Physiology of Water and Dehydration

What defines a hydrophilic substance?

It is water-soluble and polar

• It possesses a partial electrical charge

What defines a hydrophobic substance?

It is fat-soluble and non-polar

What gives a polar molecule (e.g. water) its electrical charge?

Its specific asymmetry in electron distribution

How does water’s polarity assist in dissolving substances?

It attracts target molecules using its partial charges, helping to break molecules apart into charged ions

How does water participate in a hydrolysis reaction?

Water is added to chemically break down molecular bonds in hydrolysis

How does water participate in a dehydration synthesis reaction?

Water is removed to form new molecular bonds

How does water function as a transporter within the circulatory system?

It forms the main volume of plasma, allowing it to circulate nutrients, proteins, and wastes throughout the body

How does water function as a transporter within the renal system?

It acts as the vehicle inside the kidneys to transport filtered cellular wastes out of the body

What is meant by water having a high heat capacity?

It can absorb large amounts of heat before its actual temperature experiences a change

What is meant by water having a high heat of vaporization, and what is a bodily example?

When water converts to gas, it carries a massive amount of heat away with it

• e.g. sweating to cool down the body’s core temperature

How does water act as a physical lubricant inside the body, and what are 3 examples?

Water is a major component of our body fluids — it reduces friction as membranes and organs slide around one another

• e.g. synovial fluid (joints), pericardial fluid (heart), pleural fluid (lungs)

What total proportion of human body weight is accounted for by water?

Water accounts for 60% of our body weight

What proportion of body water is extracellular fluid (ECF), and what are its 2 sub-components"?

Extracellular fluid makes up 20% of total body fluid

• Plasma

• Interstitial fluid

What proportion of body water is intracellular fluid (ICF), and what is its primary physical requirement?

Intracellular fluid makes up 40% of total body fluid

• Crucial for allowing solute diffusion to occur across cell membranes

How do male and female body water proportions differ, and what tissue dictates this?

• Males — total body mass is 60% water

  • Higher muscle mass

• Females — total body mass is 50% water

  • Lower muscle mass

What are the specific water content percentages of muscle mass vs. fat mass?

• Muscle mass is 75% water

• Fat mass is 25% water

Higher muscle mass and lower body fat increases a person’s overall body water percentage

What direct intake rate triggers acute water poisoning / extreme waterloading?

Consuming several litres of water within a single hour

What happens to plasma osmolarity during extreme waterloading?

The extracellular water becomes too high

• Lowers blood plasma osmolarity — low concentration of disolved particles

• Plasma becomes highly dilute and hypotonic

How do tissue cells react physically to the hypotonic plasma caused by waterloading?

Water rushes into the cell toward the higher solute concentration, causing RBCs and tissues to swell and potentially rupture

What are 2 complications of severe cellular swelling during waterloading?

• Direct hemolysis of RBCs

• Brain swelling — exerts pressure on the brain stem

Can be fatal

What happens to plasma osmolarity and fluid movement during dehydration?

The extracellular water drops

• Elevates blood plasma osmolarity — high concentration of dissolved particles

• Plasma becomes highly concentrated and hypertonic

  • This pulls water out of the cells

How do RBCs and blood dynamics react to dehydration?

RBCs crenate (shrivel)

• Slows down overall blood flow

• Decreases O₂ delivery to organs

  • Affects brain and organ function

Can be fatal

What 2 physiological systems are responsible for controlling body water levels?

• The kidneys (primary control)

• The thirst reflex

What are the body’s 2 primary and immediate responses to dehydration?

• Secretes high levels of aldosterone and ADH — conserves fluids by lowering urine output

• Activates the thirst reflex

What are the body’s 2 primary and immediate responses to extreme waterloading?

• Secretes high levels of ANP — reduces sodium reabsorbtion in the kidney

• Produces a high urine output

What trigger releases aldosterone, and how does it lower urine output?

Aldosterone is activated by low blood volume

• It forces the kidneys to reabsorb sodium, and water follows the sodium back into the blood

What does ADH specifically work in the kidney to assist aldosterone?

It acts directly on the collecting ducts of the kidneys to maximize water reabsorbtion

What trigger releases ANP, and what is its specific mechanism in the kidney?

ANP is triggered by high blood volume physically stretching the atria of the heart

• It reduces sodium reabsorbtion in the kidney, so water leaves the body as urine

What is the numerical definition of how dehydration occurs?

It occurs when the loss of water and salts is greater than the gain

What is the average value for the gain and loss of water per day under normal conditions?

2.5L per day

At what cellular threshold does general water loss officially become classified as dehydration?

When it causes intracellular water loss — when it produces hypertonicity of the extracellular fluid and causes cells to shrink

What is volume depletion, and when does it occur relative to true dehydration?

A deficit specifically in extracellular fluid volume — compromises blood volume

• It occurs before dehyrdation

What are some causes of dehydration, and the biggest clinical cause of fluid loss that leads to dehydration?

• Loss of too much fluid

  • Vomiting (biggest cause)

  • Diarrhea

• Not taking in enough fluids

• Less common causes include

  • Fever, haemorrhage, sweating, burns, surgery, infections

  • Renal issues: diuretics, renal tubular acidosis, renal failure, diabetes insipid, hypothyroidism, adrenal insufficiency

What age group of children is most susceptible to severe volume depletion and dehydration?

Children under 4 years old are more susceptible to volume depletion and dehydration (due to vomiting and diarrhea)

• It is the leading cause of death in infants

Why is the fluid and solute turnover rate in infants and young children 3 times that of adults?

Infants and young children have:

• Higher metabolic rates

• Increased body surface area to mass ratio

• Possess a higher baseline body water content

What are the exact baseline body water percentages for infants, children, and adults?

• Infants — 70% water content

• Children — 65% water content

• Adults — 60% water content

What are the general, cascading dangers of leaving severe dehydration untreated?

• Muscle cramps

• Headaches

• Fever

• Hallucinations

• Worsening vomitting and diarrhea

And ultimately death

What are the weight loss percentages that define mild, moderate, and severe dehydration?

• Mild — less than 5% body weight loss

• Moderate — 5% to 10% body weight loss

• Severe — 10% to 15% body weight loss

  • Requires immediate medical attention — can lead to death

At what total fluid loss percentage do initial symptoms of dehydration appear, and what are the symptoms (6)?

Initial symptoms of dehydration appear when the body has lost approximately 2% of its total fluid

• Thirst

• Dry skin and mouth

• Dark urine

• Fatigue and weakness

• Chills

• Dizziness and head rushes

At what percentage of total fluid loss is the threshold for moderate dehydration reached, and what are the symptoms (9)?

Symptoms occur when total fluid loss has reached 5%

• Increased heart rate

• Increased respiration

• Increased body temperature

• Decreased sweating

• Decreased urnation

• Extreme fatigue

• Muscle cramps and tingling in limbs

• Headaches

• Nausea

At what percentage of total fluid loss is the threshold for severe dehydration reached, and what are the symptoms (9)?

Symptoms occur once body has reached 10% fluid loss — anything 10% or above is often fatal

• Muscle spasms and seizures

• Vomiting

• Painful urination

• Racing pulse

• Difficulty breathing

• Shrivled skin

• Dim vision

• Confusion / unconsiousness

• Chest / abdominal pain

How does dehydration affect blood volume and blood pressure, and how does this change heart rate?

It decreases blood volume and decreases blood pressure

• This triggers the baroreceptor reflex — increases heart rate

Why and how do systemic blood flow changes during dehydration cause nausea and body temperature changes?

Caused by vasocontriciton (reduced bloodflow) to non-essential organs

• Nausea — caused by a decrease in blood flow to the gut

• Temperature rise — caused by a decrease in blood flow to the skin, which decreases sweating

Why does dehydration cause severe headaches?

The vascular system responds to dehydration with vasodilation to increase blood flow to the brain, causing headaches

How does dehydration decrease urine output by the kidneys?

An increase in blood osmolarity (due to a loss of blood plasma) causes ADH and aldosterone release

• These hormones increase water reabsorbtion in the kidney, which decreases urine output

How does dehydration affect kidney filtration and waste clearance?

Low blood pressure and low blood volume decrease renal blood flow, which decreases kidney filtration (urine output)

• This causes body waste to increase to toxic levels

What 4 distinct metabolic / renal failures cause metabolic acidosis to develop during dehydration?

Metabolic acidosis develops due to

• Bicarbonate (HCO₃^-) loss in stool

• Ketone production

  • Caused by altered metabolism

• Increased lactic acid production

  • Caused by decreased tissue perfusion (decrease in O₂ delivery)

• Decreased hydrogen ion (H⁺) excretion in the urine

  • Caused by decreased renal perfusion (decrease in O₂ delivery)

What are the 2 types of sodium imbalances and 1 potassium imbalance that occurs with fluid loss?

• Hyponatremia — low sodium in the blood

• Hypernatremia — high sodium in the blood

• Hypokalemia — potassium deficit

What clinical scenario with fluid loss causes hyponatremia (low blood sodium)?

Hyponatremia occurs when sweat loss is replaced with pure water without replacing missing salts

What clinical scenario causes hypernatremia (high blood sodium), and what symptoms can this trigger?

Hypernatremia occurs with a loss of pure water only — e.g. diabetes insipidus

• Can trigger seizures, cerebral shrinkage, coma, muscle spasms

How does potassium deficit (hypokalemia) occur, and what are its clinical consequences and respiratory results?

Potassium is lost with sweating — slowly moves out of cells

• Can trigger muscle spasms, weakness, pain, cardiac arrythmias

  • Affects anything that produces action potentials

• Causes a reflex increase in respiration

Why does an increased breathing rate during dehydration happen, and why does this create a dangerous feedback loop?

Breathing rate will increase due to a:

• Increase in blood acidosis

• Decrease in potassium levels

Reflexive increases in respiration cause more water loss due to evaporation, which worstens the dehydration state

How does the hypothalamus alter homeostatic controls during severe dehydration?

It relaxes body temperature control, allowing body temperature to increase in order to decreases excess water loss from sweating

How does blood flow to the CNS change with dehydration, and how does this manifest in symptoms?

As dehydration worsens, blood flow to the CNS will decrease, which causes:

• Mental function becomes compromised after 5% body water loss

• Can lead to seizures, coma

What are some causes of vomiting that lead to dehydration?

• CNS - infections, lesions

• GastroIntestinal - gastroenteritis, obstruction, hepatitis, liver failure, appendicitis, peritonitis, pyloric stenosis, toxicity (ingestion, overdose, drug effects)

• Endocrine - diabetic ketoacidosis, addison’s disease

• Renal - infection, pylonphritis, renal failure, renal tubular acidosis