Renal Physiology
Posted by admin on July 28th, 2010Renal blood flow: Blood flow through the kidneys progressively decreases from 1200 mL/min. in young adulthood to 600 mL/min by age 80 yr. The primary underlying factor is the decreased renovascular bed. However, the reduction in flow does not simply reflect decreased renal mass, since Row per gram of tissue falls progressively after the fourth decade. The age-related decline in renal blood flow is due to fixed anatomic changes rather than to reversible vasospasm, as shown by sludies with vasoactive agents. Of significance, cortical blood flow decreases and medullary flow is preserved, a finding consistent with histologic studies that show selective loss of cortical vasculature with age. These vascular changes probably account for the patchy cortical defects commonly seen on renal scans in healthy elderly adults.
Glomerular filtration rate: The major clinically relevant functional defect arising from the age-related anatomic and histologic changes described above is a progressive decline in the glomerular filtration rale as measured by creatinine clearance (see formula under Drug Doses, below). Creatinine clearance is stable until the middle of the fourth decade, when it declines linearly at about S mL/min/1.73 m2/decade.
Longitudinal studies indicate that aging’s effect on creatinine clearance varies substantially, with as many as ‘A of older persons showing no decline in glomerular filtration rale. This variability suggests that factors other than aging may be responsible for the apparent loss of renal function. For example, increases in blood pressure still within ihe normolensive range arc associated with an accelerated, age-related loss of renal function.
Tubular function: The argument that enlirc nephrons disappear with advancing age is supported by the striking parallel decline in glomerular filtration rate and several proximal tubular functions, including maximal excretion of p-aminohippurate and iodopyracet and maximal absorption of glucose. The renal threshold for glycosuria, which relates inversely to the degree of splay in rcabsorplive capacity of individual nephrons, increases with aye. Thus, glucose generally spills into the urine at a lower blood glucose level in a young diabetic patient than in an elderly one.
Renal Changes and Disorders 111
Drug Doses
Although muscle mass, from which creatinine is derived, decreases with age. serum creatinine level does not usually decline with normal aging because the reduction in renal function (ie, the decline in glomerular filtration rate and creatinine clearance) balances the decreased production of creatinine. For example, the average healthy 80-yr-okl man has a creatinine clearance 32 mL/min less than that of his 30-yr-old counterpart but the same serum creatinine value. Depression of glomerular filtration sufficient to raise the serum creatinine level above 1.5 mgAIL is rarely due to normal aging and therefore indicates renal disease.
In clinical practice, (he doses of many drugs excreted primarily by the kidneys (eg, digoxin preparations and aminoglycoside antibiotics) require adjustment to compensate for changes in renal function (see also Ch. 21). However, adjustments are too often based on serum creatinine values, with the resultant predictable drug overdose in elderly patients. Ideally, dose adjustments should be based on creatinine clearance, which docs not require absolute 24-h urine collection but can be estimated using a sample collected after 8 h (eg. overnight).
Tags: Renal Physiology
