The glomerular filtrate contains most inorganic ions and low-molecular-weight organic solutes in virtually the same concentrations as in the plasma. It also contains small plasma peptides and a very limited amount of albumin. Filtered fluid must pass through a 3-layered glomerular filtration barrier. The first layer, the endothelial cells of the capillaries, is perforated by many large fenestrae (“windows”), like a slice of Swiss cheese, which occupy about 10% of the endothelial surface area. They are freely permeable to everything in the blood except cells and platelets. The middle layer, the capillary basement membrane, is a gel-like acellular meshwork of glycoproteins and proteoglycans, with a structure like a kitchen sponge. The third layer consists of epithelial cells (podocytes) that surround the capillaries and rest on the capillary basement membrane. The podocytes have an unusual octopus-like structure. Small “fingers,” called pedicels (or foot processes), extend from each arm of the podocyte and are embedded in the basement membrane (see Figure 1–5D). Pedicels from a given podocyte interdigitate with the pedicels from adjacent podocytes. The pedicels are coated by a thick layer of extracellular material, which partially occludes the slits. Extremely thin processes called slit diaphragms bridge the slits between the pedicels. Slit diaphragms are widened versions of the tight junctions and adhering junctions that link all contiguous epithelial cells together and are like miniature ladders. The pedicels form the sides of the ladder, and the slit diaphragms are the rungs. Spaces between slit diaphragms constitute the path through which the filtrate, once it has passed through the endothelial cells and basement membrane, travels to enter Bowman's space.
Both the slit diaphragms and basement membrane are composed of an array of proteins, and while the basement membrane may contribute to the selectivity of the filtration barrier, integrity of the slit diaphragms is essential to prevent excessive leak of plasma protein (albumin). Some protein-wasting diseases are associated with abnormal slit diaphragm structure.
The selectivity of the filtration barrier is crucial for renal function. The barrier has to be leaky enough to permit free passage of everything that should be filtered, such as organic waste, yet restrictive to plasma proteins that should not be filtered. Selectivity of the barrier is based on both molecular size and electrical charge. Let us look first at size.
The filtration barrier of the renal corpuscle provides no hindrance to the movement of molecules with molecular weights less than 7000 Da (ie, solutes this small are all freely filtered). This includes all small ions, glucose, urea, amino acids, and many hormones. The filtration barrier almost totally excludes plasma albumin (molecular weight of approximately 66,000 Da). (For simplicity we use molecular weight as the reference for size; in reality, it is molecular radius and shape that is critical.) The hindrance to plasma albumin is not 100%, however, and the glomerular filtrate does contain extremely small quantities of albumin, on the order of 10 mg/L or less. This is only about 0.02% of the concentration of albumin in plasma and is the reason for the use of the phrase “nearly protein-free” earlier. Some small substances are partly or mostly bound to large plasma proteins and are thus not free to be filtered, even though the unbound fractions can easily move through the filtration barrier. This includes hydrophobic hormones of the steroid and thyroid categories and about 40% of the calcium in the blood.
Fixed negative charges in the extracellular matrix of the filtration barrier restrict passage of negatively charged plasma proteins.
For molecules with a molecular weight ranging from 7000 to 70,000 Da, the amount filtered becomes progressively smaller as the molecule becomes larger (Figure 2–3). Thus, many normally occurring small- and medium-sized plasma peptides and proteins are actually filtered to a significant degree. Moreover, when certain small proteins appear in the plasma because of disease (eg, hemoglobin released from damaged erythrocytes or myoglobin released from damaged muscles), considerable filtration of these may occur as well.
A, As molecular weight (and therefore size) increases, filterability declines, so that proteins with a molecular weight above 70,000 Da are hardly filtered at all. B, For any given molecular size, negatively charged molecules are restricted far more than neutral molecules, whereas positively charged molecules are restricted less. (Reproduced with permission from Kibble J, Halsey CR. The Big Picture: Medical Physiology. New York: McGraw-Hill; 2009.)
Electrical charge is the second variable determining filterability of macromolecules. For any given size, negatively charged macromolecules are filtered to a lesser extent, and positively charged macromolecules to a greater extent, than neutral molecules. This is because the surfaces of all the components of the filtration barrier (the cell coats of the endothelium, the basement membrane, and the cell coats of the slit diaphragms) contain fixed polyanions, which repel negatively charged macromolecules during filtration. Because almost all plasma proteins bear net negative charge, this electrical repulsion plays a very important restrictive role, enhancing that of purely size hindrance. In other words, if either albumin or the filtration barrier were not charged, even albumin would be filtered to a considerable degree (see Figure 2–3B). Certain diseases that cause glomerular capillaries to become “leaky” to protein do so because negative charges in the membranes are lost.
It must be emphasized that the negative charges in the filtration membranes act as a hindrance only to macromolecules, not to mineral anions or low-molecular-weight organic anions. Thus, chloride and bicarbonate ions, despite their negative charge, are freely filtered.
Direct Determinants of Glomerular Filtration Rate
The value of glomerular filtration rate (GFR) is a crucial determinant of renal function. It affects the excretion of waste products, and because its normal value is so large, it affects the excretion of all the substances that are handled by downstream tubular elements, particularly salt and water. Regulation of the GFR is straightforward in terms of physical principles but very complex functionally because there are many signals impinging on the controllable elements.
The rate of filtration in any capillary bed, including the glomeruli, is determined by the hydraulic permeability of the capillaries (including in this case, all elements of the filtration barrier), their surface area, and the net filtration pressure (NFP) acting across them.
Because it is difficult to estimate the surface area of a capillary bed, a parameter called the filtration coefficient (Kf) is used to denote the product of the hydraulic permeability and surface area.
The NFP is the algebraic sum of the hydrostatic pressures and the osmotic pressures resulting from protein—the oncotic, or colloid osmotic pressures—on the 2 sides of the capillary wall. There are 4 pressures to consider: 2 hydrostatic pressures and 2 oncotic pressures. These are known as Starling forces. In the glomerular capillaries
where PGC is glomerular capillary hydrostatic pressure, πBC is the oncotic pressure of fluid in Bowman's capsule, PBC is the hydrostatic pressure in Bowman's capsule, and πGC is the oncotic pressure in glomerular capillary plasma, shown schematically in Figure 2–4, along with typical average values.
Forces involved in glomerular filtration as described in the text. (Reproduced with permission from Widmaier EP, Raff H, Strang KT. Vander's Human Physiology. 11th ed. McGraw-Hill; 2008.)
Because there is normally little total protein in Bowman's capsule, πBC may be taken as zero and not considered in our analysis. Accordingly, the overall equation for GFR becomes
Figure 2–5 shows that the hydrostatic pressure is nearly constant within the glomeruli. This is because there are so many capillaries in parallel, and collectively they provide only a small resistance to flow, but the oncotic pressure in the glomerular capillaries does change substantially along the length of the glomeruli. As water is filtered out of the vascular space it leaves most of the protein behind, thereby increasing protein concentration and, hence, the oncotic pressure of the unfiltered plasma remaining in the glomerular capillaries. Mainly because of this large increase in oncotic pressure, the NFP decreases from the beginning of the glomerular capillaries to the end (Table 2–1).
Forces affecting glomerular filtration along the length of the glomerular capillaries. Note that the oncotic pressure within the capillaries (πGC) rises due to loss of water and that the net filtration pressure (shaded region) decreases as a result.
Table 2–1.Estimated forces involved in glomerular filtration in humans ||Download (.pdf) Table 2–1. Estimated forces involved in glomerular filtration in humans
|Forces ||Afferent end of glomerular capillary (mm Hg) ||Efferent end of glomerular capillary (mm Hg) |
1. Favoring filtration
hydraulic pressure, PGC
|60 ||58 |
2. Opposing filtration
| || |
3. Net filtration pressure (1 − 2)
|24 ||10 |
The NFP when averaged over the whole length of the glomerulus is about 16 mm Hg. This average NFP is higher than that found in most nonrenal capillary beds. Taken together with a very high value for Kf, it accounts for the enormous filtration of 180 L of fluid/day (compared with 3 L/day or so in all other capillary beds combined).
The GFR is not constant but shows fluctuations in differing physiological states and in disease. Its value must be tightly controlled. To understand control of the GFR, it is essential to see how a change in any one factor affects GFR under the assumption that all other factors are held constant.
Table 2–2 presents a summary of these factors. It provides a checklist to review when trying to understand how diseases or vasoactive chemical messengers and drugs change GFR. It should be noted that the major cause of decreased GFR in renal disease is not a change in these parameters within individual nephrons, but rather a decrease in the number of functioning nephrons. This reduces Kf.
Table 2–2.Summary of direct GFR determinants and factors that influence them ||Download (.pdf) Table 2–2. Summary of direct GFR determinants and factors that influence them
|Direct determinants of GFR: GFR = Kf (PGC − PBC − πGC) ||Major factors that tend to increase the magnitude of the direct determinant |
Glomerular surface area (because of relaxation of glomerular mesangial cells)
Result: ↑ GFR
Renal arterial pressure Afferent-arteriolar resistance (afferent dilation)
Efferent-arteriolar resistance (efferent constriction)
Result: ↑ GFR
Intratubular pressure because of obstruction of tubule or extrarenal urinary system
Result: ↓ GFR
Systemic-plasma oncotic pressure (sets πGC at beginning of glomerular capillaries)
Renal plasma flow (causes increased rise of πGC along glomerular capillaries)
Result: ↓ GFR
Filtration Coefficient (Kf)
Changes in Kf are caused most often by glomerular disease, but also by normal physiological control. The details are still not completely clear, but chemical messengers released within the kidneys cause contraction of glomerular mesangial cells. Such contraction may restrict flow through some of the capillary loops, effectively reducing the area available for filtration, Kf, and, hence GFR.
Glomerular Capillary Hydrostatic Pressure (PGC)
Hydrostatic pressure in the glomerular capillaries is influenced by many factors. We can depict the situation using the analogy of a leaking garden hose. If pressure in the pipes feeding the hose changes, the pressure in the hose and, hence, the rate of leak will be altered. Resistances in the hose also affect the leak. If we kink the hose upstream from the leak, pressure at the region of leak decreases and less water leaks out. However, if we kink the hose beyond the leak, this increases pressure at the region of leak and increases the leak rate. These same principles apply to PGC and GFR. First, a change in renal arterial pressure causes a change in PGC in the same direction. If resistances remain constant, PGC rises and falls as renal artery pressure rises and falls. This is a crucial point because arterial blood pressure shows considerable variability. Second, changes in the resistance of the afferent and EAs have opposite effects on PGC. An increase in afferent arteriolar resistance, which is upstream from the glomerulus, is like kinking the hose above the leak—it decreases PGC. An increase in efferent arteriolar resistance is downstream from the glomerulus and is like kinking the hose beyond the leak—it increases PGC. Of course dilation of the AA raises PGC, and hence GFR, whereas dilation of the EA lowers PGC and GFR. It should also be clear that when the afferent and efferent arteriolar resistances both change in the same direction (ie, they both increase or decrease), they exert opposing effects on PGC.
What is the significance of this? It means the kidney can regulate PGC and, hence, GFR independently of RBF. The effects of changes in arteriolar resistances are summarized in Figure 2–6.
Effect of changes in resistance on GFR. A–D, Constricting the afferent arteriole (AA) or dilating the efferent arteriole (EA) leads to a decreased GFR, while dilating the AA or constricting the EA leads to increased GFR. (Reproduced with permission from Widmaier EP, Raff H, Strang KT. Vander's Human Physiology. 11th ed. McGraw-Hill; 2008.)
Hydrostatic Pressure in Bowman's Capsule (PBC)
Changes in pressure within Bowman's space are usually of very minor importance. However, obstruction anywhere along the tubule or in the external portions of the urinary system (eg, the ureter) increases the tubular pressure everywhere proximal to the occlusion, all the way back to Bowman's capsule. The result is to decrease GFR.
Oncotic Pressure in Glomerular Capillary Plasma (πGC)
Oncotic pressure in the plasma at the very beginning of the glomerular capillaries is the oncotic pressure of systemic arterial plasma. Accordingly, a decrease in arterial plasma protein concentration, as occurs, for example, in liver disease, decreases arterial oncotic pressure and tends to increase GFR, whereas increased arterial oncotic pressure tends to reduce GFR.
However, recall that πGC is the same as arterial oncotic pressure only at the very beginning of the glomerular capillaries; πGC then increases somewhat along the glomerular capillaries as protein-free fluid filters out of the capillary, concentrating the protein left behind. This means that NFP and, hence, filtration progressively decrease along the capillary length. Accordingly, anything that causes a steeper increase in πGC tends to lower average NFP and hence GFR.
Such a steep increase in oncotic pressure occurs in conditions of low RBF. When RBF is low, the filtration process removes a relatively larger fraction of the plasma, leaving a smaller volume of plasma behind in the glomeruli still containing all the plasma protein. The πGC reaches a final value at the end of the glomerular capillaries that is higher than normal. This lowers average NFP and, hence, GFR. Conversely, a high RBF, all other factors remaining constant, causes πGC to increase less steeply and reach a final value at the end of the capillaries that is less than normal, which increases the GFR. Table 2–1 shows typical values of the forces affecting glomerular filtration under normal conditions.
As blood is composed of cells and plasma, we can describe the flow of plasma per se, the renal plasma flow (RPF). Variations in the relative amounts of plasma that are filtered can be expressed as a filtration fraction: the ratio RPF/GFR, which is normally about 20%, that is, about 20% of the plasma entering the kidney is removed from the blood and put into the Bowman's space. The increase in πGC along the glomerular capillaries is directly proportional to the filtration fraction (ie, if relatively more of the plasma is filtered, the increase in πGC is greater). If the filtration fraction has changed, it is certain that there has also been a proportional change in πGC and that this has played a role in altering GFR.
A term we use in other chapters is filtered load. It is the amount of substance that is filtered per unit time. For freely filtered substances, the filtered load is the product of GFR and plasma concentration. Consider sodium. Its normal plasma concentration is 140 mEq/L, or 0.14 mEq/mL. A normal GFR in healthy young adult males is 125 mL/min, so the filtered load of sodium is 0.14 mEq/mL × 125 mL/min = 17.5 mEq/min. We can do the same calculation for any other substance, being careful in each case to be aware of the unit of measure in which concentration is expressed. The filtered load is what is presented to the rest of the nephron to handle. The filtered load varies with plasma concentration and GFR. An increase in GFR, at constant plasma concentration, increases the filtered load, as does an increase in plasma concentration at constant GFR. Variations in filtered load play a major role in the renal handling of many substances.