Nephron, urine formation

The most potent type of diuretic, loop diuretics are named after the loop of Henle, a component of a nephron. The nephrons are the filtering units of the kidney, and are responsible for moving fluids and waste out of the bloodstream, resulting in urine formation. The loop of Henle is a branch within each nephron where sodium and potassium are reabsorbed back into the bloodstream instead of being filtered into the urine. Loop diuretics inhibit this action and promote excretion of the sodium and potassium instead, along with calcium and magnesium. Since excess sodium causes excess fluid build-up, this results in fluid loss. Furosemide (Lasix), bumetanide (Bumex), torsemide (Demadex), and ethacryinic acid (Edecrin) are all loop diuretics. 

Figure 12.1 illustrates the main structure of a nephron [1], The measurements to be reported in this chapter were performed on rats. A rat kidney contains approximately 30000 nephrons as compared to the one million nephrons in a human kidney. The process of urine formation starts with the filtration of plasma in the glomerulus, a system of 20-40 capillary loops. The presence of a relatively high hydrostatic pressure in this system allows water, salts and small molecules to pass out through the capillary wall and into the proximal tubule. Blood cells and proteins are retained, and the filtration process saturates when the protein osmotic pressure balances the hydrostatic pressure difference between the blood and the filtrate in the tubule. For superficial nephrons, the proximal tubule is visible in the surface of the kidney and easily accessible for pressure measurements by means of a thin glass pipette. 

The diuretics currently in use today (Table 27.1) are classified by their chemical class (thiazides), mechanism of action (carbonic anhydrase inhibitors and osmotics), site of action (loop diuretics), or effects on urine contents (potassium-sparing diuretics). These drugs vary widely in their efficacy (i.e., their ability to increase the rate of urine formation) and their site of action within the nephron. Efficacy often is measured as the ability of the diuretic to increase the excretion of sodium ions filtered at the glomerulus (i.e., the filtered load of sodium) and should not be confused with potency, which is the amount of the diuretic required to produce a specific diuretic response. 

Although the glomerular filtrate is identical to plasma except for the absence of protein in the filtrate, the final composition of the urine is very different from that of plasma. Urine may be diluted or concentrated. Various segments of the nephron participate in different ways in urine formation. 

After epithelialization and the formation of the S-shaped tubule, there is still much that needs to occur in order for the nephron to function. Cells destined to form the podocyte layer of the glomerulus flatten out and lose some of the markers that characterized their earlier transition to epithelium, including c-MYC, HOX-c9, LFB-1 and LFB-3, while keeping a high level of WT1. Expression of more classical mesenchymal markers such as vimentin takes place, but the cells also keep a number of epithelial proteins such as desmosomal components. The result is a tissue that is more organized than most connective tissue but leakier than most epithelium, the optimum design for urine filtration. 

The main functions of the human kidney are the formation and excretion of urine, and control of the composition of body fluids. Details of the structure and functions ofthe human kidney may be found in textbooks of physiology [1] or biomedical engineering [13]. Each ofthe two human kidneys contains approximately one million units of tubules (nephrons), each 20-30 pm in diameter, and with a total... 

Figure 45-16 Diagrammatic representation of the interplay of factors involved in kidney stone formation. High or low pH may act as a promoter or inhibitor of stone formation depending on the stone type in question (e.g., calcium stone formation is favored by inadequate acidification while urate is less soluble in acidic urine). Controversy exists as to whether formed stones become trapped as they pass through the nephron ( free particle theory ) or whether stone formation occurs at damaged sites on the tubule wall ( fixed particle theory ).

Two important functions of the kidney are 1) to maintain a homeostatic balance of electrolytes and water and 2) to excrete water-soluble end products of metabolism. The kidney accompiishes these functions through the formation of urine by the nephrons (Fig. 27.1). Each kidney contains approximately 1 million nephrons and is capable of forming urine independentiy. The nephrons are composed of a specialized capillary bed called the glomerulus and a long tubule divided anatomically and functionally into the proximai tubuie, ioop of Henie, and distal tubule. Each component of the nephron contributes to the normal functions of the kidney in a unique manner thus, all are... 

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