Fluid and Water Balance

These vocabulary flashcards cover the fundamentals of fluid and water balance, including body water distribution, electrolyte concentrations, capillary exchange forces, tonicity, and hormonal regulation via ADH.

Total Body Water (Infants)

$$75\%$$

Total Body Water (Adults)

$$50-60\%$$

Total Body Water (Older Adults)

$$45\%$$

Water Content: Brain and Kidneys

$$80-85\%$$

Water Content: Heart and Lungs

$$75-80\%$$

Water Content: Liver, Muscles, and Skin

$$70-75\%$$

Water Content: Blood

$$50\%$$

Water Content: Bones

$$20-25\%$$

Water Content: Teeth

$$8-10\%$$

Fluid Compartments

Consist of Intracellular fluid (ICF) and Extracellular fluid (ECF), which include interstitial spaces, plasma (vascular), and the transcellular compartment.

Transcellular Compartment

A sub-compartment of extracellular fluid including cerebral spinal fluid and fluid in body cavities such as peritoneal, pleural, and joints.

Primary Extracellular Fluid (ECF) Electrolytes

Sodium ($Na+$) and Chloride ($Cl-$) are found in large quantities.

Primary Intracellular Fluid (ICF) Electrolyte

Potassium ($K+$) is found in large quantities.

Potassium Concentration Ratio

$K+$ is approximately $28$ times more concentrated in Intracellular fluid (ICF) than in Extracellular fluid (ECF).

Osmosis

Movement of water down its concentration gradient across a semipermeable membrane.

Hydrostatic Pressure

The force exerted by a fluid against a wall, causing movement of fluid between compartments.

Osmotic Pressure

The hydrostatic pressure needed to oppose the movement of water across a membrane.

Tonicity

The tension or effect a solution with impermeable solutes exerts on cell size due to water movement across the cell membrane.

Isotonic Solution

A solution with the same solute and water concentration compared to body fluids; results in no change in cell size.

Hypotonic Solution

A solution with low solute concentration and high water concentration; causes cellular swelling ($\uparrow$ cell size).

Hypertonic Solution

A solution with high solute concentration and low water concentration; causes cellular dehydration ($\downarrow$ cell size).

Capillary Filtration Pressure (Hydrostatic)

Force that pushes water out of the capillary and into the tissue.

Capillary Colloidal/Osmotic Pressure

Force that pulls water into the capillary from the tissue; primarily due to plasma proteins like Albumin.

Interstitial Hydrostatic Pressure

Force that pushes water out of the tissue and into the capillary.

Interstitial Colloidal Osmotic Pressure

Force that pulls water into the tissue and out of the capillary.

Capillary Hydrostatic Pressure: Arterial vs. Venous

$30\,mmHg$ at the arterial end and $10\,mmHg$ at the venous end.

Interstitial Fluid Pressure

$$-3\,mmHg$$

Net Filtration

Occurs near the arterial end of the capillary because capillary hydrostatic pressure is greater than blood colloidal osmotic pressure.

Net Reabsorption

Occurs near the venous end of the capillary because blood colloidal osmotic pressure is greater than capillary hydrostatic pressure.

Osmolality

The ratio of solutes in a solution to a volume of solvent in a solution.

Antidiuretic Hormone (ADH)

Also known as Vasopressin; produced by the hypothalamus and released by the posterior pituitary to increase water reabsorption in the kidneys and constrict arterioles.

Systemic Response to High Osmolarity

Triggers thirst (increasing water intake) and ADH release (reabsorbing water from urine) to dilute the blood.

Systemic Response to Low Osmolarity

Triggers a lack of thirst (decreasing water intake) and decreased ADH release (losing water in urine) to concentrate the blood.