Chapter 19 Blood Vessels – Arteries, Capillaries, Veins

Arteries: General Classification and Function

• Three primary arterial groups distinguished by diameter, wall composition, and hemodynamic role:

  • Elastic (conducting) arteries

  • Muscular (distributing) arteries

  • Arterioles (resistance vessels)

Elastic (Conducting) Arteries

• Location & examples: aorta and its major branches—closest vessels to the heart.

• Size range: 2.5\,\text{cm}\;\text{to}\;1\,\text{cm} in diameter (largest lumens among arteries).

• Structural highlights:

  • Wall contains highest elastin content of any vessel type; elastin present in all three tunics, but especially abundant in the tunica media.

  • Tunica media exhibits concentric “holey” elastic sheets (Swiss-cheese appearance) layered between smooth-muscle cells.

  • Smooth muscle present but relatively inactive in vasoconstriction—vessel acts like a passive elastic tube.

• Functional role = “pressure reservoirs”:

  • Expand during ventricular systole, recoil during diastole, smoothing pulsatile output of the heart → near-continuous downstream flow.

  • Analogy: compliant garden hose—without elasticity (e.g., atherosclerosis), flow becomes intermittent, downstream pressures spike, predisposing vessels to aneurysm or rupture.

• Pathophysiological note: Loss of elasticity (atherosclerosis) → higher systolic pressures, vessel wall stress, potential aneurysm (ballooning) or rupture.

Muscular (Distributing) Arteries

• Distal to elastic arteries; deliver blood to specific organs (hence “distributing”).

• Laboratory relevance: comprise most named arteries (e.g., femoral, brachial).

• Diameter range: roughly that of a little finger to a pencil lead.

• Structural hallmarks:

  • Thickest tunica media relative to lumen size of any vessel type.

  • High smooth-muscle content, reduced elastin; elastic membrane on each side of tunica media (internal & external elastic laminae).

• Function:

  • Highly capable of vasoconstriction/vasodilation → regulate regional blood distribution.

  • Less stretchable than elastic arteries.

• “Thick-wall” fact: despite smaller overall caliber than elastic arteries, muscular arteries are mechanically stronger in active diameter control.

Arterioles (Resistance Vessels)

• Smallest arteries; diameter 0.3\,\text{mm}\;\text{to}\;10\,\mu\text{m}.

• Structural gradient:

  • Larger arterioles possess all three tunics; tunica media dominated by smooth muscle with sparse elastin.

  • Terminal arterioles approaching capillary beds may consist of single smooth-muscle cell layer spiraling around endothelium.

• Functional role:

  • Major determinants of peripheral resistance and minute-to-minute flow into individual capillary beds.

  • Respond to neural (sympathetic), hormonal (e.g., epinephrine, angiotensin II), and local chemical factors (e.g., \text{O}2, \text{CO}2, H⁺).

  • Constriction → tissues bypassed; dilation → dramatic local perfusion increase.

• Clinical/physiological tie-in: critical regulator of systemic blood pressure via total peripheral resistance (TPR).

Capillaries: Microscopic Exchange Vessels

• Smallest blood vessels; average length \approx 1\,\text{mm}, lumen diameter 8\text{–}10\,\mu\text{m} (RBCs pass single-file).

• Wall composition: single endothelial cell layer (tunica intima) + basement membrane.

• Pericytes:

  • Spider-shaped contractile stem cells on outer surface.

  • Functions: stabilize wall, regulate permeability, contribute to vessel repair or scar formation.

• Tissue distribution:

  • Rich networks in metabolically active tissues.

  • Poorly vascularized: tendons, ligaments (slow healing).

  • Avascular: cartilage, epithelium—receive nutrients via diffusion from neighboring connective tissue; cornea & lens nourished by aqueous humor.

• Analogy: if arteries/arterioles = highways, capillaries = back alleys/driveways providing direct cellular access.

• Primary role = exchange of gases, nutrients, hormones, and wastes between blood and interstitial fluid.

Structural Types of Capillaries

• All capillaries share tight junctions between endothelial cells, but junctions are often incomplete, creating intercellular clefts for limited passage.

Continuous Capillaries

• Least permeable, most common; found in skin, muscles, lungs, CNS.

• Features:

  • Continuous endothelial lining with small intercellular clefts.

  • Pinocytotic vesicles shuttle fluids across cells.

  • In brain: tight junctions are continuous → no clefts → anatomical basis of blood-brain barrier.

• Associated pericytes present.

Fenestrated Capillaries

• Have large fenestrations (pores) that increase permeability.

• Occur in areas of active filtration (kidney glomeruli), absorption (small intestine), and endocrine secretion.

• Fenestrations resemble Swiss-cheese holes, usually covered by thin glycoprotein diaphragm.

• In digestive tract, number of fenestrations ↑ during active nutrient absorption.

Sinusoid (Discontinuous) Capillaries

• Most permeable; limited distribution (liver, bone marrow, spleen, adrenal medulla).

• Characteristics:

  • Large irregular lumens, tortuous channels → slow blood flow.

  • Large intercellular clefts and fenestrations; few tight junctions.

  • Incomplete or absent basement membrane.

  • Permit passage of large molecules & cells (e.g., plasma proteins, blood cells).

  • Macrophages may extend processes into lumen (liver Kupffer cells form part of wall).

Capillary Beds & Microcirculation

• Microcirculation: arteriole → capillary bed → venule pathway.

• Typical arrangement:

  • A single terminal arteriole branches into 10\text{–}20 true capillaries.

  • Drain into post-capillary venule.

• Blood-flow regulation:

  • Diameter of terminal arteriole + upstream arterioles.

  • Influenced by local chemicals (metabolic signals) & arteriolar vasomotor nerve fibers.

  • Capillary bed may be fully perfused or largely bypassed depending on tissue demand.

• Physiological scenarios:

  • Post-meal rest: GI capillaries open to absorb nutrients.

  • Vigorous exercise: GI capillaries close; skeletal muscle capillaries open → explains post-meal cramps during exercise.

Special Mesenteric Arrangement

• Vascular shunt (metarteriole + thoroughfare channel) directly connects terminal arteriole to post-capillary venule.

• True capillaries branch off metarteriole; each entry guarded by precapillary sphincter (smooth-muscle cuff).

  • Sphincters respond ONLY to local chemical conditions; not innervated.

  • States:

  • Sphincters open → blood flows through true capillaries (diagram 19.4a).

  • Sphincters closed → blood bypasses capillaries via shunt (diagram 19.4b).

Venous System: Return & Reservoir Functions

• Overall pathway: capillaries → venules → veins → heart.

• Progressive increase in lumen diameter and wall thickness from venules to large veins.

Venules

• Formed by convergence of capillaries; diameter 8\text{–}100\,\mu\text{m}.

• Post-capillary venules:

  • Consist solely of endothelium plus pericytes.

  • Highly porous—fluid & WBCs readily pass through (important in inflammation; WBCs adhere then migrate through wall).

• Larger venules: possess 1–2 layers of smooth muscle in tunica media; thin tunica externa.

Veins

• Formed by union of venules; three tunics present, but:

  • Walls thinner, lumens larger than corresponding arteries (often appear collapsed histologically).

  • Tunica media: little smooth muscle or elastin; poorly developed.

  • Tunica externa (adventitia): thickest layer, composed of longitudinal collagen bundles & elastic fibers; in venae cavae, contains additional longitudinal smooth-muscle bands.

• Hemodynamic role:

  • Capacitance vessels/blood reservoirs—can hold ≈ 65\% of total blood volume (Figure 19.6).

  • Large lumens + high distensibility allow volume accommodation at low pressure.

• Pressure considerations:

  • Venous pressure low; hence thin walls safe from rupture.

  • Low pressure necessitates adaptations for efficient return to heart.

Venous Adaptations for Return

1. Large-diameter lumens → minimal resistance.

2. Venous valves:

   • Intimal folds resembling heart semilunar valves; prevent backflow.

   • Most abundant in limbs where gravity opposes upward flow.

   • Absent in thoracic & abdominal cavities.

   • Demonstration: Manual emptying of a dorsal hand vein followed by collapse persistence until distal finger is released.

3. (Mentioned elsewhere in text) Muscular pump, respiratory pump, sympathetic venoconstriction—assist return (contextual connection to future chapters).

Venous Sinuses

• Specialized, flattened veins composed solely of endothelium; supported by surrounding tissues.

• Examples (not listed explicitly in excerpt but classically): coronary sinus, dural venous sinuses.

Clinical Correlation: Varicose Veins & Hemorrhoids (Homeostatic Imbalance 19.1)

• Definition: dilated, tortuous veins due to incompetent (leaky) valves.

• Prevalence: >15\% of adults; typically lower limbs.

• Predisposing factors:

  • Heredity; prolonged standing; obesity; pregnancy.

  • Elevated intra-abdominal pressure (pregnancy “potbelly,” obesity) → impaired venous return, valve weakening, wall stretching.

  • Superficial veins more vulnerable (limited surrounding support).

• Hemorrhoids: varicosities of anal veins from straining during defecation or childbirth (↑ intra-abdominal pressure).

Check-Your-Understanding Prompts (Self-Test)

• Arterial types matching functions:

  1. Dampen pulsatile pressure → Elastic arteries.

  2. Local flow control via vasodilation/constriction → Arterioles.

  3. Thickest tunica media relative to lumen → Muscular arteries.

• Capillary bed state in exercising calf (Figure 19.4): arterioles dilated, sphincters open → diagram (a).

Integrative / Foundational Connections

• Elastic recoil concept parallels Windkessel effect in hemodynamics.

• Arteriole resistance control is foundational to systemic blood-pressure regulation (Ohm’s law: \Delta P = Q \times R).

• Blood-brain barrier structure in continuous CNS capillaries ties to neurophysiology (Chapter 12).

• Capillary exchange principles (Starling forces) to be discussed in later chapters; structural variations foreshadow functional differences in permeability.

• Clinical conditions (atherosclerosis, aneurysm, varicose veins) link anatomy to pathology and public health.