Blood Vessels: Structure, Function, and Physiology

Chapter 18: The Blood Vessels

The cardiovascular system is composed of the heart, blood vessels, and blood. Blood vessels form an extensive network that transports blood throughout the body, facilitating the exchange of gases, nutrients, and wastes. This chapter explores the structure and function of blood vessels, as well as the physiological principles governing blood flow and pressure.

Components and Functions of Blood Vessels

• Blood Vessels: Tubular structures that transport blood between the heart and tissues.

• Functions:

  • Transport blood to and from tissues

  • Exchange gases, nutrients, and wastes

  • Regulate blood flow to tissues

  • Control blood pressure

  • Secrete various chemicals

Module 18.1: Overview of Arteries and Veins

Blood vessels are classified based on their structure and function within the circulatory system.

• Arteries: Distribution system; carry blood away from the heart.

• Arterioles: Smaller arteries that deliver blood to tissues/organs.

• Capillaries: Exchange system; form branching networks (capillary beds) for exchange of substances.

• Venules: Small veins that collect blood from capillaries.

• Veins: Collection system; return blood to the heart.

Tunics of the Blood Vessel Wall

Large blood vessels are composed of several tissue layers, known as tunics, surrounding a central lumen.

• Tunica Intima: Innermost layer; consists of endothelium and subendothelial connective tissue.

• Tunica Media: Middle layer; composed mainly of smooth muscle and elastic fibers, responsible for vasoconstriction and vasodilation.

• Tunica Externa (Adventitia): Outermost layer; made of connective tissue that provides structural support and protection.

Example: The aorta, a large artery, has a thick tunica media to withstand high pressure.

Types of Arteries and Veins

Blood vessels vary in structure and function depending on their location and role in circulation.

Type	Structure	Function
Elastic Arteries	Largest diameter, abundant elastic fibers	Conduct blood from heart; withstand high pressure
Muscular Arteries	Thick tunica media, more smooth muscle	Distribute blood to organs
Arterioles	Small diameter, thin walls	Regulate blood flow into capillaries
Capillaries	Single layer of endothelium	Exchange of gases, nutrients, and wastes
Venules	Small, thin-walled	Collect blood from capillaries
Veins	Thinner walls, larger lumen, valves present	Return blood to the heart

Artery Walls vs Vein Walls

Arteries and veins have distinct structural differences that reflect their functions.

• Arteries: Thicker tunica media, more elastic and muscular tissue, smaller lumen, withstand higher pressure.

• Veins: Thinner walls, larger lumen, less smooth muscle, contain valves to prevent backflow.

Example: The femoral artery has a thicker wall than the femoral vein due to higher pressure.

Blood Distribution in the Cardiovascular System

Blood is distributed unevenly among the various vessels in the body.

• Systemic veins: 55% (major blood reservoir)

• Pulmonary arteries: 15%

• Pulmonary veins: 5%

• Systemic arteries: 10%

• Capillaries: 5%

• Heart: 8%

Vascular Anastomoses

Vascular anastomoses are connections between blood vessels that provide alternate pathways for blood flow.

• Arterial Anastomoses: Found in organs like the heart and brain; allow for collateral circulation.

• Venous Anastomoses: Common; neighboring veins connected by small collaterals.

• Arteriovenous Anastomoses: Direct connection between artery and vein, bypassing capillaries.

Example: The circle of Willis in the brain is an arterial anastomosis.

Module 18.2: Physiology of Blood Flow

Hemodynamics is the study of blood flow in the cardiovascular system, governed by pressure gradients and resistance.

• Pressure Gradient: Blood flows from areas of high pressure to low pressure.

• Blood Pressure: The force exerted by blood on vessel walls, measured in mm Hg.

• Blood Flow: Volume of blood moving per minute; directly proportional to pressure gradient and inversely proportional to resistance.

• Resistance: Any impedance to blood flow, affected by vessel diameter, blood viscosity, and vessel length.

Key Equation:

$\text{Blood Flow} = \frac{\text{Pressure Gradient}}{\text{Resistance}}$

Factors That Determine Blood Pressure

Blood pressure is influenced by several physiological factors:

• Cardiac Output (CO): Volume of blood pumped by the heart per minute.

• Peripheral Resistance: Resistance of the arteries to blood flow.

• Blood Volume: Total amount of blood in the circulatory system.

• Elasticity of Vessel Walls: Ability of vessels to expand and recoil.

• Blood Viscosity: Thickness of blood, influenced by hematocrit.

Key Equation:

$\text{Blood Pressure} = \text{Cardiac Output} \times \text{Peripheral Resistance}$

Blood Pressure in Different Portions of the Circulation

Blood pressure varies throughout the circulatory system, being highest in the arteries and lowest in the veins.

Vessel Type	Pressure (mm Hg)
Aorta	~95-100
Arteries	80-95
Arterioles	35-80
Capillaries	15-35
Venules	10-15
Veins	0-10

Additional info: Pulmonary circuit pressures are much lower than systemic circuit pressures.

Systemic Arterial Pressure

• Mean Arterial Pressure (MAP): Average pressure in arteries during a cardiac cycle.

  • Calculated as $\text{MAP} = \text{Diastolic Pressure} + \frac{1}{3}(\text{Systolic Pressure} - \text{Diastolic Pressure})$

  • Normal MAP: ~95 mm Hg

• Systolic Pressure: Pressure during ventricular contraction (typically 110-120 mm Hg).

• Diastolic Pressure: Pressure during ventricular relaxation (typically 70-80 mm Hg).

• Pulse Pressure: Difference between systolic and diastolic pressures (about 40 mm Hg).

Measuring Arterial Blood Pressure

• Measured using a sphygmomanometer and stethoscope, usually at the brachial artery.

• A cuff is inflated to stop blood flow, then slowly deflated while listening for Korotkoff sounds.

• Systolic Pressure: First appearance of sounds.

• Diastolic Pressure: Disappearance of sounds.

Systemic Capillary and Venous Pressure

• Systemic Capillary Pressure: Declines from about 35 mm Hg at the arterial end to 15 mm Hg at the venous end.

• Systemic Venous Pressure: Drops further in veins, reaching as low as 0 mm Hg in the right atrium.

• Low pressure in veins is due to high compliance and low resistance.

Mechanisms of Venous Return

• Venous Valves: Prevent backflow of blood in veins.

• Smooth Muscle in Vein Walls: Contracts under sympathetic stimulation to increase venous return.

• Skeletal Muscle Pump: Contraction of skeletal muscles compresses veins, propelling blood toward the heart.

• Respiratory Pump: Changes in thoracic pressure during breathing help draw blood toward the heart.

Example: Walking activates the skeletal muscle pump in the legs, aiding venous return.