Fluids and electrolytes: Maintaining the balance

Focus Topic: Fluids and electrolytes

Various elements and processes in the body work together to maintain fluid and electrolyte balance. Because one problem can affect the entire fluid-electrolyte maintenance system, it’s important to keep all problems in check. Here’s a closer look at what makes this balancing act possible.

Fluids and electrolytes: Kidneys

Focus Topic: Fluids and electrolytes

The kidneys play a vital role in fluid and electrolyte balance. If the kidneys don’t work properly, the body has great difficulty controlling fluid balance. The workhorse of the kidney is the nephron, which forms urine. The body puts the nephrons through their paces every day.

A nephron consists of a glomerulus and a tubule. The tubule, sometimes convoluted, ends in a collecting duct. The glomerulus is a cluster of capillaries that filters blood. Like a vascular cradle, Bowman’s capsule surrounds the glomerulus.

Capillary blood pressure forces fluid through the capillary walls and into Bowman’s capsule at the proximal end of the tubule. Along the length of the tubule, water and electrolytes are either excreted or retained according to the body’s needs. If the body needs more fluid, for instance, it retains more. If it needs less fluid, less is reabsorbed and more is excreted. Electrolytes, such as sodium and potassium, are either filtered or reabsorbed throughout the same area. The resulting filtrate, which eventually becomes urine, flows through the tubule into the collecting ducts and eventually into the bladder as urine.

Fluids and electrolytes: Superabsorbent

Focus Topic: Fluids and electrolytes

Nephrons filter about 125 ml of blood every minute, or about 180 L/day. That rate, called the glomerular filtration rate, leads to the production of 1 to 2 L of urine per day. The nephrons reabsorb the remaining 178 L or more of fluid, an amount equivalent to more than 30 oil changes for the family car!

Fluids and electrolytes: A strict conservationist

Focus Topic: Fluids and electrolytes

If the body loses even 1% to 2% of its fluid, the kidneys take steps to conserve water. Perhaps the most important step involves reabsorbing more water from the filtrate, which produces a more concentrated urine.

The kidneys must continue to excrete at least 20 ml of urine every hour (500 ml/day) to eliminate body wastes. A urine excretion rate that’s less than 20 ml/hour usually indicates renal pathology. The minimum excretion rate varies with age.

The kidneys respond to fluid excesses by excreting a more dilute urine, which rids the body of fluid and conserves electrolytes.

Fluids and electrolytes: Other organs and glands

Focus Topic: Fluids and electrolytes

In addition to the kidneys, other organs and glands are essential to maintaining fluid and electrolyte balance. Sodium, potassium, chloride, and water are lost from the GI tract; however, electrolytes and fluid are also absorbed from the GI tract.

The parathyroid glands also play a role in electrolyte balance, specifically the balance of calcium and phosphorus. The thyroid gland is also involved by balancing the body’s calcium level.

Fluids and electrolytes: Antidiuretic hormone

Focus Topic: Fluids and electrolytes

Several hormones affect fluid balance, among them a water retainer called antidiuretic hormone (ADH). (You may also hear this hormone called vasopressin.) The hypothalamus produces ADH, but the posterior pituitary gland stores and releases it. If you can remember what ADH stands for, you can remember its job: to restore blood volume by reducing diuresis and increasing water retention.

Fluids and electrolytes: Sensitive to changes

Focus Topic: Fluids and electrolytes

Increased serum osmolality or decreased blood volume can stimulate the release of ADH, which in turn increases the kidneys’ reabsorption of water. The increased reabsorption of water results in more concentrated urine.

Likewise, decreased serum osmolality or increased blood volume inhibits the release of ADH and causes less water to be reabsorbed, making the urine less concentrated. The amount of ADH released varies throughout the day, depending on the body’s needs.

This up-and-down cycle of ADH release keeps fluid levels in balance all day long. Like a dam on a river, the body holds water when fluid levels drop and releases it when fluid levels rise.

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Fluids and electrolytes: Renin and angiotensin

Focus Topic: Fluids and electrolytes

To help maintain a balance of sodium and water in the body as well as to maintain a healthy blood volume and blood pressure, special cells (juxtaglomerular cells) near each glomerulus secrete an enzyme called renin. Through a complex series of steps, renin leads to the production of angio ten sin II, a powerful vasoconstrictor.

Angiotensin II causes peripheral vasoconstriction and stimulates the production of aldo sterone. Both actions raise blood pressure. (See Aldosterone production.)

As soon as the blood pressure reaches a normal level, the body stops releasing renin and this feedback cycle of renin to angio tensin to aldosterone stops.

Fluids and electrolytes: The ups and downs of renin

Focus Topic: Fluids and electrolytes

The amount of renin secreted depends on blood flow and the level of sodium in the bloodstream. If blood flow to the kidneys diminishes, as happens in a patient who’s hemorrhaging, or if the amount of sodium reaching the glomerulus drops, the juxtaglomerular cells secrete more renin. The renin causes vasoconstriction and a subsequent increase in blood pressure.

Aldosterone production

Fluids and electrolytes

Conversely, if blood flow to the kidneys increases, or if the amount of sodium reaching the glomerulus increases, juxtaglomerular cells secrete less renin. A drop-off in renin secretion reduces vasoconstriction and helps to normalize blood pressure.

Fluids and electrolytes: Aldosterone

Focus Topic: Fluids and electrolytes

The hormone aldosterone also plays a role in maintaining blood pressure and fluid and electrolyte balance. Secreted by the adrenal cortex, aldosterone regulates the reabsorption of sodium and water within the nephron.

Fluids and electrolytes: Triggering active transport

Focus Topic: Fluids and electrolytes

When blood volume drops, aldosterone initiates the active transport of sodium from the distal tubules and the collecting ducts into the bloodstream. That active transport forces sodium back into the bloodstream. When sodium is forced into the bloodstream, more water is reabsorbed and blood volume expands.

Fluids and electrolytes: Atrial natriuretic peptide

Focus Topic: Fluids and electrolytes

The renin-angiotensin system isn’t the only factor at work balancing fluids in the body. A cardiac hormone called atrial natriuretic peptide (ANP) also helps keep that balance. Stored in the cells of the atria, ANP is released when atrial pressure increases. The hormone opposes the renin-angiotensin system by decreasing blood pressure and reducing intravascular blood volume.

This powerful hormone:

  • suppresses serum renin levels
  • decreases aldosterone release from the adrenal glands
  • increases glomerular filtration, which increases urine excretion of sodium and water
  • decreases ADH release from the posterior pituitary gland
  • reduces vascular resistance by causing vasodilation
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