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Clinical Application

The Nephrotic Syndrome

The nephrotic syndrome is a set of symptoms that often appears in patients with renal diseases. It causes considerable loss of plasma proteins into the urine (proteinuria), widespread edema, and increased susceptibility to infections.

Plasma proteins are lost into the urine because of increased permeability of the glomerular membranes, which accompanies renal disorders such as glomerulonephritis. As a consequence of a decreasing plasma protein concentration (hy-poproteinemia), the plasma osmotic pressure falls, increasing filtration pressure in capillaries throughout the body. This may lead to widespread, severe edema as a large volume of fluid accumulates in the interstitial spaces within the tissues and in body spaces such as the abdominal cavity, pleural cavity, pericardial cavity, and joint cavities.

Also, as edema develops, blood volume decreases and blood pressure drops. These changes may activate the renin-angiotensin system, leading to the release of aldosterone from the adrenal cortex (see chapter 13, pp. 527-529), which, in turn, stimulates the kidneys to conserve sodium ions and water. This action reduces the urine output and may aggravate the edema.

The nephrotic syndrome sometimes appears in young children who have lipoid nephrosis. The cause of this condition is unknown, but it alters the epithelial cells of the glomeruli so that the glomerular membranes enlarge and distort, allowing proteins to leak through. ■

The proximal convoluted tubule reabsorbs about 70% of the filtered sodium, other ions, and water. By the end of the proximal convoluted tubule, osmotic equilibrium is reached, and the remaining tubular fluid is isotonic (fig. 20.20).

Active transport continues to reabsorb sodium ions as the tubular fluid moves through the nephron loop, the distal convoluted tubule, and the collecting duct. Consequently, almost all of the sodium and water (97%-99%) that enters the renal tubules as part of the glomerular filtrate may be reabsorbed before the urine is excreted. However, aldosterone controls sodium reabsorption, and antidiuretic hormone controls water reabsorption. Under the influence of these hormones, reabsorption of sodium and water can change to keep conditions in the body fluids constant. Chapter 21 (pp. 861 and 865) discusses the specific effects of these hormones.

Recall that the kidneys filter an extremely large volume of fluid (180 liters) each day. Thus, if 99% of the glomerular filtrate is reabsorbed, the remaining 1% excreted includes a relatively large amount of sodium and water (table 20.2). On the other hand, if sodium and water reabsorption decrease to 97% of the amount filtered, the amount excreted triples! Therefore, small changes in the

Figure 20.20

Water reabsorption by osmosis occurs in response to the reabsorption of sodium and other solutes by active transport in the proximal convoluted tubule.

Blood flow

Glomerular capsule

Glomerular filtrate

Proximal convoluted tubule

Isotonic tubular fluid

Blood flow

Glomerular filtrate

Glomerular capsule

Proximal convoluted tubule

Blood flow

Peritubular capillary

Blood flow

Blood flow

(1) Sodium ions are reabsorbed by active transport

(2) Negatively charged ions are attracted to positively charged ions

(3) As concentration of ions (solute) increases in plasma, osmotic pressure increases

(4) Water moves from proximal tubule to capillary by osmosis

Peritubular capillary

Blood flow

table

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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