Anatomy chapters
Blood vessels and anastomoses
The structure of arteries, veins and capillaries, end arteries, and the arterial and portosystemic anastomoses that keep tissues alive when a vessel is blocked.
- Vessel walls have three coats: intima, media and adventitia.
- An anastomosis links two vessels so blood can bypass a block; an end artery has no useful links.
- Exam favourites: end arteries (retina, brain, kidney, spleen) and portosystemic anastomoses.
On this page
Overview
Blood vessels carry blood from the heart to the tissues and back. There are arteries, arterioles, capillaries, venules and veins.
An anastomosis is a connection between two vessels. It lets blood take another route if one vessel is blocked. An end artery has no such connection, so blocking it kills the tissue it supplies.
Structure of vessel walls
All vessels except capillaries have three layers:
- Tunica intima: endothelium on a thin layer of connective tissue.
- Tunica media: smooth muscle and elastic tissue.
- Tunica adventitia: outer connective tissue.
Large vessels have their own small vessels, the vasa vasorum, to supply the outer wall.
Arteries
Elastic arteries
These are the aorta and its large branches, such as the brachiocephalic, common carotid and subclavian arteries. Their media is full of elastic sheets.
They stretch during systole and recoil during diastole, so blood keeps flowing between beats.
Muscular arteries
Medium-sized arteries such as the radial, femoral and coronary arteries. Their media is mainly smooth muscle. They control how much blood goes to each organ.
Arterioles
The smallest arteries. Their muscular walls control peripheral resistance and so blood pressure.
Arterioles are the main site of peripheral resistance.
Capillaries and sinusoids
Capillaries are a single layer of endothelium. Exchange of gases, nutrients and waste happens here.
- Continuous: muscle, skin, lungs, brain.
- Fenestrated: kidney glomeruli, gut, endocrine glands.
- Sinusoids: wide, irregular and leaky; found in the liver, spleen and bone marrow.
Veins
Veins have thinner walls and wider lumens than the matching arteries. They hold most of the blood volume.
Many veins, especially in the limbs, have valves that stop backflow. Muscle contraction squeezes the deep veins and pushes blood towards the heart (the muscle pump).
The veins of the head and neck, the venae cavae, the portal vein and the vertebral venous plexus have no valves or very few.
End arteries
An end artery is the only supply to a region and does not connect with its neighbours.
- True (anatomical) end artery: no anastomosis at all. The classic example is the central artery of the retina.
- Functional end artery: anastomoses exist but are too small to save the tissue if the main vessel is blocked suddenly. Examples are the coronary, renal, splenic and cerebral (central) branches.
Blocking the central artery of the retina causes sudden, painless loss of vision in that eye.
The coronary arteries do anastomose, but the links are too small to help after a sudden block. That is why they are functional, not true, end arteries.
Arterial anastomoses
Around joints
Most large joints have a ring of anastomosing branches around them. Blood can still reach the limb when the joint is bent and one vessel is kinked.
Classic examples are the anastomoses around the elbow (branches of the brachial artery with recurrent branches of the radial and ulnar arteries) and the knee (genicular branches of the popliteal artery with branches of the femoral, anterior tibial and fibular arteries).
Around the scapula
This links the subclavian artery (suprascapular and dorsal scapular branches) with the axillary artery (subscapular artery and its circumflex scapular branch).
If the axillary artery is tied above the origin of the subscapular artery, the scapular anastomosis keeps the arm alive.
Other important anastomoses
- Circle of Willis: links the internal carotid and vertebral systems at the base of the brain.
- Palmar arches: link the radial and ulnar arteries in the hand.
- Cruciate and trochanteric anastomoses: around the hip.
- Marginal artery: links the superior and inferior mesenteric arteries along the colon.
Portosystemic anastomoses
The portal vein drains the gut to the liver. At a few sites, small portal veins meet small systemic veins. If the liver is scarred (cirrhosis), portal pressure rises and these links swell.
| Site | Portal side | Systemic side | What you see |
|---|---|---|---|
| Lower oesophagus | Left gastric vein | Oesophageal veins (azygos system) | Oesophageal varices, vomiting blood |
| Anal canal | Superior rectal vein | Middle and inferior rectal veins | Anorectal varices |
| Umbilicus | Paraumbilical veins | Superficial veins of the abdominal wall | Caput medusae |
| Retroperitoneum | Colic veins | Lumbar and renal veins | Usually silent |
Oesophageal varices are the most dangerous site, because they can bleed heavily.
Clinical relevance
Collateral circulation after a slow block
How it happens: an artery narrows slowly, for example by atherosclerosis.
What you see: anastomoses have time to enlarge, so the tissue may survive. In coarctation of the aorta, enlarged intercostal arteries notch the ribs on an X-ray.
Infarction after a sudden block
How it happens: a clot or embolus blocks an end artery.
What you see: death of the tissue it supplies: myocardial infarction, renal or splenic infarct, or stroke.
Varicose veins
How it happens: valves in the superficial leg veins fail.
What you see: dilated, twisted veins in the legs, worse on standing.
Examiners ask you to name the true end artery, list functional end arteries, list the vessels in the scapular anastomosis and match each portosystemic site with its portal and systemic veins. Caput medusae and oesophageal varices are the clinical hooks.
Quick revision
- Three coats: intima, media, adventitia.
- Elastic arteries: aorta and large branches. Arterioles: peripheral resistance.
- Sinusoids: liver, spleen, bone marrow.
- True end artery: central artery of retina. Functional: coronary, renal, splenic, cerebral.
- Scapular anastomosis: subclavian with axillary artery.
- Portosystemic sites: lower oesophagus, anal canal, umbilicus, retroperitoneum.
Written by The House Job from standard anatomy references (Snell's Clinical Anatomy by Regions is the book named in the UHS PGET syllabus).
FAQ
What is an end artery?
An artery that is the only blood supply to a region and has no useful connections with nearby arteries, so blocking it causes infarction.
Which is the classic true end artery?
The central artery of the retina.
What causes caput medusae?
Portal hypertension, which opens the links between the paraumbilical veins and the veins of the abdominal wall.
Why does the arm survive if the axillary artery is tied?
Blood reaches it through the scapular anastomosis, as long as the tie is placed between the thyrocervical trunk and the subscapular artery.