Basic renal processes

The maintenance of plasma volume and plasma osmolarity occurs through regulation of the renal excretion of sodium, chloride, and water. Each of these substances is freely filtered from the glomerulus and reabsorbed from the tubule none is secreted. Because salt and water intake in the diet may vary widely, the renal excretion of these substances is also highly variable. In other words, the kidneys must be able to produce a wide range of urine concentrations and urine volumes. The most dilute urine produced by humans is 65 to 70 mOsm/1 and the most concentrated the urine can be is 1200 mOsm/1 (recall that the plasma osmolarity is 290 mOsm/1). The volume of urine produced per day depends largely upon fluid intake. As fluid intake increases, urine output increases to excrete the excess water. Conversely, as fluid intake decreases or as an individual becomes dehydrated, urine output decreases in order to conserve water. 

On average, 500 mOsm of waste products must be excreted in the urine per day. The minimum volume of water in which these solutes can be dissolved is determined by the ability of the kidney to produce a maximally concentrated urine of 1200 mOsm/1  

This volume, referred to as obligatory water loss, is 420 ml water/day. In other words, 420 ml of water will be lost in the urine each day in order to excrete metabolic waste products regardless of water intake. 

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Figure 19.2 Basic renal processes. These processes include filtration, reabsorption, and secretion. (1) Filtration is the movement of fluid and solutes from the glomerular capillaries into Bowman s capsule. (2) Reabsorption, which takes place throughout the nephron, is the movement of filtered substances out of the tubule and into the surrounding peritubular capillaries. (3) Secretion is the movement of selected unfiltered substances from the peritubular capillaries into the renal tubule for excretion. Any substance that is filtered or secreted, but not reabsorbed, is excreted in the urine.

The nephron performs three basic renal processes (see Figure 19.2) ... 

As a clearance route the renal route has attractive features for the design of drugs. For instance clearance rates, certainly for neutral compounds, are low. Moreover, the clearance process by filtration is not saturable and tubular secretion is only saturated at high doses with acidic and basic compounds. In a similar vein drug interactions... 

Interleukin-8 (IL-8) is a potent neutrophil and lymphocyte chemotactic cytokine It is one of a family of 13 human CXC chemokines [244]. These small basic heparin binding proteins are proinflammatory and mainly involved in the initiation and amplification of acute inflammatory reactions and in chronic inflammatory processes. In the urinary tract infection model, the epithelial cells of the renal tract were shown to secrete chemokines and IL-8 was identified as the main chem-okine involved in transepithelial neutrophil migration. Urinary levels of immunoreactive IL-8 may be elevated... 

The basic anatomic unit of renal function is the nephron (Fig. 30.4A). Basic components are glomemlus, proximal tubule, loop of Henle, distal tubule, and collecting tubule. The renal excretion of dmgs involves one or more of the processes of glomemlar filtration, tubular reabsorption and active tubular secretion (Fig. 30.4B). 

Our urology model went through several iterations to get the shape and the material properties of the UPJ correct. Several attempts were made to create a reahstic shape for a dilated renal pelvis from a set of basic geometric structures. Feedback from an experienced urologist was h bly valuable in this endeavor. Another consideration when creating the mold was how easily it could be extracted. An initial process involved creating the components separately and then joining them with sflicone. This was tedious... 

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