Vein Structure and Function Quiz

Structure of Veins

• Veins are thin-walled, collapsible, and low-pressure vessels
• Contain semi-lunar valves
• Normally, veins have low pressure and are partially filled with blood, with a flattened or hourglass cross-section
• During exercise, veins change shape to oval and round, allowing them to accommodate more blood and increasing blood flow to the heart

Blood Flow in Veins

• Blood flow in veins is maintained by the combined effect of the muscle pump, respiration, and venous valves
• Hydrostatic pressure is the pressure of the weight of the blood pressing on a vessel, measured at a height above or below heart level
• Hydrostatic pressure is 0 at the level of the heart, decreasing above and increasing below the level of the heart
• Hydrostatic pressure increases intramural pressure, resulting in blood pooling, which needs to be compensated to facilitate venous return

Breathing and Venous Flow

• Venous flow system is a low-pressure system
• Muscle-related respiration changes pressures in the thorax and abdomen
• During inspiration:
  • Diaphragm descends, increasing pressure in the abdominal cavity and decreasing venous flow in the legs
  • Pressure in the thorax decreases, increasing venous flow in the head, arms, and vena cava
• During expiration:
  • Diaphragm ascends, increasing pressure in the thoracic cavity and decreasing venous flow in the head, arms, and vena cava
  • Venous flow in the legs increases due to decreased abdominal pressure
• Changes in venous flow to the heart are opposite to venous flow in the legs

Response to Valsalva's Maneuver

• Deep inspiration followed by bearing down results in cessation of blood flow in both upper and lower extremity veins
• No flow occurs during Valsalva's maneuver
• Increased velocities are seen after completing the Valsalva, confirming the competency of the valves system

Evaluation of Lower Extremity Veins

• Divided into two categories: non-invasive and invasive
• Non-invasive venous evaluation:
  • Duplex sonography
  • Technical aspects of venous duplex imaging:
    • Carried out in both transverse and sagittal planes
    • Transverse plane: venous lumen is evaluated by compressions on gray scale imaging
    • Sagittal plane: spectral Doppler wave form is studied using Valsalva's maneuver, proximal compression, and distal augmentation

Test your knowledge on the structure and function of veins, including their thin-walled nature, presence of semi-lunar valves, and changes in shape during exercise. Learn about how veins accommodate more blood and maintain blood flow during normal and active states.