Veins are blood vessels that carry blood toward the heart. They form a low-pressure, high-capacitance system that holds approximately 60-70% of the total blood volume.
Classification of Veins
| Type | Diameter | Wall Thickness | Examples |
|---|---|---|---|
| Postcapillary venules | 10-50 microns | Thin | Microcirculation |
| Collecting venules | 50-200 microns | Thin | Microcirculation |
| Small veins | 0.2-1 mm | Thin | Tributaries |
| Medium veins | 1-10 mm | Moderate | Radial, saphenous, mesenteric |
| Large veins | > 10 mm | Thick wall, large lumen | SVC, IVC, portal, subclavian |
Postcapillary Venules
Postcapillary venules are the smallest venous vessels, formed by the confluence of capillaries.
Structure
Wall:
- Endothelium (similar to capillaries)
- Pericytes (more abundant than on capillaries)
- No smooth muscle (initially)
- Thin basement membrane
Diameter: 10-50 microns
Function:
- Primary site of white blood cell migration (diapedesis)
- Important in inflammation (P-selectin, ICAM-1 expression)
- Fluid reabsorption (venous end of Starling forces)
Collecting Venules
As postcapillary venules merge, they acquire smooth muscle:
- Diameter: 50-200 microns
- First smooth muscle cells appear
- Pericytes transition to smooth muscle
- Vasoactive (respond to inflammatory mediators)
Role in Inflammation
Postcapillary venules are the primary site of inflammatory cell emigration:
- Margination: Leukocytes move to vessel wall
- Rolling: Selectin-mediated adhesion
- Adhesion: Integrin-mediated firm adhesion
- Transmigration: Diapedesis through endothelial junctions
- Migration: Chemotaxis through interstitium
Small and Medium Veins
Small and medium veins collect blood from the microcirculation and transport it toward the large veins.
Structure
Tunica intima:
- Endothelium
- Thin subendothelial layer
- No internal elastic lamina (or very thin)
- Valves (in many medium veins)
Tunica media:
- 2-3 layers of smooth muscle (thin relative to arteries)
- Circularly arranged muscle
- Collagen and elastic fibers
- Less elastin than arteries
Tunica adventitia:
- Thickest layer in most veins
- Collagen, elastic fibers
- Smooth muscle bundles (longitudinal in some veins)
- Vasa vasorum
- Nerves
Comparison with Arteries of Similar Size
| Feature | Medium Vein | Medium Artery |
|---|---|---|
| Wall thickness | Thin | Thick |
| Lumen diameter | Larger | Smaller |
| Smooth muscle | 2-3 layers | 10-40 layers |
| Elastic fibers | Sparse | Abundant |
| Internal elastic lamina | Absent | Prominent |
| Valves | Present (many) | Absent |
| Shape when empty | Collapsed | Round (patent) |
| Relative compliance | High | Low |
Valves
Venous valves are bicuspid folds of the tunica intima that prevent retrograde blood flow.
Structure
- Leaflets: Two semilunar cusps (similar to semilunar valves)
- Composition: Collagen core covered by endothelium
- Attachment: To the vein wall (sinus behind each cusp)
- Orientation: Point toward the heart
Distribution
| Location | Valve Presence |
|---|---|
| Lower extremities (deep and superficial) | Numerous |
| Upper extremities | Moderate |
| Trunk (SVC, IVC, portal) | Absent |
| Head and neck (internal jugular) | One valve (often incompetent) |
| Veins of the brain | Absent (valveless) |
| Visceral veins | Variable |
Mechanism
- Flow toward heart: Leaflets pressed against the wall (open)
- Flow away from heart: Leaflets fill with blood, coapt (closed)
- Competence: Requires intact leaflets and normal wall diameter
Large Veins
Large veins have the most complex structure:
Superior Vena Cava
- No valves
- Partial myocardial sleeve at the atrial junction
- Thin wall relative to the aorta
Inferior Vena Cava
- No valves (except the Eustachian valve at the right atrial orifice)
- Larger diameter than SVC
- Longer course
Portal Vein
- Valveless (allows free flow between portal and systemic circulations)
- Receives blood from splanchnic circulation
- Delivers blood to liver for processing
Pulmonary Veins
- Myocardial sleeves at the left atrial junction
- Triggers for atrial fibrillation (arrhythmogenic foci)
- No valves
Venous Return Mechanisms
Muscular Venous Pump
Skeletal muscle contraction compresses adjacent veins, propelling blood toward the heart:
- Valves prevent retrograde flow
- Effective during walking (calf muscle pump)
- Inactivity leads to venous pooling
Respiratory Pump
Changes in intrathoracic pressure affect venous return:
- Inspiration: Increased negative intrathoracic pressure, increased venous return
- Expiration: Decreased negative pressure, decreased venous return
- Valsalva: Increased intrathoracic pressure, decreased venous return
Cardiac Suction
- Ventricular contraction pulls the AV valve annulus toward the apex
- Creates negative atrial pressure
- Suctions blood from the great veins
Gravity
- Below the heart: Gravity impedes venous return (requires valves and muscle pump)
- Above the heart: Gravity assists venous return
Venomotor Tone
Sympathetic activation causes venoconstriction:
- Mobilizes blood from capacitance vessels
- Increases venous return
- Maintains cardiac output during hypovolemia
Venous Pressure
| Location | Normal Pressure (mmHg) |
|---|---|
| Right atrium (CVP) | 0-6 |
| Peripheral veins (supine) | 5-10 |
| Peripheral veins (standing) | 40-80 (foot) |
| Portal vein | 5-10 |
| Hepatic vein | 5-10 |
| Jugular vein | 5-10 |
Clinical Measurement
Jugular venous pressure (JVP):
- Assessed by examining the internal jugular vein
- Normal: < 4 cm above sternal angle
- Increased in: Heart failure, SVC obstruction, pericardial disease, tricuspid regurgitation
Central venous pressure (CVP):
- Measured via central venous line
- Reflects right atrial pressure
- Normal: 0-6 mmHg
Venous Compliance
Veins are 20-30 times more compliant than arteries:
- Contain 60-70% of blood volume
- Can accommodate large volume changes with minimal pressure change
- Sympathetic constriction can mobilize 500-1000 mL of blood
Stress volume vs. unstressed volume:
- Unstressed volume: Volume in veins at zero transmural pressure (~70% of blood volume)
- Stressed volume: Volume that distends the veins, generating pressure (~30% of blood volume)
- Venoconstriction converts unstressed to stressed volume
Clinical Significance
Chronic Venous Insufficiency
Failure of venous valves leading to venous hypertension:
Causes:
- Primary valve incompetence
- Post-thrombotic syndrome (deep vein thrombosis damage)
- Obesity, prolonged standing
Staging (CEAP classification):
| Class | Description |
|---|---|
| C0 | No visible venous disease |
| C1 | Telangiectasias, reticular veins |
| C2 | Varicose veins |
| C3 | Edema |
| C4 | Skin changes (lipodermatosclerosis, pigmentation) |
| C5 | Healed ulcer |
| C6 | Active venous ulcer |
Deep Vein Thrombosis (DVT)
Thrombus formation in deep veins:
- Most common in lower extremities
- Risk: Immobility, surgery, hypercoagulability
- Complication: Pulmonary embolism
- Valves are often damaged by the thrombus
Varicose Veins
Dilated, tortuous superficial veins:
- Incidence: 15-25% of adults
- Risk factors: Female, obesity, pregnancy, prolonged standing
- Mechanism: Valve incompetence, vein wall weakness
- Symptoms: Heaviness, pain, itching
- Treatment: Compression, sclerotherapy, ablation, stripping
Superior Vena Cava Syndrome
Obstruction of the SVC:
- Most common cause: Lung cancer (small cell, squamous)
- Other: Lymphoma, thrombosis, mediastinal fibrosis
- Symptoms: Facial swelling, dyspnea, distended neck veins
- Emergency: Airway compromise, cerebral edema
Portal Hypertension
Increased pressure in the portal venous system (> 10 mmHg):
- Prehepatic: Portal vein thrombosis
- Intrahepatic: Cirrhosis (most common)
- Posthepatic: Budd-Chiari syndrome
Consequences:
- Esophageal varices (rupture -> life-threatening bleed)
- Ascites
- Splenomegaly
- Caput medusae (abdominal wall collaterals)
Venous Access
Common sites for venous cannulation:
- Peripheral: Cephalic, basilic, median cubital
- Central: Internal jugular, subclavian, femoral
- Long-term: PICC (peripherally inserted central catheter)
Aging of the Venous System
- Valve degeneration and incompetence
- Vein wall dilation and tortuosity
- Reduced venous compliance
- Impaired muscle pump function
- Increased risk of DVT and varicose veins