Info

(mEq/L (milliequivalents per liter) is a commonly used measure of concentration based on how many charges an ion carries. For a substance with a charge of 1, such as Cl-, a mEq is equal to a millimole.)

(mEq/L (milliequivalents per liter) is a commonly used measure of concentration based on how many charges an ion carries. For a substance with a charge of 1, such as Cl-, a mEq is equal to a millimole.)

Concentrations (mg/100 mL)

Glomerular Filtrate Urine

100 0

26 1,820

4 53

1 196

Afferent arteriole Efferent arteriole

Afferent arteriole Efferent arteriole

Figure

(a) The first step in urine formation is filtration of substances through the glomerular membrane into the glomerular capsule.

(b) The glomerular filtrate passes through fenestrae of the capillary endothelium.

Figure

(a) The first step in urine formation is filtration of substances through the glomerular membrane into the glomerular capsule.

(b) The glomerular filtrate passes through fenestrae of the capillary endothelium.

^1 Reconnect to chapter 15, Exchanges in the Capillaries, pages 605-606 The glomerular capsule receives the resulting glomerular filtrate, which has about the same composition as the filtrate that becomes tissue fluid elsewhere in the body. That is, glomerular filtrate is mostly water and the same solutes as in blood plasma, except for the larger protein molecules. More specifically, glomerular filtrate contains water, glucose, amino acids, urea, uric acid, creatine, creatinine, and sodium, chloride, potassium, calcium, bicarbonate, phosphate, and sulfate ions. Table 20.1 shows the relative concentrations of some of the substances in the blood plasma, glomerular filtrate, and urine.

Filtration Pressure

The main force that moves substances by filtration through the glomerular capillary wall is the hydrostatic pressure of the blood inside, as in the case for other capillaries. (Recall that glomerular capillary pressure is high compared to other capillaries.) Glomerular filtration is also influenced by the osmotic pressure of the blood plasma in the glomerulus and by the hydrostatic pressure inside the glomerular capsule.

The colloid osmotic pressure of the plasma due to the plasma proteins is always higher than that of the glomerular filtrate (except in some kinds of kidney disease). This tends to draw water back into the glomerular capillaries, thus opposing filtration. Any increase in glomerular capsule hydrostatic pressure would also oppose filtration.

The net effect of all of these forces is called net filtration pressure, and it is normally always positive, favoring filtration at the glomerulus. It can be calculated as follows:

Net filtration pressure =

force favoring filtration - forces opposing filtration

(glomerular capillary (capsular hydrostatic hydrostatic pressure) pressure and glomerular capillary osmotic pressure)

Hydrostatic pressure of blood

Hydrostatic pressure of blood

Plasma collid osmotic pressure opposes outward flow (about 32 mm Hg)

Capsular hydrostatic pressure opposes outward flow (about 18 mm Hg)

Glomerular hydrostatic pressure promotes outward flow (about 60 mm Hg)

Figure 20.16

The rate of glomerular filtration is affected by the hydrostatic and osmotic pressure of the plasma and the hydrostatic pressure of the fluid in the glomerular capsule.

Plasma collid osmotic pressure opposes outward flow (about 32 mm Hg)

Glomerular hydrostatic pressure promotes outward flow (about 60 mm Hg)

Capsular hydrostatic pressure opposes outward flow (about 18 mm Hg)

Figure 20.16

The rate of glomerular filtration is affected by the hydrostatic and osmotic pressure of the plasma and the hydrostatic pressure of the fluid in the glomerular capsule.

and filtration rate rises. Vasodilation of these vessels produces opposite effects.

The concentrations of certain components of the blood plasma can be used to evaluate kidney functions. For example, if the kidneys are functioning inadequately, the plasma concentrations of urea (as indicated by a blood urea nitrogen test) and of creatinine may increase as much as tenfold above normal.

Filtration Rate

The glomerular filtration rate (GFR) is directly proportional to the net filtration pressure. Consequently, the factors that affect the glomerular hydrostatic pressure, glomerular plasma osmotic pressure, or hydrostatic pressure in the glomerular capsule will also affect the rate of filtration (fig 20.16).

Normally, glomerular hydrostatic pressure is the most important factor determining net filtration pressure and GFR. Since each glomerular capillary is located between two arterioles—the afferent and efferent arterioles— any change in the diameters of these vessels is likely to change glomerular hydrostatic pressure, changing glomerular filtration rate. The afferent arteriole, through which the blood enters the glomerulus, may vasocon-strict in response to stimulation by sympathetic nerve impulses. If this occurs, net filtration pressure in that glomerulus decreases, and filtration rate drops. If, on the other hand, the efferent arteriole (through which the blood leaves the glomerulus) vasoconstricts, blood backs up into the glomerulus, net filtration pressure increases,

If arterial blood pressure drops drastically, as may occur during shock, the glomerular hydrostatic pressure may fall below the level required for filtration, leading to acute renal failure. At the same time, the epithelial cells of the renal tubules may fail to receive sufficient nutrients to maintain their high rates of metabolism. As a result, cells may die (tubular necrosis), and renal functions may be lost permanently, resulting in chronic renal failure.

The colloid osmotic pressure of the glomerular plasma also influences net filtration pressure and the rate of filtration. In other systemic capillaries, filtration occurs at the beginning of the capillary, but the osmotic effect of the plasma proteins predominates at the capillary, and most filtered fluid is thus reabsorbed. The small excess remaining eventually becomes lymph.

Because of the relatively high hydrostatic pressure in the glomerular capillaries, much more fluid is filtered than by capillaries elsewhere. In fact, as filtration occurs through the capillary wall, proteins remaining in the plasma raise the colloid osmotic pressure within the glomerular capillaries. Despite this, the glomerular capillary hydrostatic pressure is sufficiently great that the net filtration pressure is normally positive. That is, the forces favoring filtration in the glomerular capillaries always predominate. Of course, conditions that lower plasma colloid osmotic pressure, such as a decrease in plasma protein concentration, would increase filtration rate.

Essentials of Human Physiology

Essentials of Human Physiology

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