Regulation of body fluids

Total body water

Makes up approximately 60% of body weight in males, 50% in females, and 75% in infants.

Intracellular fluid (ICF)

Fluid found inside the cells (cytosol); makes up 2/3 of total body water.

Extracellular fluid (ECF)

Fluid found outside the cells; makes up 1/3 of total body water.

Blood plasma

The intravascular component of extracellular fluid (1/4 of ECF).

Interstitial fluid

Also known as tissue fluid; the fluid between cells (3/4 of ECF).

Transcellular fluid

Fluid in specific regions like the brain (CSF), eyes, joints, and around the heart.

Osmotic pressure

The tendency of a solution to take in water; higher solute concentration leads to higher osmotic pressure.

Fluid balance

The state where fluid gain (liquids, food, metabolic water) equals fluid loss (urine, sweat, lungs, faeces).

Metabolic water

Water produced as a byproduct of chemical reactions (cellular respiration) within cells.

Excretion

The removal of metabolic waste products (CO2, urea, lactic acid) to prevent toxicity.

Osmoreceptors

Cells in the hypothalamus that detect increases in osmotic pressure (concentration) of the blood.

Antidiuretic Hormone (ADH)

Produced by the hypothalamus and released by the posterior pituitary; increases water reabsorption in the kidneys.

ADH Mechanism

Increases the permeability of the distal convoluted tubule and collecting duct to water.

High ADH levels

Results in high permeability, more water reabsorption, and small volumes of concentrated urine.

Low ADH levels

Results in low permeability, less water reabsorption, and large volumes of dilute urine.

Aldosterone

A hormone secreted by the adrenal cortex that regulates sodium and potassium levels.

Aldosterone stimuli

Secreted in response to low blood pressure, low sodium, or high potassium levels.

Sodium-Potassium Pump

Mechanism in the kidney tubules that reabsorbs 3 sodium ions into the blood for every 2 potassium ions secreted into urine.

Effect of Aldosterone

Increases sodium reabsorption, which causes water to follow via osmosis, increasing blood volume/pressure.

Thirst centre

Located in the hypothalamus; stimulated by osmoreceptors when osmotic pressure is high.

Dehydration

Occurs when water loss exceeds intake; symptoms include low blood pressure, dizziness, and headache.

Water intoxication

A condition where body fluids become too dilute (usually from replacing salt/water loss with plain water); can lead to collapse.

Alimentary canal

The site where ingested water is absorbed into the bloodstream to reduce osmotic pressure.