Lecture 8: Water, electrolytes, and body fluids

Dehydration

Inadequate fluid intake → excessive fluid loss

Severe symptoms of dehydration

shrinkage of brain cells → stretching of vascular connections to the skull → intercranial bleeding

Why does dehydration especially affect the elderly?

Less muscle mass → less thirst reflex

Overhydration

Excessive water intake → excessive water entry into body cells → Cell swelling

Symptoms of overhydration

Increase in space occupied in skull → increased intercranial pressure → impaired blood flow

Body fluid compartments

Body fluid is divided into different fluid compartments separated by membranes/blood vessel walls

Permeability of capillary vs membranes

Capillaries are much more permeable than plasma membranes

Total Body Water (TBW)

Total volume of water (L) in a person’s body, expressed as a percentage of body weight

TBW value

60% of person’s body weight

Composition of TBW

1/3 Extracellular fluid, 2/3 Intracellular Fluid

Blood volume

Plasma volume / (1- Hematocrit)

Paracellular movement

Passage of water between cells through tight junctions

Transcellular movement

Usage of water channels (aquaporins) to enter cell or diffuse through lipid bilayer

Water movement

Low to high solute concentration

Relationship between osmotic pressure and solute concentration

Proportional

Effective osmoles

Non-penetrating solutes

Ineffective osmoles

Penetrating solutes

Electrolytes

Substances that dissociate into ions in water

Osmolarity

Measure of osmotic pressure in a solution

Normal ECF osmolarity

285-295 mOsm/L

Major determinant of plasma osmolarity

Na+ concentration

Plasma osmolarity formula

Solute / water

Activation of thirst reflex

Increased plasma osmolarity

Monitoring of plasma osmolarity

Hypothalamic osmoregulators

ADH/vasopressin

Stimulates insertion of aquaporin-2 in epithelial mebranes of collecting duct in kidney

Albumin

Effective osmoles that draw water into plasma compartment

Filtration

hydrostatic pressure forces fluid out of the capillary

Absorption

oncotic pressure draws fluid back into capillary

Fluid movement at arteriole end of capillary

filtration > absorption

Fluid movement at Venule end of capillary

Absorption > filtration

Lymphathic system

Drains excess water and solutes from tissues and returns to circulatory system

Edema

Accumulation of excessive fluid in interstitial space