Accurate assessment of kidney function in the critically
- 1. 01 INTRODUCTION Glomerular filtration is the first step in the complex process of urine formation. For filtration to occur, a rapid renal blood flow (RBF) at a consistent pressure is essential. There are many factors that can alter RBF and, thus, the rate of glomerular filtrate generation. At any given time, especially under the condition of stress, multiple factors act and counteract to maintain a normal glomerular filtration rate (GFR) despite changes in RBF. This article will examine the unique characteristics of the renal circulation, describe the physiology of glomerular filtration, review the extrinsic and intrinsic factors that can alter renal hemodynamics, and discuss a clinical situation in which multiple factors are interacting in an effort to maintain the RBF and GFR despite systemic pressure change. Fig: 01 Glomerulus structure and its anatomy with Blood Capillaries. RENAL CIRCULATION Blood flows into the kidneys at a rate of about 1,000-1,200 ml per minute, representing approximately 20%-25% of the cardiac output. This rapid blood flow rate exceeds the metabolic and oxygen needs of the kidneys but facilitates efficient clearance of metabolic waste products. To understand glomerular filtration, it is essential to consider the special characteristics of the renal circulation.
- 2. 02 The renal artery pressure is approximately 100 mmHg. This high pressure is maintained up to the afferent arteriole, the location of the first major point of vascular resistance. Across the afferent arteriole, the arterial pressure falls to about 40-60 mmHg. Although this is a significantly lower pressure than present in the systemic circulation, this pressure is higher than that in the glomerular capillary bed. This pressure is referred to as hydrostatic pressure. Maintaining a hydrostatic pressure of about 50 mmHg is the key to glomerular filtration, as it is needed to overcome other opposing pressures present in the glomerular capillaries and Bowman’s space. The glomerulus is a bundle of capillaries that are highly porous compared to systemic capillaries. The portion of the blood that is not filtered across the filtration barriers in the glomerular capillaries returns to the central circulation via the peritubular capillary (PTC) network. Fig:02 Glomerular Blood Flow And its Pressure GLOMERULAR FILTRATION Glomerular anatomy. The porous glomerular capillaries rest between the afferent and efferent arterioles (see Figure 2). Their function is to filter large quantities of water and solutes from the plasma. As blood flows through the glomerulus a portion is sieved through the filtering layers of the glomerular capillaries into the Bowman’s space. The filtration barrier is composed of three layers that allow for the filtration of solutes (eg., blood urea nitrogen, creatinine, electrolytes) and water, but prevent the loss of blood components such as red and white blood cells and plasma proteins. The three layers are the glomerular capillary endothelium, glomerular basement membrane, and visceral layer of Bowman’s capsule (epithelial cell layer).
- 3. 03 The first layer of the filtration barrier is the capillary endothelium, which has large fenestrations that allow the free filtration of substances with diameters up to 100 nm, thus excluding blood cells and large plasma proteins. The surface of the endothelial cells has a negative charge that inhibits the movement of negatively charged substances such as plasma proteins, as like-charges repel each other. The second layer, the glomerular basement membrane, represents the major barrier to the filtration of macromolecules. The glomerular basement membrane is made of fibrous proteins such as collagen, fibrin, and laminin, which intertwine to form a meshwork. As the fibers cross each other, small openings are created through which selective filtration occurs. The crossed fibers act as a size barrier and restrict the filtration of large molecules. This layer also contains anionic sialoproteins that further inhibit filtration by repelling other negatively charged ions. The third layer, composed of epithelial cells, is the visceral layer of the Bowman’s capsule. These epithelial cells, called podocytes, are attached directly to the exterior surface of the basement membrane. The podocytes branch into multiple finger-like projections called foot processes. These foot processes, which cover the outer surface of the basement membrane, are in close proximity to each other forming narrow elongated, slit-type openings about 25-60 nm wide. These openings, called slit pores, are covered by thin diaphragms. The foot processes have anionic sialoproteins on their borders that form the slit pores, generating a highly negatively charged region through which the filtrate must pass. These negative charges assist in preventing plasma proteins from entering the tubular fluid since plasma proteins carry negative charges. These narrow slits combined with the negative charges of the podocytes provide the final barrier to molecule movement through the glomerular membrane. The glomerulus is a selective filtration membrane. The factors that determine which molecules are filtered are molecular size, electrical charge, protein binding, configuration, and rigidity. Small molecules with molecular weights (MW) less than 7,000 Daltons (eg., water, MW 18; and all ionsincluding sodium, potassium, chloride, phosphate, magnesium, and calcium) are filtered without restriction. Larger molecules, such as myoglobin with a MW of 17,000 Daltons, are filtered to a lesser degree. Very large molecules, such as plasma proteins with molecular weights approaching 70,000 Daltons, are restricted from passing through the normal glomerulus.
- 4. 04 Fig:03 Glomerular filteration. As the filtration barrier has a net negative electrical charge, the movement of large negatively charged molecules is restricted more than molecules with a positive or neutral charge. As a result, proteins, which are negatively charged, are not freely filtered by the glomerulus. Likewise, drugs, ions, or small molecules bound to protein are not filtered. Round molecules do not filter as easily as ellipsoid molecules. The more rigid a molecule, the less easily it filters. Normal glomerular filtrate is essentially protein free but contains crystalloids (eg., sodium, chloride, creatinine, urea, uric acid, and phosphate) in the same concentration as plasma. A final anatomical aspect of the glomerulus is the mesangial cells, which are located between the capillary loops. They support the capillary structures and carry out some phagocytic activities. They also demonstrate contractile properties and can alter the total filtration surface area. Mesangial cells contract when exposed to vasoconstrictive substances, such as angiotensin II, thus decreasing the effective filtration surface area and glomerular filtration rate. The special filtering characteristics of the glomerulus coupled with the unique renal circulation allow for effective glomerular filtration to occur.
- 5. 05 GLOMERULAR FILTRATION RATE AND FILTRATION FRACTION Glomerular filtrate moves into the Bowman’s space and then into the tubular component of the nephron. In the average 70 kg adult, glomerular filtration rate (GFR) is approximately 125 ml/minute. This means that about 180 L of glomerular filtrate is produced in a 24-hour period, which is more than 30 times the average total blood volume. All but one to two liters are reabsorbed from the nephron into the peritubular capillaryand vasa recta network. The formation of such a large amount of filtrate assures adequate filtration of plasma, but requires very efficient reabsorptive processes to prevent volume and electrolyte depletion. GFR, which indicates the volume of filtrate moving from the glomerular capillaries into Bowman’s space per unit of time, is calculated by determining the renal clearance of a marker substance. Clearance (CL) is defined as the volume of plasma from which a substance is completely removed or cleared by the kidneys per unit of time. The ideal marker for measuring CL does not bind to proteins, is freely filtered at the glomerulus, and is neither reabsorbed, secreted, synthesized, nor metabolized by the tubules. Substances that do not meet these requirements can result in falsely elevated or decreased values of GFR. As there is no naturally occurring ideal marker, endogenous creatinine (Cr) often is used to measure clearance; however, since a small amount of creatinine is secreted into the tubule, GFR measured with creatinine clearance (CrCL) will be overestimated. Since individual GFRs vary widely, a change in GFR over time is more important than the absolute value of the GFR. Thus, if the CrCL method is used, it should be used for all measurements of GFR to allow for comparison of values over time. Inulin is a marker that meets all the requirements of an ideal marker for measuring GFR, but is not often used because it is an exogenous substance that must be infused for several hours at a constant rate making it both impractical and costly. A more practical method to determine the CrCL involves the collection of a 24-hour urine and midpoint plasma Cr (PCr). CrCL is calculated using Equation 1 where UCr is the urine creatinine concentration, mg/dl; Uv is the average urine flow rate, ml/min; and PCr is the plasma creatinine concentration, mg/dl.
- 6. 06 Fig;04 Glomerular Filteration Rate Pressures influencing normal glomerular filtration. Glomerular filtration is controlled by four pressures, the algebraic sum of which defines the net filtration pressure. These pressures are: 1. Glomerular capillary hydrostatic pressure (PGC), which is the pressure exerted against the capillary wall by fluid within the capillary lumen. This pressure is generated by the blood pressure. This value ranges from 40-60 mmHg and favors the movement of fluid from the capillary lumen to the Bowman’s space (see Figure 3). 2. Glomerular capillary colloid osmotic pressure (ΠGC), which is about 18 mmHg. This force, generated by plasma proteins within the capillaries, retards the movement of fluid out of the capillary lumen. 3. Bowman’s space hydrostatic pressure (PBS), which is the pressure exerted against the outer layer of the capillary walls by fluids within Bowman’s space. This value is about 10 mmHg and also retards the movement of fluid out of the capillary lumen. 4. Bowman’s space colloid osmotic pressure (ΠBS), which is normally 0 mmHg because normal filtrate is void of protein. Net filtration pressure (NFP), which is the sum of these negative and positive pressures, can be calculated with the following formula (see Equation 4). An average PGC of 45 mmHg is used.
- 7. 07 These pressures are based on animal models but are thought to be similar to those in humans. Therefore, for adequate filtration to occur, an NFP of approximately 17 mmHg is required to produce approximately 125 ml/min of glomerular filtrate. As blood progresses through the glomerulus toward the efferent arteriole, the ΠGC increases from 18 to approximately 35 mmHg, reflecting the removal of protein free fluid from the glomerular capillary. PGC, PBS, and ΠBS remain essentially unchanged along the capillary loop. As a result, NFP decreases along the length of the capillary and in some species is zero before or at the end of the capillary. The point at which NFP falls to zero is called filtration equilibrium and is the point at which glomerular filtration ceases. In normal resting man the glomerular filtration rate (GFR) averages 125 cc/min. The ultrafiltrate is derived from an average total renal plasma flow (RPF) of 600 cc/min. The ratio of glomerular ultrafiltrate to renal plasma flow is referred to as the filtration fraction (FF). where, . The FF represents the fraction of plasma which is filtered across the glomerular capillary bed. Alterations in the FF can have an important effect on proximal tubular reabsorption of fluid. The glomerular filtration rate is several fold that of fluid movement in capillary beds of other tissues, principally because the surface area available for diffusion plus the permeability of the glomerular capillary membrane (Kf) is approximately 100 fold that of other capillary beds. Glomerular Filtration Reabsorb: NaCl , Water , amino acids , sugars Secrete: Organic ions , drugs ,Passive water and NaCl movement Secrete: K+ and H+ Reabsorb: NaCl , water Proximal Tubule Distal Tubule Glomerular tuft thin loop FF GFR = -R----P----F--- RPF = RBF × (1 – Hct)
- 8. 08 CONDITIONS ALTERING THE GLOMERULAR FILTRATION RATE The GFR can be altered by changes in any of the above four pressures or the ultrafiltration coefficient. Theoretically, there is a direct relationship between the renal plasma flow rate (the primary determinant of PGC) and the GFR. If RPF increases due to volume expansion or other causes, GFR should also rise. However, a change in RBF rarely occurs in isolation and is usually accompanied by changes in afferent or efferent arteriolar resistance designed to maintain a consistent PGC and, therefore, GFR at normal levels. Through a process called autoregulation, the kidney is adept at maintaining a normal GFR over a wide range of vascular pressures. Changes in the glomerular colloid osmotic pressure (ΠGC) can also alter GFR. Serum protein is the major determinant in colloid osmotic pressure. If ΠGC increases due to increased protein concentration, as in hypovolemic states, NFP and, thus, GFR will fall in an effort to prevent further intravascular volume loss. If the ΠGC decreases due to protein losses, as in severe malnutrition or hypoalbuminemic states, GFR can increase. When PBS increases, as in urinary tract obstruction, GFR falls. GFR decreases as glomerular filtrate outflow from the tubule is obstructed and the pressure of the increased filtrate volume in the Bowman’s space opposes the forces favoring filtration. If ΠBS rises above 0 mmHg, as it does in nephrotic conditions where protein is filtered into the Bowman’s space because of changes in the filtering membrane, GFR can increase. The presence of plasma proteins in Bowman’s space favors increased filtration due to the increased colloid osmotic pressure within that compartment. A final factor that can alter GFR is a change in the ultrafiltration coefficient (Kf) of the glomerular capillary membrane. The Kf is the product of the surface area and hydraulic perme- ability of the glomerular membrane. Vasoactive substances such as angiotensin II can cause the mesangial cells to contract thereby decreasing the available surface area for filtration resulting in a decreased GFR. Glomerular filtration is the critical initial step in urine formation. In order for adequate solute and water removal to occur, glomerular filtrate must be generated at a constant rate. Changes in RPF, hydrostatic and osmotic pressures, available filtering surface area, and glomerular membrane permeability can upset the internal environment by increasing or
- 9. 09 decreasing GFR. Fortunately there exists a complicated system where factors both extrinsic and intrinsic to the kidneys act simultaneously to minimize the effects of these changes. CONCLUSION The formation of urineis a process that beginswithglomerularfiltration and is greatly influenced bychangesin renal hemodynamics. Selective filtration of the blood is possible because of the unique characteristics of the glomerulus and renal circulation. Many factors interact to maintain a consistent blood flow allowingfiltration and urine formation to continue despite systemic changes in blood pressure. Factors that impact on renal hemodynamics include the autoregulatory mechanism, the renin-angiotensin mechanism, eicosanoids, kinins, the sympathetic nervous system (SNS), catecholamines, antidiuretic hormone, endothelin, nitric oxide, atrial natriuretic peptide, and dopamine. Knowledge of the effects of these factors will allow the nephrology nurse to predict, identify, and assist in the treatment of clinical conditions that can alter renal hemodynamics and glomerular filtration.