Vascular System: Arteries, Veins, and Capillaries (Chapter 19)

There are three different types of blood vessels: arteries, capillaries, and veins.
Arteries: vessels that take blood away from the heart.
- Direction of flow defines arteries, not the oxygen content of the blood.
- In systemic circulation, arteries carry well-oxygenated blood; in pulmonary circulation, they carry oxygen-depleted blood.
Veins: vessels that return blood back to the heart.
- In systemic circulation, veins may be low in oxygen; in pulmonary circulation, they are rich in oxygen.
Capillaries: the smallest vessels with the slowest blood flow to enable exchange of gases and nutrients with surrounding tissues.
In essence: blood vessels are hollow tubes that are filled with blood.

Arteries and veins have three wall layers; capillaries do not:
- Tunica interna (tunica intima) – innermost layer
- Tunica media – middle layer
- Tunica externa (adventitia) – outer layer
Capillaries are simply composed of the tunica interna (endothelium).
- Thick walls in capillaries would hinder exchange; thus capillaries lack a thick wall.

Also called tunica intima.
Made up of an endothelial layer that lines the vessel from the inside and provides a very smooth surface.
Smooth surface minimizes friction and promotes smooth blood flow.
Rough endothelial surfaces can cause turbulence and slow flow, which may lead to blood clotting.
Overall function: promote smooth flow and reduce clotting risk.

Contains two major components:
- Elastic fibers – provide recoil after stretching due to pulsatile flow.
- Smooth muscle – regulates lumen size via contraction/relaxation.
Contraction of smooth muscle causes vasoconstriction (lumen narrows).
Relaxation causes vasodilation (lumen widens).
Smooth muscles are innervated by the sympathetic nervous system.
- When sympathetic fibers stimulate
  $ext{α}_{1}$ receptors, smooth muscles constrict, reducing blood flow in that segment.
Parasympathetic innervation to vessels: generally absent. When sympathetic activity is low, vessels relax, and the parasympathetic system does not participate.

Outer layer composed of dense irregular connective tissue rich in collagen arranged in multiple directions.
Collagen provides stretch resistance and helps prevent tearing under mechanical stress.
In thicker vessel walls, diffusion from the lumen is too slow to supply outer layers; vessels have their own vessels.
Vasa vasorum (vessels of the vessels) supply the outer parts of thick-walled vessels.
- Note: term is
  $ext{vasa vasorum}$ .

Capillaries have only the tunica intima (endothelial lining).
Purpose: exchange of gases (O₂, CO₂) and nutrients between blood and surrounding tissues.
Thick walls would impede exchange; hence only a single endothelial layer.

All arteries and veins have tunica intima, tunica media, and tunica externa; capillaries have only tunica intima.
Cross-sectional appearance:
- Arteries typically have thicker walls than veins.
- The tunica media is much thicker in arteries, making them more efficient at regulating lumen size.
- The tunica externa is present in both, but is thicker in veins.
Veins serve as reservoirs/storage for blood; therefore, the adventitia is relatively thicker to prevent overfilling and tearing.
Veins typically have fewer smooth muscle cells in their walls compared to arteries.
Venous valves are formed by the tunica intima to prevent backflow, similar in concept to heart valves.

Sympathetic nervous system regulates vascular tone via
$ext{α}_{1}$ receptors on smooth muscle -> vasoconstriction.
Parasympathetic innervation to vessels is generally absent; there is no parasympathetic control of most peripheral vessels.
Baseline vessel tone is maintained without parasympathetic input; when sympathetic input decreases, vessels relax.

Thick-walled arteries/veins rely on vasa vasorum to supply nutrients and gases to the outer layers.
This diffusion from the lumen is insufficient for the outer layers due to wall thickness; thus the need for internal microvascular networks.

Venous valves are formed by the tunica intima to prevent backflow and assist one-way blood return to the heart.
Valves are especially important in limbs where gravitational effects can impede retrograde flow.

The structural differences reflect function:
- Arteries: thick walls and prominent tunica media allow strong, regulated blood delivery under high pressure.
- Veins: thinner walls and thicker adventitia accommodate high capacity and reservoir function; valves prevent backflow.
- Capillaries: thin walls maximize exchange efficiency.
Endothelial health is crucial for smooth flow and clot prevention; rough endothelium increases thrombosis risk.
Autonomic control (primarily sympathetic) modulates blood distribution according to tissue needs; parasympathetic input to most vessels is minimal or absent.
Vasa vasorum ensures the viability of large vessels by providing nourishment to the vessel wall itself.

Links to cardiovascular physiology: how blood pressure is maintained, how flow is regulated, and how vessel structure supports function under different hemodynamic conditions.
Therapeutic relevance: understanding arterial vs venous disorders, importance of endothelial health, and the role of vascular smooth muscle tone in conditions like hypertension.
Ethical/philosophical/practical implications: recognizing that smooth muscle tone and endothelial integrity are dynamic properties influenced by lifestyle, medications, and disease states.

Arteries: carry blood away from the heart; thick-walled with a prominent tunica media.
Veins: return blood to the heart; thinner walls; thicker adventitia; contain valves.
Capillaries: site of exchange; only tunica intima.
Tunica interna (intima): endothelial lining; smooth surface.
Tunica media: elastic fibers + smooth muscle; regulates lumen size.
Tunica externa (adventitia): dense irregular connective tissue; collagen.
Vasa vasorum: small vessels supplying the walls of larger vessels.
Vasoconstriction: narrowing of the lumen via smooth muscle contraction ( $ext{constriction}$ ).