Answer;
Blood pressure in the glomerular capillaries.
Step-by-step explanation;
-The glomerulus is a tuft of small blood vessels called capillaries located within Bowman's capsule within the kidney.
-The process by which glomerular filtration occurs is called renal ultrafiltration. The force of hydrostatic pressure in the glomerulus (the force of pressure exerted from the pressure of the blood vessel itself) is the driving force that pushes filtrate out of the capillaries and into the slits in the nephron.