Cardiovascular & Lymphatic Histology – Comprehensive Bullet Notes

Course Structure

• Midterm coverage:
  • Cardiovascular System
  • Lymphatic System
  • Digestive System
  • Integumentary System
• Final-term coverage:
  • Respiratory System
  • Urinary System
  • Male Reproductive System
  • Female Reproductive System
  • Endocrine System

Foundational Definitions

• Cell – basic structural & functional unit of all organisms.
• Tissue – aggregate of cells with similar structure & function.
• Organ – fully differentiated unit specialized for a particular function.
• System – group of physiologically or anatomically related organs/parts.
• Systemic Histology – microscopic study of tissues & organs approached system-by-system.

Cardiovascular System – Global Overview

• Subdivided into two functional parts:
  1. Blood vascular system
  • Distributes nutrients, gases, hormones; removes metabolic waste.
  • Contains a continuum of vessels (arteries → arterioles → capillaries → venules → veins) + a muscular pump (heart).
  • Circulating fluid = blood.
  1. Lymph vascular system (detailed later)

Blood Vascular System – Hierarchy of Vessels (visual “tree” in slides)

• Large (elastic) artery → Muscular artery → Arteriole → Metarteriole → Capillary bed → Post-capillary venule → Venule → Medium-sized vein → Large vein
• Arteriovenous anastomoses allow direct artery-to-vein shunts.
• Capillary subtypes: continuous, fenestrated (± diaphragms), sinusoidal.

Lymph Vascular System – Overview

• Collects excess tissue fluid → returns to blood system.
• Components:
  • Blind-ended lymphatic capillaries.
  • Conduits (lymphatic vessels) that connect to venous angles.
  • Lymphoid organs (e.g., lymph nodes).
• Circulating fluid = lymph (composition in small vessels ≈ interstitial fluid).

General Organization of Blood-Vessel Wall

1. Tunica intima (luminal)
   • Endothelium: simple squamous cells; tight-junctions; Weibel-Palade bodies storing factor VIII.
   • Subendothelial loose fibrous connective tissue (FECT); scattered smooth muscle in large vessels.
   • Internal elastic membrane (IEM) demarcates intima–media junction (distinct in most muscular arteries).
2. Tunica media
   • Concentric layers of either elastic laminae or circular smooth muscle.
   • May house elastic lamellae, collagen, fibroblasts, amorphous ECM.
   • External elastic membrane (EEM) – if present – separates media from adventitia.
3. Tunica adventitia
   • Loose → moderately dense FECT; ± longitudinal smooth muscle.
   • Vasa vasorum (blood supply to vessel wall) & nervi vasorum (sympathetic plexus) reside here in large vessels.

Elastic (Conducting) Arteries

• Examples: aorta, pulmonary trunk, brachiocephalic, carotids.
• Diameter: largest in body; wall built to dampen pressure oscillations.
• Tunica intima
  • Relatively thin vs. other layers.
  • Endothelial “brick-shaped” cells.
  • Thick subendothelial layer in large animals; smooth muscle + longitudinal collagen & fine elastic fibers.
  • IEM present but difficult to distinguish (blends with elastic-rich media).
• Tunica media (dominant layer)
  • $40!–!70$ concentric, fenestrated elastic laminae/lamellae.
  • Interlamellar matrix: fibroblasts, elastic & collagen fibers, spiral→circular smooth muscle, ground substance.
• Tunica adventitia
  • Thin relative to diameter; loose→moderately dense FECT; vasa vasorum & nervi vasorum present.
• Functional significance
  • Windkessel effect: during systole elastic wall stretches; during diastole recoil converts stored energy → hydrostatic pressure maintaining continuous flow.

Muscular (Distributing) Arteries

• Examples: brachial, radial, femoral, superior mesenteric.
• Tunica intima
  • Thin endothelium + scant subendothelial CT.
  • IEM very prominent, often wavy/folded.
• Tunica media (thick)
  • $3!–!40$ circular smooth-muscle layers.
  • Small amounts of collagen & longitudinal elastic fibers.
  • Thickness diminishes with decreasing vessel caliber.
  • External elastic lamina may be obvious in large-medium muscular arteries, indistinct in smaller ones.
• Tunica adventitia
  • Thick loose FECT.
• Function: distribute blood to regional, smaller arteries and protect from high systemic pressure.

Arterioles

• Diameter: ≤ $0.5\,\text{mm}$.
• Tunica intima: endothelium only; IEM usually present except in the smallest.
• Tunica media: $1!–!3$ smooth-muscle layers ± elastic fibers.
• Tunica adventitia: very thin longitudinal collagen & elastic fibers.
• Physiologic role
  • Major regulators of systemic blood pressure (change radius → alter peripheral resistance; Poiseuille’s law $R\propto\tfrac{1}{r^4}$).
  • Govern flow distribution into capillary beds.

Metarterioles & Pre-capillary Sphincters

• Intermediate between arteriole & capillary.
• Incomplete single layer of smooth muscle scattered along wall.
• Pre-capillary sphincter = circumferential ring of smooth muscle at capillary origin that can shut down local flow → mechanism for thermoregulation & blood-flow redistribution.

Capillaries

• Structure: diameter ≈ RBC; consists of endothelial tube ± pericytes (contractile cells sharing basal lamina).
• Continuous Capillaries (most organs, muscle, CNS)
  • Thick endothelial cytoplasm, continuous basal lamina.
  • Tight junctions (zonulae occludentes) restrict paracellular passage; transport via transcytosis/diffusion.
• Fenestrated (Visceral) Capillaries
  • Extremely thin cytoplasm; pores $60!–!80\,\mu m$ with diaphragms (exception: renal glomerulus—no diaphragms).
  • Found in GI tract, endocrine glands, renal cortex.
• Sinusoidal (Discontinuous) Capillaries
  • Larger diameter ≈ $30\,\mu m$, tortuous lumen, large intercellular gaps.
  • Discontinuous or absent basal lamina; allows free passage of macromolecules & cells.
  • Locations: liver, spleen, bone marrow, certain endocrine organs.
  • Often associated with resident macrophages (e.g., Kupffer cells in liver).
• Functional Summary
  • Primary site for exchange of gases, nutrients, metabolic waste.
  • Leucocyte diapedesis possible (more common in venules).

Venous Side of Circulation

Venules

1. Post-capillary venules (≈ $10\,\mu m$)
   • Endothelium + pericytes.
   • Highly responsive to vasoactive mediators (histamine, serotonin) → ↑ permeability (source of tissue edema).
   • Main site for leukocyte extravasation.
2. Muscular venules (up to $1\,\text{mm}$)
   • Tunica intima: endothelium + outer collagen sheath.
   • Tunica media: $1!–!3$ circular smooth-muscle layers with collagen/elastic fibers.
   • Tunica adventitia: thick; longitudinal collagen, elastic fibers, fibroblasts.
   • Function: collect blood from post-capillary venules.

Small & Medium Veins

• Tunica intima
  • Endothelium + thin subendothelial layer; may invaginate as valves (paired folds of intima reinforced by fibro-elastic core) to prevent retrograde flow.
• Tunica media: thin circular smooth muscle + collagen; few elastic fibers.
• Tunica adventitia: well developed; longitudinal collagen & elastic fibers, occasional longitudinal smooth-muscle bundles.
• Function: conduit toward larger veins; capacitance reservoir.

Large Veins (e.g., superior & inferior venae cavae)

• Tunica intima: thicker; endothelium + thin subendothelial CT; valves present only in lower-pressure large veins.
• IEM usually recognizable.
• Tunica media: thin, mostly CT; sparse smooth muscle.
• Tunica adventitia (dominant layer)
  • Very thick, moderately dense CT with spirally arranged collagen, elastic laminae.
  • Prominent longitudinal smooth-muscle bundles → reinforce wall against distention (blood at low pressure but high volume).
• Role: final collectors returning blood to heart.

Artery vs. Vein – Key Contrasts

• Pressure: arteries high; veins low.
• Tunica media:
  • Arteries dominate by smooth muscle/elastic laminae; arranged circularly.
  • Veins contain more collagen; components oriented longitudinally.
• Elastic infrastructure: highly developed in arterial media (especially elastic arteries); sparse in veins.
• Valves: present only in veins (especially limbs) to prevent backflow.

Lymphatic Vessels

Lymph Capillaries

• Blind-ended; endothelial wall lacks continuous tight junctions & basal lamina.
• Resemble post-capillary venules; anchoring filaments tether wall to ECM preventing collapse during interstitial edema.
• Function: absorb excess interstitial fluid, macromolecules, & cells.

Small / Medium Lymphatic Vessels

• Structural echo of veins of next smaller size.
• Smaller collecting vessels
  • Endothelium + collagen, elastic fibers, few smooth-muscle cells.
• Medium collecting vessels
  • Tunica intima: thin; may form valves.
  • Tunica media: thin; helically arranged smooth muscle + elastic fibers.
  • Tunica adventitia: thicker CT.
• Function: drain lymph from capillaries.

Large Lymphatic Vessels (Thoracic duct, Right lymphatic duct)

• Tunica intima: thin endothelium; subendothelial collagen/elastic fibers + longitudinal smooth muscle.
• Tunica media: thickest layer; longitudinal & circular smooth-muscle bundles within loose FECT (resembling medium vein wall).
• Tunica adventitia: poorly developed; coarse collagen ± longitudinal smooth muscle.
• Function: return lymph to venous angles (junction of internal jugular & subclavian veins).

General Notes on Lymphatics

• Lumen often appears empty or contains lightly eosinophilic proteinaceous fluid ± lymphocytes; never RBCs under normal conditions.
• Intrinsic segmental contractions of smooth muscle plus surrounding skeletal-muscle squeeze & one-way valves propel lymph.

Integrated Functional & Clinical Correlates

• Windkessel Function: Elastic recoil maintains diastolic BP; loss of elasticity (arteriosclerosis) → widened pulse pressure.
• Peripheral Resistance: Arteriolar constriction/dilation key target for antihypertensives (e.g., α-agonists, calcium-channel blockers).
• Edema & Inflammation: Post-capillary venules and lymphatics regulate fluid balance; histamine-mediated gaps → plasma leakage → swelling.
• Atherosclerosis: Begins in tunica intima (fatty streak) of large & medium arteries; importance of endothelial integrity.
• Varicose Veins: Valve incompetence → backflow & venous distention; adventitial smooth muscle unable to offset prolonged hydrostatic load.
• Lymphedema: Obstruction of lymphatics (e.g., filarial infection, post-mastectomy) → tissue swelling due to impaired drainage.

Numeric & Structural Quick Reference (all dimensions approximate)

• Elastic laminae in aorta: $40!–!70$ layers.
• Fenestration pore diameter: $60!–!80\,\mu m$.
• Sinusoidal capillary diameter: $\sim 30\,\mu m$; larger & irregular.
• Post-capillary venule lumen: $\ge 10\,\mu m$.
• Muscular venule diameter: up to $1\,\text{mm}$.
• Arteriole diameter: $\le 0.5\,\text{mm}$; media $1!–!3$ muscle layers.
• Metarteriole smooth-muscle cuff: incomplete single layer.

Layer-by-Layer “Cheat Sheet”

• Elastic Artery
  • Intima: thick subendothelium; indistinct IEM.
  • Media: concentric elastic lamellae >> smooth muscle.
  • Adventitia: thin.
• Muscular Artery
  • Intima: wavy IEM.
  • Media: multi-layer circular smooth muscle.
  • Adventitia: substantial.
• Arteriole
  • Intima: minimal; ± IEM.
  • Media: $1!–!3$ muscle layers.
  • Adventitia: sparse.
• Capillary: only endothelium ± pericytes.
• Post-capillary Venule: endothelium + pericytes, no media.
• Muscular Venule/Small Vein
  • Intima: thin.
  • Media: thin circular smooth muscle.
  • Adventitia: thick CT.
• Large Vein
  • Intima: valves.
  • Media: scant.
  • Adventitia: thick with longitudinal muscle.
• Lymphatic Vessel: similar to veins but thinner wall, more valves, no RBCs.