Veins, Vascular anastomoses, and physiology of circulation

Veins Overview

• Formation: Veins are formed from the convergence of venules, becoming larger vessels.

• Structure:

  • Thinner walls compared to arteries.

  • Larger lumens, allowing for more blood flow.

  • Histological Comparison: Thick tunica media in arteries vs. thin tunica media in veins.

  • Due to the thinner walls and larger lumens, veins struggle to maintain high blood pressure.

• Blood Pressure: Blood pressure in veins is always lower than in arteries. This is crucial for understanding blood flow dynamics.

• Tunica Externa: Composed of collagen fibers and elastic networks, aiding in structural support.

Functionality of Veins

• Capacitance Vessels: Veins are referred to as capacitance vessels or blood reservoirs due to their ability to hold larger volumes of blood without significant pressure changes.

• Blood Distribution:

  • Approximately 60% of the body's blood is contained in veins and venules.

  • The remaining blood is distributed between systemic arteries, the heart, pulmonary vessels, and capillaries.

• Valves:

  • Unique to veins; valves prevent backflow of blood, particularly in limb veins where gravity affects blood movement.

  • Most valves are located in legs and arms, where they counteract gravity to ensure blood flows back to the heart.

  • Venous Sinuses:

    • Flattened veins with thin walls (e.g., coronary sinus); allow for easier blood flow due to lower resistance.

Blood Vessel Arrangement

• Typical Pattern: Blood vessels generally follow a pattern starting from arteries, branching into arterioles, and then capillaries, which merge into venules, forming veins.

• Layer Comparison:

  • Both arteries and veins consist of a tunica intima (innermost layer), tunica media (muscle layer), and tunica externa (outer layer).

  • Comparative Thickness: Arteries have a thicker tunica media compared to the thin media in veins.

Vascular Anastomosis

• Definition: Connections between blood vessels allowing for alternative routes.

• Types:

  • Arterial Anastomosis: Connection between arteries; provides collateral circulation if one artery is blocked.

  • Found in areas like joints, abdominal organs, the brain, and heart; absent in the retina, kidneys, and spleen.

  • Venous Anastomosis: Fewer in number due to more veins than arteries; allows for redundancy in venous drainage.

  • Classic example includes the joining of the superior and inferior vena cava.

  • Vascular Shunt: Direct connection from arterioles to venules, bypassing capillaries.

Blood Flow and Blood Pressure

• Blood Flow: The volume of blood passing through a vessel per time, measured in milliliters per minute or liters per minute.

  • Cardiac Output Relationship: Blood flow is equivalent to cardiac output (heart rate x stroke volume).

• Blood Pressure:

  • Defined as the force exerted by blood on the vessel walls, measured in millimeters of mercury (mmHg).

  • Higher pressure in arteries than veins; high blood pressure can cause damage to vessel walls, while low pressure indicates insufficient blood flow.

  • Pressure is primarily measured in systemic arteries.