Compact vs Spongy Bone — Study Pack Summary & Study Notes

🦴 Overview

Compact bone and spongy bone are two structural types found in most bones. Compact bone is dense and solid, forming the outer shell and providing strength. Spongy bone (cancellous bone) forms a porous lattice that reduces weight and houses marrow.

🧩 Structure & Location

Compact bone is arranged into tightly packed cylinders called osteons and is found mainly in the diaphysis (shaft) of long bones. Spongy bone is composed of trabeculae, a network of plates and struts, and is concentrated in the epiphyses (ends) and interior of short and flat bones.

⚙️ Structural Units & Microscopic Features

The key unit of compact bone is the osteon (Haversian system) — concentric lamellae surrounding a central Haversian canal. Tiny spaces called lacunae house osteocytes, and canaliculi connect lacunae to allow nutrient transfer. Spongy bone lacks osteons; its trabeculae align along stress lines and contain marrow between them.

❤️ Blood Supply & Marrow

Compact bone receives blood via Haversian canals (longitudinal) and Volkmann's canals (transverse), which link to the periosteum. Spongy bone is supplied from the surrounding marrow cavity. Yellow marrow in compact regions stores fat; red marrow in spongy regions produces blood cells.

💪 Function & Mechanical Properties

Compact bone provides support, protection, and strong leverage for movement; it is heavy and very strong. Spongy bone reduces skeletal weight, absorbs shock, and stores marrow, making bones lighter and better at dissipating forces.

📝 Exam Tips — How to score full marks (10/10)

For each feature compare both bones clearly (e.g., Compact = osteons; Spongy = trabeculae). Aim to cover at least five features such as structure, unit, location, marrow type, blood supply, and function. Use clear paired statements: "Compact = X; Spongy = Y." Each correct feature stated earns marks.

🔑 Key Terms to Memorize

Osteon / Haversian system, Lamellae, Lacunae, Canaliculi, Haversian canal, Volkmann's canals, Trabeculae, Periosteum, Endosteum, Diaphysis, Epiphysis, Red marrow, Yellow marrow. Know definitions and where each is found or what it does.

✍️ Practical Tip

When asked to draw or label a long bone, emphasize the outer compact layer, inner spongy region at the ends, the marrow cavity, and label periosteum, endosteum, osteons (in shaft), and trabeculae (in epiphyses). Clear labels and at least five feature comparisons will maximize marks.