Anatomy of the urinary system

The renal system is designed both structurally and functionally to handle fluids within the body in a highly efficient way. It enables precisely controlled, variable excretion of a wide range of ions and small molecules from the extracellular fluid (ECF).
The ECF space is made up of approximately 12 L of interstitial fluid (between cells), and 3 L of circulating plasma. NB: plasma contains cells and large organic molecules but interstitial fluid normally does not. The predominant cation (+) in the ECF space is sodium, and the predominating anion (-) is chloride, but potassium is also an important cation, and bicarbonate is another important anion. The kidneys affect directly the concentration of ions and small molecules in the ECF space and thus have indirect effects on intra-cellular composition.
Water is the most abundant component of the human body with adults being about 60% water. In the body, water is distributed between the ECF and intra-cellular spaces in the ratio of about 1: 2.5. There is also a small volume in ‘trans-cellular’ fluid such as CSF (cerebrospinal fluid) and gastric secretions. The physiochemical processes responsible for maintaining fluid and electrolyte homeostasis in the human body are vital.
The kidneys function by producing very large quantities (about 180 L per day) of an ultra-filtrate of plasma (i.e. plasma without the cells and large organic molecules).
The filtering occurs at specialised leaky capillaries found in the kidney cortex called glomeruli. Specialised afferent and efferent arterioles that bring blood into and carry it away, respectively, from these glomeruli maintain a relatively high constant filtration pressure. This pressure drives water and small molecules out of the plasma at a rate of about 125 ml/min (or 180 L per day). This rate of filtration is the glomerular filtration rate (GFR).