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Starling's equation only refers to fluid movement across the capillary membrane that occurs as a result of filtration. The Starling equation illustrates the role of hydrostatic and colloid osmotic forces (the so-called Starling forces) in the movement of fluid across capillary membranes. Modern evidence shows that in most cases venular blood pressure exceeds the opposing pressure, thus maintaining a positive outward force. This indicates that capillaries are normally in a state of filtration along their entire length.
The blood is filtered by nephrons, which are the functional units of the kidneys. Each nephron begins in a renal corpuscle, which is composed of a glomerulus enclosed in a Bowman's capsule. Cells, proteins, and other large molecules are filtered out of the glomerulus by a process of ultrafiltration, leaving an ultrafiltrate that resembles plasma (except that the ultrafiltrate has negligible plasma proteins) to enter the Bowman's space. Filtration is driven by Starling forces.
Diagram showing the basic physiologic mechanisms of the kidney and the three steps involved in urine formation.
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 (and not absorption) because these substances have already been absorbed once (particularly in the intestines). Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium (the connective tissue that surrounds the nephrons), and then the transport of these substances from the interstitium into the bloodstream. These transport processes are driven by Starling forces, diffusion, and active transport.
higher pressure at the arterial end of the capillary causes filtration into the interstitial fluid, lower pressure at the venous end of the capillary causes reabsorption from the interstitial fluid, most of the water that was filtered into the interstitial fluid is reabsorbed along with wastes, and most of the water that was filtered into the interstitial fluid flows into lymph vessels