THORAX ABDOMEN AND PELVIS

Detailed explanation-1: -The kidney is divided into two parts: the outer cortex and the inner medulla. In the cortex, the osmolarity is relatively low (think less salty), around 300 mOsm/L or so, but as you travel further down into the medulla, the osmolarity gets much higher (think more salty).

Detailed explanation-2: -The filtrate and surrounding interstitial fluid are at their highest osmolarities at the bottom of the loop of Henle. As the filtrate continues on, it enters the thin ascending limb of the loop of Henle, which is impermeable to water.

Detailed explanation-3: -The late distal tubule and the collecting tubules are made permeable to water by the presence in the circulation of antidiuretic hormone (ADH) released from the posterior pituitary gland (p. 212). The early distal tubule is not permeable to water and its permeability is not changed by ADH.

Detailed explanation-4: -Water flows from the filtrate to the interstitial fluid, so osmolality inside the limb increases as it descends into the renal medulla. At the bottom, the osmolality is higher inside the loop than in the interstitial fluid.