9.1 Vascular Anatomy and Physiology

Blood vessels are the highways of our body, transporting vital substances and maintaining homeostasis. They come in three types: arteries, veins, and capillaries, each with unique structures suited to their functions.

The vascular system does more than just move blood around. It regulates blood pressure, helps control body temperature, and plays a role in immune responses. Understanding these functions is key to engineering artificial blood vessels.

Vascular Structure and Function

Anatomical structures of blood vessels

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• Blood vessels categorized into arteries carry oxygenated blood away from heart, veins return deoxygenated blood to heart, and capillaries facilitate exchange of nutrients and gases
• Vessel wall comprises three layers:
  • Tunica intima forms innermost layer with endothelium and basement membrane regulating permeability and preventing thrombosis
  • Tunica media contains smooth muscle cells and elastic fibers controlling vessel diameter and blood flow
  • Tunica adventitia consists of connective tissue providing structural support and anchoring vessels to surrounding tissues
• Structural variations between vessel types:
  • Arteries feature thick walls with abundant elastic fibers withstanding high pressure (aorta)
  • Veins possess thinner walls and valves preventing backflow of blood (saphenous vein)
  • Capillaries composed of single endothelial layer facilitating efficient exchange (alveolar capillaries)

Functions of vascular system

• Blood circulation transports oxygen and nutrients to tissues while removing metabolic waste products (CO2, urea)
• Blood pressure regulation occurs through vasoconstriction and vasodilation controlled by autonomic nervous system and local factors (nitric oxide)
• Thermoregulation achieved by altering blood flow to skin capillaries dissipating or conserving heat
• Immune response facilitated by leukocyte trafficking allowing white blood cells to migrate to sites of infection or injury
• Endocrine function involves transport of hormones from endocrine glands to target tissues (insulin, thyroid hormones)
• Hemostasis maintained through platelet aggregation and activation of coagulation cascade preventing excessive blood loss (fibrin clot formation)

Endothelial Function and Vascular Mechanics

Endothelial cells in vascular homeostasis

• Barrier function maintains selective permeability controlling passage of molecules between blood and tissues
• Regulation of vascular tone through production of vasodilators (nitric oxide) and vasoconstrictors (endothelin)
• Angiogenesis process involves formation of new blood vessels from existing ones crucial for tissue growth and repair
• Inflammation modulation by expressing adhesion molecules facilitating leukocyte recruitment to sites of injury
• Thrombosis prevention achieved by producing anticoagulant factors (heparin sulfate, thrombomodulin)
• Lipid metabolism influenced by endothelial cells through lipoprotein lipase activity breaking down triglycerides

Mechanical properties for tissue engineering

• Elasticity refers to vessel's ability to return to original shape after deformation measured by compliance and distensibility
• Tensile strength determined by collagen content in vessel walls resisting rupture under high pressure
• Viscoelasticity describes time-dependent mechanical behavior combining elastic and viscous properties
• Stress-strain relationship exhibits non-linear behavior due to complex structure of vessel walls
• Tissue engineering applications require:
  1. Matching mechanical properties of engineered vessels to native tissue ensuring proper function
  2. Designing scaffolds with appropriate stiffness supporting cell growth and differentiation
  3. Considering flow-induced shear stress on cells mimicking physiological conditions
• Mechanotransduction involves cellular response to mechanical stimuli converting physical forces into biochemical signals
• Remodeling capacity allows blood vessels to adapt to changes in mechanical load through structural modifications