Systemic Arteries

The aorta is a single continuous tube divided into four functional segments: ascending (coronaries only), arch (three great vessels to head and arms), thoracic descending (intercostals and bronchials), and abdominal (solid organs and bowel, bifurcating at L4). Three key openings pierce the diaphragm — T8 for the IVC (+ right phrenic nerve), T10 for the oesophagus (+ both vagal trunks), T12 for the aorta (+ thoracic duct + azygos vein) — memorised as "I Eat Apples." The aortic hiatus is a fibro-osseous gap (not muscular), so unlike the oesophageal hiatus it does not herniate, but it is the level where traumatic aortic transection occurs most commonly in deceleration injuries.

The arch of aorta gives three branches in order from right to left: brachiocephalic trunk (first and largest, dividing behind the right sternoclavicular joint into right common carotid and right subclavian), left common carotid, and left subclavian. The subclavian artery's branches are remembered as "VIT C D" — Vertebral (→ basilar → Circle of Willis), Internal thoracic (→ anterior intercostals; harvested for CABG as the gold-standard left ITA-to-LAD graft), Thyrocervical trunk, Costocervical trunk, Dorsal scapular. Subclavian steal syndrome occurs when proximal stenosis reverses vertebral artery flow during arm exercise, causing vertebrobasilar symptoms — a BP difference >15 mmHg between arms is the bedside clue.

The thoracic aorta supplies posterior intercostal arteries to spaces 3–11 (spaces 1–2 are supplied by the costocervical trunk of the subclavian), bronchial arteries to the airway walls, and oesophageal branches to the middle oesophagus. The key clinical application is coarctation: narrowing at the ligamentum arteriosum forces blood through intercostal collaterals that enlarge and erode the inferior rib margins — rib notching on CXR — a sign that only appears after age 5–6 when collaterals have had time to hypertrophy. The lower oesophageal arteries connect the left gastric artery (portal system) with the azygos (systemic), making the lower oesophageal mucosa a site of portosystemic varices in portal hypertension — and a cause of life-threatening haematemesis.

Fig. 4.164 — Abdominal aorta: unpaired midline visceral branches (coeliac, SMA, IMA), paired visceral branches (suprarenal, renal, gonadal) and paired parietal branches, ending in the bifurcation at L4.

Gray's Anatomy for Students, 4e

The abdominal aorta's ventral branches supply the gut in three embryological blocks: coeliac (T12/L1) for foregut, SMA (L1) for midgut up to the splenic flexure, IMA (L3) for hindgut. The critical watershed between SMA and IMA territory sits at the splenic flexure — its tenuous arc of Riolan collateral makes this the commonest site of ischaemic colitis after AAA repair or low-flow states. Renal arteries are end-artery systems with no cortical anastomosis, which is why bilateral renal artery stenosis combined with ACE inhibitor therapy — which removes the efferent arteriolar tone needed to maintain glomerular filtration pressure — precipitates acute kidney injury.

Fig. 4.121 — Anterior (visceral) branches of the abdominal aorta: the coeliac trunk (foregut), superior mesenteric artery (midgut) and inferior mesenteric artery (hindgut).

Gray's Anatomy for Students, 4e

Four organs dominate exam questions on visceral blood supply: thyroid, stomach, suprarenal, and ductus arteriosus. The thyroid is surgically treacherous because two major nerves — the external branch of the superior laryngeal nerve (voice pitch) and the recurrent laryngeal nerve (voice at all) — cross the superior and inferior thyroid arteries respectively, so ligation technique dictates whether the patient wakes up hoarse or speechless. The stomach's six arteries all trace back to the coeliac trunk via a rich anastomotic ring along both curvatures, making ischaemia rare — but portal hypertension exploits the left gastric–oesophageal anastomosis as a collateral channel, producing fatal variceal bleeds. The suprarenal gland's triple arterial supply (phrenic, aorta, renal — P-A-R top to bottom) explains why it survives AAA surgery; its venous drainage is the asymmetry that mirrors the gonadal vein pattern.

Slide review questions (Prof. Ping Zhang): "Describe the blood supply of thyroid gland, stomach, hand, suprarenal gland." Hand covered in §11.9. Below: thyroid, stomach, suprarenal, and ductus arteriosus.

The aorta bifurcates at L4 — the level of the iliac crests, the same bony landmark you palpate when performing a lumbar puncture. From there, two common iliac arteries split into internal (pelvic viscera) and external (lower limb) branches before the sacroiliac joint. The external iliac dives under the inguinal ligament to become the femoral artery — this transition point is where cardiac catheterisation gains arterial access. The internal iliac (hypogastric) bifurcates into anterior and posterior divisions: the posterior division is the safe one (gluteal, iliolumbar, lateral sacral), while the anterior division supplies every pelvic viscus — uterus, bladder, rectum, and genitalia — meaning that internal iliac ligation in obstetric haemorrhage is the last resort to stop pelvic bleeding without hysterectomy. The femoral triangle's "NAVY" arrangement — nerve lateral, artery, vein medial, Y-fronts (lymphatics) — and the midpoint-of-inguinal-ligament pulse are core exam points every clinical year.

The abdominal aorta bifurcates at L4 (level of iliac crest / umbilicus) into right and left common iliac arteries. Each divides anterior to the sacroiliac joint into:

Both coronary arteries arise from the aortic sinuses — the concavities just above the aortic valve cusps — in diastole, when the valve leaflets fall back and blood pools behind them to fill the coronary ostia. This timing means tachycardia is inherently dangerous for the ischaemic heart: the shorter diastole is, the less coronary filling time there is. The LAD is the single most clinically important artery in the body, supplying the anterior two-thirds of the interventricular septum (where the bundle of His runs), the anterior left ventricle, and the apex — its occlusion causes the largest MIs and the highest mortality. The RCA dominates cardiac electrophysiology: it supplies the SA node (60% of people) and the AV node (80–90%), which is why inferior MI characteristically causes bradycardia and heart block rather than pump failure. "Dominance" means which artery gives the posterior descending artery — right in 85% — and matters only when identifying the culprit vessel in a posterior MI.

The carotid system is where neurology, vascular surgery, and head-and-neck anatomy converge most dangerously. The common carotid bifurcates at C4 — the level of the thyroid cartilage notch — and the carotid sinus sitting right at that bifurcation acts as the body's main blood pressure sensor; a tight collar or a tumour here can trigger a vagal faint simply by mechanical compression. The internal carotid gives no branches in the neck — a key distinguishing feature — then supplies the cerebral hemisphere via the circle of Willis, whose anastomotic ring at the base of the brain is the most common place in the body to develop berry aneurysms. The circle's anterior communicating artery carries 30–35% of all aneurysms; a posterior communicating artery aneurysm announces itself with a painful, pupil-dilating third nerve palsy before rupture — a warning sign that is a neurosurgical emergency. The vertebrobasilar system feeding the brainstem generates the most distinctive stroke syndromes: Wallenberg's (PICA), locked-in (basilar), and the lateral pontine syndromes — all with the hallmark of crossed signs (ipsilateral cranial nerve, contralateral limb).

11.7.2 — Circle of Willis: Full Anatomy, Aneurysms & Stroke Syndromes ★★★

Fig. 8.38 — Arterial supply to the brain. A. The cerebral arterial circle (of Willis): anterior + posterior cerebral, communicating, internal carotid and basilar arteries. B–C. MR and CT angiograms.

Gray's Anatomy for Students, 4e

The Circle of Willis (circulus arteriosus cerebri) is an anastomotic polygon at the base of the brain formed by: 2 ACAs + AComm (anterior) + 2 ICAs + 2 PComms + 2 PCAs (from basilar tip).

Artery	Territory	Occlusion Syndrome
Anterior cerebral artery (ACA)	Medial frontal + parietal cortex (leg area of homunculus); corpus callosum	Contralateral leg > arm weakness + sensory loss; urinary incontinence; abulia (frontal lobe)
Middle cerebral artery (MCA)	Lateral hemisphere (face + arm area > leg); Broca's area (dominant F3); Wernicke's area (dominant T1)	Contralateral face + arm > leg weakness + sensory loss; dominant = aphasia (Broca's = expressive; Wernicke's = receptive); non-dominant = neglect + apraxia; eye deviates TOWARD lesion (frontal eye field)
Posterior cerebral artery (PCA)	Occipital lobe (primary visual cortex); thalamus; midbrain	Contralateral homonymous hemianopia with macular sparing (MCA collaterals preserve macular cortex); thalamic pain syndrome; Weber's syndrome (ipsilateral CN III palsy + contralateral hemiplegia)
Anterior communicating artery (AComm)	Connects 2 ACAs; no territory	Most common site for berry aneurysm (30–35%); rupture can cause bitemporal visual field defects or ACA infarction
Posterior communicating artery (PComm)	Connects ICA to PCA; joins anterior + posterior circulations	Aneurysm here (25% of berry aneurysms) → compresses CN III → painful complete CN III palsy (pupil-involving = surgical emergency; mydriasis = pupillary dilation is FIRST sign)

⚠ Berry (Saccular) Aneurysms & Subarachnoid Haemorrhage

Sites (order of frequency): AComm (30–35%) → PComm–ICA junction (25%) → MCA bifurcation (20%) → basilar tip (5–10%) → other. Multiple aneurysms in 20%. Associated conditions: ADPKD (autosomal dominant polycystic kidney disease), Marfan's syndrome, Ehlers-Danlos, coarctation of aorta, first-degree family history.

Rupture → subarachnoid haemorrhage (SAH): "thunderclap headache" = sudden-onset worst headache of life (peak intensity in seconds); ± photophobia, neck stiffness (meningism from blood in CSF), loss of consciousness (in severe cases). Investigation: CT head (blood in basal cisterns/sulci); if negative but strong suspicion = lumbar puncture at >6 hours (xanthochromia = yellow CSF = breakdown products of blood = confirms SAH). CT angiogram or digital subtraction angiography (DSA) to find aneurysm. Treatment: neurosurgical clipping or endovascular coiling (ISAT trial: coiling = better short-term outcomes for suitable anatomy); nimodipine (calcium channel blocker → prevents vasospasm).

Fig. 7.50 — Branches of the axillary artery (subclavian → axillary → brachial), divided into three parts by pectoralis minor.

Gray's Anatomy for Students, 4e

The upper limb arterial chain — subclavian → axillary → brachial → radial/ulnar → palmar arches → digital arteries — is the route surgeons trace when a hand goes cold. The subclavian becomes the axillary at the lateral border of the first rib, and the axillary becomes the brachial at the lower border of teres major — these transition points are landmarks, not anatomical changes in the vessel wall. The brachial artery runs medial to the biceps tendon in the cubital fossa, which is why you position your stethoscope there to auscultate Korotkoff sounds when measuring blood pressure. Distally, the radial artery travels through the anatomical snuffbox, whose floor is the scaphoid — so snuffbox tenderness after a fall on an outstretched hand is scaphoid fracture until proven otherwise, even with a normal X-ray. The dual palmar arches provide crucial collateral protection: Allen's test validates this collateral before you commit to an arterial line in the radial artery.

Fig. 6.37 — Arteries of the lower limb: external iliac → femoral → popliteal → anterior & posterior tibial / fibular arteries.

Gray's Anatomy for Students, 4e

The lower limb arterial tree is the territory of peripheral vascular disease — the same atherosclerotic process that blocks coronary arteries progressively narrows the femoral, popliteal, and tibial vessels, producing the classic progression from claudication to rest pain to gangrene. The femoral artery enters the thigh at the midpoint of the inguinal ligament and travels through the adductor canal to become the popliteal artery at the adductor hiatus — knowing this anatomy explains why a tight adductor canal can compress the vessel in cyclists and runners. In the popliteal fossa the structures run deepest-to-most-superficial as PAT: popliteal Artery (deepest, against the bone), then popliteal Vein, then Tibial nerve — the reverse of the femoral triangle. Below the knee, three arteries reach the foot: anterior tibial (→ dorsalis pedis, palpable between 1st and 2nd metatarsals), posterior tibial (behind the medial malleolus, palpable in Tom Dick ANd Harry's groove), and peroneal (not palpable). Checking all five lower-limb pulses — femoral, popliteal, DP, PT, and comparing ABPI — is the minimum vascular examination every student must do fluently.

An aneurysm is a permanent, localised dilation of an artery to more than 1.5 times its normal diameter — for the infrarenal aorta, that means ≥3 cm. AAAs grow silently and rupture without warning: the classic triad of severe tearing back pain, pulsatile abdominal mass, and hypotension is a surgical emergency with 80% overall mortality, but only 50% of patients even reach hospital. Size is the dominant rupture predictor — below 5.5 cm in men the annual risk stays low enough to watch; above it, elective repair (EVAR for most, open for complex anatomy) saves lives. Aortic dissection is a different catastrophe: an intimal tear lets blood into the media, creating a false lumen that can propagate the length of the aorta in seconds. The Stanford classification is the clinical one — Type A (ascending aorta involved) goes immediately to theatre because it can tear back into the pericardium (tamponade), shear off the coronary ostia (MI), or destroy the aortic valve (acute AR); Type B (descending only) gets β-blockers to reduce shear stress on the wall, targeting heart rate below 60 and systolic below 120 mmHg.