20.1

Importance of blood vessels in transporting blood
Role of blood vessels in gas, nutrient, and substance exchange with body cells
Consequences of reduced vessel functioning:
- Ineffective circulation of blood-borne substances
- Tissue injury
- Impaired metabolism
- Threatened functions of bodily systems

Blood vessels transport blood via:
- Systemic circuit: Arteries carry oxygen-rich blood to tissues; veins return oxygen-poor blood to the heart.
- Pulmonary circuit: Arteries carry oxygen-poor blood to lungs; veins return oxygen-rich blood to heart.

Different blood vessel types have unique structures but share general features.
Arteries and arterioles:
- Thicker walls than veins due to higher pressure near the heart.
- Smaller lumens support blood pressure maintenance.
Veins and venules:
- Thinner walls and larger lumens for less resistance.
- Many contain valves for unidirectional blood flow, particularly in limbs.
Vasa vasorum:
- Small blood vessels within larger vessel walls for nourishment and waste removal.
Nervi vasorum:
- Minute nerves controlling smooth muscle contraction and dilation in vessels.

Tunica Intima:
- Lined by an endothelial layer (simple squamous epithelium).
- Continuous throughout the vascular system; damage to the endothelium leads to clot formation.
- Recent studies show endothelium regulates:
- Capillary exchange
- Blood flow (via endothelins)
Tunica Media:
- Middle layer; thickest in arteries.
- Contains smooth muscle and elastic fibers.
- Vasoconstriction: Reduces lumen diameter, increases blood pressure.
- Vasodilation: Increases lumen diameter, lowers blood pressure.
Tunica Externa:
- Outermost layer; tends to be thinner in arteries, thicker in veins.
- Composed of collage and elastic fibers; stabilizes the vessel's position.

General Appearance:
- Arteries: Thick walls, small lumens, rounded cross-section.
- Veins: Thin walls, large lumens, generally flattened.
Tunica Intima Appearance:
- Arteries: Wavy endothelial lining, internal elastic membrane present.
- Veins: Smooth endothelial lining, no internal elastic membrane.
Tunica Media:
- Arteries: Thickest layer, predominately smooth muscle and elastic fibers.
- Veins: Thinner than tunica externa, predominately smooth muscle and collagenous fibers.
Tunica Externa:
- Arteries: Thinner than tunica media (except large ones).
- Veins: Thickest layer, contains collagen and elastic fibers.

Arteries conduct blood away from the heart and have thick walls to withstand high pressure.
Elastic arteries:
- Largest arteries, more than 10 mm diameter. High elastic fiber percentage facilitates expansion and recoil.
- Function as conducting arteries, maintaining pressure to drive blood.
Muscular arteries:
- Include arteries with diameters from 0.1 mm to 10 mm; more smooth muscle than elastic fibers.
- Function as distributing arteries, actively regulating blood flow via vasoconstriction.

Arterioles are very small arteries leading to capillaries.
Tunics are thinner and each contains a critical endothelial lining.
Only 1-2 smooth muscle layers exist in the tunica media.
Average diamater less than 30 micrometers; determinants of blood flow and pressure regulation.

Microscopic vessels that supply blood to tissues (perfusion).
Diameter ranges from 5-10 micrometers; walls are composed of endothelium and basement membrane with occasional smooth muscle.
Types of Capillaries:
- Continuous Capillaries:
- Most common, found in almost all tissues.
- Complete endothelial lining with tight junctions and intercellular clefts allowing small molecules to pass.
- Fenestrated Capillaries:
- Have pores (fenestrations) making them permeable to larger molecules, commonly found in intestines and kidneys.
- Sinusoid Capillaries:
- Have extensive intercellular gaps and large openings for large molecules; found in liver and spleen.

Metarteriole:
- Combines properties of arterioles and capillaries with sphincters regulating blood flow.
Precapillary sphincters allow selective blood flow.
Avascular shunt: Direct bypass route if sphincters are all closed.
Vasomotion: Pulsatile blood flow, regulated by local chemical signals based on tissue needs.

Venules are small veins formed from the merging of capillaries.
Average diameter ranges from 8 to 100 micrometers, walls contain endothelium and thin layers of muscle and connective tissue.
Significant site for diapedesis, allowing white blood cells to enter tissue from the bloodstream.

Direction of Blood Flow:
- Arteries carry away from the heart, veins carry toward it.
Wall Thickness:
- Arteries have thick walls; veins have thin walls.
Pressure Levels:
- Arteries: High, veins: low.
Valves:
- Absence in arteries; presence in many veins.

Edema:
- Caused by pooled blood increasing tissue pressure, leading to fluid accumulation in interstitial tissues.
- Requires treatment of underlying causes.
Varicose Veins:
- Develop due to valve defects allowing blood accumulation; visible and often painful.
- Common in lower limbs, especially during pregnancy.
- Treatment may involve support hose, surgery, or laser procedures.

Veins as capacitance vessels store approximately 64% of total blood volume.
Venoconstriction:
- Mediated by sympathetic stimulation to regulate blood flow as needed.
- Increases pressure and returns blood to heart efficiently.

Vascular surgery focuses on vascular system diseases, including various procedures to repair vessels.
Vascular technicians assist with imaging and treatment of cardiovascular disorders.
Projected growth in this field indicates increasing demand for vascular care specialists.

Interactive links provided for further learning on vascular surgery and imaging technology.