Vascular Anatomy and Physiology: Normal Flow and Collateral Pathways
Vascular Anatomy and Physiology tests normal arterial, venous, and cerebrovascular anatomy, the pressure-flow relationships that produce normal waveforms, and the collateral routes that explain findings after occlusion.
Arterial and venous organization
The aortic arch gives the right brachiocephalic (right subclavian and right CCA), left CCA, and left subclavian. The CCA bifurcates into ICA and ECA; the ICA has no cervical branches and supplies the brain, while the ECA supplies face and scalp. Vertebrals arise from the subclavicles and join to form the basilar artery. Lower extremity inflow is common iliac to external iliac to common femoral, then superficial femoral (SFA) and profunda femoris; the SFA passes through the adductor canal to become the popliteal, which divides into anterior tibial and tibioperoneal trunk, then posterior tibial and peroneal. Upper limb flow is subclavian to axillary to brachial to radial and ulnar with a palmar arch. Deep veins accompany arteries; superficial great and small saphenous veins connect via perforators; valves direct flow from superficial to deep and from calf to heart.
Physiology rules
Flow is governed by pressure gradient and resistance; Poiseuille resistance rises with viscosity and vessel length and falls with the fourth power of radius. Laminar flow has a parabolic profile; turbulence occurs at high velocity or irregular geometry. High-resistance peripheral arteries show triphasic flow; low-resistance beds show continuous diastolic flow. Venous flow is phasic with respiration and augments with distal compression; the calf muscle pump and valves prevent reflux. Collaterals enlarge after occlusion, often producing damped, monophasic downstream waveforms.
Worked example
A mid-SFA occlusion is suspected. Duplex shows absent color in the mid-thigh SFA, but the popliteal artery reconstitutes with low-velocity, monophasic flow. Reasoning: the profunda femoris and genicular branches bypass the occluded segment; the popliteal signal is not normal triphasic because it receives collateral flow. The correct interpretation is occlusion with distal reconstitution, not a popliteal stenosis.
Traps that cost marks
- Calling the ECA the ICA; the ICA has no cervical branches and low-resistance flow, while the ECA has branches and higher-resistance flow.
- Assuming absent flow equals occlusion without checking gain, scale, and collaterals.
- Reversing perforator direction; perforators normally flow superficial to deep.
- Forgetting that respiratory phasicity is lost with proximal venous obstruction.
- Labeling the peroneal as posterior tibial; the peroneal lies more laterally.