Q4 LONG ANSWER 10M ★ High-yield

Describe the types of blood vessels. Define anastomosis and end artery with examples, and add a note on portosystemic anastomoses.

Anatomy · 1st Professional — Pre-Clinical

Vessel	Structure	Function
Elastic (conducting) artery	Abundant elastic laminae in the media	Aorta and its main branches; convert pulsatile into steady flow — the Windkessel effect
Muscular (distributing) artery	Smooth muscle predominates	Named arteries; regulate regional flow
Arteriole	Thick muscle relative to lumen	The chief site of peripheral resistance
Capillary	Endothelium and basement membrane only	Exchange
Vein	Thin media, wide lumen, valves	Capacitance — holds 65% of blood volume

All vessels have three coats — tunica intima, media and adventitia
Vasa vasorum supply the outer coats of large vessels; nervi vasorum are the sympathetic vasomotor nerves

Anastomosis = a communication between two or more vessels, providing an alternative (collateral) route for blood.

Type	Description	Examples
Arterio-arterial	Between arteries	Circle of Willis, palmar arches, mesenteric arcades
Veno-venous	Between veins	Dorsal venous arch; vertebral venous plexus
Arteriovenous	Direct communication bypassing capillaries	Skin of the nose, ear, fingertips — for temperature regulation
Portosystemic	Between portal and systemic venous systems	Lower oesophagus, anal canal, umbilicus, retroperitoneum

Actual anastomosis — between vessels of appreciable size, giving effective collateral circulation
Potential anastomosis — between very small branches; opens only slowly and may be inadequate for sudden occlusion

End artery = an artery that is the sole source of supply to a region, with no effective anastomosis.

Type	Definition	Examples
Anatomical (absolute) end artery	NO anastomosis at all; occlusion causes certain necrosis	Central artery of the retina, arteries of the brain, spleen, kidney, vasa recta of the intestine
Functional end artery	Anastomoses exist but are too small to be effective	Coronary arteries, arteries of the brain, kidney, spleen, intestine

Site	Vessels	Clinical use
Circle of Willis	Internal carotid and vertebral systems	Protects the brain; one carotid can be occluded slowly without infarction
Scapular	Subclavian branches with the subscapular artery	Permits ligation of the third part of the subclavian or first part of the axillary artery
Around the elbow	Brachial and profunda with the recurrent arteries	Ligation of the brachial artery
Around the knee	Genicular branches of the femoral and popliteal	Ligation of the femoral artery
Trochanteric and cruciate	Gluteal and circumflex femoral arteries	Supply to the head of femur
Marginal artery of Drummond	Superior and inferior mesenteric arteries	Collateral supply to the colon

Site	Portal tributary	Systemic tributary	Consequence of hypertension
Lower oesophagus	Left gastric vein	Oesophageal veins (azygos)	Oesophageal varices — may bleed fatally
Anal canal	Superior rectal vein	Middle and inferior rectal veins	Haemorrhoids
Umbilicus	Para-umbilical veins	Superficial epigastric veins	Caput medusae
Retroperitoneal	Colic veins	Retroperitoneal veins	Retroperitoneal varices
Bare area of liver	Portal branches	Phrenic veins	—

Valves are folds of intima that prevent backflow; numerous in the limbs, especially the lower, and absent in the venae cavae, portal vein, cerebral and vertebral veins
Venae comitantes — paired veins accompanying a deep artery within a common sheath; arterial pulsation aids venous return
Venous sinuses — channels with no muscular wall, as in the dura mater
Perforating veins connect superficial to deep veins in the limbs, with valves directing flow inward; their incompetence causes varicose veins
The vertebral venous plexus (Batson) is valveless and connects the pelvis to the skull — a route for the spread of prostatic and breast carcinoma to the vertebrae and brain

Portal hypertension opens these channels; the oesophageal varices are the most dangerous, because the veins are submucosal and unsupported
Central retinal artery occlusion causes sudden painless permanent blindness — the retina tolerates ischaemia for only a few minutes
Coronary arteries are functional end arteries, which is why occlusion infarcts myocardium; gradual narrowing allows some collateral development
Ligation of a vessel is safe only where anastomosis is good — the reason the classical sites of ligation are chosen where they are
Arteriovenous fistula for dialysis is created surgically, exploiting the principle that a direct communication carries high flow
Varicose veins result from incompetent valves in the perforating veins, so blood is forced outward into the superficial system on muscular contraction
Deep vein thrombosis follows Virchow triad — stasis, endothelial injury and hypercoagulability; the calf veins are the usual site
Watershed areas between two arterial territories are the first to infarct in hypotension — the splenic flexure of the colon is the classic example
Atherosclerosis affects elastic and large muscular arteries preferentially, sparing the arterioles, which is why it causes infarction rather than diffuse ischaemia

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