Urine output and gfr relationship

Renal function - Wikipedia

urine output and gfr relationship

Glomerular filtration rate (GFR) is the volume of fluid by lesser vasoconstriction of the output or efferent arteriole. GFR. Increased GFR, urine production, and decreased FC of Na were (2) to assess the correlation between the renal ER and FC of solutes; and (3). cardiac output and renal function in a HF population with careful cardiac and Both authors have reported that they have no relationships relevant to the FIGURE 1 FF Is More Important Than GFR in Heart Failure. Relative resistance. 1 .

Remote organs damage has been suggested in several experimental studies [ 1112 ]. Ischemic-induced AKI has been found to induce myocardial apoptosis [ 13 ], to activate lung inflammatory and apoptotic pathways, and to increase lung water permeability [ 14 ]. Surprisingly, even a small increase of serum creatinine after cardiac surgery or transient i. Although fluid resuscitation and optimization of renal perfusion pressure are central to the prevention and treatment of AKI, excessive fluid resuscitation may be harmful in some critically ill patients.

First, aggressive fluid resuscitation, although increasing renal blood flow, can be ineffective in restoring renal microvascular oxygenation due to hemodilution with no increase in blood-oxygen carriage capacities [ 18 ]. Second, positive fluid balance can deteriorate cell oxygenation and prolong mechanical ventilation [ 19 ]. Finally, fluid overload may lead to central venous congestion and decrease of renal perfusion pressure [ 20 ], which will promote the development of AKI in patients with acute heart failure [ 21 ] or sepsis [ 22 ].

The type of fluid used also can have a role with "renal toxicity" associated with the use of colloids. Urine output and definition of acute kidney injury In clinical research, more than 30 definitions of acute renal failure have been used before the release of the RIFLE criteria by the Acute Dialysis Quality Initiative group in [ 23 ].

The first merit of this classification was to introduce a standard and simple definition of AKI for clinical research purposes but also to stratify the severity of AKI based on serum creatinine level, creatinine clearance, or urine output.

urine output and gfr relationship

A part from the change in nomenclature Risk, Injury, and Failure were replaced by stage 1, 2, and 3, the categories Loss and Endstage disappearedan absolute increase of serum creatinine of 0. Finally, the AKIN criteria should be applied "after following adequate resuscitation when applicable" with the purpose of excluding patients with pure renal pre-azotemia.

The introduction of the RIFLE and AKIN definitions were a crucial step forward in the development of clinical research and have since been widely accepted by the medical community. Using these classifications, a patient with decrease of urine output will be classified as "AKI. This would be the case if decreased urine output is not associated with a decline of creatinine clearance.

Although severe acute renal failure with oliguria or anuria has been reported to be associated with a worse outcome compared with patients with preserved urine output, the use of urine output as a criterion to classify AKI severity may be misleading.

Understanding urine output in critically ill patients

It was reported that the combination of creatinine and urinary output for classifying the patient's risk of death was more stringent than urinary output alone for classifying patients [ 724 ]. One can conclude that patients classified according to the urine output criterion only might be less severe than those classified according to the combination of creatinine and urine output [ 25 ].

On the other hand, severe tubular dysfunction can lead to increased urine output despite low GFR. Urine output therefore seems to be a nonspecific and poor parameter for classifying of AKI in critically ill patients. Because there is less circulating protein, principally albumin, the osmotic pressure of the blood falls. Less osmotic pressure pulling water into the capillaries tips the balance towards hydrostatic pressure, which tends to push it out of the capillaries.

GFR and renal physiology

The net effect is that water is lost from the circulation to interstitial tissues and cells. It is determined as follows: Intuitively, you should realize that minor changes in osmolarity of the blood or changes in capillary blood pressure result in major changes in the amount of filtrate formed at any given point in time.

urine output and gfr relationship

The kidney is able to cope with a wide range of blood pressures. In large part, this is due to the autoregulatory nature of smooth muscle. When you stretch it, it contracts.

urine output and gfr relationship

Thus, when blood pressure goes up, smooth muscle in the afferent capillaries contracts to limit any increase in blood flow and filtration rate. When blood pressure drops, the same capillaries relax to maintain blood flow and filtration rate.

The net result is a relatively steady flow of blood into the glomerulus and a relatively steady filtration rate in spite of significant systemic blood pressure changes. One third of this is 10, and when you add this to the diastolic pressure of 80, you arrive at a calculated mean arterial pressure of 90 mm Hg. Therefore, if you use mean arterial pressure for the GBHP in the formula for calculating NFP, you can determine that as long as mean arterial pressure is above approximately 60 mm Hg, the pressure will be adequate to maintain glomerular filtration.

Blood pressures below this level will impair renal function and cause systemic disorders that are severe enough to threaten survival. This condition is called shock. This is more than just an academic exercise.

Renal function

Since many drugs are excreted in the urine, a decline in renal function can lead to toxic accumulations. Additionally, administration of appropriate drug dosages for those drugs primarily excreted by the kidney requires an accurate assessment of GFR.

GFR can be estimated closely by intravenous administration of inulin. Inulin is a plant polysaccharide that is neither reabsorbed nor secreted by the kidney.

urine output and gfr relationship

Its appearance in the urine is directly proportional to the rate at which it is filtered by the renal corpuscle.

However, since measuring inulin clearance is cumbersome in the clinical setting, most often, the GFR is estimated by measuring naturally occurring creatinine, a protein-derived molecule produced by muscle metabolism that is not reabsorbed and only slightly secreted by the nephron. Chapter Review The entire volume of the blood is filtered through the kidneys about times per day, and 99 percent of the water filtered is recovered.