blood vessels

What are the three main types of blood vessels and their functions?

• Arteries: Carry blood away from the heart

• Capillaries: Exchange nutrients and gases with tissue cells

• Veins: Carry blood toward the heart

What are the structural layers of blood vessels?

• All vessels have a lumen (central blood-filled space)

• All except capillaries have 3 layers:

  1. Tunica intima

  2. Tunica media

  3. Tunica externa

• Capillaries consist of only endothelium (simple squamous) and a sparse basal lamina

Tunica intima

Innermost layer that is in “intimate” contact with blood

Made of endothelium (simple squamous epithelium)

Has a subendothelial layer (CT basement membrane) in vessels >1 mm

Tunica Media

• Middle layer made of smooth muscle and elastin

• Controlled by sympathetic vasomotor nerves

• Regulates:

  • Vasoconstriction (↓ lumen diameter)

  • Vasodilation (↑ lumen diameter)

• Thickest layer, helps maintain blood flow and pressure

Tunica Externa

• Outermost layer of a blood vessel wall

• Made of loose collagen fibers for protection and anchoring

• Contains nerve fibers and lymphatic vessels

• Large veins may have elastic fibers in this layer

• vasa vasorum—tiny vessels that nourish the outer wall of large blood vessels

When vascular smooth muscle contracts, what happens to the diameter of the blood vessel? What is this called?

The diameter decreases, and this is called vasoconstriction.

Arteries

Carry blood away from heart

Divided into three groups based on size and function

• Elastic arteries

• Muscular arteries

• Arterioles

Elastic Arteries

Large arteries (like the aorta) with thick walls and elastin; they act as pressure reservoirs to maintain blood flow between heartbeats.

Muscular Arteries

Deliver blood to organs

Most common type of artery

Thick tunica media with lots of smooth muscle

Less elastic tissue, but active in vasoconstriction

Arterioles

• Smallest arteries

• Larger ones have all 3 tunics; smaller ones mostly smooth muscle and endothelium

• Regulate blood flow into capillary beds via vasodilation and vasoconstriction

• Lead directly to capillary beds

Capillaries

Function: Exchange gases, nutrients, wastes, and hormones with interstitial fluid

Structure: Endothelial cells joined by tight junctions with intercellular clefts for fluid/solute passage

Types: Continuous, fenestrated, sinusoidal

Continuous Capillaries

skin, muscles, lungs, and CNS

In the brain, they form the blood-brain barrier with tight junctions and no intercellular clefts

Fenestrated Capillaries

• Found in kidneys, intestines, and endocrine glands

• Have fenestrations (pores) in endothelial cells

• Allow increased permeability

• Pores often covered by a thin glycoprotein diaphragm

Sinusoidal Capillaries

• large lumens, large clefts

• Found in liver, bone marrow, spleen, and adrenal medulla

• slow blood flow

• Contain macrophages

Name the type of artery that matches each description

A. Major role in dampening the pulsatile pressure of the

heart contractions

B. Vasodilation or constriction determines blood flow to

individual capillary beds

C. Have the thickest tunica media relative to their lumen size

A → Elastic artery
B → Arteriole
C → Muscular artery

Capillary Beds

Vascular shunt – a direct path from arteriole to venule (metarteriole → thoroughfare channel), bypassing true capillaries

Precapillary sphincter – smooth muscle cuff that controls blood flow into true capillaries; regulated by local chemicals, not nerves

Types of veins

Carry blood toward the heart

Formation begins when capillary beds unite downstream
• Two groups

Venules

Veins

Venules

• Formed when capillaries unite

• Made of endothelium and a few pericytes

• Very porous—allow fluid and WBC movement

• Larger venules may have 1–2 layers of smooth muscle

  a very small vein, especially one collecting blood from the capillaries.

Veins

• Formed when venules merge

• Have all three tunics, but with thinner walls and larger lumens than arteries

• Thin tunica media, thick tunica externa

• Contain collagen fibers and elastic networks

• Act as blood reservoirs due to large lumen and thin walls

Venous valves

•Prevent backflow of blood

•Most abundant in veins of limbs

Venous sinuses

•Flattened veins with extremely thin walls

•Composed only of endothelium

•Examples: coronary sinus of the heart and dural sinuses of the brain

What are the key differences between arteries and veins?

• Arteries:

  • Carry blood away from the heart

  • Thick walls, especially tunica media

  • Smaller lumen

  • High pressure

  • No valves (except in the heart)

• Veins:

  • Carry blood toward the heart

  • Thinner walls, especially tunica media

  • Larger lumen

  • Low pressure

  • Have valves to prevent backflow

What is the function of venous valves? What layer of the vessel are these valves composed of?

• Function: Prevent the backflow of blood, especially in limbs

• Layer: Made from folds of the tunica intima