Most urinary tract infections are believed to arise by the ascending route after entry via the urethral meatus. This is by far the most common route of infection in the female and in association with instrumentation, in both sexes. It now generally accepted that bacteria ascend the urinary stream by simple Brownian movement and can reach the kidney in the absence of alteration of urine flow.

Some workers have postulated that there may be a pathway from the intestines to the kidney by way of lymphatic channels. According to Beeson’s review, direct lymphatic channels have not been convincingly demonstrated between the appendix and cecum and right kidney. Some investigators report passage of Indian ink particles from the lower to upper tract in animals but this remains to be confirmed.

Beeson furthers points out that lymphatic drainage of a region generally follows its blood supply and venous drainage. The arteries and veins of the urinary tract are segmentally distributed even at different levels of the ureter. Drainage from the bladder wall and ureter is not toward to the kidney, but into the common iliac glands,. Therefore, it is unlikely that the postulated pathway from lower to upper tract via the lymphatics exist at all, in his view.

Hematogenous, or blood-borne, transport of bacteria occurs much less commonly than by the ascending route, but at times is extremely important. The kidney is well supplied by blood vessels and at any one time receives one quarter of the cardiac output. Thus, any systemic bacterial infection  can lead to seeding of the kidney with bacteria. Staphylococcal bacteremia is commonly associated with spread to the kidney. This organism commonly produces cortical abscesses which may extend to the perirenal fat. Systemic candidiasis is also often associated with renal infection. Occasionally a hematogenous focus may arise from the kidney itself.