Water in the body
- Water is essential to life
- Body fluids consist largely of water together with dissolved solutes
- Biochemical reactions occur in water
- Sources of water include
◦ Ingested food and fluids
◦ Cellular metabolism (small amount only)
Total body water
- Influenced by age, body mass, sex and body fat
- In adults, water accounts for
- ~ 60% body weight in males
- ~ 50% body weight in females
- Infants ~ 75% body weight in water
- Less body fat and bone mass
- Amounts reflect differences in
- Adipose Cssue (10% water)
- Muscle Cssue(75% water)
- Total body water declines with age
Fluid compartments
Two main fluid compartments
1. Intracellular fluid (ICF)
2. Extracellular fluid (ECF)
Three locations in the body
Intracellular– within the cells(cytosol)
Extracellular– outside the cells
◦ Interstitial fluid (IF)
◦ Plasma
Composition of body fluids
Water – the universal solvent
Body water contains a number of solutes
1. Electrolytes – these dissociate in solution NaCl èNa+ + Cl
2. Non electrolytes - stay together in solution, e.g. glucose, urea.
Electrolytes have greater osmotic power – contribute more than one particle to solution
Each of the fluid compartments has a distinct pattern of solutes
Extracellular fluid
◦ Major cation Na+ ◦ Major anion Cl
Intracellular fluid
◦ major cation K+ ◦ Major anion HPO4 2-
Movement of fluid between compartments
- There is continuous mixing of fluids between compartments
- Membranes are freely permeable to water but not to solutes
- Fluid moves readily across capillary wall. Facilitated by thin, single layer of endothelial cells.
- Regulated by osmotic and hydrostatic pressures.
- There is a continuous exchange and mixing of body fluids between fluid compartments.
- This is regulated by osmotic and hydrostatic pressures.
- Water can move freely between compartments along osmotic gradients.
- Normally osmolarities of all body fluids are equal. Changes in solute concentrations are followed by osmotic changes.
- Anything that changes solute concentrations in any compartment leads to net water flow.
- In order to reach the intracellular compartment, fluids must pass through both the plasma and the interstitial fluid.
- Exchanges occur between all these compartments almost continuously, leading to compensatory shifts of fluid from one compartment to another.
- So net water movement is equal in all directions.
- Exchange of gases, nutrients, water and wastes occurs.
Fluid and electrolyte balance
- Adequate cell functioning relies on homeostasis of surrounding fluids.
- Maintenance of fluid homeostasis includes two major tasks:
1. Maintaining water balance within the cells (ICF). This ensures adequate functioning of cells.
2. Maintaining water balance in the plasma. This ensures the tissues of the body are adequately perfused. - Both are achieved by maintaining the correct level of water in the ECF.
- ECF composition and volume maintained within narrow limits.
- Water balance
- Water intake = water output Must maintain solute concentration
- Must replace water losses
Water output
- Certain amounts of water losses are uncontrollable
- Losses from lungs, skin and digestive tract.
- Kidneys must excrete wastes and some water must accompany them
Water input
- 60% fluids (approx 1500mls),
- 30% food (approx 750mls),
- plus 10% from cellular metabolism
Regulation of water intake
- Thirst mechanism the driving force for water intake
- Controlled by the hypothalamus
- Hypothalamic thirst centre stimulated by:
- 1-2% change in osmolarity
- 5-10% change in plasma volume
- Baroreceptor inputs
- Angiotensin II activation
- Feedback mechanisms inhibiting thirst:
- Moistening of mucosa
- Stretch receptors stomach, intestine
- Decrease in osmolarity
Influence of hormones on the ECF
- Antidiuretic hormone (ADH) inhibits diuresis
- Stimulates water conservation at the kidneys.
- High sodium concentration (increased osmolarity) leads to ADH acts to increase water reabsorption at collecting duct of kidney.
- This conservation of water will dilute Na+ leads to a decrease in osmolarity.
- ADH made by hypothalamus. Stored and released from posterior pituitary.
- ADH works to maintain electrolyte balance of the ECF by conservation of water at the kidneys g decreased urine output.
- Water and electrolyte (in particular Na+ and K+) blood levels are continually adjusted to maintain fluid homeostasis
Disorders of water balance
- Dehydration- when water output exceed water intake.
- Extracellular fluid loss due to:
◦ haemorrhage
◦ severe burns
◦ severe vomiting and diarrhoea
◦ profuse sweating
◦ low water intake
◦ endocrine disturbances e.g. diabetes insipidus, diabetes mellitus
Dehydration
- Water is lost from the extracellular fluid.
- Water depletion exceeds Na+ depletion.
- Decreased total body water increases plasma osmolarity (solute concentration).
- Results in hypernatremia (high Na+ in the ECF)
◦ Does not occur in haemorrhage where whole blood is lost (fluids and solutes are lost equally) - Water moves from ICF to ECF via osmosis
- Aim- to equalise osmolarity Cells dehydrate and crenate (shrink)
- Cell functioning is impaired
Disorders of water balance
- Hypotonic hydration -when water intake exceeds water output.
- Water is added to the ECF, osmolarity decreases.
- Normally ADH would be inhibited → urine output would increase.
- Also called water intoxication or overhydration.
- Occurs with:
◦ renal insufficiency
◦ rapid excess water ingestion
Hypotonic hydration
- Too much water in the ECF results in relative decrease in solute concentration
- Results in hyponatremia (low Na+ in ECF)
- Water moves from ECF to ICF via osmosis
- Aim - to equalise osmolarity Cells swell, with lysis possible.
- Leads to severe metabolic disturbances
◦ Nausea, vomiting, muscle cramps - Cerebral oedema possible
◦ Disorientation, convulsions, coma, death
Oedema
- An abnormal accumulation of fluid in the interstitial space.
- A fluid distribution problem where fluid moves from blood to interstitial fluid (IF)
- Caused by:
◦ increases in capillary hydrostatic pressure → Lt or Rt sided heart failure, lymphatic obstruction
◦ increases in capillary permeability → infection
◦ decreases in colloid osmotic pressure →low plasma proteins associated with protein malnutrition, liver disease, kidney disease - Results in swollen tissues, puffy feet, swollen ankles, puffy eyes.
- Can impair tissue functioning because of increased distance oxygen nutrients and wastes must diffuse between cells and blood.
- Fluid shift may severely decrease circulating blood volume.
- Can result in compromised cardiovascular functioning.
- Lymphoedema – accumulation of IF due to surgically removed or blocked lymph glands
pH regulation
Three mechanism are able to alter the free H+ ion concentration of body fluids.
pH is regulated by
◦ Chemical buffer systems
◦ Respiratory regulation
◦ Renal mechanisms
Chemical buffer systems
◦ The most rapid
◦ Temporarily tie up H+ ions
Respiratory regulation
◦ Acts within minutes
Renal mechanisms
◦ Slowest, taking hours to days
◦ Most potent mechanism
Respiratory regulation and renal mechanisms are able to correct H+ ion imbalances and are called physiological buffer systems.
Chemical buffer systems
A buffer is a substance or solution that minimizes changes in pH by binding or releasing free H+ ions.
Chemical buffers → Bind H+ ions if pH drops (when solution is acidic)
Release H+ ions if pH increases (when solution is alkaline)
Three main chemical buffer systems in the body
1. Carbonic acid-bicarbonate ion buffer system
2. Phosphate buffer system
3. Protein buffer system
We will discuss the Carbonic acid-bicarbonate ion buffer system only. Also called the bicarbonate buffer system.
Respiratory regulation of pH
- Acts more slowly, but has more capacity than the chemical buffering system.
- CO2 generated in the tissues is transported in the blood and removed by the lungs
Renal mechanism of pH
- The ultimate pH (acid-base ) balance of the blood is achieved by the kidneys.
- Slowest mechanism, but is the most potent.
- Kidneys work to:
◦ remove acids produced by metabolic reactions in the body
◦ conserve or renew acids when needed - The renal mechanism is able to
◦ conserve HCO3- and generate new HCO3-
◦ excrete HCO3-
→ This occurs in renal tubule of nephron and collecting ducts - Changes in pH alter renal tubule reabsorption and secretion of H+ ions
- If pH drops (acidic)
◦ There is an increase in H+ ion concentration in the blood
◦ This excess in H+ ions needs to be secreted by the kidneys
◦ HCO3- will be reabsorbed and generated (alkaline reserve) - If pH increases (alkaline)
◦ There is a decrease in H+ ion concentration in blood
◦ H+ ions need to be absorbed by the kidneys
◦ HCO3- secreted
RESPIRATORY ALKALOSIS
- Respiratory alkalosis is often caused by hyperventilation in association with stress or pain.
- Occurs when hyperventilation causes excessive CO2 exhalation causing a reduction in carbonic acid and a rise in pH.
- Causes include panic attacks with hyperventilation, early salicylate overdose and fever (particularly in infants).
METABOLIC ALKALOSIS
- Metabolic alkalosis due to an excess of bases or a deficit of acids in the body leading to a increase in blood pH.
- Typical causes are:
◦ Vomiting
◦ Intake of excess base (e.g. antacid overdose)
◦ Constipation (excessive bicarbonate reabsorbed)
◦ Excessive diuresis
Blood pH above 7.8 → stimulation of nervous system → muscle tetany, convulsions, respiratory arrest
RESPIRATORY ACIDOSIS
*Occurs when hypoventilation causes retention of CO2.
*Leads to carbonic acid excess and a decrease in pH.
*Seen where gas exchange is hampered by diseases such as pneumonia, cystic fibrosis, or emphysema, or with shallow breathing.
*Acute causes include Status Asthmaticus, central nervous system depression, pulmonary oedema, pneumothorax.
METABOLIC ACIDOSIS
*Metabolic acidosis is the second most common cause of acid-base imbalance.
*Occurs with an excess of fixed acids in the body leading to decrease in pH.
*Causes include renal disease, excessive ingestion of acids (alcohol), lactic acid production seen in shock or cardiac arrest, excessive loss of bicarbonate ions (persistent diarrhoea), ketosis, starvation.
Blood pH below 6.8 → Nervous system shock, coma and death
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