6D Edrolo - Regulation of water balance

Why is regulating water balance important?

Helps prevent the loss of water and crenation of a cell, or the gain of water and swelling of the cell, by regulating the osmolarity of the extracellular fluid.

How is the osmolarity of the extracellular fluid regulated?

It's easier for the body to alter the amount of water in the extracellular fluid that it is to change the amount of solutes. Therefore, to keep the osmolarity within the optimal range, the amount of water present is regulated.

More water added to the extracellular fluid, the osmolarity will decrease (the extracellular fluid is becoming hypotonic).

Water remove from the extracellular fluid, the osmolarity will increase (the extracellular fluid becomes hypertonic).

What is osmoregulation?

The homeostatic regulation of osmolarity in the body via the alteration of water and solute balance.

Why else is water important for the body?

- production of urine

- removal of waste via evaporation of sweat

- maintenance of blood glucose

- protection of the brain and central nervous system

What is the general stimulus-response model of regulating water balance in the body?

1. change in water volume, causing change in osmolarity, blood volume and blood pressure

2. baroreceptors, or 2. osmoreceptors

3. cells in the kidneys, or 3. the hypothalamus and pituitary gland

4. cells in the distal convoluted tubule and collecting duct (for both modulators), or 4. the hypothalamus (only for the second modulator)

5. alteration in the reabsorption of water which changes the osmolarity of blood and blood pressure and volume

What happens when there is a decrease in water levels?

A decrease in water levels causes:

- an increase in osmolarity in blood,

- a decrease in blood pressure and volume.

Osmoreceptors and the hypothalamus detects an increase in osmolarity, they initiate the antidiuretic hormone pathway.

Decreases in blood pressure and volume are detected by baroreceptors, stimulating the release of renin.

What is the stimulus-response model for decrease in water levels?

1. decrease in water levels

2. baroreceptors, or 2. osmoreceptors

3. kidney cells (release renin), or 3. hypothalamus (releases ADH)

4. activation of sodium potassium pumps (increase reabsorption of sodium), or 4. aquaporins inserted into distal convoluted tubule and collecting duct, or 4. stimulation of cells in the thirst centre

5. increased reabsorption of water from kidney filtrate, or 5. increased consumption of fluid (relates only to the last effector).

What is renin?

An enzyme secreted by the kidneys in response to low blood pressure and volume. Initiates the process which increase the reabsorption of water and odium, and increases the excretion of potassium in a nephron.

What is ADH?

A molecule secreted by the pituitary gland in response to high solute concentrations in the blood. ADH increases the amount of water reabsorbed by the distal convoluted tubule and collecting duct, hence the amount of water conserved by the body.

What happens when there is an increase in water levels?

Causes:

- decrease in osmolarity

- increase in blood pressure and volume

Receptors signal the hypothalamus to decrease the release of ADH. Therefore, fewer aquaporins will be inserted into the distal convoluted tubules and collecting duct, solute concentration surrounding nephrons will not increase. More water will be excreted in urine.

What is the stimulus-response model for increase in water levels?

1. increase in water levels

2. baroreceptors, or 2. osmoreceptors

3. hypothalamus

4. decrease in insertion of aquaporins in distal convoluted tubule and collecting duct), or 4. thirst centre is suppressed

5. decrease in reabsorption of water in urine, or 5. decrease in consumption of fluid