Fluid Compartments and Water Balance

Solvent

Dissolves a substance

Solute

The substance being dissolved

What does the permeability of a selectively permeable membrane depend on?

1. Lipid solubility

2. Size of a molecule

3. Charge of the molecule

Aquaporins

Channels on membrane that allow water to pass through

What are two major fluid compartments of the body

1. Intracellular fluid (ICF)

2. Extracellular fluid (ECF)

Intracellular fluid

Inside the cells and part of the cytoplasm

Extracellular fluid

Outside the cells and has subcompartments

What are the subcompartments of the ECF?

1. Interstitial fluid

2. Plasma

3. Cerebrospinal fluid, synovial fluid, perilymph, endolymph

What do electrolytes do?

Plays a main role in governing body’s water distribution and total water content

Osmolarity

Concentration of solutes in a volume of solvent. This raises with the more solutes there are

Osmosis

Movement of water between the ECF and ICF through diffusion from LOW SOLUTE CONCENTRATION to HIGH SOLUTE concentration until they are equal

What is the most abundant solute in the ECF

Sodium (Na+) concentration

What is the most abundant solute in the ICF

Potassium (K+) concentration

Fluid balance

When daily gains and losses are equal. Homeostatic mechanisms balance the gains and losses

How is fluid gained?

1. Water and food intake

2. Byproduct of aerobic metabolism

How is fluid lost?

1. Urine and feces

2. Sweat (sensible perspiration)

3. Vapor from skin (insensible perspiration)

Simple diffusion

The movement of solute down the concentration gradient (higher to lower)

Osmotic equilibrium

No net movement of water across the membrane

Osmotic pressure

The pressure required to prevent movement of water. Greater concentration = greater pressure. Applied by plasma proteins in the blood vessel

Tonicity

ability of a solution in the ECF to affect fluid volume and pressure in a cell - solutes CANNOT cross membrane (the only thing moving is water)

Isotonic

No net fluid movement (indicates that the concentration of solute and solvent is equal in the ICF and ECF). 0.9% NaCl solution

Hypotonic

Has less solutes and more water in the ECF than inside the cell. Since the cell has more solutes, the fluid will move into the cell to reach equilibrium

Hypertonic

Has more solutes than water in the ECF than the ICF. Fluid in the ICF will move outside the cell to reach equilibrium

Dehydration causes

Decreased drinking, increased sweating, increased urine and fecal output, hormonal or electrolyte problem

Hypovolemia and causes

Blood volume depletion. There is equal loss of water and electrolytes → No net change in osmolarity. Caused by hemorrhage or severe burns

Edema

Sequestration (isolation) of water. Pockets of fluid accumulate in the ECF. Caused by kidneys not working properly (renal insufficiency), blocked lymphatic vessels, hormonal/electrolyte problem

How does change in the ECF affect blood volume and pressure?

If ECF volume rises, the blood volume and blood pressure go up

If ECF volume drops, the blood volume and blood pressure do down

How is blood volume and pressure regulated with urine volume

Increasing or decreasing reabsorption of Na+ and/or water in kidneys directly affects urine volume

What are three hormones that regulated BP by increasing blood volume and decreasing urine volume?

1. Angiotensin II (AII)

2. Aldosterone

3. ADH

Angiotensin II

Stimulated by low renal blood pressure

Aldosterone

Stimulated by AII, low Na+, and K+. → causes Na+ to be reabsorbed in the kidney and water follows

ADH

Stimulated by osmoreceptors in hypothalamus or AII and causes an increase in water reabsorption in the kidney by installing more aquaporins

What hormone regulates BP by decreasing blood volume and increases urine volume?

Atrial natriuretic peptide

Atrial natriuretic peptide

Stimulated by elevated BP and does the OPPOSITE of AII, ADH, and andosterone

Describe Renin-angiotensin stimulation

Angiotensin → Renin converts it into → Angiotensin I → Angiotensin converting enzyme converts it into → Angiotensin II

What happens after dehydration causes reduction in blood volume and bp?

Stimulates production of AII → Stimulates ADH and aldosterone

What happens when dehydration causes an increase in blood osmolarity

Stimulates osmoreceptors in hypothalamus → Stimulates ADH release

What happens when dehydration causes thirst?

Intense sense of this with a 2-3% increase in plasma osmolarity OR 10-15% blood loss