Water Balance Flashcards
What is the role of water in physiology?
As part of blood volume (and hence BP), water transports nutrients and O2
Precursor of body fluids: saliva, joint lubrication, bile etc.
Solvent for metabolic and biochemical processes
Temperature regulation
- High specific heat: rises slowly, resists temperature fluctuations
- Sweat: 99% H2O. Heat energy from skin evaporates sweat
Waste products removal: compounds are made water soluble (conjugated in enterocytes and hepatocytes) for filtering by kidney and clearance via urine as soon as possible
Describe the mechanisms by which the body increases water volume and thus increased blood pressure
- Renin is an enzyme secreted by the kidney when sodium or blood volume are decreased, leading to decreased BP
- Renin converts Angiotensinogen (from liver) to Angiotensin I (inactive)
- Angiotensin I is converted to angiotensin II be angiotensin converting enzyme (ACE)
- Angiotensin II causes restriction of small blood vessels resulting in increased BP
- Angiotensin II also promotes release of aldosterone from the adrenal cortex
- Aldosterone promotes Na+ reabsorption
- Results in increased ECT osmolality
- Promotes fluid retention (water follow Na+)
- Increases BP by increased blood volume
- The output of urine is also controlled by the hypothalamus, which triggers the posterior pituitary gland to secrete ADH (vasopressin)
- ADH is released when hydration is low and blood solute concentration is high (=BP is low)
- ADH increases water permeability of the distal convoluted tubule and the collecting duct in the kidneys
- ADH therefore promotes water reabsorption/reduce urine output, resulting in raised blood volume and BP
Describe the mechanisms by which the body decreases water volume and thus reduces blood pressure
- Atrial natriuretic factor (ANF) release by atrial myocytes (heart) in response to raised BP
- ANF acts to reduce water and sodium load in the circulation by promoting sodium and water excretion, to return BP to normal
- ANF also promotes blood vessel dilation to reduce BP (more space = less pressure against the blood volume)
Alcohol inhibits the action of ADH
Detail how water is compartmentalised in the body
ICF = fluid found inside cells (63% of fluid 25L) ECF = interstitial (fluid between cells 11L) and intravascular (blood and lymph 4L) 37%
Based on a 73kg adult. Distribution changes with growing and ageing
What are electrolytes? Which electrolytes are found in the ECF and in the ICF?
ICF major cations = potassium (K+), Mg2+
ICF major anions = phosphate, sulphate (SO4-)
ECF major cations = Na+, Ca2+
ECF major anions = Chloride (Cl-), bicarbonate (HCO3-)
What are effects of dehydration and hyper-hydration on the body?
Body’s response to dehydration involves ADH, RAS, ALD pathways
Symptoms include thirst, flushed skin, nausea, dizziness, muscle cramps, failing kidney function
Body’s response to hyper-hydration includes ANF
Concentration of electrolytes is diluted, especially sodium => hyponatremia => water from diluted blood is pulled into cells by osmosis
Diluted blood and swollen cells can cause headache, blurred vision, muscle cramps
What is the NRV for water?
No RDI but an AI = 15 cups (male), 11 cups (female) per day (1 cup = 250mL)
AI based on water coming 80% from fluid and 20% from food
What is the difference between osmolarity and osmolality?
Osmoles = moles of solute that contribute to osmotic pressure Osmolality = the osmoles of solute per kg of solvent Osmolarity = the concentration of a solution as total number of solute particles per litre of solution
Osmotic pressure = the amount of force needed to prevent dilution of the compartment containing the higher particle concentration
Osmosis = passive diffusion of water across a semi-permeable membrane from areas of low solute concentration to high solute concentration
Blood proteins confer higher osmotic pressure on plasma as compared to interstitial fluid (remember how low circulating blood protein resulting in peripheral oedema)
