Basic renal physiology: the aim of urine production is to maintain homeostasis of volume and blood composition. To achieve this goal, three processes happen in the kidneys.
Filtration happens in the glomeruli and is dependent on perfusion and its pressure. Before ending up as filtrate in Bowman's capsule, fluid has to pass three barriers: the endothelial cells of glomerular capillaries, the basement membrane, and the podocytes. Molecules of more than 70 kDa will not pass, molecules with a size between 7-70 kDa are filtered to proportional extent, and molecules smaller than 7 kDa are freely filtered. Charge of the molecules is an important factor for filtration.
Reabsorption is the re-uptake of water and solutes from the filtrate back to the peritubular fluid (blood). Reabsorption happens in the tubules via different mechanisms: active transport (ATP dependent), diffusion, ion channels, cotransport, and countertransport.
In the proximal tubule 70% of the sodium ions is resorbed and 60% of the water. More water (28%) and sodium ions (20%) are resorbed in the loop of Henle. In the ascending loop also salt is resorbed. In the distal tubule 7% of the sodium ions is returned in the circulation and 5% of water. In the collecting duct the last part of sodium ions and water is resorbed. The amount of water resorbed depends on the presence of the antidiuretic hormone ADH.
Secretion is the transport of solutes from the peritubular fluid into the tubular fluid. This is a way of secreting materials, which cannot pass the capsule of Bowman. Many drugs e.g. are excreted via secretion.
Regulation of the volume of water in the body is directly linked to:
Extra info: Excretion and regulation of water and salt are linked. High blood volume does not necessarily result in high blood pressure. Osmolality depends on more solutes than just sodium.
Regarding body water all are true EXCEPT…
Extra info: Water moves by a passive process. Plasma water = 3 liters; interstitial water = 9 liters; intracellular water = 27 liters. These amounts are approximate in a healthy 65 Kg male.