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 closely 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.

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

Structure of Blood Vessel Walls

General Structure

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). Provides a slick surface to reduce friction.

• Tunica media: Middle layer, mostly smooth muscle and sheets of elastin. Regulated by sympathetic vasomotor nerve fibers and chemicals. Responsible for vasoconstriction (decreasing lumen diameter) and vasodilation (increasing lumen diameter). Maintains blood flow and pressure.

• Tunica externa (adventitia): Outermost layer, composed mainly of collagen fibers that protect, reinforce, and anchor the vessel. Contains nerve fibers, lymphatic vessels, and in larger vessels, a network of blood vessels called vasa vasorum that nourish the external tissues of the vessel wall.

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

Types of Blood Vessels

Arteries

• Elastic arteries: Thick-walled arteries 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. Contain more elastin than other vessels.

• Muscular arteries: Distribute blood to specific body 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 are the primary sites for exchange between blood and tissues. Their thin walls consist of only the tunica intima, sometimes just a single endothelial cell.

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

• Fenestrated capillaries: Have pores (fenestrations) that increase permeability. Found in areas of active filtration (kidneys), absorption (intestines), and hormone secretion (endocrine glands).

• Sinusoidal capillaries: Most permeable, with large gaps and incomplete basement membranes. Found in liver, bone marrow, spleen, and adrenal medulla. Allow passage of large molecules and blood cells.

Veins

Veins carry blood from capillary beds toward the heart. As venous vessels join, they form larger veins with thinner walls and larger lumens compared to arteries. Veins are called capacitance vessels or blood reservoirs because they can contain up to 65% of the blood supply at any time.

• 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. Adaptations include venous valves (prevent backflow, most abundant in limbs) and venous sinuses (specialized, flattened veins with thin walls, e.g., coronary sinus).

Blood Flow, Pressure, and Resistance

Definitions

• 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 a vessel wall by the blood, measured in mm Hg. Systemic arterial BP is measured in large arteries near the heart.

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

Three main sources of resistance:

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

• Blood vessel length: Longer vessels have greater resistance.

• Blood vessel diameter: Most important factor. Resistance varies inversely with the fourth power of the vessel radius (). Small changes in diameter cause large changes in resistance.

Relationship Between Flow, Pressure, and Resistance

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

• In the body, vessel diameter (especially arterioles) is the major determinant of resistance and thus local blood flow.

Blood Pressure Regulation

Arterial Blood Pressure

• Systolic pressure: Pressure during ventricular contraction (average 120 mm Hg in adults).

• Diastolic pressure: Pressure during ventricular relaxation (average 70–80 mm Hg).

• Pulse pressure: Difference between systolic and diastolic pressure.

• Mean arterial pressure (MAP): Pressure that propels blood to tissues. Calculated as:

• MAP and pulse pressure decline with distance from the heart.

Venous Blood Pressure

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

• Adaptations to aid venous return include the muscular pump, respiratory pump, and sympathetic venoconstriction.

Short- and Long-Term Regulation of Blood Pressure

• Blood pressure is regulated by cardiac output (CO), total peripheral resistance (TPR), and blood volume:

• Short-term regulation: Neural and hormonal controls alter CO and TPR.

• Long-term regulation: Renal mechanisms control blood volume.

Neural Controls

• Baroreceptor reflexes: Stretch receptors in carotid sinuses, aortic arch, and large arteries detect changes in BP and adjust vessel diameter and heart rate accordingly.

• Chemoreceptor reflexes: Respond to changes in blood CO2, pH, and O2 to adjust CO and TPR.

• Higher brain centers (hypothalamus, cortex) can influence BP during stress or exercise.

Hormonal Controls

• Epinephrine and norepinephrine increase CO and vasoconstriction.

• Angiotensin II stimulates vasoconstriction and release of aldosterone and ADH, increasing blood volume.

• Atrial natriuretic peptide (ANP) decreases blood volume and causes vasodilation.

• Antidiuretic hormone (ADH, vasopressin) conserves water and causes vasoconstriction.

Renal Mechanisms

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

• Indirect mechanism: Renin-angiotensin-aldosterone system (RAAS) increases BP by vasoconstriction and increasing blood volume.

Capillary Exchange and Bulk Flow

Capillary Transport Mechanisms

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

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

• Bulk flow: Movement of fluid driven by pressure differences.

Bulk Flow: Filtration and Reabsorption

• Fluid is forced out of capillaries at the arterial end (filtration) and most returns at the venous end (reabsorption).

• Bulk flow is determined by the balance of hydrostatic and osmotic pressures:

• HPc: Capillary hydrostatic pressure (pushes fluid out)

• OPc: Capillary colloid osmotic pressure (pulls fluid in)

• HPi: Interstitial fluid hydrostatic pressure (usually negligible)

• OPi: Interstitial fluid colloid osmotic pressure (usually low)

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

Regulation of Blood Flow

Intrinsic and Extrinsic Controls

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

• Extrinsic: Sympathetic nervous system and hormones adjust blood flow to maintain MAP, often by redirecting blood from less active to more active tissues (e.g., during exercise).

Water Balance and ADH

Water Intake and Output

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

• Osmolality is maintained around 280–300 mOsm/kg.

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

Regulation of Water Output

• Obligatory water losses include insensible loss (skin, airways) and sensible loss (urine, sweat, feces).

• ADH increases water reabsorption in the kidneys, reducing urine output and increasing body fluid volume.

• ADH release is stimulated by increased osmolality or decreased blood volume/pressure.

Summary Table: Comparison of Arteries, Veins, and Capillaries

Example: During exercise, intrinsic controls dilate arterioles in skeletal muscle to increase blood flow, while extrinsic controls constrict arterioles in the digestive tract to maintain MAP.

Additional info: These notes are based on Marieb Human Anatomy & Physiology, Chapter 19, and are suitable for college-level Anatomy & Physiology students preparing for exams on the cardiovascular system and blood vessels.