Blood Vessel Structure and Function

Overview of Blood Vessels

Blood vessels form a closed delivery system that begins and ends at the heart. They are dynamic structures that can constrict, relax, and even multiply, working in conjunction with the lymphatic system to circulate blood throughout the body.

• Arteries carry blood away from the heart.

• Veins carry blood toward the heart.

• Capillaries are exchange vessels, directly serving cellular needs by allowing substances to move across their walls between tissue cells and blood.

Additional info: Systemic arteries carry oxygen-rich blood; systemic veins carry oxygen-poor blood. Pulmonary arteries and veins have the opposite oxygenation status.

Structure of Blood Vessel Walls

Layers of Blood Vessel Walls

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 (basement membrane and loose connective tissue in larger vessels).

• Tunica media: Middle layer, primarily smooth muscle cells and elastic fibers. Regulated by sympathetic vasomotor nerve fibers and chemicals. Responsible for vasoconstriction (decreasing lumen diameter) and vasodilation (increasing lumen diameter).

• Tunica externa (adventitia): Outermost layer, mainly collagen fibers that protect, reinforce, and anchor the vessel to surrounding structures. Contains nerve fibers, lymphatic vessels, and vasa vasorum (small vessels that nourish the external tissues of larger vessel walls).

Capillary walls consist only of endothelium with a sparse basal lamina, allowing for efficient exchange.

Types of Blood Vessels

Arteries

Arteries are classified based on their size and function:

• Elastic arteries: Thick-walled arteries near the heart (e.g., aorta). Large lumen provides low-resistance to blood flow. Act as pressure reservoirs, expanding and recoiling as blood is ejected from the heart.

• Muscular arteries: Distributing arteries that deliver blood to specific organs. Have the thickest tunica media, more smooth muscle, and less elastic tissue than elastic arteries. More active in vasoconstriction.

• Arterioles: Smallest arteries leading into capillary beds. Larger arterioles have all three tunics; smaller ones are mostly smooth muscle around endothelium. Control flow into capillary beds via vasodilation and vasoconstriction, and are major determinants of total peripheral resistance (TPR).

Capillaries

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

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

• Fenestrated capillaries: Found in areas of active filtration (kidneys), absorption (intestines), and endocrine hormone secretion. Endothelial cells contain pores (fenestrations) for increased permeability.

• Sinusoid capillaries: Most permeable, least common. Found in liver, bone marrow, spleen, and adrenal medulla. Large intercellular clefts, fenestrations, and incomplete basement membranes allow passage of large molecules and blood cells.

Veins

Veins carry blood from capillary beds back toward the heart. They have thinner walls and larger lumens compared to arteries, and often appear collapsed in histological preparations.

• Venules: Formed when capillaries unite. Smallest venules consist only of endothelium and pericytes; larger venules have thin tunica media and externa.

• Veins: Have all three tunics, but thinner walls and larger lumens. Serve as blood reservoirs, containing up to 65% of the blood supply at any time. Adaptations include venous valves (prevent backflow, most abundant in limbs) and venous sinuses (specialized, flattened veins with thin walls).

Flow, Pressure, and Resistance

Definitions and Relationships

Blood flow, pressure, and resistance are key concepts in cardiovascular physiology.

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

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

• Resistance (R): Opposition to flow, mainly due to friction between blood and vessel walls. Most resistance is encountered in peripheral circulation (TPR).

Three important sources of resistance:

• Blood viscosity: Thickness or "stickiness" of blood. Increased viscosity increases resistance.

• Blood vessel length: Longer vessels increase resistance.

• Blood vessel diameter: Smaller diameter increases resistance dramatically (inversely proportional to the fourth power of radius).

Key equations:

• Relationship between flow, pressure, and resistance:

• Mean arterial pressure (MAP):

• Cardiac output:

• MAP in terms of stroke volume, heart rate, and resistance:

Blood Pressure Regulation

Short-Term Regulation

Short-term regulation of blood pressure involves neural and hormonal controls that alter TPR and CO.

• Neural controls: Reflexes involving baroreceptors (monitor stretch/pressure) and chemoreceptors (monitor CO2, pH, O2). The cardiovascular center in the medulla integrates these inputs to adjust heart rate, stroke volume, and vessel diameter.

• Baroreceptor reflex: Increased MAP stimulates baroreceptors, leading to vasodilation and decreased heart rate/contractility, lowering MAP. Decreased MAP inhibits baroreceptors, leading to vasoconstriction and increased heart rate/contractility, raising MAP.

• Chemoreceptor reflex: Responds to changes in blood CO2, pH, and O2, increasing CO and MAP via sympathetic stimulation.

• Hormonal controls: Epinephrine and norepinephrine increase CO and vasoconstriction; angiotensin II stimulates vasoconstriction and aldosterone/ADH release; atrial natriuretic peptide (ANP) decreases MAP by promoting vasodilation and reducing blood volume.

Long-Term Regulation

Long-term regulation of blood pressure is primarily managed by the kidneys, which control blood volume.

• Direct renal mechanism: Increased blood pressure/volume leads to increased urine output, reducing blood volume and pressure. Decreased pressure/volume leads to water conservation, increasing blood volume and pressure.

• Indirect renal mechanism (Renin-Angiotensin-Aldosterone System): Drop in MAP triggers renin release, leading to formation of angiotensin II, which increases MAP by vasoconstriction, stimulating aldosterone and ADH release, and triggering thirst.

Capillary Exchange and Bulk Flow

Capillary Transport Mechanisms

Substances move between blood and interstitial fluid via:

• Diffusion: Lipid-soluble molecules diffuse across endothelial cell membranes; water-soluble molecules pass through intercellular clefts or fenestrations.

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

Bulk Flow

Bulk flow of fluids across capillary walls is crucial for determining relative fluid volumes in blood and interstitial space. It is governed by hydrostatic and osmotic pressures.

• Hydrostatic pressure (HP): Force exerted by fluid against vessel wall. Capillary HP pushes fluid out of capillary (filtration), greater at arterial end (35 mm Hg) than venous end (17 mm Hg).

• Osmotic pressure (OP): Created by nondiffusible plasma proteins, pulling fluid into capillary (~26 mm Hg).

• Net filtration pressure (NFP): At arterial end, NFP is positive (filtration); at venous end, NFP is negative (reabsorption).

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

Regulation of Water Intake and Output

Water Balance

Water intake must equal output (~2500 ml/day). Osmolality is maintained around 260-300 mOsm.

• Intake: Metabolic water, beverages, food.

• Output: Insensible loss (skin, airways), urine, sweat, feces.

Rise in osmolality stimulates thirst and ADH release; decrease inhibits both.

Antidiuretic Hormone (ADH)

ADH regulates water reabsorption in the kidneys. Increased ADH decreases urine output and increases body fluid volume; decreased ADH has the opposite effect.

• Release stimulated by increased ECF osmolality (via hypothalamic osmoreceptors) or large drop in blood volume/BP (via baroreceptors and renin-angiotensin-aldosterone system).

Intrinsic and Extrinsic Controls of Blood Flow

Regulation of Tissue Perfusion

Tissue perfusion is regulated to match metabolic needs. Intrinsic controls (autoregulation) act locally on arterioles, while extrinsic controls (sympathetic and endocrine) maintain MAP and direct blood flow to where it is needed most.

• Intrinsic controls: Organs regulate their own blood flow by varying resistance of their arterioles.

• Extrinsic controls: SNS and hormones reduce flow to less-needed regions to maintain MAP, while intrinsic controls increase flow to active tissues.

Example: During exercise, intrinsic controls dilate skeletal muscle arterioles, increasing blood flow to muscles, while extrinsic controls constrict arterioles in kidneys and digestive organs.

Generalized Structure Comparison Table