Three separate processes contribute to the formation of urine within the nephron of the kidney. These are filtration, reabsorption and secretion. We shall be considering the first of these, filtration.
Filtration occurs in the glomerulus of the nephron. The glomerulus is a capillary bed within the nephron, and filtration of fluid out of the glomerulus occurs both as a result of the structure of the nephron, and of Starling’s forces. The glomerulus is part of the vascular structure of the nephron. It is a dense capillary bed – or capillary tuft – that emerges from an afferent arteriole, and returns to an efferent arteriole.
Glomerular Filtration Rate (GFR)
If we disregard any oncotic pressure in the Bowman’s capsule, we have in effect, three pressures to consider: glomerular hydrostatic pressure, glomerular oncotic pressure, and Bowman’s capsule hydrostatic pressure. These can be expressed as a formula that will tell us the amount of hydrostatic pressure pushing fluid out of the glomerulus:
Net glomerular pressure equals: Glomerular hydrostatic pressure minus [glomerular oncotic pressure + Bowman’s capsule hydrostatic pressure]
In order to measure the glomerular filtration rate, we must add to this measurement an estimation of glomerular permeability, and surface area – that is, how many functioning nephrons are available in the body for filtration, and how effectively the glomeruli filter fluid. A normal glomerular filtration rate is around 125mls/min, and this measurement is used to determine, and to classify, kidney function.
|RLO: Starling’s Forces||An examination of the roles of hydrostatic and oncotic pressure in movement of fluid and gases across the capillary wall.|
|RLO: Kidney Physiology||Introduction to the physiology of the kidney, examining the processes by which the kidneys filter blood, control body pH and eliminate the waste products of metabolism from the body.|
|RLO: The Anatomy of the Kidneys||Introduction to the external and internal anatomy of the kidney.|