Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. The venous circulation is a low-pressure system with much lower amounts of smooth muscle and elastic tissue, thinner walls, and larger lumens than arteries. Veins are capacitance vessels with significant compliance and the ability to distend and hold up to 70%–80% of blood volume at rest.

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Veins are tubular collections of cells transporting deoxygenated blood and waste products from capillaries in the periphery of the body back to the heart.

General characteristics

Characteristics of veins and the venous system include:

  • Veins are more abundant than arteries.
  • Compared to arteries, veins have:
    • Large lumens 
    • Thin walls
    • Less smooth muscle and elastic tissue
  • Blood passes through veins in order of increasing luminal diameter:
    • Starts with the smallest vessels (venules)
    • Ends in the largest vessel (the vena cava)
  • Veins often accompany an artery:
    • Veins surround the artery in an irregular branching network.
    • Function as a countercurrent heat exchange → allows cool blood returning from the periphery to be warmed before returning to the heart
  • Capacitance vessels:
    • Collapsed when empty, but able to distend significantly (known as compliance)
    • The venous system can hold up to 70%–80% of the blood volume at rest.

Pressure within the venous circulation

  • Venous circulation is a low-pressure system
    • Averages only 10 mm Hg (compared to 120 mm Hg in the arteries during systole)
    • Minimal fluctuations between systole and diastole
  • Pressure is affected by gravity and proximity to the heart:
    • More proximal to the heart = ↓ pressure
    • Compared to standing, the recumbent position has ↓ pressure.
    • While standing: foot pressure highest, head pressure the lowest
  • Pressure is too low to spontaneously push blood against gravity
  • Moving blood up (against gravity) requires:
    • The pumping action of contracting skeletal muscles
    • One-way venous valves preventing retrograde flow

Layers of the Vessel Wall

All veins have the same basic structure and are made up of 3 primary layers: tunica intima, tunica media, and tunica adventitia.

Structure of a vein wall

Structure of a vein wall

Image: “Structure of a vein wall” by Phil Schatz. License: CC BY 4.0
Cross-section of artery and vein

Cross-section of artery and vein

Image: “Types of Arteries and Arterioles” by Phil Schatz. License: CC BY 4.0, edited by Lecturio.

Tunica intima

  • Made of:
    • A single layer of endothelial cells
      • Simple, squamous epithelial cells
      • Endothelium in veins appears smooth.
    • Small amounts of fibrous connective tissue
    • Valves: 
      • Formed from thickened endothelium and connective tissue
      • Found in medium veins (most commonly in the limbs and veins inferior to the heart)
  • Functions:
    • Acts as a selectively permeable barrier
    • Secretes vasoactive substances
    • Provides a smooth lining to the blood vessel (prevents aggregation of platelets and/or RBCs when intact)
    • Valves help prevent retrograde blood flow, especially in the lower extremities.
  • Vascularized by direct diffusion from the lumen
  • In capillaries: the only layer present in the vessel wall
  • In veins: no internal elastic membrane (present, however, in larger arteries)

Tunica media

  • Made of: 
    • Collagen
    • Smooth muscle (much less than in arteries)
    • Elastic tissue (sparse compared to arteries)
  • Functions: provides strength and structure
  • In veins: no external elastic membrane (present, however, in larger arteries)

Tunica adventitia

  • Also called the tunica externa
  • Normally the thickest layer in veins
  • Made of: 
    • The primary component is collagen, which merges with connective tissue and surrounds neighboring vessels, nerves, and organs.
    • Longitudinal smooth muscle fibers (vena cava only)
  • Functions:
    • Strengthens the vessel wall
    • Anchors the vessel
  • Innervated by tiny nerves known as nervi vasorum
  • Vascularized by tiny vessels known as vasa vasorum

Types of Veins

Segmental differentiation distinguishes the 3 primary types of veins by overall size, function, and composition. Veins generally exist on a continuum with gradual changes in vessel morphology down the venous tree.

The 3 primary types of veins are:

  1. Small veins and venules
  2. Medium veins
  3. Large veins
Comparison of vein types

Comparison of large, medium, and small veins

Image: “Comparison of large, medium, and small veins” by Phil Schatz. License: CC BY 4.0
Intraluminal pressures of different vessels

Intraluminal pressures of different vessels

Image by Lecturio.

Venules and small veins

  • Marks the beginning of the venous circulation (drains capillaries)
  • Venules:
    • Thinnest walls relative to their lumens
    • Range: 15–100 µm in diameter
    • Can be quite porous → exchange fluid with surrounding tissue (similar to capillaries)  
    • Join together to form small veins
  • Small veins: 
    • Generally 100 µm–1 mm in diameter 
    • Can unite to form venous plexuses
    • Unnamed

Diagram of a venule

Image: “Venule” by Phil Schatz. License: CC BY 4.0

Medium veins

  • Diameter up to 1 cm
  • Drain venous plexuses
  • Accompany medium-sized (distributing) arteries
  • Embedded in skeletal muscle compartments → skeletal muscle acts as a pump, moving blood towards the heart
  • One-way venous valves prevent retrograde blood flow within veins → ensures blood keeps moving forward
  • Includes most named vessels, for example:
    • Cephalic and basilic veins (upper limb)
    • Great and small saphenous veins (lower limb)
Medium sized vein

Diagram of a medium-sized vein

Image: “Medium-sized vein” by Phil Schatz. License: CC BY 4.0

Large veins

  • Examples: 
    • Superior and inferior vena cava: 
      • Largest veins in the body
      • Both lead to the right atrium of the heart
    • Portal vein
  • Vena cava have wide bundles of longitudinal smooth muscle within their tunica adventitia → allowing elongation or shortening according to changes in posture
Large vein

Diagram of a large vein

Image: “Large vein” by Phil Schatz. License: CC BY 4.0

Venous sinuses

  • Characterized by:
    • Especially thin walls
    • Large lumens
    • No smooth muscle
  • Examples:
    • Coronary sinus of the heart
    • Dural sinuses of the brain

Clinical Relevance

  • Chronic venous insufficiency (CVI): spectrum of disorders characterized by venous dilation and/or abnormal vein function in the lower extremities, resulting from venous hypertension. More severe cases present with skin changes, which may include skin pigmentation, stasis dermatitis, lipodermatosclerosis, and (eventually) the development of ulcers. Diagnosis is usually based on exam findings alone. The mainstay of management is compression therapy, though a variety of surgical options also exist. Venous ulcers are common as the disease progresses.
  • Phlebitis and thrombophlebitis: Phlebitis refers to inflammation of a vein and is commonly due to the presence of a thrombus, which is known as thrombophlebitis. The conditions may occur in both deep and superficial veins. In certain situations, such as the postpartum period or an indwelling vascular catheter, the clot may become infected and lead to septic thrombophlebitis. Management involves anticoagulation if a clot is present, and antibiotics if the clot is infected. Isolated superficial phlebitis is typically benign and self-limited.
  • Deep vein thrombosis (DVT) and pulmonary embolism (PE): Deep vein thrombosis refers to a thrombus formed in a deep vein. A DVT most commonly forms in a femoral, popliteal, iliofemoral, or pelvic vein. Patients present with pain and swelling distal to the thrombus. A PE occurs if a portion breaks off, lodges in the pulmonary vasculature, and occludes the affected vessel. Pulmonary embolisms can be fatal. Ultrasound can visualize the thrombus and anticoagulation is the primary mode of treatment. 
  • Edema: Excess serous fluid accumulates in a body cavity or the interstitial space. Hydrostatic pressure increases within a vein leading to increased pressure within the capillaries. The increased pressure pushes fluid out of the vasculature and into the extracellular fluid, resulting in edema. Symptoms vary depending on the location of the edema and the underlying etiology. Edema is a symptom of a wide variety of illnesses.


  1. Taylor, A.M., and Bordoni, B. (2021). Histology, blood vascular system. In StatPearls. Retrieved April 29, 2021, from 
  2. Saladin, K.S., Miller, L. (2004). Anatomy and physiology. (3rd Ed., Pp. 752‒753). 
  3. Moore, K.L., and Dalley, A.F. (2006). Clinically oriented anatomy. (5th Ed., Pp 42‒43).

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