Proper function of the kidney requires that it receives and adequately filters blood. This is performed at a microscopic level by many hundreds of thousands of filtration units called renal corpuscles, each of which is composed of a glomerulus and a Bowman's capsule. A universally accepted way to assess efficient renal function is to estimate the rate of filtration, called the glomerular filtration rate (GFR).
Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. It is called reabsorption (not absorption) because these substances have already been absorbed previously, usually in the intestines.
Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubular fluid into the renal interstitium (the connective tissue that surrounds the nephrons); then these substances are transported from the interstitium into the bloodstream (Figure 1). These transport processes are driven by Starling forces, diffusion, and active transport.
Within the peritubular capillary network, molecules and ions are reabsorbed back into the blood. The sodium chloride that is reabsorbed into the system increases the osmolarity of blood in comparison to the glomerular filtrate.
This reabsorption process allows water (H2O) to pass from the glomerular filtrate back into the circulatory system. Glucose and various amino acids are also reabsorbed into the circulatory system. Glomerular filtrate is then separated into two forms: reabsorbed filtrate and non-reabsorbed filtrate. The non-reabsorbed filtrate is then known as tubular fluid as it passes through the collecting duct to be processed into urine.