Kidney #2— Functions and Mechanisms

1. Blood enters a ball of capillaries called glomerulus
2. Due to high blood pressure (65 mm Hg as opposed to 25 mm Hg), blood is filtered into the lumen of Bowman's capsule. Water and small dissolved molecules (salts, sugar, amino acid, nitrogenous waste, etc.) enter the glomerulus and become the filtrate. Blood cells, plasma proteins, and platelets are filtered out.
•  Bowman's capsule is the blind end of the nephron's tubule, and it surrounds the glomerulus
3. In the proximal tubule, hydrogen ions are secreted, and bicarbonate ions—which are important buffers— are reabsorbed. The epithelial cells also secrete ammonia to maintain a constant pH within the filtrate. In addition, drugs and other toxins are released from the peritubular capillaries, and they travel through the interstitial fluid and are secreted into the filtrate. Furthermore, important nutrients, such as glucose, amino acids, potassium ions, are actively or passively transported back to the blood. Sodium ions are pumped into the interstitial fluid, which allows water to follow by osmosis.
4. The filtrate now travels into the descending limb of the loop of Henle, from the cortex to the inner medulla. In this section, the transport epithelium is permeable to water but not salt and other solutes, so water diffuses out of the tubule.
5. The filtrate now travels up the ascending limb of the loop of Henle, from the inner medulla back to the cortex. In the thin segment that is close to the turn, NaCl is passively diffused out of the tubules, which contributes to the high osmolarity of the inner medulla. In the thick segment in the cortex, NaCl is pumped out of the tubule.
   The reason why an osmotic gradient exists within the entire loop of Henle is that the blood flows in the opposite direction as the filtrate.
6. Next, the filtrate enter the distal tubule. Potassium and hydrogen ions are secreted, and  NaCl and bicarbonate ions are reabsorbed.
7. The filtrate then enters the collecting duct, which carries the filtrate into the renal pelvis (note that the collecting duct collects from several nephrons). Here, sodium chloride is actively transported out of the duct. Also, in the inner medulla, urea diffuses into interstitial fluid. This increases the osmolarity of the interstitial fluid and enables water to be reabsorbed.