Porth's Essentials of Pathophysiology, 4e

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Structure and Function of the Kidney

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Although the exact mechanism for this increase is uncer- tain, it is thought to be related to the fact that amino acids and sodium are absorbed together in the proxi- mal tubule via secondary active transport. As a result, delivery of sodium to the macula densa is decreased, which elicits an increase in renal blood flow through the juxtaglomerular apparatus feedback mechanism. The resultant increase in blood flow and GFR allows sodium excretion to be maintained at a near-normal level while increasing the excretion of the waste products of pro- tein metabolism, such as urea. The same mechanism is thought to explain the large increases in renal blood flow and GFR that occur with high blood glucose levels in persons with uncontrolled diabetes mellitus. ■■ Urine formation, which begins with the formation of a plasma ultrafiltrate from the glomerular capillaries into Bowman capsule, serves to regulate extracellular volume and concentration of electrolytes and other solutes. ■■ Tubular mechanisms allow the kidneys to retain water, electrolytes, and nutrients that are essential and regulate their levels in the blood by altering the degree to which they are reabsorbed or secreted into the tubular fluid for elimination. ■■ The kidney’s ability to produce a dilute or concentrated urine as a means of controlling the osmolality of body fluids relies on the development of an osmotic gradient in the medullary interstitium of the thin loops of Henle and reabsorption of water from the collecting tubules under the influence of the antidiuretic hormone (ADH). ■■ The glomerular filtration rate (GFR), which is the amount of filtrate that is formed each minute as blood moves through the glomeruli, is regulated by the arterial blood pressure and renal blood flow. Feedback mechanisms, both intrarenal (e.g., autoregulation, local hormones) and extrarenal (e.g., sympathetic nervous system, blood-borne hormones), normally keep blood flow and GFR constant despite changes in arterial blood pressure. SUMMARY CONCEPTS

regulation of blood pressure, production of red blood cells, and absorption of calcium.

Elimination Functions of the Kidney The functions of the kidney focus on elimination of water, excess electrolytes, metabolic acids, and waste products from the blood. As renal function declines, there is an increase in serum levels of substances such as urea, creatinine, phosphate, and potassium. The effect of renal failure on the concentration of serum electrolytes and metabolic end products is discussed in Chapter 26. Sodium and Potassium Elimination Elimination of sodium and potassium is regulated by the GFR and by humoral agents that control their reabsorp- tion. Aldosterone, a hormone secreted by the adrenal gland, functions in the regulation of sodium and potas- sium elimination by the principal cells in the distal and collecting tubules. Sodium reabsorption in the distal and collecting tubules is highly variable and depends on the presence of aldosterone. In the presence of aldosterone, which stimulates sodium absorption and simultaneous excre- tion of potassium into the tubular fluid, almost all the sodium in the distal tubular fluid is reabsorbed, and the urine essentially becomes sodium free. In the absence of aldosterone, virtually no sodium is reabsorbed from the distal tubule and excessive amounts of sodium are lost in the urine. The remarkable ability of the distal tubular and collecting duct cells to alter sodium reabsorption in relation to changes in aldosterone allows the kidneys to excrete urine with sodium levels that range from a few tenths of a gram to 40 g/day. Atrial natriuretic peptide (ANP) is also believed to have an important role in salt and water excretion by the kidney. It is synthesized by muscle cells in the atria of the heart and released when the atria are stretched. Increased levels of this peptide directly inhibit the reab- sorption of sodium and water in the renal tubules. Atrial natriuretic peptide also inhibits renin secretion and therefore angiotensin II formation, which in turn reduces reabsorption of sodium. This decreased sodium reabsorption increases urine output and helps return blood volume to normal. Atrial natriuretic peptide lev- els, which become elevated when the atria are stretched in congestive heart failure, help to decrease vascular vol- ume by increasing urine output. Like sodium, potassium is freely filtered in the glom- erulus and reabsorbed in the proximal and distal tubule. Unlike sodium, however, potassium is both reabsorbed from and secreted into the tubular fluid. The amount of potassium that is delivered to the distal tubule each day is only about 70 mEq, yet the average person consumes that much or more in the diet. Therefore, the excess potassium that is not filtered in the glomerulus must be secreted into the tubular fluid so it can be eliminated in the urine. Potassium secretion occurs mainly in the distal and collecting tubules, with plasma potassium and aldosterone levels being the main physiological

Elimination and Endocrine Functions of the Kidney

The kidneys play a critical role in maintaining the volume and composition of body fluids through the reabsoption of water and electrolytes, as well as in ridding the body of waste products. The kidneys also have endocrine functions that are important to the