Physiology of Water and Dehydration

Hydrophilic vs Hydrophobic

Hydrophilic → water soluble / polar (easily dissolvable)

Hydrophobic → fat soluble / non-polar (not dissolvable)

The Roles of Water

Polar molecule, solvent, chemical reactions, transporter, thermal properties, lubricant

Water Structure

Polar molecule → opposite charges on opposite sides of water molecule

Water as a Solvent

Helps break molecules into charged ions

• The ions making up the molecule are attracted to the different charged sides of the water molecule and therefore break apart to move to the opposite ends

Water in Chemical Reactions (hydrolysis vs dehydration)

Water is the ideal medium

• in a hydrolysis reaction, water is added to break bonds

• In a dehydration synthesis reaction, water is removed to make bonds

Water as a Transporter (circulatory system and kidneys)

Circulatory system → water is a medium for transporting nutrients and waste via blood plasma (extracellular)

Kidney → removes wastes by being transported out of the body with water

Water’s Thermal Properties (heat capacity and vaporization)

High heat capacity → can absorb a lot of heat before it changes temperature

High heat of vaporization → when water vaporizes it carries a lot of heat with it

Water as a Lubricant

Helps reduce friction as membranes and organs slide over one antoher

• e.g. synovial fluid in joints, pericardial fluid, and pleural fluid

Body Water Characteristics

60% of body weight

Extracellular fluid → plasma, interstitial fluid (moves between cells)

Intracellular fluid → inside cells

Needed for solute diffusion

Extreme Water Loading (what it does to extra/intracellular water

Several litres within an hour

• extracellular water content is too high

  • Osmolarity is low → dilute / hypotonic (too much water, not enough solutes)

  • Water moves into cells

• Cells swell (water moves into RBCs and tissues)

  • Can cause rupture of cells → hemolysis of RBCs

  • Brain swelling can cause pressure on brain stem

• Can be fatal

Dehydration (how it happens and what it causes)

Extracellular water is low and usually hypertonic (concentrated plasma → low water + high solute)

• pulls water from intercellular space (cells)

  • RBCs crenate → causes slow blood flow and less O2 delivery

• Organ cells shrink → brain effects, organ function effects

Can be fatal

How body water is controlled

Role of kidney and the thirst reflex (and digestive system)

If dehydrated → high aldosterone and ADH (reabsorbs Na in kidneys)

• conserve fluids → reduce urine output

• Thirst reflex

If water loaded → high ANP (triggered from L atrium), low aldosterone and ADH

• high urine output → high volume, less concentrated

Dehydration vs Volume Depletion

Dehydration → happens second

• often intracellular water loss → cell shrinkage

  • Produces hypertonicity

Volume Depletion → happens first

• deficit in extracellular fluid volume → affects blood volume

Outside Causes of dehydration

Losing too much fluid → diarrhea and vomiting

Not drinking enough

Other causes → fever, haemorrhage, sweating, burns, surgery, infections, renal

Causes of vomiting and diarrhea leading to dehydration

CNS → infections, lesions

Gastro-intestinal → hepatitis, liver failure, appendicitis, etc

Endocrine → diabetic ketoacidosis, addison’s disease (low aldosterone)

Renal → infection, renal filaure, etc

Types of Dehydration

Mild → less than 5% weight loss

Moderate → 5-10% weight loss

Severe → 10-15% weight loss (needs immediate medical attention → can lead to death)

Signs of mild dehydration

Initial signs and symptoms appear when body has lost 2% of its total fluid

• thirst/loss of appetite

• Dry skin/dry mouth

• Dark coloured urine

• Fatigue and weakness

• Chills

• Head rushes/dizziness

Signs of moderate dehydration

Total fluid loss reaches 5%

• increased heart rate and respiration

• Decreased sweating and urination

• Increased body temp

• Extreme fatigue

• Muscle cramps and tingling limbs

• Headaches

• Nausea

Signs of severe dehydration

Body reaches 10% fluid loss → 10% loss and above is often fatal;

• muscle spasms and seizures

• Vomiting / painful urination

• Racing pulse / difficulty breathing

• Shrivelled skin / dim vision

• Confusion / unconsciousness

• Chest and abdominal pain

Cardiovascular effects of dehydration

Decreased blood volume and blood pressure

• barorecpetor reflex → increases heart rate

• Vasoconstriction to non-essential organs

  • Decreased flow to skin and decreased sweating → can cause increase in temp

  • Decreased flow to gu → nausea

Renal effects of dehydration

Increased blood osmolarity

Increased ADH and aldosterone → increased water absorption, decreased urine output

Decreased renal flow due to decreased BP and volume → decreased filtration → increased waste (can get to toxic levels)

Acid-Base effects of dehydration

Metabolic acidosis due to:

• bicarbonate loss in stool

• Ketone production

Decreased perfusion → increased lactic acid production

Decreased renal perfusion → decreased hydrogen ion excretion

Electrolyte effects of dehydration (sodium)

Sodium imbalance (hypo- or hypernatremia)

• loss of only water → results in

• Seizures, cerebral shrinkage, coma

• Muscle spasms

Electrolyte effects of dehydration (potassium)

Potassium deficit (hypokalemia)

• loss with sweat

• Slow shift from cells

Muscle spasms, weakness, and pain

Cardiac arrythmias

Results in reflex increase in respiration → this can cause more water loss due to evaporation

Respiratory effects of dehydration

Increasing breathing due to:

• increased acidosis

• Decreased potassium

CNS effects of dehydration

Hypothalamus:

• relaxes body temp control

• Allows body temp to increase → less water loss to sweating

Decreased blood flow to CNS as dehydration worsens

• mental function compromised after 5% water loss

• Seizures, coma