recognise arteries, veins and capillaries from microscope slides, photomicrographs and electron micrographs and make plan diagrams showing the structure of arteries and veins in transverse section (TS) and longitudinal section (LS)

Objective

By the end of this lesson you should be able to:

• Recognise arteries, veins and capillaries on microscope slides, photomicrographs and electron micrographs.
• Draw accurate plan diagrams of arteries and veins in transverse section (TS) and longitudinal section (LS).
• Explain the structural differences that relate to their functions.

1. Overview of the Circulatory System

The circulatory system transports nutrients, gases and waste products. It consists of three main vessel types:

1. Arteries – carry blood away from the heart under high pressure.
2. Veins – return blood to the heart under low pressure.
3. Capillaries – sites of exchange between blood and tissues.

Key functional relationships

Blood pressure (\$P\$) can be expressed as:

\$P = \frac{F}{A}\$

where \$F\$ is the force exerted by the heart and \$A\$ is the cross‑sectional area of the vessel. Arteries have smaller lumens and thicker walls, giving a higher \$P\$; veins have larger lumens and thinner walls, giving a lower \$P\$.

2. Microscopic Identification

2.1. Typical slide preparation

• Fixation (e.g., formalin)
• Embedding in paraffin
• Sectioning at 5–10 µm thickness
• Staining (e.g., H&E)

2.2. Features to look for

2.3. Photomicrograph clues

• Arteries: bright pink tunica media, clear elastic lamina, narrow lumen.
• Veins: larger, pale lumen, visible valves, thin wall.
• Capillaries: barely visible wall, often seen as a thin line or “halo” around red blood cells.

2.4. Electron micrograph clues

• Arteries: abundant smooth muscle cells, dense collagen bundles, internal elastic lamina as a distinct electron‑dense layer.
• Veins: fewer muscle cells, larger intercellular gaps, valves composed of endothelial cells with overlapping flaps.
• Capillaries: single layer of endothelial cells, basement membrane, pericytes on the abluminal side.

3. Drawing Plan Diagrams

3.1. General guidelines

1. Use a ruler for straight lines; keep scale consistent (e.g., 1 mm = 1 cm on paper).
2. Label each layer: tunica intima, tunica media, tunica externa (adventitia).
3. Indicate lumen, elastic laminae, smooth muscle bundles, valves (vein) and any visible nerves.
4. Use different line styles (solid, dashed) for different tissues.

3.2. Transverse Section (TS) – Artery

• Draw a small circular lumen.
• Surround it with a thin inner line (tunica intima) and a distinct internal elastic lamina.
• Next, draw several concentric rings of thick muscle fibres (tunica media).
• Outside this, add a thin outer layer (tunica externa) with scattered collagen fibres.

3.3. Transverse Section (TS) – Vein

• Draw a larger, often oval lumen.
• Include a thin tunica intima; internal elastic lamina is usually absent.
• Show a few scattered muscle fibres in the tunica media.
• Depict one or two valve leaflets projecting into the lumen.
• Enclose with a relatively thick tunica externa containing connective tissue.

3.4. Longitudinal Section (LS) – Artery

• Represent the lumen as a long tube.
• Show concentric layers as parallel bands: inner endothelial line, elastic lamina, thick muscle band, outer connective tissue.
• Indicate the direction of blood flow with an arrow.

3.5. Longitudinal Section (LS) – Vein

• Draw a wider tube.
• Include occasional valve flaps as triangular projections from the wall into the lumen.
• Show thin muscle layer and a relatively thick outer connective tissue layer.

4. Summary Table – Comparison of \cdot essel Types

5. Self‑Assessment Questions

1. List three microscopic features that distinguish an artery from a vein in a transverse section.
2. Why are valves present in veins but not in arteries?
3. Describe how the structure of a capillary wall facilitates exchange.
4. When viewing an electron micrograph, what characteristic indicates you are looking at a vein?
5. Sketch a longitudinal section of an artery, labeling at least four distinct layers.

6. Further Reading (Suggested Topics)

• Blood pressure regulation and the role of arterial compliance.
• Clinical significance of varicose veins and deep vein thrombosis.
• Microcirculation and the role of precapillary sphincters.