Blood Vessel Structure and Function

Key Terms

• Lumen: The central blood-containing space within a blood vessel.

• Vasoconstriction: The narrowing of blood vessels due to contraction of the muscular wall.

• Vasodilation: The widening of blood vessels due to relaxation of the muscular wall.

• Microcirculation: The flow of blood through the smallest vessels, including arterioles, capillaries, and venules.

• Venous sinus: A vein with a thin wall and no smooth muscle, often found in the brain and heart.

• Anastomosis: A connection or opening between two vessels that allows for alternate routes of blood flow.

• Blood flow: The movement of blood through the circulatory system.

• Blood pressure: The force exerted by circulating blood on the walls of blood vessels.

• Vasomotor tone: The baseline level of contraction in blood vessel smooth muscle, regulated by sympathetic nerve fibers.

• Systolic pressure: The pressure in arteries during ventricular contraction.

• Diastolic pressure: The pressure in arteries during ventricular relaxation.

• Pulse pressure: The difference between systolic and diastolic pressure.

• Perfusion: The process of delivering blood to capillary beds in tissue.

• Autoregulation: The ability of tissues to regulate their own blood supply.

• Vasomotion: The intermittent contraction and relaxation of precapillary sphincters, regulating blood flow through capillary beds.

Blood Flow Through Vessels

Blood flows through a series of vessels, each with distinct roles in circulation. The direction of flow and oxygenation status varies by circuit:

• Arteries: Carry blood away from the heart; systemic arteries carry oxygenated blood, pulmonary arteries carry deoxygenated blood.

• Veins: Carry blood toward the heart; systemic veins carry deoxygenated blood, pulmonary veins carry oxygenated blood.

• Capillaries: Sites of exchange between blood and tissues; blood oxygenation depends on the circuit.

Structure of Blood Vessel Walls

Layers of Vessel Walls

Blood vessels (except capillaries) have three layers:

• Tunica intima (innermost): Endothelial lining; provides a smooth surface for blood flow.

• Tunica media (middle): Smooth muscle and elastic fibers; responsible for vasoconstriction and vasodilation. Sympathetic vasomotor nerve fibers control this layer, regulating vessel diameter.

• Tunica externa (outermost): Connective tissue; provides support and protection.

The tunica media is the layer most influenced by sympathetic nerve fibers, which cause vasoconstriction or vasodilation.

Types of Arteries

Arteries are classified by size and function:

• Elastic arteries (conducting arteries): Largest; thick tunica media with many elastic fibers; close to the heart; help maintain blood pressure.

• Muscular arteries (distributing arteries): Medium-sized; thickest tunica media; distribute blood to specific organs.

• Arterioles (resistance arteries): Smallest; thin layers; regulate blood flow into capillary beds.

Order from the heart: Elastic arteries → Muscular arteries → Arterioles.

Types of Capillaries

Capillaries are the smallest blood vessels and are classified by their permeability:

• Continuous capillaries: Most common; uninterrupted endothelial lining; found in skin, muscle, and brain.

• Fenestrated capillaries: Have pores; found in kidneys, intestines, and endocrine glands; allow for rapid exchange.

• Sinusoidal capillaries: Large gaps; found in liver, bone marrow, spleen; allow passage of large molecules and cells.

Capillaries are absent in cartilage, epithelia, cornea, and lens of the eye.

Capillary Beds

A capillary bed is a network of capillaries supplied by one arteriole and drained by a venule. It consists of:

• Metarteriole: Connects arteriole to capillaries.

• True capillaries: Where exchange with tissues occurs.

• Thoroughfare channel: Direct route from arteriole to venule; used to shunt blood when exchange is not needed.

• Precapillary sphincters: Regulate blood flow into true capillaries.

Blood is shunted through the thoroughfare channel when exchange is not required.

Veins

Veins return blood to the heart and are known as capacitance vessels due to their ability to hold large volumes of blood. Order from capillary bed to heart:

• Capillaries → Venules → Veins → Heart

Adaptations for venous return include:

• Large lumens to reduce resistance

• Valves to prevent backflow

• Muscle pump (skeletal muscle contraction)

• Respiratory pump (pressure changes during breathing)

Hemodynamics: Blood Flow, Pressure, and Resistance

Sources of Resistance

Three major sources of resistance to blood flow:

• Blood viscosity: Thickness of blood; increased viscosity increases resistance.

• Vessel length: Longer vessels increase resistance.

• Vessel radius: Smaller radius increases resistance; radius has the greatest influence and is easily changed.

Arterioles are the major determinant of peripheral resistance.

Laminar Flow and Resistance

Laminar flow describes the smooth, orderly movement of blood in parallel layers. Resistance is inversely proportional to the fourth power of the vessel radius:

• If radius doubles, resistance drops to 1/16th of original value.

• If radius halves, resistance increases to 16 times original value.

Mathematically:

• If , then

• If (radius doubles), then

• If (radius reduced to half), then

Equation:

Relationship Between Flow, Pressure, and Resistance

Blood flow is determined by the pressure gradient and resistance:

• Larger pressure gradient → faster blood flow

• Larger resistance → slower blood flow

• Resistance is more important for local blood flow regulation due to its ability to change rapidly (via vessel diameter).

Equation:

Where is the pressure difference and is resistance.

Systemic Blood Pressure

Systemic blood pressure is the force exerted by blood on arterial walls throughout the body. It is generated by ventricular contraction and is highest in the aorta, with the steepest drop occurring in arterioles.

Determinants of Arterial Blood Pressure

Two main factors determine arterial blood pressure:

• Elasticity of arteries near the heart: Allows arteries to stretch and recoil, maintaining pressure.

• Blood volume ejected into arteries (Stroke Volume, SV): Greater volume increases pressure.

Average adult blood pressure: 120/80 mmHg (systolic/diastolic).

Mean Arterial Pressure (MAP)

MAP is the average pressure in arteries during one cardiac cycle and is a better indicator of organ perfusion than systolic pressure alone.

Equation:

Example: For 120/80 mmHg, mmHg

Capillary Blood Pressure

Capillary blood pressure is low to prevent rupture and to allow for effective exchange of nutrients and wastes between blood and tissues.

Venous Blood Pressure

Venous blood pressure is much lower than arterial pressure and relies on adaptations (valves, muscle pump, respiratory pump) to return blood to the heart.

Summary Table: Types of Blood Vessels

Additional info:

• Autoregulation and vasomotion are important for local control of blood flow, allowing tissues to match perfusion to metabolic needs.

• Venous sinuses are specialized veins found in the dura mater of the brain and in the heart (coronary sinus).