Nephron active transport processes

Figure 4S-6 Tubular reabsorptive mechanisms the major primary active transport processes in the proximal nephron.The renal tubular epithelium consists of a single layer of ceils. At the luminal side, adjacent cells are in contact (the tight junction), whereas toward the basal side of the cells, there are gaps between adjacent cells (lateral intercellular spaces). (From Lote CJ. Principles of renal pfiysio/ogy. 4tb ed. London Kluwer Academic Publishers, 2000 Chapter 4.)...

Direct coupling of adenosine triphosphate (ATP) hydrolysis is an example of an active transport process. The most important of these in the nephron is Nafi K -ATPase, which is located on the basolateral membranes of the tubulo-epithelial cells (Figure 45-6). This enzymatic transporter accounts for much of renal oxygen consumption and drives more than 99% of renal sodium reabsorption. Other examples of primary active transport mechanisms are a Ca " -ATPase, an H -ATPase, and an H, K -ATPase. These enzymes establish ionic gradients, polarizing cell membranes and thus driving secondary transport processes. 

The processes of selective reabsorption of nutrients and xenobiotics goes on within the complex tubule system. 98-99% of filtered materials (salts, water, sugars, amino acids) are eventually reabsorbed by passive or active transport. Biomolecules such as glucose and amino acids are entirely reabsorbed if their concentrations are within the normal range in the blood. However, should the concentrations be higher than normal, those molecules might not be completely reabsorbed because they have exceeded the ability of the nephron transport systems to accommodate them. This is referred to as exceeding the renal threshold. Urine is therefore a convenient body fluid to assay for the initial assessment of metabolic or excretory system malfunctions. 

Re-absorption is a two-step process beginning with the active or passive extraction of substances from the mbule fluid into the renal interstitium (the connective tissue that surrounds the nephrons), followed by their transport from the interstitium into the bloodstream. These transport processes are driven by hydrostatic, oncotic, diffusion and active transport. Some key regulatory hormones for re-absorption include aldosterone, which stimulates active sodium re-absorption (and water as a result), and antidiuretic hormone, which stimulates passive water re-absorption. Both hormones exert their effects principally on the collecting ducts. 

A renal clearance value greater than the GFR indicates that the drug is actively secreted, but a value less than the GFR does not preclude active secretion because tubular reabsorption takes place in the distal nephron. Competitive inhibition provides the only conclusive evidence that a transport process is carrier-mediated for example, probenecid decreases proximal tubular secretion of penicillin G. 

Excretion is the process by which a compound exits the body. Urinary excretion, fecal excretion, and exhaled air are the most common pathways. Compounds removed from the blood by the kidney are excreted in urine. Within the kidney, functional units called nephrons filter the blood. Some bases and weak acids are actively transported out of the blood. Ions are especially subject to the kidney s filtration. Large molecules are primarily excreted in fecal matter because of the size exclusion in the glomerulus of the kidney. In addition, large molecules may be secreted in the bile and therefore excreted in the feces. Gases with low solubility and Uquids with high volatility are efficiently excreted in exhaled air. Other less prominent excretion pathways include breast milk, sweat, and tears. Certain heavy metals such as Cr, Cd, Co, W, and U will accumulate in hair follicles and be detectable in hair. 

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