Arteries (general structure)

Arteriae

Also known as: artery, elastic artery, muscular artery, conducting artery, distributing artery

Arteries carry blood away from the heart under high pressure. Their walls are thick, with a prominent tunica media. They are classed as elastic (conducting) or muscular (distributing) arteries and branch eventually into arterioles.

Blood Vessel Wall

Overview

Share of blood volume≈13% (systemic arteries)

Pulse wave velocity≈4–10 m/s (rises with age)

Elastic (conducting) arteries. These are the aorta, pulmonary trunk, brachiocephalic trunk and the common carotid, subclavian and common iliac arteries. Their media contains dozens of fenestrated elastic lamellae. They stretch in systole and recoil in diastole (the Windkessel effect), which smooths the pulse and keeps blood flowing between beats.

Muscular (distributing) arteries. These include most named arteries, such as the radial, femoral, coronary and renal arteries. Their media contains up to about 40 layers of smooth muscle, bounded by distinct internal and external elastic laminae. By contracting and relaxing they control how much blood each organ receives.

Arterioles. These have only one to three layers of smooth muscle and are the main resistance vessels.

Pressure and the pulse wave. Arteries hold only about 13% of the blood volume but are under the highest pressure. The pressure wave travels along them at about 4–10 m/s, much faster than the blood itself. As arteries stiffen with age, the wave speeds up, and reflected waves return earlier and raise systolic pressure.

End arteries. Some arteries have no useful connections with their neighbours (functional end arteries). Examples are the coronary, renal, splenic, central retinal and cerebral perforating arteries. Blocking one causes an infarct.

Anatomy

Histology: Three layers (tunics). The tunica intima is endothelium and a thin subendothelial layer, with an internal elastic lamina in muscular arteries. The tunica media is smooth muscle with elastic fibres, the thickest layer in arteries. The tunica externa (adventitia) is connective tissue containing vasa vasorum and nerves. In histological sections the lumen is round and relatively small for the wall thickness, and the internal elastic lamina looks wavy.

Function

Conduct blood at high pressure to the tissues.
Store elastic energy in systole and maintain diastolic pressure and flow (elastic arteries).
Regulate the distribution of blood between organs (muscular arteries and arterioles).

Clinical significance

Atherosclerosis

Atherosclerosis begins with endothelial dysfunction. LDL builds up and is oxidised, macrophages engulf it and become foam cells, and fatty streaks form. Smooth muscle cells then lay down a fibrous cap over a necrotic lipid core. A stable plaque narrows the lumen. A vulnerable plaque, with a thin cap and a large lipid core, can rupture and trigger a thrombus. The commonest sites are the abdominal aorta, coronary arteries, popliteal arteries, carotid bifurcations and circle of Willis.

Arteriosclerosis and vasculitis

Hyaline arteriolosclerosis occurs in hypertension and diabetes. Hyperplastic, "onion-skin" arteriolosclerosis occurs in malignant hypertension. Mönckeberg medial calcific sclerosis calcifies the media without narrowing the lumen.

Vasculitis is classified by the size of vessel affected. Large-vessel vasculitis includes giant cell and Takayasu arteritis. Medium-vessel vasculitis includes polyarteritis nodosa and Kawasaki disease. Small-vessel vasculitis includes the ANCA-associated vasculitides and IgA vasculitis.

Test yourself

In most people, the atrioventricular (AV) node is supplied by a branch of which artery?

Left anterior descending artery
Circumflex artery
Right coronary artery
Left main coronary artery

Answer: Right coronary artery

The AV nodal artery arises from the dominant coronary artery at the crux of the heart. That is the RCA in about 80–90% of people. This is why inferior MIs from RCA occlusion are often complicated by bradycardia and AV block.

A patient has ST elevation in leads II, III and aVF. Which territory is infarcting, and which artery is usually responsible?

Anterior wall — LAD
Lateral wall — circumflex
Inferior wall — right coronary artery
Septum — left main stem

Answer: Inferior wall — right coronary artery

Leads II, III and aVF look at the inferior (diaphragmatic) wall. In right-dominant hearts this wall is supplied by the posterior interventricular branch of the RCA. Leads V1–V4 correspond to the LAD, and leads I, aVL, V5 and V6 to the circumflex.

In the fetus, the ductus arteriosus connects:

The pulmonary trunk (left pulmonary artery) to the aortic arch
The umbilical vein to the inferior vena cava
The right atrium to the left atrium
The ascending aorta to the superior vena cava

Answer: The pulmonary trunk (left pulmonary artery) to the aortic arch

The ductus arteriosus runs from the start of the left pulmonary artery to the underside of the aortic arch, beyond the left subclavian artery. It carries most right ventricular output past the high-resistance fetal lungs. After birth it becomes the ligamentum arteriosum.