Blood Vessel Structure and Function

Overview of Blood Vessels

Blood vessels form a closed delivery system that begins and ends at the heart, working in conjunction with the lymphatic system. They are dynamic structures capable of constriction, dilation, and even multiplication in response to physiological needs.

• 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.

Example: Systemic arteries carry oxygen-rich blood, while systemic veins carry oxygen-poor blood. Pulmonary arteries and veins have the opposite function.

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

• Tunica media: Middle layer, primarily smooth muscle cells and elastic fibers. Responsible for vasoconstriction (decreasing lumen diameter) and vasodilation (increasing lumen diameter), thus regulating blood flow and pressure.

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

Capillary walls consist only of endothelium with a sparse basal lamina, facilitating exchange.

Types of Blood Vessels

Arteries

Arteries are classified based on size and function:

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

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

• Arterioles: Smallest arteries, lead into capillary beds. Control flow into capillaries via vasodilation and vasoconstriction; major determinants of total peripheral resistance (TPR).

Capillaries

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

• 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 areas of active filtration (kidneys), absorption (intestines), and hormone secretion.

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

Veins

Veins carry blood toward the heart and act as blood reservoirs. They have thinner walls, larger lumens, and lower pressure compared to arteries.

• Venules: Formed by the union of capillaries; very porous, allowing fluids and WBCs to move into tissues.

• Veins: Have all three tunics, but thinner walls and larger lumens. Contain valves to prevent backflow, especially in limbs. Can hold up to 65% of the blood supply at any time.

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

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 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. Total peripheral resistance (TPR) is the resistance in systemic circulation.

Key factors affecting resistance:

• Blood viscosity (thickness)

• Blood vessel length

• Blood vessel diameter (most variable and influential)

Relationship:

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

Blood Pressure Regulation

Short-Term and Long-Term Controls

Blood pressure is regulated by the heart, blood vessels, and kidneys, under supervision of the brain. Regulation involves cardiac output (CO), total peripheral resistance (TPR), and blood volume.

• Short-term regulation: Alters TPR and CO via neural and hormonal controls.

• Long-term regulation: Alters blood volume via renal mechanisms.

Key equations:

• (Stroke Volume × Heart Rate)

• (Mean Arterial Pressure)

Neural Controls

• Baroreceptor reflex: Baroreceptors in carotid sinuses, aortic arch, and large arteries sense changes in stretch (pressure). Increased MAP stimulates baroreceptors, leading to vasodilation and decreased HR/SV, lowering MAP. Decreased MAP inhibits baroreceptors, leading to vasoconstriction and increased HR/SV, raising MAP.

• Chemoreceptor reflex: Chemoreceptors respond to changes in blood CO2, pH, and O2, stimulating cardiovascular centers to increase CO and TPR.

• Higher brain centers: Hypothalamus and cerebral cortex can modify arterial pressure during stress or exercise.

Hormonal Controls

• Epinephrine and norepinephrine: Increase CO and vasoconstriction.

• Angiotensin II: Potent vasoconstrictor; stimulates aldosterone and ADH release to increase blood volume.

• Atrial natriuretic peptide (ANP): Decreases MAP by promoting vasodilation and reducing blood volume.

• Antidiuretic hormone (ADH): Stimulates kidneys to conserve water and causes vasoconstriction.

Renal Mechanisms

• Direct renal mechanism: Kidneys eliminate or conserve water to regulate blood volume and pressure.

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

Capillary Exchange and Bulk Flow

Transport Mechanisms

Capillaries allow exchange of gases, nutrients, and wastes between blood and tissues via several mechanisms:

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

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

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

Bulk Flow: Hydrostatic and Osmotic Pressures

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

• Osmotic pressure (OP): Created by nondiffusible plasma proteins; pulls fluid into capillary.

Net Filtration Pressure (NFP):

• At arterial end: NFP is positive (filtration out of capillary).

• At venous end: NFP is negative (reabsorption into capillary).

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

Regulation of Blood Flow Through Tissues

Intrinsic and Extrinsic Controls

Tissue perfusion is regulated to match metabolic needs. Intrinsic controls (autoregulation) act locally on arterioles, while extrinsic controls (neural and hormonal) maintain MAP and redistribute blood flow as needed.

• Intrinsic controls: Organs regulate their own blood flow by adjusting arteriole resistance.

• Extrinsic controls: Sympathetic nervous system and hormones adjust vessel diameter to maintain MAP and direct blood flow to where it is most needed (e.g., during exercise).

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

Fluid Compartments and Water Balance

Body Fluid Compartments

Body fluids are distributed between intracellular fluid (ICF) and extracellular fluid (ECF), which includes plasma and interstitial fluid (IF). Exchange of water, nutrients, and wastes occurs between these compartments.

• Osmolality is maintained around 260-300 mOsm; regulated by thirst and ADH release.

• Water intake must equal output (~2500 ml/day).

• Water output includes insensible loss (skin, airways), urine, sweat, and feces.

Regulation of Water Intake and Output

• Thirst mechanism: Controlled by hypothalamic osmoreceptors, dry mouth, and decreased blood volume/pressure.

• ADH (antidiuretic hormone): Regulates water reabsorption in kidneys; increased ADH decreases urine output, conserving body fluids.

Summary Table: Comparison of Arteries, Veins, and Capillaries

Additional info: These notes expand on the original lecture slides by providing definitions, examples, and equations for key physiological concepts, ensuring a self-contained study guide for exam preparation.