. Inferior labial artery
6. Superior labial artery.
7. Lateral nasal artery.
8. Terminal [angular] branch.

Maxillary artery
It is the larger terminal branch of the ECA, within the substance of the parotid gland behind the neck of
the mandible. It has a wide territory of distribution and supplies - External ear, middle ear, auditory tube,
duramater, upper and lower jaws, muscles of temporal and infra-temporal region, nose and paranasal
sinuses, palate and pharynx.

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Course
Artery is divided into 3 parts by the lateral pterygoid muscle

1. First [mandibular] part – Passes between neck of the mandible and the sphenomandibular ligament.
2. Second [pterygoid] part – Runs either superficial or deep to the lower head of lateral pterygoid muscle
3. Third [pterygopalatine] part– Through the pterygo maxillary fissure it enters the pterygopalatine fossa.

Branches
First part
1. Deep auricular artery – Supplies the external auditory meatus, tympanic membrane and the
temporo-mandibular joint.
2. Anterior tympanic artery – Supplies the middle ear.
3. Middle meningeal artery – refer later.
4. Accessory meningeal artery - Enters the skull through the foramen ovale.
5. Inferior alveolar artery - See later.

Second part
1. Anterior and posterior deep temporal branches.
2. Pterygoid branches
3. Masseteric artery.
4. Buccal artery.

Third part
1. Posterior superior alveolar artery
2. Infra-orbital artery.
3. Greater palatine artery.
4. Pharyngeal artery.
5. Artery of the pterygoid canal.
6. Sphenopalatine artery.

Middle meningeal artery

It is a branch from the first part of the maxillary artery, given off in the infra-temporal fossa.
Enters the cranial cavity [middle cranial fossa] by passing through the foramen spinosum along with the
meningeal branch [nervus spinosus] of the mandibular nerve. It runs in the extradural space in a groove on
the squamous part of the temporal bone and divides into a frontal [anterior] and a parietal [posterior]
branch. Frontal branch is larger runs parallel to and a little in front of the central sulcus of the brain.

Branches
1. Ganglionic branches – to the trigeminal ganglion.
2. Petrosal branch.
3. Superior tympanic branch.
4. Temporal branches.

Note: Middle meningeal artery supplies the cranial bones more than the meninges.

Applied Anatomy
Middle meningeal artery may be damaged in head injuries resulting in extra-dural hemorrhage. Frontal
branch is more commonly involved, hematoma presses on the motor area resulting in hemiplegia of the
opposite side. Rarely parietal branch is involved leading to contra lateral deafness.

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Inferior alveolar artery
It is accompanied by the corresponding nerve, enters the mandibular foramen, runs within the mandibular
canal. It gives out branches to molar and premolar teeth and the adjoining gums before dividing into
mental and incisive branches.
Before entering the mandibular canal, artery gives a lingual and mylohyoid branch.

Internal carotid artery [ICA]

It is one of the terminal branches of CCA. ICA ascends within the carotid sheath, enters the skull through
the carotid canal in the petrous part of the temporal bone and terminates in the middle cranial fossa by
dividing into anterior and middle cerebral arteries. Artery can be described in 4 parts – Cervical, Petrous,
Cavernous and Cerebral.

Cervical part – Within the carotid sheath [along with internal jugular vein and vagus nerve], it does not
give any branches.

Petrous part - lies within the carotid canal of the petrous part of the temporal bone, gives a carotico-
tympanic branch which enters the middle ear.

Cavernous part – Within the cavernous sinus, gives superior and inferior hypophyseal branches,
cavernous branches and a meningeal branch.

Cerebral part – ICA pierces the dural roof of the cavernous sinus and then runs upwards to reach the
medial end of the lateral sulcus of the brain, where it divides into anterior and middle cerebral arteries.

Cerebral part has the following branches

1. Ophthalmic artery.
2. Posterior communicating artery.
3. Anterior choroidal artery.
4. Anterior cerebral artery.
5. Middle cerebral artery.

™ ICA supplies the hypophysis cerebri, orbit and a major part of the brain.
™ ICA has a ‘U’ shaped bend, convex forwards while passing through and above the cavernous sinus,
this loop is clinically referred to as the carotid siphon, which damps down the pulsations of the artery
and maintains a regular flow of blood to the brain [branches 1, 2 and 3 are from the upper limb of the
carotid siphon].
™ Posterior communicating artery connects the ICA with the posterior cerebral artery, to complete the
circle of Willis [Connection between ICA and Vertebral arteries].

Ophthalmic artery
Branch of ICA, enters the orbit through the optic canal infero-lateral to the optic nerve [Both are enclosed
within a common dural sheath].
In the orbit, artery crosses the nerve, from lateral to medial side, passes medially along the medial wall of
the orbit, between the superior oblique and medial rectus muscles. It terminates near the medial angle of
the eye by dividing into supra-trochlear and dorsal nasal branches.

Branches
A. To the eyeball
1. Central artery of retina – Pierces the dural sheath of the optic nerve about 1.25 cm behind the
eyeball. Artery reaches the optic disc and supplies the optic nerve and 6-7 layers of retina. Intra
asdsouza 6
 ocular part of the artery is visualized in the living through an ophthalmoscope. This artery is an end
artery, obstruction produces blindness.
2. Posterior ciliary arteries – Consists of two sets, long and short, both of which pierce sclera around
the optic nerve.

B. Muscular branches - supply the extra-ocular muscles.

C. Lacrimal artery – passes laterally along the upper border of the lateral rectus and supplies the lacrimal
gland, eyelids and conjunctiva.

D. Branches along the medial wall of the orbit

1. Posterior ethmoidal artery
2. Anterior ethmoidal artery.
3. Medial palpebral arteries
4. Supra orbital artery.
5. Supra trochlear artery.
6. Dorsal nasal artery

Anastomoses between the (1) dorsal nasal branch of the ophthalmic and the terminal branch of the facial,
(2) supratrochlear and supraorbital with that of superficial temporal and (3) that between the lacrimal and
middle meningeal are the communications between the ICA and ECA.

Subclavian artery
¾ This is the principal artery of the upper limb, also supplies a considerable part of the neck and brain
through its branches.
¾ The two subclavian arteries differ in their origin – left is a branch of arch of aorta and right is a branch
of brachiocephalic trunk. Both have a similar course in the neck [left has an additional portion in the
thorax and hence is longer than the right].
¾ Artery extends from the sternoclavicular joint to the outer border of the first rib, has a curved course
with an upward convexity. For descriptive purposes artery is divided into 3 parts by the scalenus
anterior muscle.
¾ Second part lies deep to the scalenus anterior and superficial to the scalenus medius. First part of the
artery is hooked round by ansa subclavia on both the sides and by the recurrent laryngeal nerve on the
right side only.

Branches
1. Vertebral artery.
2. Internal thoracic artery.
3. Thyrocervical trunk
[All the above are from the first part].
4. Costocervical trunk [from the first part on the left side and from the second part on the right side].
5. Dorsal scapular artery [from the third part].

Applied Anatomy
1. A cervical rib can compress upon the subclavian artery, in which case radial pulse is diminished or
absent on turning the head upwards and to the affected side after a deep breath.
2. An aneurysm in the third part of the subclavian artery may press on the brachial plexus causing pain,
numbness, weakness etc. in the upper limb.
3. Right subclavian artery may arise from the descending thoracic aorta, then passing posterior to the
oesophagus. Dysphagia [difficulty in swallowing] may be caused by this abnormal origin of the
artery.
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4. Subclavian steal syndrome – (Obstruction of the subclavian artery prior to the origin of the vertebral
artery), in order to provide collateral circulation to the affected arm, some amount of blood is ‘stolen’
from the vertebral artery of the opposite side [which should have gone to the brain]. This may result in
ischaemic neurological symptoms.

Vertebral artery
It arises from the upper surface of the first part of the subclavian artery. After its course in the neck, it
enters the cranial cavity by passing through the foramen magnum. At the lower border of the pons it
unites with that of the opposite side to form the basilar artery.

Artery is divided into 4 parts

1. First part – Extends upwards and slightly backwards from its origin to the foramen transversarium of
C6.
2. Second part – Extends from the foramen transversarium of C6 to that of C1, surrounded by a plexus of
sympathetic nerves and vertebral vein.
3. Third part – Present in the sub-occipital triangle, lodged in a groove on the posterior arch of the atlas,
dorsal ramus of C1 intervening.
4. Fourth part – Pierces the dura and arachnoid mater, passes upwards and medially through the foramen
magnum. At the lower border of the pons it joins with the fellow of the opposite side to form the
basilar artery.

Branches
In the neck
1. Spinal branches – enter the vertebral canal through inter-vertebral foramina and supply the upper 5-6
cervical segments of the spinal cord.
2. Muscular branches – Arise from the third part of the artery and supply the sub occipital muscles.

In the cranial cavity

1. Meningeal branches – supply the meninges of the posterior cranial fossa.
2. Posterior spinal artery.
3. Anterior spinal artery.
4. Medullary branches.
5. Posterior inferior cerebellar artery.

Anterior spinal artery

Each arises from the vertebral artery, unites as it descends with that of the other side, to form a median
trunk. This artery supplies the medial part of the medulla oblongata [including pyramidal tract and
hypoglossal nuclei] apart from the spinal cord.
Lesion of the anterior spinal artery leads to ‘medial medullary syndrome’ - manifested by impairment of
voluntary movements [due to the involvement of pyramidal tract] on the contra lateral side and ipsi lateral
loss of tongue movements and wasting of tongue musculature [due to the involvement of the hypoglossal
nucleus].

Posterior inferior cerebellar artery

It is the largest branch of the vertebral artery and has a tortuous course. It supplies – (a) Lateral part of the
medulla oblongata including the nucleus ambiguus, lateral spinothalamic tract and nucleus and spinal tract
of trigeminal nerve (b) Inferior vermis and infero-lateral surface of the cerebellar hemisphere. (c) Forms
the choroid plexus of the fourth ventricle.

asdsouza 8
Thrombosis of the this artery produces lateral medullary syndrome, which is characterized by loss of pain
and temperature sensation of the same side of the face and opposite half of the body, paralysis of vocal
cords, soft palate and pharyngeal muscles of the same side.

Thyrocervical trunk
It arises from the superior aspect of the first part of the subclavian artery distal to the origin of the
vertebral artery. The arterial trunk immediately divides into three branches - inferior thyroid, superficial
cervical and suprascapular.

Inferior thyroid artery – Runs upwards and medially, passes behind the carotid sheath. Close to the lower
pole of the lateral lobe of the thyroid gland, artery [accompanied by the recurrent laryngeal nerve] divides
into terminal glandular branches.
During surgery of the thyroid gland, inferior thyroid artery is ligated away from the gland to save the
recurrent laryngeal nerve

Branches:
1. Ascending cervical artery.
2. Inferior laryngeal artery: runs along with the recurrent laryngeal nerve and supplies the laryngeal
muscles and mucous membrane below the vocal folds.
3. Tracheal, oesophageal and pharyngeal: supply the corresponding structures.
4. Glandular branches: 4 - 5 in number supply the lower part of the gland, one of the branches ascends
upwards and anastomoses with the branch of superior thyroid artery.

Costocervical trunk
It arises from the back of the subclavian artery; arches backwards over the cervical pleura, close to the
neck of the first rib divides into superior inter costal and deep cervical arteries.
Superior inter costal artery descends anterior to the neck of the first rib and divides into first and second
posterior inter costal arteries.
Note: In about 35% of the subjects, dorsal scapular artery and superficial cervical artery take a common
origin [which is called as the transverse cervical artery], from the thyrocervical trunk.

External jugular vein

It drains blood from the greater part of the face and scalp.
Formation: formed below the angle of the mandible or within the parotid gland by the union of the
posterior division of the retro-mandibular vein and posterior auricular vein.

Course: Vein runs downwards superficial to the sternocleidomastoid and deep to the platysma, pierces the
investing layer of the deep fascia about 2.5cm above the clavicle, to enter the supra-clavicular triangle.

Termination: ends in the subclavian vein.

The vein is provided with two pairs of valves, one at its termination into the subclavian vein and other one
about 4cm above the clavicle. In between these valves the vein may show a dilatation called as sinus.
These valves however do not prevent regurgitation.

Tributaries
1. Formative tributaries – posterior auricular and posterior division of the retro-mandibular veins.
2. Transverse cervical vein
3. Suprascapular vein.
asdsouza 9
4. Anterior jugular vein.
5. Posterior external jugular vein.
6. [Occasionally] Occipital vein.

Applied Anatomy
External jugular vein serves as a useful venous manometer. Dilated vein due to rise in venous pressure
may be seen in patients with right heart failure, obstruction to SVC etc.

Anterior jugular vein

It begins below the chin by the union of small veins in the sub-mental region.
¾ Runs downwards close to the anterior midline of the neck
¾ It pierces the deep cervical fascia and turns laterally immediately above the clavicle, deep to the
sternocleidomastoid and ends by joining the external jugular vein close to its termination.
¾ In the supra sternal space, the anterior jugular veins of the two sides are connected to each other by a
transverse arch – jugular venous arch. Sometimes anterior jugular veins of the two sides are united to
form a single trunk along the median plane.
¾ In size the anterior jugular vein is usually inverse to the external jugular vein.

Internal jugular vein

It drains venous blood from the brain, superficial part of the face and neck.

It begins as a continuation of the sigmoid sinus at the posterior compartment of the jugular foramen. At its
commencement the vein presents a dilatation known as the superior bulb, which is lodged in the jugular
fossa of the temporal bone. IJV runs downwards within the carotid sheath, behind the medial end of the
clavicle it unites with the subclavian vein to form the brachiocephalic vein. Close to its termination it
presents another dilatation, inferior bulb, which is guarded by valves. On either side of the IJV is the
vertical chain of deep cervical lymph nodes.

Tributaries
1. Inferior petrosal sinus – communicates the superior bulb of IJV with the cavernous sinus.
2. Common facial vein [formed by the fusion of facial vein and the anterior division of retro-mandibular
vein]
3. Lingual vein.
4. Pharyngeal veins from the pharyngeal plexus.
5. Superior thyroid vein.
6. Middle thyroid vein.

• The thoracic duct opens into the angle of union between the left IJV and left subclavian vein. Right
lymphatic duct opens similarly on the right side.
• It may communicate with the external jugular vein through the oblique jugular vein.
• In congestive cardiac failure or any other condition where venous pressure is raised, the IJV is
markedly dilated.
• IJV is accessible in between and deep to the two heads of sternocleidomastoid.
• Under normal condition the rate of flow of CSF is increased during lumbar puncture when both IJV s
are compressed. Absence of pressure rise upon compression of veins indicates a complete or partial
block in the spinal subarachnoid space.

asdsouza 10
Brachiocephalic vein
1. Brachiocephalic veins are formed behind the sternoclavicular joint, by the union of IJV and
subclavian vein.
2. Right vein is shorter [2.5cm] than the left [6cm].
3. Right runs vertically down and the left runs obliquely [downwards and to the right] behind the
upper half of the manubrium sterni.
4. The two- brachiocephalic veins unite at the lower border of the right first costal cartilage to form
the superior vena cava.
Tributaries
1. Vertebral vein
2. Internal thoracic vein
3. Inferior thyroid vein
4. First posterior inter costal vein

Left brachiocephalic vein in addition receives left superior intercostal vein, pericardial and thymic veins.

Superior Vena Cava [SVC]

It is a large vein drains blood from the upper part of the body into the right atrium. It is devoid of valves.

Formation: It is formed by the union of two brachiocephalic veins behind the lower border of the right
first costal cartilage close to the sternum. SVC measures 7cm in length and passes downwards behind the
first and second right inter- costal spaces and ends in the upper and posterior part of the right atrium
behind the 3rd right costal cartilage.
Extra pericardial part of SVC is in the superior mediastinum and the intra pericardial part is in the middle
mediastinum [SVC pierces the fibrous pericardium opposite the 2nd right costal cartilage].

Relations
Anterior – Anterior margins of the right lung and pleura; right internal thoracic vessels
Posterior – Trachea, root of the right lung and right vagus nerve.
Right side – Right lung and pleura; phrenic nerve and pericardiophrenic vessels
Left side – Ascending aorta and brachiocephalic trunk

Tributaries
1. Formative – right and left brachiocephalic veins.
2. Azygos vein – opens into the SVC opposite the right second costal cartilage.
3. Several smaller pericardial and mediastinal veins.

Development
1. Upper [extra pericardial] part – from the lower part of the right anterior cardinal vein.
2. Lower [intra pericardial] part – from the right common cardinal vein.

Applied Anatomy
SVC may be obstructed above or below the termination of the azygos vein. Blood is drained by inferior
vena cava or by azygos vein by collateral connections.

[Some of the communications between SVC and IVC - Superior and inferior epigastric veins; superficial
epigastric and lateral thoracic veins; azygos vein; and vertebral venous plexus. All these connections are
dilated in obstruction of SVC].

asdsouza 11
EMBRYOLOGY OF THE HEART Dr. A. S. D’ Souza

1. Fibrous pericardium – septum transversum 11. Inter-atrial septum

a. Upper part – septum secundum
2. Parietal pericardium – Somatopleural b. Lower part – septum primum
mesoderm c. Lowest part – septum intermedium#
[# Fused ventral and dorsal endocardial
3. Visceral pericardium – Splanchnopleural cushions]
mesoderm [myoepicardial mantle]
Fossa ovalis – upper part of the septum
4. Pericardial cavity – Intra-embryonic coelom primum
Limbus fossa ovalis – lower free margin of
5. Transverse sinus – breakdown of the dorsal the septum secundum
mesocardium. [Due to folding of the heart tube
and approximation of arterial and venous ends of 12. Coronary sinus – left horn and body of the
the heart tube]. sinus venosus.

6. Oblique sinus – absorption [& rearrangement] 13. Superior vena cava

of pulmonary veins into the left atrium. a. Upper part [extrapericardial part] till the
joining of azygos vein – right anterior cardinal
7. Ascending aorta – truncus arteriosus vein
b. Lower part [intrapericardial part] below the
8. Pulmonary trunk – truncus arteriosus joining of azygos vein – right common cardinal
vein
9. Right atrium
a. Posterior smooth part – right horn of the 14. Right ventricle
sinus venosus a. Inflowing part – bulbus cordis and right
b. Anterior rough part [including the right part of the primitive ventricle
auricle] –Right half of the primitive b. Out flowing part – from the bulbus cordis
atrium
c. Ventral part close to atrio-ventricular 15. Left ventricle – left portions of the
orifice –from the right atrio-ventricular corresponding parts [as above].
canal
d. Crista terminalis – upper part of the right 16. Atrio-ventricular valves – Proliferation of
venous valve. mesenchymal cells around A-V canals.
e. Valve of inferior vena cava and valve of
coronary sinus – lower part of the right 17. Inter ventricular septum
venous valve. a. Lower muscular part – from the original
inter ventricular septum
10. Left atrium b. Upper membranous part – proliferation of
a. Posterior smooth part – Incorporation of endocardial [AV] cushions.
pulmonary veins
b. Anterior part [rough] including left 18. Musculature [myocardium] of the heart from
auricle – left half of primitive atrium the myo-epicardial mantle [splanchnopleural
c. Ventral part close to the atrio-ventricular mesoderm].
canal – from left half of the atrio-ventricular
canal 19. Oblique vein of the left atrium – derived
from the left common cardinal vein

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Development of the Heart
Prior to the formation of embryonic folds two longitudinal vessels the dorsal aortae appear on each side
of the notochord, along the dorsal wall of the yolk sac. At the cephalic end the dorsal aortae invade the
cardiogenic area as the endothelial heart tubes, which develop in the splanchnopleuric mesoderm. The
endothelial heart tubes are separated from the floor of the pericardial sac by a thickened plate of
mesoderm - the myoepicardial mantle [MEM].

After the formation of embryonic folds, the pericardial sac lies in the ventral wall of the foregut. Two
endothelial heart tubes lie between the foregut and the splanchno-pleuric layer of the pericardial sac.
However the heart tubes are separated from the pericardial sac by the MEM. Initially MEM is separated
from the heart tubes by the cardiac jelly, which is subsequently replaced by the myocardium.

The cranial end of each heart tube is connected with the corresponding dorsal aorta along the lateral wall
of the foregut through the first aortic arch artery.

Two para-median endothelial heart tubes lie ventral to the foregut and project into the dorsal wall of the
pericardial sac covered by the MEM. Cephalic and lateral folding of the embryo, place the two tubes in
the thoracic region, where they meet and fuse in a cranio-caudal direction and form a single heart tube.
Cranial end is the arterial end, since it is continuous with the dorsal aorta, through the first aortic arch
artery. Caudal end is the venous end, at first partly embedded in the mesenchyme of the septum
transversum.

Note: All the components of heart and blood vessels [endothelium, muscle and connective tissue] are
mesodermal in origin. MEM forms the epicardium and myocardium; endothelial heart tube persists as the
endocardium.

Single heart tube formed shows four dilatations, which are named cranio-caudally as follows:
1. Bulbus cordis – gets subdivided into two parts – conus [bulbus cordis proper], that is continuous with
the ventricle [caudally] and truncus arteriosus [cranially], which is continuous with the aorta.
2. Primitive ventricle – along with the conus gives rise to both the ventricles.
3. Primitive atrium – partitioned to form the right and left atria.
4. Sinus venosus - presents right and left horns and a body [floor]. Each horn receives [from the
corresponding half of embryo] a vitelline vein from the yolk sac, one umbilical vein from the placenta
and a common cardinal vein from the body wall.

Formation of the cardiac loop

As the development progresses, longitudinally placed heart tube grows rapidly in a limited space, dorsal
to the pericardial sac. As a result, tubular heart tube bulges ventrally [and slightly caudally], into the
pericardial sac in a ‘U’ shaped manner. At this stage caudal part of the tube is embedded in the substance
of the septum transversum and the part of the heart tube within the pericardial sac consists of bulbus
cordis and the ventricle [bulbo-ventricular loop].

Subsequently as the caudal end is freed from the septum transversum, atrium and sinus venosus come to
lie behind and above the ventricle and the heart tube assumes ‘S’ shape. Bulbo- ventricular loop is
suspended from the dorsal wall of the pericardial sac by a bilaminar fold the dorsal mesocardium.
Continuous growth of the loop produces degeneration of the dorsal mesocardium, with its eventual
rupture [transverse sinus is so formed between the arterial and venous ends of the tube].

Bulbo-ventricular ridge [sulcus] – Separates the bulbus cordis and the primitive ventricle. In the later
stages ridge disappears and a common bulbo-ventricular cavity is formed, which communicates with the
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truncus arteriosus and atrium. Primitive atrium is placed on the dorsal aspect that grows transversely
between the developing oesophagus and truncus arteriosus. The sinus venosus communicates with the
primitive atrium and is dorsal and caudal to it.

Fate of the sinus venosus

Sinus venosus communicates with the atrium through the sinu-atrial orifice, which initially occupies the
median position and is guarded by right and left venous valves. A sickle shaped sinu atrial fold grows
from the left side of the orifice; this fold reduces the size of the orifice and shifts it to the right side of the
primitive atrium. Thus the blood conveyed by the sinus venosus now reaches right half of the primitive
atrium. Consequently left horn of the sinus venosus and its tributaries become much reduced in size.

With the appearance of the hepatic bud in the septum transversum, the supra hepatic parts of the right and
left umbilical and left vitelline veins disappear. Supra hepatic part of the right vitelline vein persists as the
terminal part of the IVC. Degeneration of the left anterior cardinal vein and left common cardinal vein,
leads most of the blood into the right horn of the sinus venosus. [Right common cardinal vein and the
terminal part of right anterior cardinal vein form the SVC].

Thus the entire blood from various parts of the body reaches the right half of the primitive atrium. Due to
this differential work load right horn of the sinus venosus enlarges and later gets incorporated into the
posterior part of the right atrium [after the formation of the inter-atrial septum]. The left horn remains
narrow and persists as the coronary sinus.

Right and left venous valves separate from each other, left fuses with the inter-atrial septum. Right venous
valve gets subdivided into three parts, which form crista terminalis, valve of the IVC and valve of
coronary sinus.

Primitive Atrium
The transversely oriented common atrial chamber communicates with the bulbo-ventricular cavity
through the AV canal. From the ventral and dorsal walls of the AV canal, mesenchymal cells proliferate
and form ventral and dorsal endocardial AV cushions. The AV cushions gradually fuse with each other to
form an anteroposterior partition - septum intermedium, dividing the AV canal into right and left AV
orifices.

During 5th week the dorsal wall of primitive atrium receives the sinu-atrial orifice, which is guarded by
right and left venous valves. The valves unite above and extend to the roof of the atrium as an endothelial
ridge - septum spurium.

Inter atrial septum

When the formation of AV cushions is in progress, separation of the primitive atrium into right and left
atria takes place.

Septum primum - arises from the roof of the atrial chamber to the left of the opening of sinus venosus
[close to the middle of primitive atrial chamber]. For some time, the lower edge of this septum is free and
concave and is separated from the septum intermedium by a gap - foramen [ostium] primum. Progressive
growth of the septum primum culminates in the closure of the foramen primum when the caudal edge of
the septum primum fuses with the atrial surface of the septum intermedium.

Associated with the [before complete] closure of foramen primum, another opening foramen [ostium]
secundum appears [during the 7th week] due to degeneration of the cephalic part of the septum primum.
During later part of the 7th week, a thick septum secundum grows downwards from the atrial roof just to
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the right of the original attachment of the septum primum [to the left of septum spurium]. Caudal edge of
the septum secundum grows to overlap the foramen secundum and cephalic free margin of the septum
primum. However a valvular opening communicates the right and left atria called as the foramen ovale.
The lower edge of the septum secundum is thick and firm but the cranial end of the septum primum is thin
and mobile like a flap.

Foramen ovale remains patent throughout fetal life. During this period, right atrial pressure is higher than
the left atrial pressure, since the latter receives negligible amount of blood from the lungs. Most of the
blood of the right atrium [especially that from IVC] passes to the left atrium through foramen ovale. With
the advent of pulmonary circulation after birth, more blood reaches left atrium from the lungs and the left
atrial pressure rises. As a result septum primum is pushed to right towards the septum secundum and
gradually undergoes structural fusion.

Left atrium
Interatrial septum divides the primitive atrium into left and right atria. The dorsal wall of left atrium
initially receives a single pulmonary vein. This vein divides into right and left branches, each of which
again bifurcates into upper and lower branches, to drain into the corresponding lung buds. Later parts of
these pulmonary veins close to the left atrium are absorbed [into the atrium], so that four veins [two from
each lung] come to open into the left atrium.

Fate of the bulbus cordis

It is divided into two parts – Conus [bulbus cordis proper] and Truncus arteriosus [TA].
A spiral septum appears within the TA and subdivides it into ascending aorta and pulmonary trunk. Conus
merges with the primitive ventricle to form the bulbo-ventricular cavity.

Truncus arteriosus
A spiral shaped septum divides the truncus arteriosus into two vessels - ascending aorta and pulmonary
trunk. Beyond the pericardial sac TA is continuous with the aortic sac. Aortic sac divides into right and
left horns, each of which is connected to the corresponding dorsal aorta by six pairs of aortic arch arteries.

The cephalic end of spiral septum fuses with the dorsal wall of the aortic sac, between 5th and 6th pairs of
aortic arch arteries. As a result, upper five pairs of aortic arch arteries are connected to the ascending aorta
and the sixth pair is continuous with the pulmonary trunk.

Bulboventricular cavity
This consists of a dilated part that communicates posteriorly with the atria [through right and left AV
canals] and a conical upper part communicating with the truncus arteriosus [ascending aorta & pulmonary
trunk].

Inter-ventricular septum
While septation of the atrium is in progress, the inter-ventricular septum exhibits its appearance in the
early part of 6th week. The septum grows upwards from the floor of the bulboventricular cavity [so
divides the lower dilated part of this cavity into right and left parts] and partially fuses with the AV
cushions [septum intermedium]. The cephalic margin of IV septum is free and concave in the middle
[fusion with the AV cushions is on the dorsal and ventral ends], so that a ventricular foramen exists that
communicates the two ventricles.

Two ridges – right and left bulbar ridges appear in the conical upper part of the bulboventricular cavity.
The ridges grow towards each other and fuse to form the bulbar septum. Bulbar septum grows down and
meets the AV cushions. The gap between the upper edge of the IV septum and lower edge of the bulbar
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septum is filled by proliferation of tissue from the AV cushions, thus closing the ventricular foramen
during 8th week.

Conducting System
Cardiac muscle fibres are differentiated from the myo-epicardial mantle and envelop the heart tube.
Myocardium starts beating as early as the 4th week of embryonic life, even before nerve fibres reach the
heart. Muscle fibres appear first in the ventricles and begin to beat, and then in the atria, which begin to
beat at a faster rate. Atrial region causes the ventricular rate to speed up by the development of the AV
node and bundle, which appear on the dorsal AV cushion of the AV canal during 6th week. The sinus
venosus then commences to beat at a still faster rate, and acts as a pace maker by the development of the
SA node. SA node appears during 3rd month of foetal life in the sinus venosus of right atrium close to the
SVC orifice. Fetal heart beats at a faster rate, but the flow is sluggish as the cardiac orifices are large,
valves are incompetent and the peripheral resistance of blood vessels is poor due to less musculature of
the vessel wall.

Sequence [chronology] of events during development of the heart

a. Development of the cardiovascular system starts during the early part of 3rd week.
b. Heart starts beating on the 22nd day, begins to pump blood from the 24th day.
c. Fusion of the heart tubes and formation of cardiac loop during 4th week.
d. Various septa and definitive heart are formed between 5th and 8th week [septum primum appears
during 6th week and septum secundum during 7th week].

Congenital Anomalies

A. Defects in position
1. Dextro-cardia – Chambers and blood vessels are placed on the opposite sides [structures that normally
lie on the right side are on the left side and vice versa].
2. Situs inversus – This is associated with general transposition of the viscera due to reverse rotation,
including the abdominal viscera.

B. Defect in the Thoracic wall – Heart is exposed, and can be seen form outside due to defective
development of the chest wall [fault in the lateral folding of embryo].

C. Defects in the inter atrial septum [A.S.D.]

1. Patent foramen ovale – Septum primum and secundum develop normally but valvular passage fails to
close.
2. Probe patency of foramen ovale – In this condition, foramen ovale is closed functionally but remains
patent anatomically, so that a probe can be passed between the septum secundum and septum primum.
This is observed in about 25 % of individuals without any trans-septal blood flow.
3. Persistent foramen secundum – is due to the incomplete development of the septum secundum, so it
fails to overlap the septum primum. Blood shunts between right and left atria.
4. Septum primum fails to reach the AV cushions resulting in foramen primum defect.
5. Prenatal closure of foramen ovale – Closure of foramen ovale in fetal life, right half becomes
hypertrophied and the left half poorly develops. Child usually dies shortly after birth.
6. Cor-triloculare – Complete absence of the IA septum [heart is three chambered – Bi-ventricular
monoatrial].

D. Defect in the IV septum [V.S.D.]

1. Persistent IV foramen - This anomaly usually affects the membranous part of the septum

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 2. Cor-triloculare – Complete absence of the IV septum [heart is 3 chambered with two atria and a single
ventricle - Bi atrial monoventricular].

E. Defects in the AV canal – May lead to a condition in which all chambers of the heart
intercommunicate.

F. Defects in the TA and Bulbus cordis

1. Undivided truncus arteriosus – is due to agenesis of the spiral [aorto-pulmonary] septum.

2. Transposition of the ascending aorta and pulmonary trunk – Due to reverse attachment of the spiral
septum.
3. Atresia or stenosis – any of the orifices of the heart may be too narrow [stenosis] or none at all
[atresia].
Eg. Valvular stenosis of the pulmonary orifice – Semilunar valves may be fused, Valvular stenosis of
aortic orifice.

G. Combined defects

1. Fallot’s tetralogy – It is a combination of four cardiac anomalies, which include pulmonary stenosis, IV
septal defect [VSD], displacement of the aortic orifice to the right to override the ventricular septum and
hypertrophy of the right ventricle. [When the classical Fallot’s tetralogy is associated with ASD, it is
known as Fallot’s pentalogy]. Fallot’s tetralogy causes cyanosis when the venous blood shunts from the
right to left ventricles. Cyanosis fails develop as long as the blood flow is from left to right.

2. Eisenmenger’s complex – It includes pulmonary hypertension [due to hypoplasia of the pulmonary

capillary plexus], dilatation of the pulmonary trunk and hypertrophy of the right ventricle. Sometimes
Eisenmenger’s complex may be associated with ASD / VSD / PDA, then referred to as ‘Eisenmenger’s
syndrome’.

Note: Heart and vascular congenital malformations account for the largest category of human birth defects
[VSD is the commonest].

Development of the major arteries

Two longitudinal vessels, primitive aortae appear in the embryo prior to the formation of embryonic folds.
Primitive aortae are continuous with the endocardial heart tubes. With the formation of the head fold of
the embryo, the heart tube lies ventral to the foregut and its cephalic end is connected to the primitive
aortae. The primitive aortae bend ventrally along the lateral wall of the foregut passing through the
mesodermal core of the first pharyngeal arches.

Therefore at this stage each primitive aorta consists of three portions

1. Ventral aorta – lying ventral to the foregut,
2. First aortic arch arteries – passing through the first pharyngeal arches.
3. Dorsal aorta - lying dorsal to the foregut and on either side of the notochord.

After the fusion of two heart tubes, the two ventral aortae fuse partially to form the aortic sac, which is
continuous with the truncus arteriosus. Un-fused parts remain as the right and left horns or limbs of the
aortic sac, each of which is connected to the corresponding dorsal aorta through the first pair of aortic arch
arteries. When five more pharyngeal arches appear on the lateral wall of the pharynx, successive arterial
arches too appear in a cranio-caudal sequence. Altogether six pairs of aortic arch arteries embrace the lateral
wall of the pharynx and extend from the ventral horn of the aortic sac to the dorsal aortae - right and left.
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Following changes take place

1. Greater parts of the first and second arch arteries disappear [smaller portions remain in adults and are
represented by the maxillary artery and stapedial artery respectively].
2. 5th arch artery disappears on both sides.
3. Each horn of the aortic sac is therefore connected only with 3rd, 4th and 6th arch arteries. The spiral
septum, which divides the truncus arteriosus into ascending aorta and pulmonary trunk, extends into the
aortic sac. 3rd and 4th arch arteries open into the ventral part [which forms ascending aorta] and the sixth
into the dorsal part [which forms the pulmonary trunk].
4. The two dorsal aortae grow cranially beyond the point of attachment of the first arch artery and
caudally fuse with each other caudal to the 4th thoracic segment.
5. The portion of dorsal aorta between the attachment of 3rd and 4th arch arteries [ductus caroticus]
disappears on both the sides.
6. Portion of the right dorsal aorta, between the point of attachment of the 4th arch artery and the point of
fusion of the two dorsal aortae disappears.
7. Each 6th arch artery gives off an artery to the developing lung bud. On the right side, part of the 6th
arch artery between this bud and dorsal aorta disappears and on the left side remains as the ductus
arteriosus.
8. Dorsal aorta gives off series of lateral inter-segmental branches [called as inter-segmental arteries] to
the body wall.

Summary of development of the major arteries

1. Ascending aorta and pulmonary trunk - derived from the truncus arteriosus.
2. Arch of aorta – is derived from ventral part of the aortic sac, left horn of aortic sac and left fourth
arch artery.
3. Descending aorta – formed from the left dorsal aorta below the attachment of the 4th arch artery and
fused parts of the dorsal aortae till the 4th lumbar segment.
4. Left subclavian artery – Left 7th cervical inter-segmental artery [7th inter-segmental artery is
attached to the dorsal aorta opposite to the attachment of the 4th arch artery].
5. Right subclavian artery – derived from right 4th arch artery and 7th cervical inter-segmental artery.
6. External carotid artery – grows out as a bud from the 3rd arch artery.
7. Common carotid artery – Part of the 3rd arch artery prior to the external carotid bud
8. Internal carotid artery – Portion of the 3rd arch artery after the external carotid artery
9. Pulmonary artery – derived from the 6th arch artery.

Development of other arteries

Dorsal aorta gives off three groups of branches
1. Ventral splanchnic arteries – these supply the gut. They are coeliac artery [to the infra diaphragmatic
part of the fore gut], superior mesenteric [artery of the midgut], inferior mesenteric [to the structures
derived from the hind gut] and in the thoracic region oesophageal and bronchial to the supra
diaphragmatic part of the foregut.
2. Lateral or intermediate splanchnic arteries – supply structures derived from the intermediate
mesoderm [renal, suprarenal, inferior phrenic and gonadal - testicular /ovarian arteries].
3. Dorso-lateral [somatic inter-segmental] branches- run between the adjacent segments. Some of them
persist and retain their original inter segmental position – e.g. inter costal and lumbar arteries.

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