, pub-7771400403364887, DIRECT, f08c47fec0942fa0 REGULATION OF FLUID AND ELECTROLYTE BALANCE

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Have you ever noticed the need for a drink after eating that large bucket of popcorn at the 

movies? Or on television, patients entering the ER with substantial blood loss are immediately 

given intravenous fluids (an IV)? Both scenarios relate to fluid and electrolyte balance. What do download full PDF here

these terms mean? Fluid refers to water. For water balance to occur, water intake through 
ingested liquids and foods and cellular metabolism must equal water output via sweating, urine, 

feces, and breathing. Water balance is essential for the body to be properly hydrated, avoiding 

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both dehydration and over-hydration. Electrolytes are inorganic compounds that dissociate in 

water to form ions. They get their name because they can conduct an electrical current in 

solution. Sodium is the most abundant ion of the extracellular fluid and is the main contributor 

to the osmolarity or solute concentration of blood. Get full notes in pdf

One of the key tasks of the kidneys is to regulate fluid and electrolyte balance by 

controlling the volume and composition of the urine. These adjustments are essential because the 

osmolarity of body fluids must be around 300 milliosmols/liter. There are three hormones that 

play key roles in regulating fluid and electrolyte balance: 1) antidiuretic hormone, released from 

the posterior pituitary; 2) aldosterone, secreted from the adrenal cortex; and 3) atrial natriuretic 

peptide, produced by the heart. We will consider the role of each in turn.

Antidiuretic Hormone (ADH) is a hormone that prevents fluid loss and promotes the 

conservation of body water. The term antidiuretic is derived from anti, meaning against, and 

diuresis, which refers to fluid loss. The primary stimulus for ADH release from the posterior pdf

pituitary gland is an increase in blood osmolarity (that is, increased solute concentration and 

decreased water concentration). The elevation in blood osmolarity is detected in the 

hypothalamus by specialized neurons called osmoreceptors. ADH acts by increasing the 

reabsorption of water in the distal convoluted tubules and collecting ducts of the nephrons in the 

kidney. The net result of this mechanism is that water is conserved. Under these conditions a 

small volume of highly concentrated (hypertonic) urine is excreted. Another action of ADH is to 

stimulate thirst. This results in an increase in water intake, which lowers blood osmolarity and 

helps to restore homeostasis. If ADH is absent, as occurs in the disorder called diabetes 

insipidus, water reabsorption in the kidney is decreased dramatically and large volumes of dilute 

urine are excreted, up to 25 liters per day!

Aldosterone is a hormone that regulates blood sodium levels. Aldosterone specifically 

increases sodium reabsorption in the distal convoluted tubule and collecting duct of the nephrons 

in the kidneys. The result of this mechanism is to conserve sodium. Because “water follows salt,” 

this may also lead to water retention when ADH is present.  Pdf Another action of aldosterone is to 

increase the secretion of potassium by the kidney resulting in its decrease in the blood and 

increase in the urine.

Aldosterone release from the adrenal cortex is triggered directly by an 

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increase in potassium (primarily) or a decrease in sodium in the blood reaching the adrenal 

cortex. Aldosterone release is also stimulated by the activation of the renin-angiotensin system. 

In this mechanism, the juxtaglomerular cells of the kidneys release renin in response to a 

decrease in blood volume, a reduction in blood pressure, or stimulation by the sympathetic 

nervous system. Renin is an enzyme that converts a plasma protein called angiotensinogen to 

angiotensin I.  Pdf Angiotensin I is in turn acted upon by angiotensin converting enzyme (ACE) to 

form Angiotensin II. Angiotensin II has two major actions: 1) it stimulates aldosterone release 

from the adrenal cortex, which increases sodium reabsorption and results in sodium 

conservation; and 2) it causes vasoconstriction, which elevates blood pressure. As a result of these mechanisms, homeostasis is restored.

Atrial natriuretic peptide (ANP) is a hormone that promotes both fluid and sodium loss 

by the kidneys. The name natriuretic actually means “salt excreting.” ANP release from the atria 

is stimulated when blood volume and pressure are elevated. ANP has three major effects: 1) it 

decreases aldosterone release, resulting in a decrease in sodium reabsorption and increased 

sodium loss in the urine; 2) it decreases ADH release, which decreases water reabsorption and 

increases water loss to lower blood volume and pressure; and 3) it decreases thirst.



The volume and composition of urine reflect one's state of hydration. For example, if 

Phil Physiology has a low fluid intake and becomes dehydrated, he will excrete a small volume 

of concentrated urine. His body is trying to conserve water. Concentrated urine has a high 

specific gravity. Specific gravity is the ratio of the weight of a substance to the weight of an 

equal volume of water. Water has a specific gravity of 1.000, since equal volumes of water have 

equal weights at equal temperatures. The specific gravity of normal urine ranges from 1.001 to 

1.035 and depends on the amount of solutes. The greater the concentration of solutes, the higher 

the specific gravity will be. At the other extreme, Anna Anatomy has a high fluid intake and is 

over-hydrated. She will excrete a large volume of dilute urine having a low specific gravity. 

Let’s explore four specific cases in which blood volume and/or blood osmolarity has been 

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perturbed. Use your understanding of the factors that regulate hormone release and the 

subsequent actions of the hormones to predict the effects of these perturbations on urine volume 

and osmolarity. 

ACTIVITY: For each case, work with your team to predict how the parameters listed would be 

altered (increased, decreased, or not changed.) Put an , , or nc (no change) on each line. The 

information in the introduction to this lab will be helpful!

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