Unit D

Module 8 ~ Lesson 5

Osmotic Balance

The body's water or osmotic balance is critical to the maintenance of blood plasma solute concentration and blood volume. Both of these factors play roles in the movement of materials between the cells and the blood as well as blood pressure. When the balance between blood volume and blood plasma solute concentration are disrupted, the body's systems may begin to fail, with possibly disastrous consequences.

Water intoxication is an interesting example of how your body can react to imbalance. Water intoxication occurs when the normal balance of electrolytes in the body become severely altered due to an excess of water consumption.

The concentration gradient between ions such as sodium and outside cells is lowered. Osmotic pressure is reversed, and water moves into the cells in an effort to balance concentration. Electrolyte imbalance and tissue swelling can cause fluid in the lungs and an irregular heartbeat. Swelling puts pressure on the brain and nerves, which can produce behaviour resembling alcohol intoxication.

Instances of water intoxication are much less common than the opposite extreme, dehydration. Your body has a mechanism to respond to high osmotic pressure in your bloodstream that occurs under dehydrated conditions. High osmotic pressure occurs when there are low water and high solute (salt) concentrations.

Alison Keating. (2013). Retrieved June 10, 2018 from https://waldthesisproject.wordpress.com/2013/11/21/dehydration/. Used under the Fair Dealings provision of the Canadian Copyright Act.

Antidiuretic Hormone

Osmotic pressure is controlled by the hypothalamus and its interaction with the posterior pituitary gland of the brain. The hypothalamus contains osmoreceptors that detect osmotic pressure. If osmotic pressure is high (high concentration of solute in the blood), the pituitary is stimulated to release a hormone called antidiuretic hormone (ADH). ADH increases the permeability of the distal tubule and collecting duct of the nephron to water. ADH is sometimes referred to as vasopressin due to its secondary effect of causing a moderate increase in blood pressure through vasoconstriction. By increasing the permeability of these regions of the nephron, water is reabsorbed back into the bloodstream. This lowers the osmotic pressure.

A number of circumstances can decrease ADH release from the pituitary. Individuals with diabetes insipidus are unable to produce sufficient quantities of ADH to allow for normal water reabsorption. These people release large quantities of dilute urine, leading to dehydration. The consumption of alcoholic beverages, as well as caffeinated drinks, temporarily blocks the release of ADH. This also leads to increased urine output and subsequent dehydration.


The kidneys reabsorb sodium ions at the proximal tubule of the nephron. A further reabsorption of sodium ions also occurs at the distal tubule and collecting duct when sodium levels in the bloodstream drop abnormally. Sodium ion reabsorption is effected by the hormone aldosterone.

When blood pressure drops or sodium ion concentration in the blood is reduced, it stimulates the release of aldosterone from a triangular-shaped gland located on top of each kidney. Aldosterone stimulates the distal tubules and collecting ducts to reabsorb sodium ions. Negatively charged chlorine ions and water are also reabsorbed, which has the net effect of retaining salt and water. Aldosterone also stimulates the secretion of potassium ions into the distal tubes and collecting ducts if the concentration of potassium ions in the blood is high.


Osmoreceptors are sensitive to osmotic pressure and will stimulate or inhibit the release of ADH from the pituitary.
ADH increases the permeability of the distal tubule and the collecting duct to allow more water to be reabsorbed into the blood. ADH is secreted under conditions of dehydration so that your body conserves water.


Read "Regulating Reabsorption of Water", "Reabsorption of Salts", "Maintaining the Blood pH", and "Upsetting the Balance of the Excretory System" on pages 316 to 318 of your textbook.