Learning Objectives
- Identify arteries, arterioles, capillaries, venules and veins on light‑microscope slides, photomicrographs and electron micrographs.
- Recognise the four principal vessels of the systemic and pulmonary circuits (aorta, pulmonary artery, pulmonary vein, vena cava).
- Draw accurate plan (cross‑section) and longitudinal diagrams of arteries and veins, labelling the three tunics (tunica intima, tunica media, tunica externa) and any special structures (elastic laminae, valves).
- Explain how the structure of each vessel type relates to its function, including the role of water in blood and the formation of tissue fluid.
- Identify the major blood‑cell types (RBC, neutrophil, monocyte, lymphocyte) in microscopic images.
1. The Mammalian Circulatory System
1.1. Closed double circulation
In mammals the heart pumps blood through two separate, closed circuits:
| Circuit | Direction of flow | Major vessels (away from heart) | Major vessels (toward heart) |
|---|
| Systemic | Left ventricle → body → right atrium | aorta (elastic artery) | vena cava (superior & inferior) |
| Pulmonary | Right ventricle → lungs → left atrium | pulmonary artery (elastic artery) | pulmonary vein (thin‑walled vein) |
1.2. Main components of the transport system
- Heart – muscular pump that creates the pressure gradient driving blood flow.
- Blood – ~90 % water; plasma contains albumin, globulins and fibrinogen; cells (RBC, WBC, platelets) account for the remaining ~10 %.
- Blood vessels – a continuous network of arteries, arterioles, capillaries, venules and veins.
2. Vessel Types – Structure, Function and Microscopic Identification
2.1. Overview of the six vessel categories
| Vessel type | Typical lumen diameter | Relative wall composition (tunics) | Principal functional role |
|---|
| Elastic artery (e.g., aorta) | Large (≈10 mm) | Intima thin; prominent internal elastic lamina; very thick media rich in elastic fibres and smooth muscle; distinct external elastic lamina; thin externa. | Absorbs pressure pulses and maintains continuous flow. |
| Muscular artery (e.g., brachial artery) | Medium (≈2–5 mm) | Thin intima; clear internal elastic lamina; thick media dominated by smooth‑muscle cells; little external elastic lamina; thin externa. | Regulates blood flow to specific regions by vasoconstriction/dilation. |
| Arteriole | Very small (10–30 µm) | Thin intima; faint/internal elastic lamina; 1–2 layers of smooth‑muscle in the media; very thin externa. | Principal site of resistance; controls arterial pressure. |
| Capillary | 5–10 µm (just wide enough for one RBC) | Single endothelial layer (intima) + basement membrane; no media or externa; pericytes on the abluminal side. | Site of exchange of gases, nutrients, wastes and many hormones. |
| Venule | 15–30 µm | Thin intima; few smooth‑muscle cells in a very thin media; thin externa. | Collects blood from capillary beds and begins low‑pressure return to the heart. |
| Large vein (e.g., vena cava) | Large, often oval | Thin intima; internal elastic lamina absent or indistinct; scant smooth‑muscle in the media; thick collagen‑rich externa; valves formed from endothelium. | Returns blood to the heart under low pressure; valves prevent back‑flow. |
2.2. Comparison of vessel types (Cambridge‑style summary)
| Feature | Elastic artery | Muscular artery | Arteriole | Capillary | Venule | Large vein |
|---|
| Wall thickness (relative to lumen) | Very thick | Thick | Thin‑moderate | Very thin (single endothelium) | Thin | Thin |
| Lumen size | Small | Medium | Very small (10–30 µm) | 5–10 µm (fits one RBC) | 15–30 µm | Large, often oval |
| Elastic laminae | Prominent internal & external | Prominent internal, faint external | Faint internal (often invisible) | Absent | Absent | Absent (or very faint internal) |
| Main functional role | Buffers pressure pulses | Regulates regional blood flow | Controls total peripheral resistance | Exchange of gases, nutrients, wastes | Collects capillary blood | Returns blood to heart; valves prevent reflux |
2.3. Microscopic clues for each vessel type
2.3.1. Light‑microscope (H&E) slides
- Elastic artery: Bright pink, very thick wall; multiple concentric smooth‑muscle layers; distinct thin eosinophilic internal elastic lamina; lumen relatively small.
- Muscular artery: Similar pink wall but muscle layers dominate; internal elastic lamina visible; external elastic lamina faint.
- Arteriole: Narrow lumen; 1–2 thin smooth‑muscle layers; internal elastic lamina may appear as a faint line.
- Capillary: Extremely thin wall; appears as a faint halo around a single RBC; no smooth muscle.
- Venule: Slightly larger lumen than arterioles; thin wall with few muscle fibres; endothelial nuclei often more prominent.
- Large vein: Wide, often oval lumen; thin wall; valve leaflets visible as thin, translucent flaps projecting into the lumen; thick outer collagenous layer.
2.3.2. Photomicrographs (digital images)
- Arteries – intense pink tunica media, crisp internal elastic lamina, narrow circular lumen.
- Veins – pale pink wall, wide lumen, visible valve leaflets (“flap‑like” structures).
- Capillaries – thin dark lines or “halo” around RBCs; sometimes only endothelial nuclei are seen.
2.3.3. Electron micrographs
- Elastic artery: Electron‑dense internal elastic lamina, concentric smooth‑muscle cells, abundant elastic fibres, thick basal lamina.
- Muscular artery: Similar but with more contractile filaments and fewer elastic fibres.
- Arteriole: Thin smooth‑muscle layer, faint elastic lamina, tight endothelial junctions.
- Capillary: Single endothelial cell layer, ~50 nm basement membrane, pericyte processes abluminally, numerous pinocytic vesicles.
- Venule: Endothelial cells with larger intercellular gaps, sparse smooth muscle, thin basal lamina.
- Vein (valve leaflet): Overlapping endothelial cells forming a thin flap; few smooth‑muscle cells; thick collagenous adventitia.
2.4. Recognising the major blood‑cell types
| Cell type | Key H&E features | Electron‑microscopic clues |
|---|
| Red blood cell (RBC) | Biconcave disc, pink cytoplasm, no nucleus, uniform ~7 µm diameter. | Thin flexible membrane, central pallor, absence of organelles. |
| Neutrophil | Multi‑lobed nucleus, pale pink granular cytoplasm. | Numerous lysosome‑like granules; segmented nucleus. |
| Monocyte | Large, kidney‑shaped nucleus; abundant grey‑blue cytoplasm. | Large vacuoles, occasional phagolysosomes. |
| Lymphocyte | Small, round nucleus occupying most of the cell; scant cytoplasm. | Dense chromatin, few organelles. |
3. Drawing Plan (Cross‑Section) and Longitudinal Diagrams
3.1. General drawing guidelines
- Choose a consistent scale (e.g., 1 cm = 1 mm) and use a ruler for straight lines.
- Label the three tunics exactly as tunica intima, tunica media, tunica externa. Add internal elastic lamina and external elastic lamina where appropriate.
- Show the lumen as an empty space; indicate direction of blood flow with an arrow.
- Use different line styles or colours for each tissue layer (solid = intima, dashed = media, dotted = externa, etc.).
- For veins, draw at least one valve leaflet; for arteries, illustrate the thickness of the media with several concentric rings.
3.2. Transverse (plan) diagrams
3.2.1. Elastic artery (e.g., aorta)
- Small, roughly circular lumen.
- Thin inner line = tunica intima (endothelium).
- Thin bright line = internal elastic lamina.
- Multiple thick concentric bands = thick tunica media (smooth muscle + elastic fibres).
- Faint outer line = external elastic lamina.
- Outer thin band = tunica externa (collagen).
3.2.2. Muscular artery (e.g., brachial artery)
- Circular lumen, slightly larger than in elastic arteries.
- Thin tunica intima.
- Clear internal elastic lamina.
- 3–4 thick concentric bands of smooth‑muscle = tunica media.
- Thin tunica externa of connective tissue.
3.2.3. Arteriole
- Very small, circular lumen.
- Thin tunica intima.
- Faint internal elastic lamina (often a thin line).
- One or two thin layers of smooth‑muscle = tunica media.
- Very thin tunica externa.
3.2.4. Capillary
- Extremely narrow lumen (just fits one RBC).
- Single thin line = endothelial layer (tunica intima).
- Surrounding halo = basement membrane.
- No media or externa.
3.2.5. Venule
- Oval or slightly irregular lumen.
- Thin tunica intima.
- Scattered smooth‑muscle cells = very thin tunica media.
- Thin tunica externa.
3.2.6. Large vein (e.g., vena cava)
- Large, often oval lumen.
- Thin tunica intima.
- Internal elastic lamina absent or indistinct.
- Few smooth‑muscle fibres in a thin tunica media.
- One or more thin valve leaflets projecting into the lumen.
- Relatively thick, collagen‑rich tunica externa.
3.3. Longitudinal diagrams
3.3.1. Elastic artery (longitudinal view)
- Long tube representing the lumen.
- Inner thin line = tunica intima.
- Parallel band = internal elastic lamina.
- Broad dark band = thick tunica media (concentric smooth‑muscle + elastic fibres).
- Outer thin band = external elastic lamina (optional).
- Outermost band = tunica externa (collagen).
- Arrow showing blood flow away from the heart.
3.3.2. Muscular artery (longitudinal view)
- Similar to elastic artery but the media band is narrower and shows distinct bundles of smooth‑muscle fibres.
3.3.3. Vein (longitudinal view)
- Wide tube with thin wall.
- Inner line = tunica intima.
- Scattered smooth‑muscle cells in a thin media band.
- One or more triangular valve flaps drawn as thin projections from the intima into the lumen.
- Thick outer band = tunica externa.
- Arrow indicating blood flow toward the heart.
4. Water, Blood and Tissue Fluid
- Water as the main plasma component – ≈90 % of plasma; provides a solvent for ions, nutrients and gases, carries heat, and contributes to blood volume.
- Plasma proteins – albumin (maintains osmotic pressure), globulins (immune function), fibrinogen (clotting).
- Specific heat capacity of water – stabilises body temperature during rapid heat exchange in capillary beds.
- Formation of tissue fluid (interstitial fluid) – Plasma filters out of capillaries under hydrostatic pressure; most of it re‑enters the capillary under colloid osmotic pressure (Starling forces).
- Functions of tissue fluid – Delivers oxygen and nutrients to cells, removes metabolic wastes, and serves as the medium for immune‑cell movement.
5. Summary – Quick Reference Sheet
| Vessel | Key tunics & special structures | Typical lumen size | Main function |
|---|
| Elastic artery | Intima + prominent internal & external elastic laminae + very thick media (elastic + smooth muscle) + thin externa | Large (≈10 mm) | Buffers pressure pulses; maintains continuous flow. |
| Muscular artery | Intima + clear internal elastic lamina + thick media (smooth muscle dominant) + thin externa | Medium (≈2–5 mm) | Regulates regional blood flow. |
| Arteriole | Intima + faint internal elastic lamina + 1–2 thin smooth‑muscle layers + very thin externa | 10–30 µm | Controls total peripheral resistance & arterial pressure. |
| Capillary | Single endothelial layer (intima) + basement membrane; pericytes present | 5–10 µm | Site of exchange of gases, nutrients, wastes, hormones. |
| Venule | Intima + very thin media (few smooth‑muscle cells) + thin externa | 15–30 µm | Collects blood from capillary beds; begins low‑pressure return. |
| Large vein | Intima + thin media + thick collagenous externa + valve leaflets (endothelial) | Large, often oval | Returns blood to heart; valves prevent back‑flow. |