EVALUATION OF RENAL FUNCTION

27 EVALUATION OF RENAL FUNCTION



1 Briefly describe how urine is formed.


Urine represents the net effect of glomerular ultrafiltration of plasma and renal tubular excretion and reabsorption of substances into and out of the glomerular filtrate.



2 What is glomerular filtration rate (GFR)?


GFR is the rate at which the glomerulus forms the ultrafiltrate of plasma within Bowman’s space.



3 What drives GFR?


The formation of the plasma ultrafiltrate within Bowman’s space is a passive process that is driven primarily by glomerular hydrostatic pressure. The glomerular hydrostatic pressure directly reflects the hydraulic pressure generated by cardiac output and local renal vascular tone. Therefore, extrarenal factors that alter renal perfusion can affect GFR and elimination of nitrogenous wastes from the body.



4 What is the purpose of the glomerulus?


The glomerulus acts as a molecular sieve to form an acellular, low-protein ultrafiltrate of plasma that will eventually become urine.



5 Describe how the glomerulus functions as a molecular sieve.


The properly functioning glomerulus acts to exclude molecules from the ultrafiltrate based on their size, shape, and charge. Substances smaller than 68 kilodaltons (kD) may freely enter the filtrate. The concentrations of these substances within the filtrate may be modified later by renal tubular reabsorption or secretion. The glomerulus has a net negative charge that serves to repel negatively charged molecules from entering the filtrate, such as albumin. Its molecular mass (69 kD) and net negative charge prevent albumin from accumulating in the glomerular filtrate, as long as the glomeruli are functioning within normal limits.



6 How do the renal tubules participate in formation of urine?


The renal tubules function under the combinatorial regulation of multiple hormonal inputs to alter the initial glomerular filtrate in a way that maintains homeostasis and responds to the body’s needs. For example, conserved substances are reabsorbed by the renal tubules, (e.g., glucose, amino acids, water-soluble vitamins), and wastes are secreted by the tubules (e.g., protons, ammonia).



7 How does the kidney function in total body water balance?


Under the control of antidiuretic hormone (ADH), aldosterone, and other hormones, the renal tubules modify the glomerular filtrate either by retention of water, which results in a concentrated urine, or by water excretion, which results in a dilute urine.



8 Define renal threshold.


Certain molecules (e.g., glucose) that readily enter the glomerular filtrate are almost entirely reabsorbed by the renal tubules, as long as the concentration within the filtrate is not excessive. Renal threshold refers to the maximal concentration of a given substance that the renal tubules can reabsorb. The substance will begin to appear in the urine when its concentration in the plasma, and therefore in the glomerular filtrate, exceeds this threshold reabsorptive capacity.



9 What is urea?


Urea is used as a serum biochemical marker of nitrogenous waste retention by the kidneys. Urea represents the endpoint of protein catabolism. During the process of protein catabolism, ammonium is generated. Within the liver, two molecules of ammonium are combined to form urea during the urea cycle in an energy-dependent reaction.



10 How does urea enter the blood?


The major source of blood urea comes from endogenous synthesis, primarily by the liver. A small portion of the urea that is filtered through the glomerulus is reabsorbed by the kidney and may reenter the general circulation. Additionally, a small amount of urea is found in the gut due to pancreatic secretions and movement of urea-containing water into the gut. However, little of this urea is directly absorbed as urea. Instead, it is converted to ammonia by enteric bacteria, which, in nonruminants, can then be absorbed and reenter the urea cycle.



11 How does protein catabolism affect blood urea nitrogen concentration?


Because urea is produced as the end product of protein metabolism, the blood urea nitrogen (BUN) concentration is directly proportional to the rate of protein catabolism. Therefore, diseases or physiologic states that alter the rate of protein catabolism can affect BUN.



12 List diseases or physiologic states that increase the rate of protein catabolism, and therefore may increase BUN concentration, in the absence of renal dysfunction.



a. Consumption of high-protein diet

b. Gastrointestinal (GI) hemorrhage

c. Prolonged strenuous exercise

d. Fever

e. Seizures

f. Acidosis

g. Hyperadrenocorticism/corticosteroid administration

h. Infection

i. Burns

j. Starvation

k. Possibly tetracycline administration


13 What happens to urea once it enters the bloodstream?


Once produced by the liver, urea enters the bloodstream and distributes throughout the total body water. Most urea is excreted from the body by the kidneys. A portion (at least 25% in humans) is removed from the bloodstream and metabolized to ammonia and carbon dioxide by the action of urease-producing enteric bacteria. The ammonia generated by enteric bacteria may then be absorbed and reenter the urea cycle. This results in a futile cycling of urea between the gut and the liver in nonruminants. (Controlling this generation of ammonia by enteric bacteria is especially important when managing hepatoencephalopathy.) In ruminants the ammonia generated by enteric bacteria is used as a substrate for amino acid synthesis by enteric organisms.



14 What is the fate of urea once it is presented to the glomerulus?


Because of the filtration pressure at the glomerulus, urea is passively moved across the glomerular basement membrane during the formation of the glomerular filtrate. The concentration of urea within the initial glomerular filtrate passively equilibrates with the concentration of urea in the blood. Therefore the blood urea concentration is equal to that contained within the initial glomerular filtrate. The filtrate’s urea concentration is modified during its movement through distal portions of the nephron, such that the final concentration of urea within the urine is different than that in the initial glomerular filtrate.



15 Where do the renal tubules modify the concentration of urea in the initial glomerular filtrate?


As the filtrate moves through the tubules, the urea concentration is passively modified at the following three locations in the nephron:


a. Proximal tubule: urea passively reabsorbed from the filtrate

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Aug 26, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on EVALUATION OF RENAL FUNCTION

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