Final answer:
The effector in the regulation of blood cell count is erythropoietin.Erythropoietin is the effector in the negative feedback system that regulates red blood cell count by inhibiting its production when sufficient red blood cells are present.
Step-by-step explanation:
An effector is the component in a negative feedback system that causes a change to reverse the situation and return the physiological value to the normal range. In the regulation of blood cell count, the hormone erythropoietin (b) is the effector. When the blood has plenty of red blood cells, erythropoietin is no longer released from the kidney, preventing the overproduction of red blood cells.