Structure and Function of Blood Vessels

Five Types of Blood Vessels

Structure of a Blood Vessel Wall

The wall of a blood vessel consists of three layers called tunics.

Tunica Interna (Intima)

• Forms the inner lining in direct contact with blood
• Blood flows through the lumen (interior opening)
• Innermost layer is the endothelium
• Endothelial cells secrete fluids to mediate contractile state and assist with capillary permeability
• Basement membrane provides physical support
• Collagen fibers provide tensile strength, stretch, and recoil
• Internal elastic lamina — thin sheet of elastic fibers with window-like openings facilitating diffusion

Tunica Media

• Middle layer; shows greatest variation among vessel types
• Comprised mainly of smooth muscle and elastic fibers
• Primary role: regulate the diameter of the lumen
• Vasoconstriction — decrease in lumen diameter
• Vasodilation — increase in lumen diameter
• Plays important role in vascular spasm after vessel damage (limits blood loss)

Tunica Externa

• Outer covering
• Contains nerves and tiny blood vessels (vasa vasorum) that supply the vessel wall with oxygen and nutrients
• Helps anchor the vessel in place

Arteries

Elastic Arteries

• Largest arteries (aorta to finger-sized branches)
• Largest diameter but relatively thin walls for their size
• Characterized by well-defined internal and external elastic laminae
• Thicker tunica media dominated by elastic fibers (elastic lamellae)
• As blood is ejected in, walls stretch and store mechanical energy
• Stretched fibers recoil, continuing blood flow to the body (elastic recoil)

Muscular Arteries

• Contain more smooth muscle than elastic tissue
• Capable of greater vasoconstriction and vasodilation
• Known as distributing vessels — distribute blood to the rest of the body
• Important in maintaining vascular tone, vessel pressures, and efficient blood flow

Anastomoses

The union of branches of two or more arteries supplying the same body region. Allows collateral circulation if a vessel becomes blocked or damaged.

Arterioles

• Literally "small arteries"
• Regulate blood flow into capillary networks of tissue
• Regulate resistance — decreasing internal diameter increases friction, resistance, and pressure

Capillaries

• Smallest blood vessels (capillus = "little hair")
• Exchange vessels — network forms an enormous surface area for diffusion
• Lack a tunica media and tunica externa

Capillary Beds

• During low metabolic use, capillary beds constrict and restrict blood flow
• During increased activity, capillary beds dilate, increasing blood flow and delivery of substances
• Precapillary sphincters — constrict and relax to allow blood flow into specific capillary beds

Venules

• Thin walls that do not readily maintain their shape
• Drain capillary blood and begin return flow to heart
• Postcapillary venules — receive blood from capillaries; extremely porous and significant sites of nutrient exchange
• Muscular venules — most distensible elements of the vascular system; expand and serve as an excellent reservoir for accumulating large volumes of blood

Veins

• Relatively thin walls compared to diameter
• Tunica externa is the thickest layer (collagen and elastic fibers)
• Blood pressure within veins is much lower than in arteries
• Connect like rungs on a ladder, creating anastomoses between deep and superficial veins
• Skeletal-muscle pump helps squeeze blood back to the heart
• Valves prevent blood from flowing backward to distal parts

Blood Distribution at Rest

Systemic veins and venules act as a blood reserve. During hemorrhage or exercise, vasoconstriction of these vessels redistributes blood to maintain survivable blood pressure and deliver oxygen to skeletal muscle.