Interventional procedures on the carotid arteries are performed through a transcervical access while retrograde blood flow is established from the internal carotid artery to a venous or external location. A system (100) for use in accessing and treating a carotid artery comprises an arterial access device (110) comprising a sheath having an internal lumen, the sheath adapted to be introduced into a common carotid artery via a transcervical incision and receive blood flow into the internal lumen from the common carotid artery. The system further comprises a shunt (120) fluidly connected to the arterial access device, wherein the shunt has an internal lumen that provides a pathway for blood to flow from the internal lumen of the sheath toward a return site (115), a proximal extension having a distal end connected to a proximal end of the sheath at a connection location, the proximal extension having an internal lumen in fluid communication with an internal lumen of the sheath, a hemostasis valve located at the proximal end of the proximal extension, the hemostasis valve adapted to inhibit blood flow out of the internal lumen of the proximal extension while allowing catheter introduction into the internal lumen of the proximal extension and into internal lumen of the sheath and a flow control assembly (125) coupled to the shunt and adapted to regulate blood flow through the shunt between at least a first blood flow state and at least a second blood flow state. The flow control assembly includes one or more components that interact with the blood flow through the shunt.
