Blood Vessel Structure and Function

Overview of the Cardiovascular System

The cardiovascular system is a closed delivery system composed of the heart and blood vessels. Blood vessels transport blood throughout the body, working closely with the lymphatic system to maintain fluid balance and support tissue health.

• Arteries carry blood away from the heart.

• Veins return blood toward the heart.

• Capillaries are exchange vessels, directly serving cellular needs by allowing substances to move between blood and tissues.

Systemic arteries usually carry oxygen-rich blood, while systemic veins carry oxygen-poor blood. The pulmonary circuit is the exception, with pulmonary arteries carrying oxygen-poor blood to the lungs and pulmonary veins returning oxygen-rich blood to the heart.

Structure of Blood Vessel Walls

General Organization

Except for capillaries, blood vessel walls consist of three layers, or tunics, surrounding a central blood-containing space called the lumen:

• Tunica intima: Innermost layer, in direct contact with blood. Composed of endothelium (simple squamous epithelium) and a subendothelial layer (in larger vessels).

• Tunica media: Middle layer, primarily smooth muscle and elastic fibers. Responsible for vasoconstriction and vasodilation, regulating blood flow and pressure.

• Tunica externa (adventitia): Outermost layer, mainly collagen fibers that protect, reinforce, and anchor the vessel. Contains nerves, lymphatic vessels, and vasa vasorum (small vessels that nourish the outer wall).

Capillaries have only a thin tunica intima (endothelium with sparse basal lamina), allowing efficient exchange of materials.

Types of Blood Vessels

Arteries

• Elastic arteries: Thick-walled, near the heart (e.g., aorta). Act as pressure reservoirs, expanding and recoiling to maintain continuous blood flow.

• Muscular arteries: Distribute blood to specific organs. Have the thickest tunica media, more smooth muscle, and are more active in vasoconstriction.

• Arterioles: Smallest arteries, leading into capillary beds. Control blood flow into capillaries via vasodilation and vasoconstriction; major determinants of peripheral resistance.

Capillaries

Capillaries are the smallest blood vessels and serve as the primary sites for exchange between blood and tissues. Their thin walls facilitate the movement of gases, nutrients, and wastes.

• Continuous capillaries: Least permeable, most common. Found in skin, muscles, lungs, and CNS. Endothelial cells joined by tight junctions with intercellular clefts for limited passage.

• Fenestrated capillaries: Have pores (fenestrations) for increased permeability. Found in kidneys, intestines, and endocrine glands.

• Sinusoidal capillaries: Most permeable, with large gaps and incomplete basement membranes. Found in liver, bone marrow, spleen, and adrenal medulla.

Veins

Veins return blood to the heart and act as blood reservoirs. Their walls are thinner and lumens larger than arteries, allowing them to accommodate more blood at lower pressure.

• Venules: Smallest veins, formed by the union of capillaries. Very porous, allowing fluid and white blood cells to move into tissues.

• Veins: Formed by the convergence of venules. Have thinner walls, less smooth muscle, and larger lumens than arteries. Contain valves to prevent backflow, especially in limbs.

• Venous sinuses: Specialized, flattened veins with extremely thin walls (e.g., coronary sinus, dural sinuses).

Blood Flow, Pressure, and Resistance

Definitions and Relationships

• Blood flow (F): Volume of blood flowing through a vessel, organ, or circulation per unit time. For the entire vascular system, it equals cardiac output (CO).

• Blood pressure (BP): Force per unit area exerted on a vessel wall by the blood, measured in mm Hg.

• Resistance (R): Opposition to flow, mainly due to friction between blood and vessel walls. Also called total peripheral resistance (TPR).

Key factors affecting resistance:

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

• Vessel length: Longer vessels increase resistance.

• Vessel diameter: Most important and frequently changed; smaller diameter increases resistance exponentially.

Relationship:

• Flow is directly proportional to the pressure gradient () and inversely proportional to resistance ():

Blood Pressure Throughout the Circulation

• Pressure is highest in the aorta and declines through the systemic circuit, reaching 0 mm Hg in the right atrium.

• The steepest drop occurs in arterioles, the main resistance vessels.

Arterial Blood Pressure

• Systolic pressure: Peak pressure during ventricular contraction (about 120 mm Hg in adults).

• Diastolic pressure: Lowest pressure during ventricular relaxation (about 80 mm Hg in adults).

• Pulse pressure: Difference between systolic and diastolic pressure.

• Mean arterial pressure (MAP): Average pressure propelling blood to tissues. Calculated as:

Venous Blood Pressure

• Venous pressure is steady and much lower than arterial pressure (about 15 mm Hg gradient from venules to heart).

• Venous return is aided by the muscular pump, respiratory pump, and sympathetic venoconstriction.

Regulation of Blood Pressure

Short-Term Regulation: Neural and Hormonal Controls

• Neural controls (via the cardiovascular center in the medulla) adjust cardiac output and vessel diameter using baroreceptors and chemoreceptors.

• Baroreceptor reflexes respond to changes in arterial pressure by adjusting sympathetic and parasympathetic output.

• Chemoreceptor reflexes respond to changes in blood CO2, pH, and O2 levels.

• Hormones such as epinephrine, norepinephrine, angiotensin II, aldosterone, antidiuretic hormone (ADH), and atrial natriuretic peptide (ANP) regulate blood pressure by affecting cardiac output, vessel diameter, and blood volume.

Long-Term Regulation: Renal Mechanisms

• Kidneys regulate blood volume, thus influencing blood pressure over the long term.

• Direct renal mechanism: Increased BP/volume leads to increased urine output; decreased BP/volume leads to water conservation.

• Indirect renal mechanism (renin-angiotensin-aldosterone system): Low BP triggers renin release, leading to angiotensin II formation, which increases BP by vasoconstriction and stimulating aldosterone and ADH release.

Capillary Exchange and Bulk Flow

Capillary Transport Mechanisms

• Diffusion: Movement of substances down their concentration gradients (e.g., O2, CO2).

• Vesicular transport: Larger molecules (e.g., proteins) are transported via endocytosis and exocytosis.

• Bulk flow: Movement of fluid driven by pressure differences; important for regulating fluid volumes between blood and interstitial space.

Forces Governing Bulk Flow

• Hydrostatic pressure (HP): Force exerted by fluid against a wall; capillary HP pushes fluid out of capillaries.

• Colloid osmotic pressure (OP): Created by plasma proteins; pulls fluid into capillaries.

• Net filtration pressure (NFP): Determines direction of fluid movement:

• At the arterial end, NFP is positive (filtration out); at the venous end, NFP is negative (reabsorption in).

• Excess interstitial fluid is returned to the blood via the lymphatic system.

Regulation of Blood Flow

Intrinsic and Extrinsic Controls

• Intrinsic (autoregulation): Local regulation of blood flow by tissues, adjusting arteriole resistance to match metabolic needs.

• Extrinsic controls: Sympathetic nervous system and hormones adjust blood flow to maintain systemic blood pressure and direct flow to essential organs during stress or exercise.

Example: During exercise, intrinsic controls increase blood flow to skeletal muscles, while extrinsic controls reduce flow to less active organs to maintain MAP.

Summary Table: Comparison of Arteries, Veins, and Capillaries

Key Equations

• Flow, pressure, and resistance:

• Mean arterial pressure:

• Cardiac output:

• Relationship to MAP:

Additional info:

• These notes are based on Marieb Human Anatomy & Physiology, Chapter 19, and are suitable for college-level Anatomy & Physiology students.

• For further study, review textbook sections 19.1–19.10 and related figures for detailed diagrams and clinical correlations.