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, dilate, 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).

• Tunica media: Middle layer, mostly smooth muscle and sheets of elastin. Responsible for vasoconstriction (decreasing diameter) and vasodilation (increasing diameter), regulated by sympathetic nerve fibers and chemicals. This layer is thickest in arteries.

• 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, its own blood supply (vasa vasorum).

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

Types of Blood Vessels

Arteries

• Elastic arteries: Thick-walled, near the heart (e.g., aorta). Large lumen, low resistance, act as pressure reservoirs. Contain more elastin, allowing expansion and recoil.

• Muscular arteries: Distribute blood to specific organs. Thickest tunica media, more smooth muscle, less elastic tissue. Actively change diameter to regulate blood flow.

• 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 are the primary sites for exchange between blood and tissues. Their thin walls consist of only endothelium, sometimes a single cell thick.

• 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 of fluids and small solutes.

• 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, 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 cells.

Veins

Veins carry blood from capillary beds back toward the heart. They have thinner walls and larger lumens than arteries, and often appear collapsed in histological sections. Veins are called capacitance vessels because they can hold up to 65% of the body's blood supply at any time.

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

• Veins: Formed by the joining of venules. Have all three tunics, but thinner walls and larger lumens. Adaptations include valves (prevent backflow, especially 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)

• Blood vessel length (longer = more resistance)

• Blood vessel diameter (smaller diameter = more resistance; resistance varies inversely with the fourth power of the radius)

Relationship:

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

Blood Pressure Regulation

Arterial Blood Pressure

• Systolic pressure: Peak pressure during ventricular contraction (average 120 mm Hg).

• Diastolic pressure: Lowest pressure during ventricular relaxation (average 80 mm Hg).

• Pulse pressure: Difference between systolic and diastolic pressure.

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

Venous Blood Pressure

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

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

Short- and Long-Term Regulation

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

• MAP is directly proportional to CO and TPR:

• CO is determined by stroke volume (SV) and heart rate (HR):

Therefore:

• 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 send signals to the cardiovascular center in the medulla to adjust vessel diameter, HR, and contractility.

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

• Higher brain centers: Hypothalamus and cortex can influence BP during stress, exercise, and temperature changes.

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 blood volume and causes vasodilation.

• Antidiuretic hormone (ADH/vasopressin): Promotes water retention and vasoconstriction.

Renal Mechanisms

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

• Indirect 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.

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) transported via endocytosis/exocytosis.

• Intercellular clefts/fenestrations: Passage of water-soluble substances (e.g., amino acids, sugars).

Bulk Flow

Bulk flow is the movement of fluid across capillary walls, driven by hydrostatic and osmotic pressures. It determines the relative fluid volumes in blood and interstitial space.

• Hydrostatic pressure (HP): Force exerted by fluid against a wall. Capillary HP pushes fluid out; interstitial HP pushes fluid in (usually negligible).

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

Net filtration pressure (NFP) determines the direction of fluid movement:

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

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

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

Regulation of Blood Flow Through Tissues

Intrinsic and Extrinsic Controls

• Intrinsic (autoregulation): Local control by tissues to match blood flow to metabolic needs, primarily by adjusting arteriole diameter.

• Extrinsic: Sympathetic nervous system and hormones adjust blood flow to maintain MAP, often by reducing flow to less active regions.

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

Water Balance and ADH

Water Intake and Output

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

• Intake: Beverages, food, metabolic water.

• Output: Urine, insensible loss (skin, lungs), sweat, feces.

Osmolality is maintained around 280–300 mOsm. Increases stimulate thirst and ADH release; decreases inhibit them.

Role of Antidiuretic Hormone (ADH)

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

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

• Decreased ADH leads to increased urine output and decreased body fluid volume.

Summary Table: Comparison of Arteries, Veins, and Capillaries