Kidney filtration capacity

Aldosterone, the most potent of the mineralocorticoids (Figure 25.43), is involved in the regulation of sodium and potassium balances in tissues. Aldosterone increases the kidney s capacity to absorb Na, Cl, and HgO from the glomerular filtrate in the kidney tubules. 

Nitrogen compounds commonly determined are creatinine, urea, and uric acid. Creatinine is an end product of the energy process occurring within the muscles, and is thus related to muscle mass. Creatinine in urine is commonly used as an indicator and correction factor of dilution in urine. Creatinine in serum is an indicator of the filtration capacity of the kidney. Urea is the end product of the nitrogen luea cycle, starting with carbon dioxide and ammonia, and is the bulk compoimd of urine. The production of uric acid is associated with the disease gout. In some cases, it appears that the excess of uric acid is a consequence of impaired renal excretion of this substance. 

It is recognized that GFR is dependent on numerous factors, one of which is protein load. Bosch suggested that an appropriate measure of renal function should reflect the filtration capacity of the kidney, and not the resting GFR. Subjects with normal renal function administered an oral or intravenous protein load prior to measurement of GFR have increased their GFR by as much as 50%. As renal function declines, the kidneys compensate by increasing SNGFR. The renal reserve, the maximal degree by which GFR can be increased, usually declines and thus may be a complementary measure of renal function for these patients. 

Potassium salts are not as abundant as sodium salts in food for two reasons first, because potassium salts are less abundant in nature than sodium salts and, second, because potassium salts are bitter and therefore are not added to food for seasoning. Leguminous plants are one of the richest sources of dietary potassium. The total potassium intake in man is much lower than the sodium intake. However, potassium intake is always sufficient to maintain potassium requirements because of the kidney s capacity to reabsorb potassium lost in the glomerular filtrate. 

The serum concentration of Pi increases with a physiological dechne in renal function associated with aging (but not renal disease per se). Healthy individuals excrete approximately 67% of their absorbed phosphate via the urine and the remainder via the gut as endogenous secretions. As the glomerular filtration capacity of the kidneys declines, the serum Pi concentration increases and more Pi is retained by the body. PTH secretions increase but the typical serum PTH concentrations, although elevated, remain within the upper limits of the normal range, at least for a decade or so. Thereafter, however, serum Pi and PTH both continue to climb as renal function declines and increased rates of bone turnover lead to measurable bone loss. This situation probably affects millions in the United States each year as they enter the 50s and proceed into the 60s many of these individuals are overweight or obese and have the metabolic syndrome, which... 

Only about 10% of the Na-i- filtered by the glomerulus is reabsorbed by the distal convoluted tubule (DCT) and therefore the capacity of the thiazide group of diuretics to influence the elimination of Na-H in the urine is limited compared to the loop agents. Thiazides can prevent the reabsorption of up to 5% of the total filtered Na+, whereas the equivalent figure for loop diuretics is about 20%. Thiazides can still produce a moderate naturesis and diuresis compared to carbonic anhydrase inhibitors and the K+-sparing agents. Most thiazides are ineffective at low glomerular filtration rates. They also hinder the ability of the kidneys to produce a dilute urine. 

Aminoglycosides have a low volume of distribution and are excreted by glomerular filtration, which is followed by active reuptake via a high-capacity, low-affinity transport system. The aminoglycosides remain in the kidney longer than in other exposed tissues such as the liver. Thus, with gentamycin, the time for the tissue concentration to be reduced by half is... 

Osmotic diuretics. Diuretics that are filtered in the glomerulus but cannot be re-absorbed. An example is mannitol. Their presence leads to an increase in the osmolarity of the filtrate to maintain osmotic balance, water is retained in the urine. Glucose, like mannitol, can also behave as an osmotic diuretic. In diabetes mellitus, the concentration of glucose in the blood exceeds the maximum resorption capacity of the kidney glucose remains in the filtrate, leading to the osmotic retention of water in the urine. 

This study showed that the filtration reserve capacity of the kidney is lost when elevated microproteinuria is present, but that there was no functional impairment at a renal Cd burden not yet causing microproteinuria. Implicitly, it validated the proposal of a CdU of 10 pg Cd/ g creatinine as biological exposure limit to prevent microproteinuria in male Cd workers. 

Roels HA, Lauwerys RR, Bernard AM, Buchet JP,Vos A, Oversteyns M. Assessment of the filtration reserve capacity ofthe kidney In workers exposed to cadmium. Br J Ind Med 1991 48 365-374. 

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