Overview

Share of blood volume≈64% (systemic veins)

Compliance vs arteries≈24×

Mean systemic filling pressure≈7 mmHg

Wall structure. Compared with an artery of the same size, a vein has a thinner intima, a thin media with few smooth muscle layers, and a relatively thick adventitia, which is often its thickest layer. Large veins such as the IVC have longitudinal bundles of smooth muscle in the adventitia. The lumen is wide and often collapsed or irregular in histological sections.

Capacitance. Veins are about 8 times more distensible than arteries and hold about 3 times the volume, so their compliance is roughly 24 times greater. When sympathetic nerves constrict them (through α1 receptors), several hundred millilitres of blood shift toward the heart. This maintains cardiac filling after a haemorrhage.

Venous return. Blood flows back to the heart because of the small pressure difference between the mean systemic filling pressure (about 7 mmHg) and right atrial pressure. Three mechanisms help it along: the skeletal muscle pump with its valves, the respiratory pump (falling pressure in the chest during inspiration), and suction created by the heart.

Deep and superficial veins. In the limbs, deep veins usually run as paired venae comitantes beside the arteries, which allows heat exchange between them. Superficial veins run in the subcutaneous tissue and help regulate body temperature.

Anatomy

Histology: Thin tunica intima, sometimes folded into valves. A thin tunica media with a few layers of smooth muscle and collagen. A thick tunica externa of collagen, often with longitudinal smooth muscle in large veins, and abundant vasa vasorum. The lumen is large and irregular.

Function

Return blood to the heart.
Act as a blood reservoir (about 64% of blood volume in systemic veins) whose capacity is adjusted by venoconstriction.
Help regulate body temperature through the superficial veins.

Clinical significance

Deep vein thrombosis and Virchow’s triad

Virchow's triad describes the three factors that promote venous thrombosis. Stasis occurs with immobility, long flights, surgery and pregnancy. Endothelial injury occurs with trauma, surgery and indwelling catheters. Hypercoagulability occurs with cancer, oestrogen, inherited thrombophilia, antiphospholipid syndrome and pregnancy. Thrombi usually start in the valve pockets of the calf veins and can spread upward and embolise to the lungs.

Pulmonary embolism

Pulmonary embolism presents with sudden breathlessness, pleuritic pain, a fast heart rate, low oxygen levels and sometimes collapse. Massive PE causes acute right heart failure and obstructive shock. CT pulmonary angiography confirms it. Treatment is anticoagulation, with thrombolysis or clot removal when the patient is haemodynamically unstable.

Test yourself

The left testicular vein usually drains into the:

Inferior vena cava
Left renal vein
Left common iliac vein
Left internal iliac vein

Answer: Left renal vein

The left gonadal vein joins the left renal vein at a right angle, while the right gonadal vein joins the IVC directly. This asymmetry is one reason why about 90% of varicoceles are on the left.

Oesophageal varices in portal hypertension develop at an anastomosis between which veins?

Left gastric vein and oesophageal tributaries of the azygos vein
Superior rectal vein and inferior rectal veins
Paraumbilical veins and superficial epigastric veins
Splenic vein and left renal vein

Answer: Left gastric vein and oesophageal tributaries of the azygos vein

The left gastric (coronary) vein belongs to the portal system and the oesophageal veins drain into the azygos (systemic) system. When portal pressure rises they dilate into oesophageal varices, which can bleed catastrophically. The other options describe the anorectal, paraumbilical and retroperitoneal anastomoses.

In the femoral triangle, which structure lies immediately medial to the femoral artery?

Femoral nerve
Femoral vein
Great saphenous vein
Inguinal ligament

Answer: Femoral vein

From lateral to medial the order is NAVY: Nerve, Artery, Vein, Y-fronts (the femoral canal). This is why femoral venous access is about 1 cm medial to the femoral pulse.