𝙒𝙝𝙮 𝘽𝙤𝙣𝙚 𝙈𝙖𝙧𝙧𝙤𝙬 𝘼𝙨𝙥𝙞𝙧𝙖𝙩𝙚𝙨 𝘽𝙚𝙘𝙤𝙢𝙚 𝘿𝙞𝙡𝙪𝙩𝙚𝙙: 𝘼 𝙃𝙚𝙢𝙤𝙙𝙮𝙣𝙖𝙢𝙞𝙘 𝙀𝙭𝙥𝙡𝙖𝙣𝙖𝙩𝙞𝙤𝙣 𝙛𝙤𝙧 𝘾𝙡𝙞𝙣𝙞𝙘𝙞𝙖𝙣𝙨

Every surgeon has seen it: the first 1–2 mL of aspirate is thick and cellular... and everything after that becomes progressively thinner until it looks like pure venous blood.

If the marrow cavity is “full of marrow,” why does peripheral blood replace it so quickly?

The answer lies in marrow structure, pressure physiology, and fluid dynamics.

𝗠𝗮𝗿𝗿𝗼𝘄 𝗦𝘁𝗿𝗼𝗺𝗮 𝗜𝘀 𝗮 𝗦𝘁𝗿𝘂𝗰𝘁𝘂𝗿𝗲, 𝗡𝗼𝘁 𝗮 𝗟𝗶𝗾𝘂𝗶𝗱

The marrow cavity is not a fluid-filled space—it’s a semi-solid, thixotropic, reticular tissue made of:

Mesenchymal stromal cells and fibroblasts

Adipocytes and macrophages

Extracellular matrix (collagen, laminin, fibronectin)

A dense network of venous sinusoids

Stroma is high-viscosity and mechanically resistant, meaning it moves slowly when suction is applied. 

This sets up a competition:

Marrow stroma: high viscosity, high resistance

Peripheral blood: low viscosity, low resistance

Once a low-resistance pathway opens, the thinner fluid always wins.

𝗧𝗵𝗲 𝗗𝗲𝗹𝗶𝗰𝗮𝘁𝗲 𝗣𝗿𝗲𝘀𝘀𝘂𝗿𝗲 𝗘𝗾𝘂𝗶𝗹𝗶𝗯𝗿𝗶𝘂𝗺

Inside trabecular bone, three pressures are normally balanced:

Marrow interstitial pressure

Sinusoidal venous pressure

Intraosseous venous pressure

Bone marrow sinusoids are fenestrated, discontinuous, extremely thin-walled, and compliant.

They are perfect for cell trafficking—but extremely vulnerable to shear stress and rapid pressure drops.

𝗛𝗼𝘄 𝗗𝗶𝗹𝘂𝘁𝗶𝗼𝗻 𝗛𝗮𝗽𝗽𝗲𝗻𝘀: 𝗧𝗵𝗲 “𝗖𝗵𝗮𝗻𝗻𝗲𝗹𝗶𝗻𝗴” 𝗠𝗲𝗰𝗵𝗮𝗻𝗶𝘀𝗺

Aspiration creates a localized low-pressure zone at the needle tip.

Phase 1: Harvest (0–2 mL)

You draw true marrow because you are sampling the tissue immediately adjacent to the needle ports.

Phase 2: Rupture (>2 mL)

As you keep pulling:

Thick stroma cannot move fast enough;

Local negative pressure spikes;

The path through the stroma has high resistance;

The path through sinusoids has low resistance;

The result: sinusoidal endothelial rupture.

Phase 3: Flood

Once a sinusoid breaks:

Low-viscosity venous blood accelerates toward low-pressure zones

Blood outpaces stromal flow by orders of magnitude

The aspirate rapidly transitions from marrow to blood

This is classic "channeling"—fluid choosing the pathway of least resistance.

𝗦𝘂𝗺𝗺𝗮𝗿𝘆

Peripheral blood contamination isn’t necessarily a technique failure (although that certainly contributes)—it’s physics:

Anatomy: Marrow is semi-solid; blood is liquid.

Physiology: Sinusoids rupture when exposed to high shear or suction.

Fluid Dynamics: Low-viscosity blood overwhelms high-viscosity stroma once a channel opens.

Engineering: Reducing point-source suction and distributing negative pressure prevents rupture and preserves true marrow content.

The most current best practices can help reduce peripheral blood. This will be a future topic or DM me for the advanced copy.