Renin is an enzyme which converts the plasma protein angiotensinogen to angiotensin I . Angiotensin II also stimulates the adrenal gland to produce aldosterone which causes water and sodium retention which together increase blood volume. This is a negative feedback system. In other words the initial stimulus is a fall in blood volume which leads to a fall in perfusion pressure in the kidneys. When blood volume, renal perfusion and GFR improve the system feeds back to switch off or turn down the response to the stimulus.
Reabsorption is by either active or passive transport.
Active transport requires energy to move solute against an electrochemical or a concentration gradient. It is the main determinant of oxygen consumption by the kidney. Passive transport is where reabsorption occurs down an electrochemical, pressure or concentration gradient. Most of the solute reabsorption is active, with water being freely permeable and therefore moving by osmosis. When the active reabsorbtion of solute from the tubule occurs, there is a fall in concentration and hence osmotic activity within the tubule.