What affect does the urinary system have on the blood pressure? What affect does the renal system have on the blood pressure?
The renal system controls blood pressure by a process known as tubuloglomerular feedback mechanism
Renal system has an intrinsic property to maintain a relatively constant renal blood flow. In a broad sense, this property helps to increase overall arterial pressure when blood pressure decreases.
I'm assuming you have a generalised idea about the anatomy of nephron. In the early distal convulated tubules of the nephron are some specialised cells called macula densa cells which have the ability to sense NaCl concentration in the filtrate. When this concentration decreases (which happens in low arterial blood pressure), the macula densa cells stimulates another specialised cells located in the afferent arterioles named juxta glomerular cells to release renin. Renin is an enzyme which converts inactive angiotensinogen to active angiotensin 1 which is then again converted to angiotensin 2 by Angiotensin Converting Enzyme.
Angiotensin 2 causes afferent arteriolar dilation as well as stimulates aldosterone secretion. Aldosterone causes efferent arteriolar contraction as well as renal retention of salt and water.
All of this leads to an increased water volume in the extracellular fluid which in turn increases the arterial blood pressure.